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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
C-series Clinac® Accelerator System Basics
Revision AF: September 2005 Copyright © 2005 Varian Medical Systems C-series Clinac® Accelerator System Basics
i
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
WARNING! Do not attempt to operate or repair the Clinac using the descriptive material in this Course Manual. Refer only to the specific operation and maintenance manuals supplied with your equipment. Uninformed or careless operation of the machine can expose the operator, patient and other persons to hazards that can cause serious injury or death. Clinacs used for training purposes are specially configured for that purpose. Specifically, protective screens and covers have been removed from some areas where dangerous voltages may be present, and safety interlocks may be disabled. Also, 3-phase primary power is present in the Power Distribution Chassis even when the Clinac is in the Emergency Off state. Do not reach into the Power Distribution Chassis without first turning off the main circuit breaker on the wall, and do not reach into any area of the Clinac until your instructor has told you that it is safe to do so.
Compiled and Edited by: Bill Kirkness Varian Medical Systems Oncology Systems Customer Support Education Department
ii
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Statement of Objectives C-series Clinac® Maintenance Course This course is provided for those personnel directly responsible for the maintenance of the Clinac. It will cover Modulator and RF System theory, beam generation and control, motion control, dosimetry, position readouts, power distribution, accessories, computer control system, diagnostics, interlocks and troubleshooting. This material is for Training Purposes Only. For on-site operation and/or maintenance procedures, refer to the specific operation and maintenance manuals supplied with your equipment.
Statement of Policy This material is the exclusive property of Varian Associates and is loaned subject to return upon demand and with the express condition that information contained herein not generally known in the trade shall be treated confidentially and shall not be reproduced, redistributed or used in any manner, directly or indirectly, detrimental to Varian’s interests. This material must be returned to Varian Medical Systems immediately upon demand at any time.This material is registered to:
Name: ________________________ Date: _________________ C-series Clinac® Accelerator System Basics
iii
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
East
Ed. Dept.
Education Department
Ed. Dept. Northern Region Service
Conf. Room
Clinac 600C/D Lab
bc
Book Room
Vision Lab
Clinac 23EX Lab
Classroom
Classroom
2
3
Clinac 2100C Lab
Classroom 1 Admin. Offices
1b
5
6
Storage Areas
7
1a
Classroom
Copy Room
Classroom
Cafeteria
Classroom
Electrical Breakers
Area 2
Clinac 2300C/D Lab
Shipping & Receiving
Open 8 am5 pm
Main Conf. Lobby Room
North
Info. Svc’s
Classroom
bc Copy Room
Accounting
b c
4 VAR iS Lab
H.R.
Support Engineering Logistics
Conf. Room
ONCOLOGY SYSTEMS CUSTOMER SUPPORT 596 ALDER DRIVE MILPITAS, CA 95035 (408) 321-9400
iv
Storage Area
S.A.P. Command Center, Help Desk, Facilities
Work Shop
Area 1
West
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Revision History Revision
Date
Description
A
May 1996
Initial Version
B
Feb 1997
General reformat and minor corrections. Added Title Page and Table of Contents to each chapter.
C
Aug 1997
Updated Chapter 1 from new Varian Safety Manual, including Lockout/Tagout and Gantry Pin Procedures.
D
Jul 1999
Added Klystron Theory section (formerly in High Energy Beam Delivery System Manual) to Chapter 5.
E
Jan 2000
Updated Title Page, added Table of Contents to this section, Tables of Illustrations to all chapters with captioned illustrations.
F
Jan 2002
General update. Increased font size for better legibility.
AA
May 2002
Added abbreviation list. Converted from WordPerfect to Adobe FrameMaker resulting in some page and paragraph re-numbering. Added index.
AB
Nov 2003
Minor corrections. Redrew illustrations with CorelDraw.
AC
Aug 2004
General update for 2004.
AD
Jan 2005
General update for 2005. Also reduced footer font size.
AE
May 2005
Updated Chapter 1 from new Clinac Safety Manual. Removed Chapter 2, “Controls & Indicators” and renumbered remaining chapters. Re-formatted and added explanatory text to Chapter 2, “Machine Physics.”
AF
September 2005
General update for consistency with Varian standards affecting notes, cautions and warnings.
C-series Clinac® Accelerator System Basics
v
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Visual Cues
This document uses the following visual cues to help you locate and identify information. This symbol identifies comments about a specific task. Some examples include notes, required data, messages, substitutions and shortcuts.
CAUTION: Describes actions or conditions that can result in minor or moderate injury or can result in damage to equipment.
WARNING:Describes actions or conditions that can result in serious injury or death. Italics: Used for emphasis, defining new terms, or book titles. Bold: Identifies menu commands, items you can select on the screen, and buttons to press.
Abbreviations
vi
The following abbreviations are used throughout this manual: Abbreviation
Meaning
AMC
Advanced Motion Controls, Inc.
BNC
A standard coaxial cable connector developed by the Berkeley Nucleonics Corp.
CPU
Central processing unit (today often used to mean “microprocessor”)
CRT
Cathode-Ray Tube Monitor
EEPROM
Electrically Erasable Programmable Read-only Memory
EPROM
Erasable Programmable Read-only Memory
EMI
Electronic Measurements, Inc.
FPGA
Field Programmable Gate Array
LED
Light-emitting diode
MCU
Microcontroller (a microprocessor especially designed for control system applications)
MPU
Microprocessor (unit)
PCB
Printed Circuit Board (sometimes referred to as a “card”)
PSA
Patient support assembly (Treatment couch)
PWM
Pulse Width Modulation
SCR
Silicon Controlled Rectifier or Thyristor
STD
Pro-Log Corp. Control System Bus: “Simple To Design”
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents Chapter 1: Emergency and Safety ........................................1-1 1. Abstract........................................................................................... 1-5 2. Introduction..................................................................................... 1-7 2.1. Overview .................................................................................. 1-7 2.2. Operators and Treatment Personnel ......................................... 1-7 2.3. Maintenance and Service Personnel.......................................... 1-8 2.4. Customer Support .................................................................... 1-8 2.5. Related Publications ................................................................. 1-9 3. Emergency Procedures..................................................................... 1-9 3.1. Terminating the Treatment Beam ............................................. 1-9 3.2. Performing an Emergency-Off ................................................. 1-10 3.3. Using the Emergency Hand Pendant....................................... 1-10 3.4. Lowering the Treatment Couch in an Emergency .................... 1-11 3.5. X-Ray and Electron Beam Radiation ....................................... 1-13 3.6. Induced Radiation in Accelerator Components........................ 1-14 3.7. Radio Frequency (RF) Radiation.............................................. 1-15 3.8. Electromagnetic Interference .................................................. 1-16 3.9. Sulfur Hexafluoride and Freon 12 Gases ................................ 1-16 3.10. Lead ..................................................................................... 1-17 3.11. Beryllium ............................................................................. 1-18 3.12. Dielectric Insulating Oil ........................................................ 1-18 3.13. Depleted Uranium (Low-Energy Clinacs) ............................... 1-19 3.14. Ozone and Oxides of Nitrogen (High-Energy Clinacs)............. 1-19 3.15. Implosion ............................................................................. 1-19 3.16. Electric Shock ...................................................................... 1-20 3.17. Service Precautions .............................................................. 1-20 3.18. Electrical Fire ....................................................................... 1-21 3.19. Remote Movements............................................................... 1-21 3.20. LaserGuard .......................................................................... 1-21 3.21. On-Board Imager Precautions............................................... 1-22 3.22. High Dose Precautions ......................................................... 1-22
Table of Contents
vii
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.23. Falling Parts or Accessories .................................................. 1-23 3.24. Deterioration of Plastic Parts from Radiation ........................ 1-23 3.25. Patient Fall from the Treatment Couch ................................. 1-23 3.26. Treatment Couch Pinch Points ............................................. 1-24 3.27. High Temperature Surfaces .................................................. 1-24 3.28. Laser Beams ........................................................................ 1-25 3.29. Software Integrity................................................................. 1-25 3.30. Microwave Tube Operating Hazards ..................................... 1-26 4. Service and Maintenance Guidelines.............................................. 1-27 4.1. Clinac Accelerator Specifications ............................................ 1-27 4.2. Lockout/Tagout Procedures ................................................... 1-28 5. Owner Guidelines .......................................................................... 1-48 5.1. Planning Operations .............................................................. 1-48 5.2. Radiation Protection Survey ................................................... 1-48 5.3. Safety and Emergency Training .............................................. 1-49 5.4. Routine Use ........................................................................... 1-49 5.5. Quality Assurance.................................................................. 1-50 5.6. Accidental Radiation Overdose ............................................... 1-50 5.7. Backup Interlocks .................................................................. 1-51 5.8. Emergency Beam Termination................................................ 1-51 5.9. Emergency Plan ..................................................................... 1-51 6. Appendix A: Venting Waveguide Gases .......................................... 1-52 6.1. Testing for Waveguide Arcing ................................................. 1-52 6.2. Prerequisites: Parts Ordering Procedure ................................. 1-52 6.3. Venting a Clinac With a Venting System................................. 1-53 6.4. Venting a Clinac Without a Venting System............................ 1-53 6.5. Retrofitting the Clinac With a New Venting System................. 1-55 6.6. Contents of 872031-01 and 872031-02 Kits ........................... 1-55
Chapter 2: Machine Physics .................................................2-1 1. Introduction .................................................................................... 2-3 2. Definitions:...................................................................................... 2-3
viii
Table of Contents
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3. Kinetic Energy Relationships............................................................ 2-3 4. Rest Energy Relationships ............................................................... 2-4 5. Total Energy Relationships............................................................... 2-4 6. Conversion of Energy to Electron Volts............................................. 2-5 7. Measurement of Energy Change....................................................... 2-6 8. Example of Simple Acceleration ....................................................... 2-8 9. The Standing Wave Accelerator ........................................................ 2-9 10. Impedance Matching.................................................................... 2-11 11. Plotting ........................................................................................ 2-12 12. Accelerator Equivalent Circuit...................................................... 2-13 13. Load Line Considerations............................................................. 2-16 14. Fill Time ...................................................................................... 2-16 15. Injection Timing ........................................................................... 2-17 16. Electron Injection and Bunching .................................................. 2-18 17. Advances in Linear Accelerator Design for Radiotherapy............... 2-19 17.1. Electron Therapy .................................................................. 2-59
Chapter 3: Modulator Theory ...............................................3-1 1. Introduction..................................................................................... 3-5 2. Basic Concepts ................................................................................ 3-5 3. DeQing Principles ............................................................................ 3-7 4. Non-resonant Transmission Line Principles...................................... 3-8 5. Pulse Shape Definitions ................................................................. 3-16 6. Line Type Modulator Load Element Principles ................................ 3-17 7. Fault Conditions ............................................................................ 3-18 8. Thyratron Theory ........................................................................... 3-19 8.1. Introduction ........................................................................... 3-19 8.2. Operating Notes on Hydrogen-filled Tubes .............................. 3-23
Table of Contents
ix
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Chapter 4: RF Theory...........................................................4-1 1. Introduction .................................................................................... 4-5 2. Traveling Waves on Transmission Lines ........................................... 4-5 3. Waveforms ...................................................................................... 4-8 4. Waveguides ................................................................................... 4-11 5. Resonant Circuits.......................................................................... 4-14 6. RF Transmission Theory ................................................................ 4-16 7. RF Waveguide Design .................................................................... 4-17 7.1. Modes .................................................................................... 4-17 7.2. The TE10 Mode ....................................................................... 4-18 7.3. Coupling ................................................................................ 4-18 7.4. Determining the TE10 Dominant Mode of a Waveguide ............ 4-19 8. Transmission Lines ....................................................................... 4-20 8.1. VSWR .................................................................................... 4-21 9. Vector Analysis – 3dB Quadrature Hybrid...................................... 4-21 10. Circulators .................................................................................. 4-22 11. Klystron Theory ........................................................................... 4-24 11.1. Theory of Klystron Operation................................................ 4-24 11.2. Associated Equipment.......................................................... 4-34 11.3. Power Supplies..................................................................... 4-35 11.4. Cooling ................................................................................ 4-39 11.5. RF Circuits .......................................................................... 4-45 11.6. Tuning ................................................................................. 4-49 11.7. Noise in Klystron Amplifiers ................................................. 4-51 11.8. Summary ............................................................................. 4-52
Chapter 5: Ion Chamber Theory ...........................................5-1 1. Introduction .................................................................................... 5-3 2. Present Configuration...................................................................... 5-3 3. Efficiency ........................................................................................ 5-5 4. Upper Limit of Dose Range (Saturation) ........................................... 5-6
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Table of Contents
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 5. Applied Electric Field ....................................................................... 5-7 6. Effects of Temperature and Pressure ................................................ 5-7 7. Beam Opacity .................................................................................. 5-8 8. Inverse Square Law.......................................................................... 5-8 9. Insulation Materials ......................................................................... 5-9 9.1. Mica ......................................................................................... 5-9 10. Pulse Shape ................................................................................. 5-10 11. Concluding Remarks.................................................................... 5-11 12. References ................................................................................... 5-12
Chapter 6: Vacuum Theory...................................................6-1 1. Introduction..................................................................................... 6-3 2. The Nature of Vacuum ..................................................................... 6-3 2.1. What Is Vacuum? ..................................................................... 6-3 2.2. What About Pressure? .............................................................. 6-4 2.3. How Is a Vacuum Produced? .................................................... 6-4 2.4. Different Types of Vacuum ....................................................... 6-4 2.5. Where Is Vacuum Used?........................................................... 6-5 2.6. Why Is Vacuum Needed? .......................................................... 6-5 3. Temperature .................................................................................... 6-6 4. Pressure .......................................................................................... 6-7 4.1. What is Gas? ............................................................................ 6-7 4.2. Atmospheric Pressure............................................................... 6-7 4.3. Pressure Measurement ............................................................. 6-8 4.4. Partial Pressure ........................................................................ 6-9 4.5. Vapor Pressure....................................................................... 6-10 4.6. Effects of Pressure.................................................................. 6-12 4.7. Pressure Ranges..................................................................... 6-12 5. Gas Particles.................................................................................. 6-13 6. Gas Laws ....................................................................................... 6-13 6.1. Avogadro’s Law....................................................................... 6-13
Table of Contents
xi
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 6.2. Boyle’s Law ............................................................................ 6-14 6.3. Gas Expansion....................................................................... 6-14 6.4. Charles’ Law .......................................................................... 6-15 6.5. Gay-Lussac’s Law .................................................................. 6-16 6.6. General Gas Law.................................................................... 6-16 7. Gas Flow ....................................................................................... 6-17 7.1. Viscous Flow.......................................................................... 6-17 7.2. Molecular Flow....................................................................... 6-17 7.3. Mean Free Path...................................................................... 6-18 8. Conductance ................................................................................. 6-18 8.1. Conductance in Viscous Flow................................................. 6-19 8.2. Conductance in Molecular Flow ............................................. 6-20 9. Review of the Nature of Gases........................................................ 6-20 10. Ion Pump .................................................................................... 6-21 10.1. Components......................................................................... 6-22 10.2. How the Pump Works........................................................... 6-22 10.3. Vacuum System Use ............................................................ 6-26 10.4. Summary ............................................................................. 6-26 11. Vacuum Gauges .......................................................................... 6-26 11.1. Thermocouple Gauge ........................................................... 6-26
Chapter 7: Glossary..............................................................7-1
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Table of Contents
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter One
Emergency and Safety
The material in this chapter is taken from the Varian Clinac Emergency and Safety Manual, and is included in this chapter for reference to supplement the Safety Lecture material presented during the Maintenance Training Course.
Emergency and Safety
1-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Abstract: .......................................................................................................................... 1-5 2. Introduction:.................................................................................................................... 1-7 2.1. Overview:.................................................................................................................. 1-7 2.2. Operators and Treatment Personnel:......................................................................... 1-7 2.3. Maintenance and Service Personnel: ......................................................................... 1-8 2.4. Customer Support: ................................................................................................... 1-8 2.5. Related Publications: ................................................................................................ 1-9 3. Emergency Procedures: .................................................................................................... 1-9 3.1. Terminating the Treatment Beam:............................................................................. 1-9 3.2. Performing an Emergency-Off: ................................................................................ 1-10 3.3. Using the Emergency Hand Pendant: ...................................................................... 1-10 3.4. Lowering the Treatment Couch in an Emergency: ................................................... 1-11 3.5. X-Ray and Electron Beam Radiation: ...................................................................... 1-13 3.6. Induced Radiation in Accelerator Components: ....................................................... 1-14 3.7. Radio Frequency (RF) Radiation: ............................................................................. 1-15 3.8. Electromagnetic Interference:.................................................................................. 1-16 3.9. Sulfur Hexafluoride and Freon 12 Gases:................................................................ 1-16 3.10. Lead: .................................................................................................................... 1-17 3.11. Beryllium:............................................................................................................. 1-18 3.12. Dielectric Insulating Oil: ....................................................................................... 1-18 3.13. Depleted Uranium (Low-Energy Clinacs): .............................................................. 1-19 3.14. Ozone and Oxides of Nitrogen (High-Energy Clinacs): ............................................ 1-19 3.15. Implosion: ............................................................................................................ 1-19 3.16. Electric Shock: ..................................................................................................... 1-20 3.17. Service Precautions:.............................................................................................. 1-20 3.18. Electrical Fire: ...................................................................................................... 1-21 3.19. Remote Movements:.............................................................................................. 1-21 3.20. LaserGuard: ......................................................................................................... 1-21 3.21. On-Board Imager Precautions: .............................................................................. 1-22 3.22. High Dose Precautions:......................................................................................... 1-22
1-2
Emergency and Safety
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.23. Falling Parts or Accessories: .................................................................................. 1-23 3.24. Deterioration of Plastic Parts from Radiation: ........................................................ 1-23 3.25. Patient Fall from the Treatment Couch: ................................................................. 1-23 3.26. Treatment Couch Pinch Points: ............................................................................. 1-24 3.27. High Temperature Surfaces: .................................................................................. 1-24 3.28. Laser Beams: ........................................................................................................ 1-25 3.29. Software Integrity:................................................................................................. 1-25 3.30. Microwave Tube Operating Hazards: ..................................................................... 1-26 4. Service and Maintenance Guidelines:.............................................................................. 1-27 4.1. Clinac Accelerator Specifications: ............................................................................ 1-27 4.2. Lockout/Tagout Procedures: ................................................................................... 1-27 5. Owner Guidelines: .......................................................................................................... 1-48 5.1. Planning Operations: .............................................................................................. 1-48 5.2. Radiation Protection Survey: ................................................................................... 1-48 5.3. Safety and Emergency Training: .............................................................................. 1-49 5.4. Routine Use: ........................................................................................................... 1-49 5.5. Quality Assurance:.................................................................................................. 1-50 5.6. Accidental Radiation Overdose: ............................................................................... 1-50 5.7. Backup Interlocks: .................................................................................................. 1-51 5.8. Emergency Beam Termination:................................................................................ 1-51 5.9. Emergency Plan: ..................................................................................................... 1-51 6. Appendix A: Venting Waveguide Gases: .......................................................................... 1-52 6.1. Testing for Waveguide Arcing: ................................................................................. 1-52 6.2. Prerequisites: Parts Ordering Procedure: ................................................................. 1-52 6.3. Venting a Clinac With a Venting System:................................................................. 1-53 6.4. Venting a Clinac Without a Venting System:............................................................ 1-53 6.5. Retrofitting the Clinac With a New Venting System:................................................. 1-55 6.6. Contents of 872031-01 and 872031-02 Kits: ........................................................... 1-55
Emergency and Safety
1-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Illustrations Figure 1.1. Console Dedicated Keyboard:............................................................................ 1-10 Figure 1.2. Emergency Pendant:......................................................................................... 1-11 Figure 1.3. Typical Lockout Devices: .................................................................................. 1-29 Figure 1.4. Main Circuit Breaker Lockout/Tagout: ............................................................. 1-30 Figure 1.5. Gantry Locking Pin with Lock and Tag:............................................................. 1-34 Figure 1.6. ETR or Exact Couch with Safety Braces:........................................................... 1-35 Figure 1.7. PSA Couch with Safety Braces: ......................................................................... 1-36 Figure 1.8. Air Hose Removed and Lockout Tag Attached: .................................................. 1-37 Figure 1.9. Handle Removed and Lockout Tag Attached: .................................................... 1-38 Figure 1.10. Water Valve with Handle Removed and Tag Attached: ..................................... 1-40
1-4
Emergency and Safety
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Abstract European Representative
Notice
The Clinac Safety Guide (P/N 1104957-03) provides reference information about safety precautions pertaining to C-Series Clinac accelerators.
Manufacturer:
European Representative:
Varian Medical Systems, Inc. 3100 Hansen Way, Bldg. 4A Palo Alto, CA 94304-1030, US
Varian Medical Systems UK Ltd. Gatwick Road, Crawley West Sussex RH10 9RG United Kingdom
Information in this document is subject to change without notice and does not represent a commitment on the part of Varian. Varian is not liable for errors contained in this document or for incidental or consequential damages in connection with furnishing or use of this material. This document contains proprietary information protected by copyright. No part of this document may be reproduced, translated, or transmitted without the express written permission of Varian Medical Systems, Inc.
FDA 21 CFR 820 Quality System Regulations (CGMPs)
Varian Medical Systems products are designed and manufactured in accordance with the requirements specified within this federal regulation.
ISO 9001 and ISO 13485
Varian Medical Systems products are designed and manufactured in accordance with the requirements specified within ISO 9001 and ISO 13485 quality standards.
CE 0086
Varian Medical Systems products meet the requirements of Council Directive MDD 93/42/EEC.
Trademarks
Clinac, Silhouette, LaserGuard, and VARiS are registered trademarks, and RMS, PortalVision, On-Board Imager, iX, Trilogy, and EDW are trademarks, of Varian Medical Systems, Inc. All other trademarks or registered trademarks are the property of their respective owners. © 1995–2004 Varian Medical Systems, Inc. All rights reserved. Printed in the United States of America.
Emergency and Safety: Abstract
1-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Important Notice This manual includes information that is critical to ensure safe operation, service, and maintenance of Varian Medical Systems medical linear accelerators. This manual is designed to assist properly trained and equipped personnel in safely completing work on the Varian Medical Systems medical linear accelerators. Comprehensive knowledge of the standards of care required for operation, service, and maintenance of the accelerator is needed to make effective use of this manual; the manual is not a substitute for such knowledge. Only properly trained personnel should operate, service, or maintain any Varian Medical Systems medical linear accelerator. Varian is not responsible for injuries or damages due to activities which do not conform to generally accepted standards of care, or which are inconsistent with specific provisions of this manual.
1-6
Emergency and Safety: Abstract
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
2. Introduction
This chapter has been reproduced from the Varian Clinac Safety Manual, P/N 1104957-03, published 12/2004. Within this chapter, the original document words “Manual” and “Chapter” have been changed accordingly.
2.1. Overview
This safety manual provides information about emergency procedures, safety precautions, service and maintenance guidelines, and owner guidelines that apply to the general operation and maintenance of the Clinac radiotherapy accelerator and associated equipment.
2.1.1. Visual Cues
This document uses the following visual cues to help you locate and identify information: Note: This symbol identifies comments about a specific task. Some examples include notes, required data, messages, substitutions, and shortcuts.
CAUTION: Describes actions or conditions that can result in minor or moderate injury or can result in damage to equipment.
WARNING: Describes actions or conditions that can result in serious injury or death. Italics: Used for emphasis, defining new terms, or book titles. Bold: Identifies menu commands, items you can select on the screen, and buttons to press. Preparing Personnel The Clinac is a sophisticated and potentially hazardous piece of equipment. Uninformed or careless operation or service can result in poor performance, equipment damage, serious and possibly fatal injuries. The owner should require the following actions of each person who operates, maintains, or is otherwise associated with the Clinac: Become thoroughly familiar with the material in this manual and the operating instructions detailed in your user documentation. Become thoroughly familiar with and follow the emergency procedures and safety precautions in this manual, as well as warnings and cautions contained in this manual and in the other books in the documentation set. Become thoroughly familiar with and follow the emergency and safety procedures established for local use by the owner.
2.2. Operators and Treatment Personnel
Varian Clinac medical linear accelerators are sophisticated and potentially hazardous pieces of equipment. Unauthorized or careless operation or service can result in poor performance, equipment damage, or serious and possibly fatal injuries. The owner must require the following actions of each person who operates, maintains, or is otherwise associated with the Clinac:
Emergency and Safety: Introduction
1-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
2.3. Maintenance and Service Personnel
!
Learn the contents of this guide and the operating instructions detailed in your user documentation.
!
Follow the emergency procedures, safety precautions, warnings, and cautions in this guide, and in all other related publications.
!
Follow emergency and safety procedures established for local use by the owner.
Maintenance and service procedures are restricted to service personnel who receive the appropriate maintenance training and are authorized by the owner.
WARNING: Authorized service personnel must become thoroughly familiar with and follow lockout/tagout safety procedures established for local use by the owner during all service and maintenance procedures. They are also required to take all precautions necessary to protect themselves, patients, and other persons from injury, and to protect equipment from damage. To ensure safe operation and maintenance conditions for use of any medical linear accelerator, the owner is responsible for establishing emergency and safety procedures. Use this manual, along with the warnings and cautions in the other books in the documentation set as the starting point for formulating local procedures.
2.4. Customer Support
If you cannot find information in this user guide, you can contact Varian in several ways: !
!
Help Desk Support •
North American toll-free support: 1.888.827.4265
•
Global telephone support:1.702.938.4807
•
Global telephone support, Treatment Planning: 1.702.938.4712
To order additional documents •
From North America:1.800.535.5350
• • !
1-8
Globally: 1.702.938.4700
World Wide Web •
!
and press 1 for “Parts” on your touch-tone phone
If you have access to the Internet, point your browser to Oncology Systems: http://www.varian.com and then select Support.
E-mail •
Information Management Systems, Digital Imaging Management Systems, and Delivery Systems: [email protected]
•
Treatment Planning Systems:
•
Brachytherapy Systems:
[email protected] [email protected]
Emergency and Safety: Introduction
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
United States mail Varian Medical Systems, Inc. 3100 Hansen Way, Bldg. 4A Palo Alto, CA 94304-1030, U.S.A.
!
European representative Varian Medical Systems, UK Ltd. Gatwick Road, Crawley West Sussex, RH102RG, England •
2.5. Related Publications
Phone: +44-1293-531-244
The following Varian publications provide further information about the Clinac and related products: !
C-Series Clinac Instructions for Use (P/N 1102903)
!
C-Series Clinac Technical Reference Guide (P/N 1106795)
!
Exact Couch User Guide/Maintenance Manual for Couch and Couch Top (P/N 1104201)
!
C-Series Clinac Custom Coding Guide (P/N 1106292)
!
MLC User Guide (P/N 1101351)
!
MLC Systems and Maintenance Guide (P/N 1101018)
!
Shaper User Guide (P/N 1101352)
!
DMLC Implementation Guide (P/N 1105417)
!
LaserGuard Clinical Reference Guide (P/N 100011555)
!
Service Technical Bulletins (STBs)
!
Customer Technical Bulletins (CTBs)
3. Emergency Procedures
This section describes procedures recommended for use in the event of an emergency. The owner is responsible for adapting these procedures locally and establishing other emergency procedures.
3.1. Terminating the Treatment Beam
To terminate the beam: From the console dedicated keyboard (Figure 1.1), press the BEAM OFF button and turn the DISABLE/ENABLE keyswitch to the DISABLE position. Remove the key and place it in a secure location.
WARNING: In the event of an emergency during operation of the Clinac, terminate the treatment beam immediately. Examples of an emergency situation requiring termination of the treatment beam include the following: ! Clinac fails to properly terminate a treatment ! Accumulated radiation dose displayed on the console monitor exceeds the dose preset for the treatment ! Fire, smoke, or gas fumes are detected ! Power failure
Emergency and Safety: Emergency Procedures
1-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 1.1. Console Dedicated Keyboard
WARNING: If the beam remains on, press the nearest Emergency Off button.
3.2. Performing an EmergencyOff
Pressing an Emergency Off button immediately turns off power to all components except the console video monitor and console computer. These remain powered on. Emergency Off buttons supplied by Varian are located on the dedicated keyboard, on both sides of the treatment couch, next to the auxiliary electronics chassis in the drive stand, and on both sides of the drive stand. Other Emergency Off buttons may be installed by the owner of the facility. For additional information about the location of Emergency Off buttons, see Section 5.1.1 on Page 1-48. To verify that the Emergency Off button has interrupted power, listen for mechanical noises (for example, running motors or fans) in the treatment room. If you hear evidence that power is still on, turn off the main facility circuit breaker.
3.3. Using the Emergency Hand Pendant 1-10
!
If a patient is receiving therapy, remove the patient from the treatment couch as soon as possible.
!
Check the dose counters, and record the cumulative number of monitor units received by the patient up to that time.
!
Do not try to operate the accelerator until service personnel have restored proper operation of the machine, including operation of the emergency-off circuits.
If couch motion controls do not function due to power failure or other emergency, use the emergency pendant to lower the couch for safe removal of a patient (Figure 1.2). You cannot use the emergency pendant to raise the
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 couch. The emergency pendant is located in the drive stand of the Clinac. It is battery-powered and remains functional after: !
Power failure.
!
Emergency Off button has been pressed.
Figure 1.2. Emergency Pendant You can also use the emergency pendant to lower the treatment couch if the couch fails to respond to a vertical motion control. The pendant is not operational unless the emergency pendant switch is enabled. Before using the emergency pendant, you must enable it by pressing an EMERGENCY PENDANT ON/OFF switch. The emergency pendant and emergency pendant on/off switch are located as follows:
3.4. Lowering the Treatment Couch in an Emergency
!
On standard Clinacs, the emergency pendant and EMERGENCY PENDANT switch are mounted in the right side of the drive stand.
!
On Silhouette high-energy models, the emergency pendant and EMERGENCY PENDANT ON/OFF switch is mounted behind the door to the immediate right of the accelerator.
You can use the emergency pendant to change the position of the couch so that the patient can be removed from the treatment room, if other controls do not function. First, enable the emergency pendant: 1. On the console dedicated keyboard, turn the DISABLE/ENABLE keyswitch to DISABLE. 2. Enter the treatment room and tell the patient to remain on the couch. 3. Open the right door of the drive stand and toggle the EMERGENCY PENDANT ON/OFF switch to ON. 4. Remove the emergency pendant from its storage location inside the drive stand and return to the treatment couch with the pendant. Do not disconnect the pendant cable from the drive stand.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Next, move the couch: 1. Check the longitudinal position of the couch to see if the patient can leave safely. 2. If the couch top is in the longitudinal position you want, proceed to step 3. If the longitudinal position of the couch top does not allow the patient to safely leave the couch: A. On the emergency pendant (Figure 1.2), rotate the NORM/OUT/DOWN switch to the OUT position. B. Press and hold down the enable button on top of the emergency pendant. C. Continue to hold down the enable button while you manually move the couch top to the desired position along the longitudinal axis. D. Release the enable button to reengage the longitudinal brake. 3. Check the height of the couch top to see if the patient can leave it safely. If the couch top is at the height you want, proceed to step 4. If the couch top is too high for the patient to leave safely, lower the couch top: A. On the emergency pendant, rotate the NORM/OUT/DOWN switch to the DOWN position. B. Press and hold down the enable button on top of the pendant. While you hold down the enable button, the couch top lowers. C. Release the button when the top reaches the height you want. 4. Help the patient from the couch and treatment room. 5. Rotate the NORM/OUT/DOWN switch to the NORM position and toggle the EMERGENCY PENDANT ON/OFF switch to the off position. 6. Return the emergency pendant to its storage location in the drive stand. 7. Safety Precautions This section identifies significant operating and maintenance hazards associated with the Clinac. This information is provided to help you avoid or control these hazards.
Note: The identification of potential hazards and suggested safety precautions described in this manual are provided to assist you in maintaining a safe workplace. Varian recommends that you refer to applicable federal, state, and other local standards for more information and specific safety requirements. Refer also to “Related Publications” on page 1-9 for additional information.
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3.5. X-Ray and Electron Beam Radiation
3.5.1. Personnel Precautions
3.5.2. DISABLE/ENABLE Key Precautions
3.5.3. Treatment Precautions
3.5.4. Cardiac Pacemaker Precautions
The Clinac can produce a lethal radiation dose in a very short time. Never operate the X-ray or electron beam without adequate X-ray shielding. Radiation exposure can cause serious illness or death, though not instantaneously. Radiation exposure may also cause certain types of cardiac pacemakers to malfunction. !
Post signs warning all persons of the radiation hazard in the area.
!
Permit no person other than the patient in the treatment room when the treatment beam is on.
!
When working on or near the machine, wear radiation monitoring devices approved by the cognizant regulatory agency.
!
Allow only trained, qualified personnel to operate or maintain the machine.
!
Before entering the treatment room, operators and service personnel must turn the DISABLE/ENABLE keyswitch to the DISABLE position. Remove the key and place it in a secure location.
!
When entering the treatment room, block the door so that it cannot close and enable the door interlock while you are inside.
!
Insert the DISABLE/ENABLE key into the keyswitch on the dedicated keyboard and turn it to the ENABLE position only when necessary to enable the beam-on condition. Immediately after the beam terminates, turn the DISABLE/ENABLE keyswitch to the DISABLE position.
!
When powering down or placing the Clinac in standby, put the DISABLE/ENABLE and power keys in a secure key-storage enclosure. Lock the enclosure to prevent unauthorized activation of the machine.
!
For each treatment plan, the physicist or dosimetrist needs to give appropriate attention to the dose correction factors resulting from scatter and attenuation through any object placed between or near the radiation source and the patient. Typical objects include devices such as shadow trays, centerspine attachments, centerspine and side rails of the treatment couch, couch panels, tennis racket panels, wedge and compensating filters, and shadow blocks.
!
Operating personnel should monitor the accumulated radiation dose readout continually during beam-on.
!
Operating personnel should terminate the beam according to the local emergency procedure if the beam fails to terminate on the preset dose.
!
Before you treat a patient wearing a pacemaker, contact the manufacturer of the pacemaker about possible hazards that may pertain to treatment by a linear accelerator.
!
Make arrangements to monitor the operation of a pacemaker worn by anyone in the treatment room or in adjacent rooms.
!
If you suspect EMI is affecting the operation of a pacemaker, stop operating the Clinac immediately.
!
Post warnings of the potential danger to individuals wearing a pacemaker.
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3.5.5. Radiation Overdose
In the event a person is exposed or thought to be exposed to excess radiation, the law in most localities of the United States requires the following steps: !
Immediately notify the appropriate local, state, and federal authorities.
!
Request an investigation by a professional qualified in the detection of radiation.
!
Consult with medical experts in radiation treatment.
For more information, see “Accidental Radiation Overdose” on page 1-50.
3.6. Induced Radiation in Accelerator Components
Copper, iron, lead, and tungsten components can become temporarily radioactive when irradiated by X-rays with energies above 10 MeV. Induction of radioactivity is caused by the photoneutron or (c,n) reaction, which produces neutrons. When the product nucleus is left in an unstable state, it becomes radioactive. The primary radioactive components are those which absorb most of the Xray energy: !
Targets
!
Collimator assemblies
!
Compensating filters
!
Other shielding and structural material surrounding the target
Except for copper, the other materials become only mildly radioactive following irradiation. The greatest concentration of radioactivity occurs in the target (tungsten and copper). Immediately after beam shutdown the target can contain up to 15% of the total activity produced in the accelerator. The target is approximately 20 times more radioactive than the next most active component. Most of the measurable radiation streams out of the primary collimator and jaw openings. The copper energy slits in the magnet system can also become highly radioactive because they come in direct contact with the beam and act as secondary targets. However, radiation from the target is wellshielded and the radiation is collimated like the primary beam; it is easy to avoid unnecessary exposure. Some radioactivity is also induced in the tungsten collimator, the tungsten or iron flattening filter, and the iron and copper in the magnet. Most of this energy is induced in the shielding within 30° from the beam axis. The tungsten collimator and jaw system are also subject to high X-ray flux. However, the relatively short half-life of the principal tungsten isotope (115 days) limits the buildup of radioactivity. Other components, such as the tungsten flattening filter and structural parts, are 20 times less active than copper when exposed to the same X-ray field.
3.6.1. Handling Precautions
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You can remove radioactive target assemblies safely, with minimal exposure, if you follow safe well-planned work practices. Use of three basic principles—time, distance, and shielding—results in doses being As Low As Reasonably Achievable (ALARA). To minimize exposure, follow these guidelines:
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3.6.2. Handling the X-Ray Target
!
Always allow the maximum time possible for radioactive decay to occur. Higher energies produce higher amounts of radioactivity. For example, an 18 MV machine requires a longer “cooling” period than a 15 MV machine.
!
Perform a radiation survey to assess the hazard. Using a dose rate meter, open the jaws and survey the nearest accessible area beneath the collimator.
!
Plastic trays, wedges, and other accessories can become radioactive following high-energy X-ray treatments. Inform personnel of the risk of residual exposure and take appropriate precautions.
!
Survey all potentially radioactive disposable components with a pancake GM survey instrument prior to disposal. Radioactive components require proper handling and disposal techniques.
The X-ray target is handled only during major service or repair. The target can become radioactive depending on the Clinac energy levels and how long it has been since the target was irradiated. A radiation survey meter is the only way to determine if the target is safe to handle. Be sure to use a radiation survey meter to detect radioactivity before handling the target.
3.7. Radio Frequency (RF) Radiation
The microwave power tube in the drive stand or gantry produces high levels of microwave energy that is supplied to the linear accelerator through a waveguide system. Since this energy does not appreciably penetrate metal, the power tube body and the entire RF system are made of metal and designed to attenuate or shield RF radiation. Avoid exposure of personnel to RF radiation and prevent anyone from being in the vicinity of open energized waveguides. Exposure to high levels of RF radiation can result in serious bodily injury, including blindness.
3.7.1. General Precautions
To minimize exposure of personnel to RF radiation: !
All input and output RF connections, waveguides, flanges, and gaskets must be tight to prevent RF leakage.
!
Do not operate the magnetron or Klystron tube unless it is properly attached to an appropriate energy-absorbing load.
!
Cardiac pacemakers may be affected.
!
If you suspect leakage of RF energy, do not attempt to operate the Clinac until service personnel have verified proper operation of the machine.
!
Do not expose any part of your body to an energized RF waveguide system that is open or loosely bolted together, or to the window of an energized magnetron or Klystron.
!
Never look into or expose any part of the body to an open waveguide while the tube is energized.
!
Permit only service personnel with the appropriate training and experience to service or repair the magnetron or Klystron tube, the RF waveguide system, or the energy-absorbing load of a Clinac.
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3.8. Electromagnetic Interference
The low levels of electromagnetic radiation present around the Clinac when the beam is on can cause electromagnetic interference (EMI) with cardiac pacemakers and surrounding hospital equipment. EMI can interfere with the operation of cardiac pacemakers or patient monitoring equipment, possibly causing serious bodily injury or death. EMI from other equipment, such as microwave hyperthermia or diathermy equipment, can interfere with the Clinac integrated dose counters, which could result in incorrect doses to patients.
3.8.1. General Precautions
3.9. Sulfur Hexafluoride and Freon 12 Gases
!
Keep covers, doors, and panels on the Clinac closed during operation.
!
If EMI appears to be interfering with the operation of the Clinac or any equipment in the vicinity, cease operation of the Clinac immediately.
!
To reduce EMI, reinstall all fasteners for covers, screens, and panels. Screws in panels and covers must be correctly and completely installed. Check to make sure that they are tight.
!
For EMI pacemaker precautions, see “Cardiac Pacemaker Precautions” on page 1-13.
Sulfur hexafluoride (SF6) and Freon 12 are colorless, odorless, nontoxic gases used in Clinacs as a dielectric to prevent arcing in the RF waveguide. These gases are stored as a liquid under pressure in a disposable metal container in the drive stand. Freon 12 is present in older model Clinacs only. SF6 is now used in Clinac production. The waveguide is a sealed unit. Exposure to breakdown products should not occur unless waveguide integrity is compromised.
Note: Freon 12 is present in Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
3.9.1. General Precautions WARNING: ! Improper handling of SF6 and Freon 12 gas cylinders can
result in the rapid release of pressurized gas, causing serious and possibly fatal injury.
! Contact with gas or liquefied gas may cause serious injuries, including burns or frostbite. ! Handle gas cylinders with care. A ruptured gas cylinder may become a projectile. !
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Although these gases are nontoxic, they can act as an asphyxiant by displacing oxygen.Avoid an inhalation hazard by venting in accordance with standard venting procedures as described in CTB42, Venting Waveguide Gases.
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3.9.2. Gas Cylinder Storage and Handling
3.9.3. First Aid for Gas Exposure
3.10. Lead
3.10.1. Precautions
3.10.2. Lead Handling and Storage
!
High-voltage arcing in the RF waveguide may cause SF6 and Freon 12 to break down into toxic compounds including hydrochloric and hydrofluoric acids; and chlorine, fluorine, and phosgene gases. Treat used waveguide pieces as though they are contaminated and handle with personal protective equipment (PPE). Always wear eye protection and chemical-resistant gloves. For information about PPE, see “Using Personal Protective Equipment” on page 1-45.
!
Attach valve caps to all stored cylinders to prevent damaging the stem and the possible release of stored energy.
!
Store in a clean, dry, and well-ventilated area where the temperature does not exceed 125° Fahrenheit (52° Celsius).
!
Ensure that all cylinders are secured to the drive stand and that all spares are secured with chains.
!
Handle cylinders with care, avoiding any collisions.
!
Do not refill or reuse damaged or dropped cylinders.
!
For inhalation, immediately remove the person to fresh air and seek medical attention.
!
In case of skin contact, flush with copious amounts of water. Treat for frostbite if necessary.
!
In case of eye contact, immediately flush with copious amounts of water. Seek medical attention.
Lead is present in the shielding, balance weights, and some wedge trays in the Clinac. !
Handling of lead shielding, balanced weights, or wedge trays may generate lead dust that can be inhaled or ingested.
!
Exposure to lead dust can cause adverse health effects such as anemia and gastrointestinal abnormalities.
!
Severe overexposure to lead dust may cause neuromuscular dysfunction, paralysis, and birth defects.
!
Observe proper hygiene and wear a disposable mask to avoid inhalation and ingestion of lead-contaminated dust.
!
Always wear gloves to protect against skin contact when handling lead. The gloves worn for lead handling should not be used for other purposes.
!
When handling lead with leather gloves, wear latex or vinyl gloves underneath the leather gloves. Lead will penetrate leather gloves that have been used repeatedly for lead. Always remove gloves and store them in a designated plastic storage bag before leaving the work area.
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After handling lead or lead-contaminated materials, wash your hands and face thoroughly.
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Do not use compressed gas to clean lead components.
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Do not use water or saturated wet cloths to clean lead. Excess moisture may cause lead to oxidize. However, you can use a moist, lintfree cloth to avoid airborne lead oxide particulates.
!
Follow proper lifting practices when lifting and handling lead.
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3.10.3. First Aid for Lead Exposure
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If lead residue contaminates the skin, wash well with soap and water.
!
If lead is ingested or inhaled, contact a physician immediately for assistance.
3.11. Beryllium
Beryllium is present in the bend-magnet window of all high-energy Clinacs. Normal service and maintenance procedures do not expose you to beryllium compounds. However, removal and manipulation of the window may cause exposure to beryllium particulates.
3.11.1. Precautions
3.11.2. First Aid for Beryllium Exposure
3.12. Dielectric Insulating Oil
!
Beryllium can be absorbed through the skin.
!
It may enter into the body through a cut or puncture, producing hard lesions with central nonhealing areas.
!
Beryllium poisoning may result in serious health problems including neuromuscular dysfunction and paralysis. Beryllium has also been known to cause cancer.
!
Wear disposable gloves to avoid skin contact.
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Wear safety glasses to prevent eye contact.
!
Observe proper personal hygiene after contact with beryllium.
!
If dust enters the eyes, flush with copious amounts of water.
!
If cut with broken glass, treat with first aid. Wash with copious amounts of water.
!
Immediately seek medical attention.
Dielectric insulating oils are present in the pulse transformer, modulator, rectifiers, and capacitors in all Clinacs. !
Dielectric insulating oil is harmful if swallowed. It is also a skin irritant.
!
Dielectric insulating oil may become hazardous over time by breaking down and becoming infused with heavy metals.
!
Dielectric oils can also produce aromatic hydrocarbons and carbon monoxide upon combustion.
3.12.1. Precautions
!
Wear gauntlet-length rubber gloves when handling oils.
!
Failure of an oil-filled component can result in smoke. If smoke is detected, ventilate the area immediately. Excessive exposure to vapors is moderately irritating to the eyes and mucous membranes, though it does not pose a significant health risk.
3.12.2. First Aid for Dielectric Insulating Oil Exposure
!
In the event of eye or skin contact, flush the affected area with copious amounts of water.
!
If ingested, seek medical attention.
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3.13. Depleted Uranium (LowEnergy Clinacs)
Depleted uranium is present in the X-ray head of older low-energy Clinacs. New production machines no longer use depleted uranium. Depleted uranium is a naturally occurring, processed radioactive material, and must be handled with caution. A major portion of the radiation emanates as alpha and beta particles, with only a small fraction being emitted as gamma rays. External radiation levels are well below those generally considered hazardous. Depleted uranium is radioactive and must be handled with caution. Uranium oxide poisoning can result from oxidation of the depleted uranium components, if the protective coating of plated components is damaged.
3.13.1. Precautions
3.14. Ozone and Oxides of Nitrogen (HighEnergy Clinacs)
!
Under no circumstances should anyone attempt to machine, file, drill, scrape, scratch, or break the protective coating (plating) of the depleted uranium components.
!
When handling depleted uranium, prevent scratching the protective coating by wearing heavy leather gloves without metal snaps or rivets. This prevents scratching the protective coating.
!
Some machines use depleted uranium alloyed with 0.75% titanium and do not have the removable contamination problem associated with earlier machines using unalloyed depleted uranium.
The interaction of the Clinac electron or X-ray beam and air produces ozone and oxides of nitrogen, although normally in negligible quantities unless high dose rates and long exposures are experienced. Because of its high radiolytic yield and chemical reactivity, ozone gas is the most toxic of the gases formed from this interaction. Pure ozone is an unstable, faintly bluish gas with a characteristically fresh, penetrating odor. The average person can detect ozone at a concentration approximately equal to the generally accepted threshold limit value of 0.1 ppm. Ozone affects the respiratory system and irritates the eyes and all mucous membranes. High ozone concentrations enhance the reactivity of combustible materials.
3.14.1. Precautions
3.15. Implosion
!
If ozone is detected, shut down the Clinac immediately and vacate the treatment room.
!
Allow sufficient time for normal room ventilation to exhaust the gas before reentering the treatment room.
!
Do not operate the Clinac again until service personnel have checked room ventilation and verified machine operation.
Because of the internal vacuum in the magnetron or Klystron and the linear accelerator, the ceramic and glass windows that separate the vacuum from the waveguide can shatter inward (implode) if struck with a hard object or subjected to mechanical shock. Other components that can implode are the thyratron tubes in the modulator and the cathode-ray tubes (CRTs) in the console video monitors. Flying debris from an implosion could result in bodily injury, including cuts and puncture wounds.
3.15.1. Precautions
When working with or near any part of the Clinac containing a vacuum, personnel should take every precaution to protect their bodies from flying debris produced by a possible implosion.
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3.16. Electric Shock
During operation, the normal voltages in some areas of the Clinac are over 25,000 Vdc. Pressing an Emergency Off button removes power from the gantry, drive stand, and parts of the console. However, voltages up to 230 Vac (380 Vac at 50 Hz) still remain in the power line from the main facility circuit breaker, the console computer, the console video monitor, the console printer, and the power line supplying these components. With the main circuit breaker turned off, locked, and tagged, many highvoltage capacitors in the Clinac continue to pose a potential shock hazard until they are discharged. Several high-voltage capacitors can recharge spontaneously to dangerous levels even after being discharged. Shorting sticks attached to the drive stand, gantry, and the modulator cabinet are designed to provide a convenient and safe means for discharging the highvoltage capacitors. Varian recommends you use shorting sticks to prevent inadvertent lack of ground. The shorting sticks must be left hanging in place to prevent capacitors from recharging. Contact with high-voltage circuits can cause serious injury or death.
3.16.1. General Precautions
3.17. Service Precautions
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!
Keep equipment covers, doors, and panels closed during operation.
!
Do not bypass interlock switches on the Clinac doors and panels.
!
Do not touch any component inside the Clinac unless you know it does not present a shock hazard. The potential for electrical shock remains until each circuit supplying the Clinac is turned off at the facility circuit breaker and high-voltage capacitors are shorted out and remain shorted out.
!
During service procedures, before you touch any high-voltage circuit component, remove power from the circuit and follow appropriate lockout/tagout procedures in accordance with OSHA 29CFR 1910.147 or equivalent local standards.
!
Hire only properly trained and experienced service personnel with a full understanding of electrical hazards and high-voltage safety practices.
!
Ground the circuit with the nearest shorting stick. Keep the stick attached to the circuit while you work on or near it. Use more than one shorting stick to eliminate the possible loss of ground.
!
Lockout/Tagout the system or subsystem that is being serviced.
!
Interlocks that must be bypassed for maintenance purposes must be restored immediately upon completion of the task. Never leave the machine unattended with any interlock bypassed without first posting appropriate warning signs.
!
Permit only service personnel with the appropriate training and experience to remove the protective housing of laser-beam units, accessories, and power supplies. For additional warnings, refer to the documentation supplied by the laser unit manufacturer.
!
Take special care and always turn off electrical current when working in confined areas of the Clinac, for example, in the Silhouette service bays located on either side of the stand. If any electrical components in these areas are not turned off, inadvertent electrical shock could occur.
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Adequate lighting is required in any area where work is performed. If necessary, provide additional lighting. For example, additional lighting is required for pin-to-pin troubleshooting in the Silhouette cabinet.
!
Turn off any electrical equipment before you move it.
3.18. Electrical Fire
Any electrical equipment carries the risk of an electrical fire. An electrical fire in the treatment room or console area could cause severe burns, asphyxiation, and other injuries or death.
3.18.1. Precautions
!
If you see smoke or smell a burning odor, follow the established local emergency procedure for a fire.
!
Never use water to fight electrical fires.
!
Permit only personnel trained in fire-fighting procedures to attempt to put out an electrical fire.
3.19. Remote Movements
The Clinac can be configured to allow couch position corrections and large couch rotation swings, as well as gantry and imager arm motions, from outside the treatment room. Some treatments (for example, stereotactic treatments) may require that the gantry move very close to the treatment couch, which increases the risk of collision. Although the gantry rotates at a maximum speed of only about one revolution per minute, substantial force is required to stop gantry motion because of the inertia of its large mass. In addition, if you have an imaging system (for example, On-Board Imager or PortalVision), extended imager arms intrude into the space near the treatment couch. 4D Console and On-Board Imager (OBI) software applications do not automatically retract the OBI arms. This presents danger of collision. For instructions on configuring motion limits, refer to the Clinac Technical Reference Guide. Use caution to prevent collisions: !
Retract all imager arms (OBI and/or PortalVision) away from the couch when not in use.
!
Always observe remote motions either directly, or using closed-circuit monitors.
!
Perform a dry-run (a pre-treatment execution of all motions prior to first treatment, or after a Plan correction) to check for potential collisions before delivering treatment.
!
Stop motions immediately if you suspect a collision may occur. The fastest way to stop motion is to release the Motion Enable bar.
If a collision occurs between the treatment couch and the gantry, call maintenance to inspect the system for damage. Do not operate the Clinac again until the machine is fully checked by service personnel. Never use the system for patient simulation until proper and safe operability has been verified by maintenance personnel.
3.20. LaserGuard
LaserGuard is an optional collision detection system for the Clinac. LaserGuard uses an invisible laser beam “shield” to detect potential collisions between the patient, treatment couch, and gantry. LaserGuard is a Class 1 laser product, and exposure for any length of time does not cause injury or damage to the eyes.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 LaserGuard does not replace existing safety systems or the vigilance of the user. You must verify all patient and equipment clearances before performing any remote motion sequence. Configuration of LaserGuard by unauthorized personnel can cause a machine malfunction that may result in damage to equipment, bodily injury, or death. Only authorized personnel should configure LaserGuard. Avoid divulging the configuration password to unauthorized individuals. For detailed information, refer to the LaserGuard Clinical Reference Guide.
3.21. On-Board Imager Precautions
The Varian On-Board Imager (OBI) allows you to take kV setup fields with two “arms” that you can rotate into position on either side of the treatment couch. One On-Board Imager arm delivers kV beam and the other arm transmits the image. Take special care to ensure that a collision does not occur between the OnBoard Imager arms and the patient, operator, or surrounding equipment. Such a collision can cause serious and possibly fatal injuries. Collisions between On-Board Imager and the treatment couch or any other fixed object can cause serious damage to both units, and serious or possible fatal injury to anyone caught between them.
3.21.1. Precautions
!
Exercise extreme caution during any motion of On-Board Imager and associated machine axes.
!
Before taking kV setup fields, rotate the On-Board Imager and associated machine axes to ensure that a collision will not occur during kV beam delivery.
!
Keep the patient under continuous observation whenever On-Board Imager and other machine axes are moved. If a collision appears possible, stop motions immediately by pressing Emergency Off.
!
Should the On-Board Imager arms collide with the treatment couch or any other equipment, shut down the Clinac immediately by pressing Emergency Off. Do not operate the Clinac again until the machine is fully checked by service personnel.
For details about On-Board Imager, refer to the On-Board Imager documentation.
3.22. High Dose Precautions
Depending on your Clinac software configuration, the Clinac allows you to deliver high-dose stereotactic fixed and arc X-rays for single surgical treatments and fractionated radiotherapy. It is important that you take appropriate precautions for delivering stereotactic treatments. When delivery high-dose treatments, Varian recommends that you: !
Use appropriate collimation (such as paraffin blocks, or a Varian MLC) to narrow the high-dose treatment field.
!
Employ reliable patient safety and immobilization precautions for stereotactic treatment: •
Immobilize the patient as comfortably as possible, and briefly explain the treatment procedure. This helps the patient to remain motionless on the treatment couch.
•
Remind the patient to stay on the treatment couch during treatment until it is safe to leave. A patient should not leave the treatment couch unless supervised by authorized personnel.
•
As with all treatments, perform a dry-run to check for potential collisions before delivering treatment.
For detailed information about Stereotactic treatments, refer to the C-Series Clinac: Instructions for Use.
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3.23. Falling Parts or Accessories
After prolonged use and its accompanying vibrations, the nuts, bolts, and other fasteners on the MLC or accessory mounts can become loose and fall from the machine. Similarly, wedge trays, shadow blocks, and other accessories can fall from the treatment head, usually as a consequence of improper installation. A part or accessory falling on a patient, operator, or other person could result in serious injury.
3.23.1. Precautions
3.24. Deterioration of Plastic Parts from Radiation
Check all nuts, bolts, and other fasteners for tightness at least once every six months. !
Take care to install accessories properly.
!
Never install or remove accessories when a patient is on the treatment couch. The only exception to this precaution is when an accessory can be installed only after the field light establishes the treatment area. In this case, use extreme caution.
!
Do not exceed the weight capacity of the accessory mount when loading shadow block trays.
Some Clinac parts are made of plastic and can lose strength as a result of age and prolonged exposure to radiation. Among these parts are the treatment couch top panels, tennis racket strings, compensator trays, and shadow block trays. Deterioration of plastic parts on the treatment couch could cause a patient to fall. Deterioration of plastic parts attached to the treatment head could allow an accessory to fall. A patient fall from the treatment couch or an accessory falling on a patient, operator, or other persons could result in severe injuries or death.
3.24.1. Precautions
3.25. Patient Fall from the Treatment Couch
!
Examine treatment couch panels monthly. Replace any panels that are cracked or show any signs of degradation. Regardless of condition, replace all couch panels after every 1000 hours of beam operation or 5 years of use, whichever comes first.
!
Inspect the strings on the tennis-racket-type panels periodically and replace upon any sign of degradation.
!
Examine all plastic parts periodically. Immediately replace any part that is cracked, discolored, or shows any signs of degradation.
Any of the following conditions could cause a patient to fall from the treatment couch: !
Collision with the gantry
!
Improper positioning or restraint of the patient
!
Top panels weakened from radiation
!
Degraded strings in the tennis-racket-type panels
!
Sudden movements or stops of the treatment couch
!
Overloading of the couch top
A patient fall from the treatment couch could result in severe injuries or death.
3.25.1. Precautions
!
Lock the couch top into position before you place the patient on it.
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3.25.2. Gantry Precautions
3.26. Treatment Couch Pinch Points
!
If you must rotate the couch top, do so before placing the patient on the couch. After rotation, make sure that the top is latched securely. Never apply weight to a couch top that is unlatched.
!
Position the patient carefully on the treatment couch. Distribute the patient's body uniformly over the couch top and secure with the restraining straps provided. For ETR and Exact Couches, the maximum safe patient weight is 400 lb. Other treatment couch weight limits may be less; for maximum patient weight, refer to the documentation for your treatment couch.
!
When the couch top is fully extended, do not exceed the maximum weight of 300 lb. allowed on the end of the couch top. Do not allow anyone to stand or sit on the end of a fully extended couch top.
!
Avoid sudden movements or stops when adjusting the treatment couch. Do not activate couch motions when brakes are released on the couch top.
!
Always keep the patient in sight when rotating the gantry. Stop the rotation immediately if a collision appears possible.
!
During patient setup, rotate the gantry from the hand pendant. Never rotate the gantry from the console.
!
Before beginning arc therapy, always perform a dry run and rotate the gantry through its entire arc to make sure that it cannot collide with the patient or equipment.
!
Always keep the patient in sight when performing setup motions. For remote motions, be sure that the camera angles provide adequate views of the patient.
The lateral and longitudinal carriages and the couch top contain potential pinch points during movement, particularly between the underside of the couch top and the lateral carriage. Additional pinch points are exposed when the protective bellows surrounding the lift mechanism must be lowered during maintenance procedures. The couch carriages, couch top, and lift mechanism contain numerous pinch points that could cause severe injury or death.
3.26.1. General Precautions
When moving the couch's lateral and longitudinal carriages or the couch top, make sure that all parts of your body are clear of the couch and that no person or equipment can be caught in the moving mechanism.
3.26.2. Service Precautions
If the bellows must be lowered for maintenance, install the safety braces and remove all electrical power to the treatment couch before working in or around the exposed lift mechanism. Follow appropriate lockout/tagout procedures. Be sure to keep hands and other body parts clear of potential pinch points.
3.27. High Temperature Surfaces
Many Clinac components operate at high temperatures, including the thyratron tubes in the modulator compartment of the drive stand, the quartz-halogen lamps in the treatment head and their accessories, and the transistor heat sinks in the power supplies. Body contact with the surface of a component operating at a high temperature can cause severe skin burns.
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3.27.1. Precautions
3.28. Laser Beams
!
Take care to prevent body contact with lamps, heat sinks, thyratron tubes, or any other component that provides a sensation of warmth upon approach.
!
Tubes or porcelain components can be damaged by the oils and moisture from your hand. When handling tubes or porcelain components, wear gloves to prevent direct contact with skin.
!
If you must handle a heat-producing component, allow a reasonable cool-down period after turning off the power.
!
Follow appropriate lockout/tagout procedures.
The optional localizer system uses up to five Class II laser beams to visually locate the Clinac isocenter and to aid in setting up the patient. Each of these beams is visible to the eye as a narrow line of bright red or green light.
WARNING: The radiation from one of these localizer laser beams can cause permanent retinal damage. The optional LaserGuard collision detection system uses an invisible laser beam ‘shield’ to detect potential collisions between the gantry head and the patient. Note: LaserGuard is a Class 1 laser product. Exposure for any length of time does not cause injury or damage to the eyes. For more information about LaserGuard, see “LaserGuard” on page 1-21.
3.28.1. Precautions
3.29. Software Integrity
!
Never stare directly into one of the visible green or red laser beams from the localizer system. Advise patients of this precaution. The invisible LaserGuard laser beams are safe and do not cause retinal damage.
!
For additional warnings, refer to the documentation supplied by the laser unit manufacturer.
Software and computer equipment included with the Clinac are installed by Varian, and developed and tested exclusively for operation of the Clinac system. This software remains the property of Varian and is licensed to the user strictly for the purpose intended. Modifying any of the software provided with the Clinac or using any other software on the Clinac computer can seriously compromise the integrity of data stored on the Clinac computer and can result in uncertain, unreliable, and potentially hazardous system operation. Any attempt to modify the Clinac computer, its operating system, or the Clinac software, or to operate non-Clinac software on the Clinac computer is considered by Varian to be a product alteration. This results in termination of the remainder of the warranty on a Clinac system. To protect the hospital, its patients, and Varian from the potential consequences of unauthorized modifications, software that has been altered shall be removed by Varian.
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3.29.1. Precautions
3.30. Microwave Tube Operating Hazards
!
Do not modify the software supplied with the Clinac, including the operating system and files placed on the fixed disk.
!
Operation of the Clinac computer is restricted to Clinac software provided by Varian. No other software is allowed.
!
Do not alter the configuration of the Clinac computer installed by Varian personnel, including interior printed-circuit boards, peripherals, and switch settings.
Serious hazards exist in the operation of microwave tubes. Safe operating practices require careful attention to hazards associated with microwave tubes. Persons who work with microwave tubes or equipment that uses them must protect themselves against serious injury. Careless operation of microwave tubes can result in damage to tubes, the linear accelerator, or other property; and potentially fatal injury. The operation of microwave tubes involves one or more of the following hazards:
3.30.1. Precautions
Microwave tubes in the Clinac operate at voltages high enough to cause fatal injury by electrical shock. !
Always break the primary circuits of the power supply and discharge high-voltage circuits when direct access to the tube is required. Follow appropriate lockout/tagout procedures.
!
Keep a safe distance from the voltages encountered.
!
Use grounded safety screens during tube operation.
3.30.2. Radio Frequency (RF) Radiation
Refer to “Radio Frequency (RF) Radiation” on page 1-15.
3.30.3. X-Ray Radiation
Never operate high-voltage tubes without adequate X-ray shielding in place.
3.30.4. Corrosive and Toxic Compounds
External output waveguides, cathodes, and high-voltage bushings of microwave tubes are sometimes operated in systems that use dielectric gas to impede microwave or high-voltage breakdown. Release of dielectric gas in the presence of moisture and arcing can create toxic and corrosive compounds. To prevent damage from dielectric gas breakdown:
3.30.5. Hot Water
!
When a leak in the dielectric system is detected or the waveguide is disassembled for service, ventilate the area and avoid breathing any fumes or touching any liquids that develop.
!
Take precautions for highly toxic and corrosive substances before permitting personnel to perform any work on or near the tube.
Extreme heat occurs in the electron collector portion of microwave tubes during operation. Water channels used for cooling reach temperatures as high as the boiling point of water (100° Celsius or above), and the hot water is under pressure (typically as high as 100 psi). A rupture of the water channel or contact with hot portions of the tube can scald or burn.
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3.30.6. Hot Surfaces
Portions of microwave and thyratron tube surfaces can reach extremely high temperatures, especially the cathode insulator and cathode/heater surfaces. All heated surfaces can remain hot for an extended time after the tube is shut off. To prevent serious burns, take care to avoid any bodily contact with these surfaces, both during tube operation and for a reasonable cool-down period after operation.
4. Service and Maintenance Guidelines
This section provides service and maintenance safety guidelines for the Clinac and related systems.
Note: Lockout/tagout procedures are required for servicing or maintaining the Clinac. Refer to OSHA regulation 29 CFR 1910.147, The Control of Hazardous Energy (Lockout/Tagout) and 29CFR1910.333, Selection and Use of Work Practices and other applicable state and federal occupational safety and health regulations (domestic US), or other local (or international) standards for additional information and requirements.
4.1. Clinac Accelerator Specifications
The following specifications must be observed for safe Clinac operation.
4.1.1. Electrical Specifications
Electrical operating specifications: !
Type of protection against electric shock: Class I
!
Degree of protection against electric shock: Type B
!
Operation: The Clinac is classified as being suitable for continuous connection to the supply main in the standby state and for specified permissible loadings.
!
The Clinac is not for use in the presence of flammable anesthetic mixtures.
!
Electrical requirements: •
Low Energy Clinac Series input voltage: 200 to 240 Vac 50 or 60 Hz 42 Amps max @208V; or 360 to 440 Vac 50 or 60 Hz 22 Amps max @ 400V.
•
High Energy Clinac Series input voltage: 200 to 240 Vac 50 or 60 Hz 125 Amps max @ 208V; or 360 to 440 Vac 50 or 60 Hz 65 Amps max @ 400V.
4.1.2. Environmental Specifications
Environmental operating requirements:
4.2. Lockout/Tagout Procedures
Lockout/tagout is required when servicing or maintaining the Clinac or a Clinac subsystem if unexpected startup or release of stored energy could cause injury to personnel. All sources of hazardous energy to which personnel could potentially be exposed, including electrical, mechanical, and
!
Humidity range: 15% to 80% relative humidity, non-condensing
!
Temperature range: 60°F to 80°F (16°C to 27°C)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 potential energy due to gravity or pressure, must be shut down and secured by authorized personnel. Depending upon the work being performed, it may be necessary to use multiple procedures to isolate all energy sources. The rules and standards established in lockout/tagout procedures apply to everyone associated with the maintenance and operation of the Clinac. Failure to follow proper lockout/tagout procedures can result in serious and possibly fatal injuries. The lockout/tagout procedures in this section are organized according to the type of energy source identified:
4.2.1. General Lockout/Tagout Procedure
!
Main Electrical Power
!
Electrical Power to Subsystems
!
Electrical Power to Facility Subsystems
!
Gantry Locking Pin
!
Patient Couch
!
Air System
!
Gas System
!
Water System
1.
The general procedure for lockout/tagout is as follows:
2.
Notify affected personnel that a lockout procedure is about to begin and that the Clinac subsystem or facility power will be shut down for service.
3.
Locate all energy sources associated with the system or subsystem.
4.
Operate the energy-isolating devices necessary to isolate the system or subsystem.
5.
Attach a lockout/tagout device to each energy-isolating device involved. Typical lockout devices are shown in Figure 1.3.
6.
Dissipate any stored energy.
7.
Verify that the system or subsystem is isolated and deenergized.
WARNING: Do not remove, bypass, or ignore lockout/ tagout devices (Figure 1.3). Only the authorized service person installing the lockout/tagout devices is permitted to remove them. Unauthorized removal can result in damage to equipment, and injury or death to patients or personnel.
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Figure 1.3. Typical Lockout Devices
4.2.2. Releasing Lockouts
4.2.2.1. Releasing Lockout During Service
This manual includes specific details of lockout/tagout procedures in the section for each lockout/tagout procedure. General steps for releasing a lockout are as follows: 1.
Prepare the system or subsystem for operation. This includes inspecting the system or subsystem to verify that all equipment components are fully assembled and operational, that all safety panels are in place, and that all tools and other nonessential items have been retrieved.
2.
Ensure that personnel are positioned safely.
3.
Remove lockout devices and tags.
4.
Notify affected personnel that the system or subsystem has been released from lockout.
A lockout/tagout procedure is not required if service and maintenance personnel will not be exposed to the unexpected release of hazardous energy. Some service or maintenance functions require equipment, or portions of the equipment, to be turned on. Under these circumstances, the work must be conducted with appropriate safeguards to prevent exposure to hazardous energy sources. Sometimes it may be necessary to remove lockout/tagout devices temporarily and turn on the machine or equipment for a limited time for testing or positioning of equipment or components. Under such circumstances, the work must be completed with appropriate safeguards to prevent exposure to hazardous energy sources. As soon as possible, once the testing is done, the authorized service personnel must again turn off the equipment and resume lockout/tagout procedures.
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4.2.3. Main Electrical Power
When service is performed that requires removing main power to the Clinac and where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout the main electrical power to the Clinac.
WARNING: The activation of main electrical power to the Clinac while someone is servicing the Clinac can result in serious and possibly fatal injury.
4.2.3.1. Lockout/Tagout Main Power
To lockout/tagout the main power: 1.
Notify affected personnel in the area that main electrical power to the Clinac will be under lockout/tagout.
2.
Place the Clinac in standby mode.
3.
Turn off the main circuit breaker, switchbox disconnect, or other energy isolating device that provides power to the Clinac.
4.
Attach an appropriate lockout/tagout device and tag to the energyisolating device (Figure 1.4).
5.
Verify that power has been removed.
Figure 1.4. Main Circuit Breaker Lockout/Tagout
4.2.3.2. Restoring Main Electrical Power
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To restore main electrical power: 1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
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4.2.3.3. Electrical Power to Subsystems
3.
Remove lockout devices and tags.
4.
Turn on the main power.
5.
Notify affected personnel that service has been completed and that the Clinac is ready for startup.
When service is performed on a subsystem within the Clinac, where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout the electrical power to the system at the applicable energy source.
WARNING: The activation of electrical power to a subsystem while someone is servicing the subsystem may result in serious and possibly fatal injury. To lockout/tagout the power to a subsystem: 1.
Notify affected personnel in the area that main electrical power to the Clinac will be under lockout/tagout.
2.
Identify and locate the source of electrical power to the individual subsystem to be worked on.
Note: For detailed information, refer to the electrical schematic diagrams in your product data book or service/systems manual.
4.2.4. Power to Facility Subsystems
3.
Turn off power to the subsystem by turning off the appropriate energy-isolating device.
4.
Attach appropriate lockout/tagout devices to the appropriate energy-isolating devices.
5.
Use shorting sticks to dissipate any remaining energy (see “Using Shorting Sticks” on page 1-41).
When service is performed on facility subsystems, where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout electrical power to the subsystem at the applicable energy source. Typical facility subsystems include but are not limited to: !
Setup lights
!
Laser positioning lights
!
Closed-circuit television
!
In-room monitors
WARNING: The activation of electrical power to a facility subsystem while someone is servicing the Clinac or subsystem may result in serious and possibly fatal injury.
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4.2.4.1. Cordand-Plug Subsystems
4.2.4.1.1. Cordand-Plug Subsystem Precautions
Lockout/tagout procedures do not apply to cord-and-plug subsystems, as long as the cord to the subsystem is pulled and appropriate safety precautions taken. Typical cord-and-plug subsystems include: 1.
Clinac control console and display.
2.
Multileaf collimator (MLC) console and display.
3.
PortalVision console and display.
4.
VARiS console and display.
5.
Respiratory Gating, LaserGuard, and other additions to the Clinac.
When servicing cord-and-plug subsystems, the following safety precautions apply: !
Ensure that the electrical subsystem is unplugged.
!
Plug must remain in the exclusive control of the person performing service or maintenance work.
!
If the plug is not under exclusive control, lock it out (with a plug cap) and tag it.
To lockout/tagout the cord-and-plug subsystems: 1.
Notify affected personnel that electrical power to the facility subsystem will be under lockout/tagout.
2.
Identify and locate the source of electrical power to the facility subsystem to be worked on.
3.
Turn off power to the subsystem by turning off the appropriate energy-isolating device.
4.
Attach appropriate lockout devices and tags to appropriate energyisolating devices.
5.
Verify that power has been removed by measuring the absence of voltage at the applicable subsystem.
To release the lockout on cord-and-plug subsystems:
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1.
Check the Clinac and facility subsystem to ensure that nonessential items (such as tools or service equipment) have been removed and that the Clinac, Clinac subsystem, or facility is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove lockout devices and tags.
4.
Turn on power to the facility subsystem.
5.
Verify proper operation of the subsystem.
6.
Notify personnel that service is complete and that the subsystem is ready for startup.
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4.2.5. Gantry
You can prevent an unexpected gantry rotation during service by locking out and tagging the gantry locking pin. Lockout/tagout is always required when servicing the gantry drive system (for example, gantry motors, drive chain, harmonic drive, and clutch). With the gantry locking pin disengaged and the drive system components intact, minor imbalances to the gantry due to the removal of fiberglass or other components may result in slow gantry rotation. The speed of rotation is directly related to the amount of the imbalance. If drive system components are not intact (for example, if the drive chain is removed), gantry imbalances may result in a more rapid rotation.
WARNING: A collision between the gantry and service personnel could cause serious and possibly fatal injuries. A collision between the gantry and the treatment couch or any other fixed object could cause serious damage to both units, and serious or possible fatal injury to anyone caught between them.
To lockout/tagout the gantry locking pin: 1.
Notify affected personnel that the gantry will be under lockout/tagout.
2.
Ensure that the gantry rotation axis is unobstructed.
3.
Rotate the gantry to one of the four locking positions (at 0, 90, 180, and 270°). Choose the angle that provides the most convenient access for the maintenance or service to be performed.
4.
Engage the locking pin.
5.
Visually confirm that the gantry locking pin is engaged. While staying outside the arc of the gantry rotation, verify that the gantry is locked in place by moving or rocking the gantry by hand, or by operating the hand pendant.
Note: On low-energy Clinacs, the gantry locking pin can be seen protruding approximately 1/4 in. to 1/2 in. past the inside surface of the gantry sprocket hub, when engaged. 6.
Attach appropriate lockout devices and tags to the gantry locking pin (Figure 1.5).
To release the gantry lockout, see “Releasing Lockouts” on page 1-29.
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Figure 1.5. Gantry Locking Pin with Lock and Tag
4.2.6. Treatment Couch Lift
There are three types of Clinac treatment couches: !
PSA couch
!
ETR couch
!
Exact couch
When service is performed on the treatment couch, where unexpected energization, startup or release of stored energy could cause injury to personnel, block the couch with safety braces and tag it.
WARNING: Unexpected movement of the patient couch can cause serious and possibly fatal injury.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 To lockout/tagout the treatment couch: 1.
Notify personnel that the treatment couch will be blocked and tagged.
2.
For the PSA couch, raise the couch skirting. For the Exact or ETR couch, lower the bellows.
3.
Raise the couch sufficiently to allow insertion of the braces.
4.
Insert the couch safety braces (Figure 1.6 and Figure 1.7).
5.
Lower the couch until it is resting against the safety braces.
6.
Attach lockout tags to the rear of the couch.
Figure 1.6. ETR or Exact Couch with Safety Braces
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Figure 1.7. PSA Couch with Safety Braces To release the lockout on the treatment couch:
4.2.7. Air System
1.
Check the Clinac and the area around the machine to ensure that nonessential items have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the couch safety braces and lockout tags.
4.
Notify affected personnel that service is complete and the patient couch is ready for use.
When service is performed on the compressed air system (high-energy Clinac models only), where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the air system.
CAUTION: The release of air pressure while someone is servicing the air system can result in injury.
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To lockout/tagout the compressed air system:
2.
Notify affected personnel that the air system will be under lockout/tagout.
3.
Turn off the air flowing to the machine.
4.
Remove the valve handle or air hose (if present).
5.
Attach an appropriate tag to the valve (Figure 1.8).
6.
Release air pressure in the line.
7.
Confirm that no air is flowing to the machine by checking that the pressure gauge reads zero.
Figure 1.8. Air Hose Removed and Lockout Tag Attached To remove the lockout: 1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout tag.
4.
Replace the valve handle or air hose.
5.
Open the valve.
6.
Verify that there are no air leaks.
7.
Notify personnel that service is complete and that the system is ready for use.
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4.2.8. Gas System
When service is performed on the SF6 or Freon 12 gas system, where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the gas system.
CAUTION: The release of gas pressure while someone is servicing the gas system can result in injury. Varian recommends the use of an electronic “sniffer” device to detect gas leaks. To lockout/tagout the gas system: 1.
Notify affected personnel that the gas system will be under lockout/tagout.
2.
Turn off the gas flowing to the machine.
3.
Remove the valve handle (Figure 1.9).
4.
Attach an appropriate lockout tag to the valve.
5.
Release any stored gas pressure or other energy in the line.
6.
Confirm that there is no gas flowing to the machine.
Figure 1.9. Handle Removed and Lockout Tag Attached
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 To remove the lockout:
4.2.9. Water System
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout tag.
4.
Replace the valve handle.
5.
Open the valve.
6.
Verify that there are no gas leaks.
7.
Notify personnel that service is complete and that the system is ready for use.
When service is performed on the water system, where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the water system.
WARNING: Release of water and water pressure on exposed electrical components while someone is servicing the water system may result in serious and possibly fatal injury.
To lockout/tagout the water system: 1.
Notify affected personnel that the water system will be under lockout/tagout.
2.
Turn off the water flowing to the machine.
3.
Turn off the water pump.
4.
Lockout/tagout the appropriate pump circuit breakers.
5.
Confirm that there is no water flowing to the machine.
6.
Remove the valve handle.
7.
Attach an appropriate lockout tag to the valve (Figure 1.10).
8.
When opening water system lines, use caution to prevent water from dripping onto, or spraying into, electronic components.
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Figure 1.10. Water Valve with Handle Removed and Tag Attached To remove the lockout:
4.2.10. Shift Changes
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout devices and tags.
4.
Replace the valve handle, if applicable.
5.
Open the valve, replace the locking pin, or otherwise restore equipment to normal operating conditions.
6.
If applicable, restore power to the system. For example, turn on the appropriate pump circuit breaker to restart the water pump.
7.
Verify that there are no leaks or other unexpected hazards.
8.
Notify affected personnel that service is complete and that the system is ready for use.
If maintenance on a Clinac extends beyond one shift, or if there is a personnel change, all additional personnel are required to place their locks on the lockout device before they begin work on the equipment. If maintenance on a Clinac requires multiple service personnel, all are required to place their locks on the lockout device.
4.2.11. Working with Contractors 1-40
When outside personnel (contractors or others) will be working on the Clinac, the on-site employer or owner and the outside contractors must inform each other of the lockout/tagout procedures they will use.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Authorized personnel responsible for performing the lockout must ensure that any affected personnel understand and comply with appropriate lockout procedures in accordance with this manual and applicable regulations. Coordinate lockout activities with all involved parties.
4.2.12. Using Shorting Sticks
When working on the modulator, gun deck, or VacIon subsystems, use shorting sticks to dissipate any residual stored energy, and leave the shorting sticks in place to prevent the reaccumulation of charge. Note: Varian recommends that you use more than one shorting stick to prevent the inadvertent loss of ground. Verify that power has been removed by measuring the absence of voltage at the subsystem. To use shorting sticks:
4.2.13. Chemical Hazards
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the Clinac, Clinac subsystem, or facility is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the shorting sticks (if in use), lockout devices, and tags.
4.
Turn on power to the subsystem.
5.
Verify proper operation of the subsystem.
The Clinac contains substances that can be hazardous to your health. Information located on container labels and material safety data sheets (MSDSs) are provided for your protection and safety. Always read the information contained on labels and MSDSs provided by the manufacturer of hazardous substances. Failure to understand and identify the presence of hazardous substances in the Clinac and its accessory products can result in serious injury. This information is provided as general guidance only. Refer to OSHA regulation 29 CFR 1910.1000 and other applicable state and federal occupational safety and health regulations (domestic US), or other local (or international) standards for additional information and requirements. The guidelines presented in the next section, “Communication Guidelines for Hazardous Chemicals,” apply to all Clinac service personnel.
4.2.13.1. Communication Guidelines for Hazardous Chemicals
The owner must develop and maintain a written hazard communication program. This program must accomplish the following: !
Inform employees about hazard-communication standards.
!
Explain implementation of the program as it applies to their specific workplace.
!
Inform and train employees how to recognize, understand, and use the labels and MSDSs provided by vendors or otherwise provided with the Clinac.
!
Inform and train employees about safety procedures for working with hazardous substances.
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4.2.13.2. Employee Responsibilities
Operators and maintenance and service personnel who come into contact with the Clinac are required to read the labels and the MSDSs provided with the Clinac. They are also required to follow pertinent instructions and warnings.
4.2.14. Material Safety Data Sheets (MSDSs)
Material safety data sheets provided with your Clinac contain the following information:
4.2.14.1. Container Labels
4.2.14.2. Hazardous Substances in the Clinac
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!
Identification of the substance, including the manufacturer’s name, address, and emergency phone number
!
Hazardous components, including the chemical ID, common names, and worker exposure limits
!
Physical and chemical characteristics, including boiling point, vapor pressure, vapor density, melting point, evaporation rate, water solubility, appearance, and odor under normal conditions
!
Physical hazards, including safe handling methods
!
Reactivity, indicating whether the substance is stable or not
!
Health hazards, including information on: how the chemical could enter the body, for instance through inhalation, skin contact, or by swallowing; whether or not the chemical is considered a carcinogen; signs and symptoms of exposure; and existing medical conditions that could be aggravated by exposure
!
Precautions for safe handling and use, including information on: what to do if the substance spills or leaks, disposal of the substance, equipment and procedures needed for cleaning up spills and leaks, how to handle the substance properly, storage, and any other precautions
Containers of hazardous chemicals are labeled with the following information: !
Chemical name
!
Manufacturer or importer information, including name, address, and emergency contacts
!
Physical hazards
!
Health hazards
!
Personal protective clothing, equipment, and procedures recommended
!
Storing or other special handling instructions
Depending upon the Clinac model, the following hazardous substances may be present: 1.
Freon 12 is present in the waveguide system in Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
2.
Sulfur hexafluoride (SF6) is present in the waveguide systems of all Clinacs other than Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
3.
Lead is present in the shielding and balance weights in all Clinacs.
4.
Beryllium (99% pure) is present in the window of the bend magnet vacuum chamber of all high-energy Clinacs.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.2.14.3. Other Substances in the Clinac
4.2.15. Preventing Exposure to Bloodborne Infections
5.
Dielectric insulating oil is present in the pulse transformer in all Clinacs.
6.
Dielektrol III capacitor oil is present in the modulator, rectifiers, and capacitors in all Clinacs.
7.
Depleted uranium is present in the X-ray head of older low energy Clinacs.
Depending upon the Clinac model, other substances, which may pose negligible health risks under normal operating conditions, may also be present. These substances may include: !
Ethylene glycol
!
Lubricants
!
Paint (touch-up)
!
Paint (black, aerosol)
!
Asbestos
Some bloodborne infections are caused by viruses carried in the bloodstream. These viruses are usually transmitted by contact with infected blood, although in some cases they may be transmitted by other bodily fluids as well. Individuals infected with bloodborne pathogens, such as the hepatitis B virus (HBV) or the human immunodeficiency virus (HIV), can develop serious health problems that could ultimately prove fatal. This information is provided for general guidance only. Refer to OSHA regulation 29 CFR 1910.1030, “Occupational Exposure to Bloodborne Pathogens” and other applicable state and federal occupational and health regulations (domestic US), or other local (or international) standards, for additional information and requirements. The guidelines presented in this section apply to all Clinac operators and maintenance and service personnel. Occupational situations that may present a risk of exposure to bloodborne pathogens include: !
Periodic maintenance inspections (PMIs).
!
Routine maintenance.
!
Cleaning of the treatment couch and the collimator.
!
Patient contact.
Since the Clinac and its accessories are located in a hospital environment, the following occupational situations may apply: !
Service-related duties that bring you into contact with human blood or other bodily fluids
!
Service-related duties that bring you into contact with hypodermic needles or other sharp instruments that might be contaminated with infected blood
!
Service-related duties that bring you into contact with materials (towels, sheets, clothing) contaminated with blood or other bodily fluids
!
Potential occupational exposure to bloodborne pathogens
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4.2.15.1. Safety Precautions
!
Providing emergency first-aid assistance to coworkers
!
Dealing with blood, unfixed tissue, or organs from humans (living or dead); tissue cultures; or solutions that contain blood or other human tissue
!
Handling experimental animals infected with HIV or HBV, or the blood, organs, or tissues from these animals
!
Assume all bodily fluids are infectious and act accordingly. Avoid contact with blood or other bodily fluids. Keep all cuts and scrapes covered.
!
Follow proper procedures when in contact with infected blood or other bodily fluids or contaminated materials. Avoid procedural shortcuts, which may save you time, but place you at risk of becoming exposed to potential bloodborne pathogens.
!
Follow proper housekeeping procedures. Recommended housekeeping guidelines include:
!
!
•
Clean and decontaminate all equipment and work surfaces that have been contaminated with blood or other potentially infectious materials. Use the disinfectant required by the employer (or, if none is specified, with a solution of 5.25% sodium hypochlorite [household bleach] diluted between 1:10 and 1:100 with water).
•
Remove and replace protective coverings, such as plastic and foil, that may have become contaminated.
•
Always use mechanical means, such as tongs, forceps, or a brush and dust pan, to pick up contaminated broken glassware. Never use your hands to pick up broken glassware even when wearing gloves.
•
Place all potentially contaminated wastes in designated disposal containers.
•
Store sharp objects in a manner that ensures safe handling.
Dispose of waste properly. Recommended waste disposal procedures include: •
Place all infectious waste in closable, leakproof containers or bags that are color-coded and appropriately marked.
•
Place all sharp items in puncture-resistant containers for disposal.
•
Double-bag infectious wastes if the outside of a bag is contaminated with blood or other potentially infectious materials.
•
Inspect and decontaminate bins, pails, and cans used for storage or disposal of potentially infectious materials on a regular basis.
Observe proper hygiene. Infected matter can enter the bloodstream through cuts and other openings in the skin, through the eyes, or through the mucous membranes of the nose and mouth. •
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It is especially important to wash hands frequently, since they are the most likely source of contact with infected blood or other bodily fluids. Wash carefully after any contact with body fluids or other potentially infectious materials and after removing gloves that may have been contaminated.
Emergency and Safety: Service and Maintenance Guidelines
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4.2.16. Using Personal Protective Equipment
•
If blood splatters in the eyes, flush thoroughly with clean water as soon as possible after the contact.
•
Wear appropriate personal protective equipment when the job involves contact with blood or potentially infected material, or when giving first aid.
Personal protective equipment (PPE) prevents blood or other potentially infectious material from coming into contact with the skin, eyes, mouth, mucous membranes, or clothing. This section details procedures for using PPE to protect yourself from possible hazards. Wear the appropriate PPE for the task you are performing. This information is provided for general guidance only. Refer to OSHA regulation 29 CFR 1910.132-1910.139 and other applicable state and federal occupational and health regulations (domestic US), or other local (or international) standards, for additional information and requirements.
4.2.16.1. Safety and Service Guidelines
4.2.16.2. Eye Protection
These guidelines for using personal protective equipment apply to all Clinac operators and maintenance and service personnel: !
Obtain PPE that fits well.
!
Inspect and clean PPE before and after each use.
!
Use the correct PPE for the situation. Depending on the hazards of the job, you may need more than one type of PPE to protect yourself.
Safety glasses or goggles must be worn while servicing the Clinac where eye hazards exist. !
Wear safety glasses fitted with side shields to provide maximum protection from flying particles or dust.
!
Keep protective eyewear clean.
!
Contact lenses are not a substitute for protective eyewear.
4.2.16.3. Foot Protection
Shoes that provide protection for the whole foot, such as work shoes, are recommended during service, maintenance, and installation of the Clinac. For electrical service on equipment involving potentials above 20 kV, wear safety-toe boots. Do not wear steel-toe boots.
4.2.16.4. Respiratory Protection
Masks should be worn whenever there is a risk of exposure to airborne substances, such as dust or chemical vapor. The mask should cover the nose and mouth.
4.2.16.5. Hand Safety
Wear protective gloves while servicing the Clinac where hazards to your hands exist, and follow these guidelines: !
Treat used waveguide pieces as though they are contaminated and handle with personal protective equipment (PPE). Always wear eye protection and chemical-resistant gloves. For information about PPE, see “Using Personal Protective Equipment” on page 1-45.
!
Leather gloves used for handling lead must be set aside and not used for other purposes. With prolonged use, leather gloves can absorb lead. Always wear disposable latex, vinyl, or nitrile gloves under leather gloves that have been used to handle lead.
Emergency and Safety: Service and Maintenance Guidelines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
Before servicing the Clinac, remove jewelry such as rings, bracelets, and watches.
!
Stay clear of pinch points and crushing hazards including, for example, motors and gears in the collimator, gantry, and couch. Lockout/tagout the couch and gantry locking pin, when appropriate, to prevent hand injuries.
!
If you anticipate coming into contact with blood or other potentially infectious materials, wear latex, vinyl, or nitrile gloves. Check items for slivers, jagged edges, or burrs before lifting.
!
Always wash your hands immediately after removing gloves.
!
Dispose of single-use gloves in the proper containers. If gloves are contaminated, do not dispose of them in regular trash.
Table 4-1 lists recommended types of gloves.
Note: For those with rubber or latex allergies, use vinyl or nitrile gloves.
Table 1.1. Hand Protection
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Operation
Type of Glove
Protect Against
Notes
Handling lead
Leather gloves with latex, vinyl, or nitrile gloves underneath
Lead particulates or dust
Latex, vinyl, or nitrile gloves prevent contamination from leaching of lead through dedicated leather gloves
Handling depleted uranium
Leather
Low-level radiation and uranium poisoning
Handling ion chamber
Latex, vinyl, or nitrile
Contaminated dust, low-level radiation, and moderate heat
Handling hot thyratron tubes
Leather
Moderate heat
Handling sharp or rough objects
Leather
Cuts and abrasions
Contact with blood or other bodily fluids
Latex, vinyl, or nitrile
Bloodborne pathogens
Cleaning couch or collimator
Latex, vinyl, or nitrile
Bloodborne pathogens
Handling waveguide parts
chemical-resistant gloves, such as rubber or neoprene
Chemical hazards
Handling chemicals
Rubber, neoprene
Chemical hazards
Also wear eye protection
Emergency and Safety: Service and Maintenance Guidelines
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4.2.17. Heavy Lifting and Handling
There are a number of situations that may require lifting or handling heavy objects during the service and maintenance of the Clinac.
4.2.17.1. Guide lines for Heavy Lifting
General guidelines for lifting and moving heavy equipment or objects safely include:
Lifting and handling heavy equipment or objects improperly can cause personal injuries, including sprains, strains, fractures, wounds, and hernias.
!
Dress for safety. Shoes with reinforced toes and nonslip soles may help to reduce the risk of injuries.
!
Think ahead. Plan a clear and unobstructed route to your destination.
!
Examine the object. Judge its weight and stability. Look for sharp edges. Decide how best to hold the object.
!
Get a good grip. Use your palms and fingers. Wear only properly fitting gloves.
!
Get help if you need it. For objects over 20 kg (44 lb.), use suitable handling devices and techniques. If you have any doubt about your ability to move the object, ask for help or use a mechanical aid.
Practice proper lifting techniques to safely protect your back and spine: !
Stand close to the load with feet wide apart.
!
Squat, bending at the hips and knees.
!
As you grip the load, curve your lower back inward by pulling your shoulders back and pushing your chest out.
!
Be sure to keep the load close to your body.
!
When you set the load down, squat, bending at the hips and knees. Keep your lower back curved inward.
If you are unable to bend your knees easily or get very close to an object, use alternative lifting techniques:
4.2.17.2. Precautions
!
Stand as close as you can.
!
Brace your knees against a solid object.
!
Bend at the hips, keeping your head and back straight.
!
Lift slowly, using your legs, buttocks, and stomach muscles.
When lifting or handling heavy objects (those weighing over 20 kg, or 44 lb.), obtain or use equipment appropriate for the task at hand. Also refer to information provided during specific product safety training. Situations that require lifting or handling heavy objects include, but are not limited to, the following: !
Lifting or handling the turntable or collimator: Use appropriate Aframe handling equipment.
!
Lifting or handling the sled assembly: Use appropriate bracket-fixture handling equipment.
!
Lifting or handling the Klystron or Klystron solenoid: Use appropriate handling equipment.
!
Lifting or handling gas cylinders (high-energy Clinacs), e-rack power supplies, RF drives, or fiberglass covers (canoes): Request assistance when necessary.
Emergency and Safety: Service and Maintenance Guidelines
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4.2.18. Tripping and Other Hazards (Silhouette Edition Clinac)
Tripping hazards exist when working in either service bay of the cabinet of a Silhouette Edition Clinac. The RF side has a step-up plate so that the hazard is decreased, but the modulator side has a stand-alone kick plate where special care must be taken. Ignoring the size and location of this plate can cause someone to trip and cause bodily injury.
5. Owner Guidelines
This section provides guidelines for establishing emergency and safety procedures for Clinac operation and maintenance.
5.1. Planning Operations
To prepare for the safety of patients and staff in the treatment room and surrounding areas, the owner is responsible for installation planning and installation of certain emergency and safety equipment.
5.1.1. Emergency-Off Buttons
Provide sufficient Emergency Off buttons in the treatment room and console area. These buttons should complement the Emergency Off buttons supplied by Varian on the dedicated keyboard, on both sides of the treatment couch, and on both sides of the drive stand. For additional information about the location of Emergency Off buttons, refer to National Council on Radiation Protection and Measurements (NCRP) Report Number 51.
5.1.2. Main Circuit Breaker
Install the main facility circuit breaker within 10 feet (3 meters) of the control console. The main circuit breaker for the Clinac should have an undervoltage trip that causes the main breaker to trip if an Emergency Off button is pressed.
5.1.3. Ventilation and Temperature Regulation
Maintain adequate room ventilation. Heat and air conditioning should be available to maintain the Clinac at room temperature (65–70° F).
5.1.4. Fire Extinguishers
Provide suitable fire extinguishers in the treatment room and near the control console. In the United States, the type of extinguisher must be approved for electrical fires by federal, state, and local codes and regulations.
5.1.5. Radio FrequencyEmitting Equipment
Ensure that nearby rooms with RF-emitting equipment have proper room shielding to prevent leakage and door interlock switches to prevent operation with doors open. Adopt operating procedures that minimize leakage potential.
5.1.6. Emergency Lighting
Provide automatic emergency lighting (with flashlights as backup) in the treatment room and console area.
5.2. Radiation Protection Survey
Before beam calibration and routine use of the Clinac, the owner must have a radiation protection survey completed by a qualified radiation expert.
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Service personnel could bump their heads on the portion of the waveguide that crosses the RF cabinet service bay. Before working in this area, Varian recommends that you first remove this portion of the waveguide, and then reinstall it when the work is completed.
Emergency and Safety: Owner Guidelines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the United States, the radiation survey report indicates if the installation meets the recommended standards of the NCRP and the applicable local, state, and federal regulations. Outside the United States, the owner is responsible for compliance with the applicable statutory and regulatory requirements. Before routine use of the Clinac, the owner must:
5.3. Safety and Emergency Training
!
Have a qualified radiological physicist calibrate the dose rate and integrated dose measured by the transmission ionization chamber.
!
Conduct checks at least daily during the first month of operation to establish that the ionization chamber response is constant within specified limits.
!
Make constancy checks during the course of each day to compare monitor response from the start to the end of the working day.
!
Conduct daily, or at least weekly, calibration checks after the constant output of the machine is established. Record all calibration measurements in a log.
Personnel who work with or near the Clinac must receive formal training on the emergency and safety procedures adopted by the owner. Each person should receive further training on a periodic basis. Training should include at least the following items: !
Location and use of Emergency Off buttons
!
Location and use of the main facility circuit breaker
!
When to use lockout/tagout procedures as required by OSHA 29 CFR 1910.147 and/or other applicable state and federal occupational safety and health regulations (domestic US) or other local (or international) standards
!
Local evacuation procedures for fire, smoke, or chemical fumes
!
Location and use of the emergency lighting system (including backup lighting such as flashlights)
!
Procedure to remove a patient from the treatment couch in an emergency
5.4. Routine Use
The owner must establish operational and maintenance safety procedures for routine use of the Clinac. Combine the following items with the other safety precautions described in this section to reduce the likelihood of injury to personnel and damage to the equipment.
5.4.1. Addressing Equipment Malfunctions
Cease machine operation immediately if any equipment malfunction is detected or suspected, and call service personnel to correct the problem.
5.4.2. Recording Observations
Record any unusual machine behavior or other observations in the log book when you perform the daily checkout procedure and throughout the treatment day.
5.4.3. Securing Keys
When the machine is placed in standby mode and left unattended, remove the DISABLE/ENABLE and POWER keys from the equipment and deposit
Emergency and Safety: Owner Guidelines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 them in a key storage enclosure. Lock the enclosure to prevent unauthorized activation of the machine.
5.4.4. Testing Emergency-Off Circuits
Test the emergency-off circuits at least once every three months to ensure proper functioning.
5.4.5. Checking Fasteners
Check all fasteners for tightness at least semiannually.
5.4.6. Posting Signs
Post signs on doors to the treatment room and in the console area to inform:
5.5. Quality Assurance
!
All persons that a radiation hazard exists in the area.
!
Personnel to wear radiation monitoring instrumentation when they enter the treatment room.
!
Operators to have the DISABLE/ENABLE key in their possession when they enter the treatment room and to make sure that the door cannot close while in the room.
!
Any person wearing a pacemaker to remain out of the area until the effect of radiation and radio-frequency interference on pacemakers is known.
Because of the importance of precisely administered treatments in radiation therapy, the owner should establish a comprehensive quality assurance (QA) program for each radiation therapy facility. Sources of errors in radiation therapy include: !
Tumor localization
!
Patient immobilization
!
Field placement
!
Human errors in calibration
!
Patient setup
!
Equipment use
A program of periodic checks can minimize many of these errors. References: “AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Committee Task Group,” Medical Physics, Vol. 21, No. 7, July 1994 “Comprehensive Quality Assurance for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group,” Medical Physics, Vol. 21, No. 4, April 1994 “Physical Aspects of Quality Assurance in Radiation Therapy,” AAPM Report No. 13, American Association of Physicists in Medicine, May 1984
5.6. Accidental Radiation Overdose
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The owner must establish the procedures to follow in case of an accidental overexposure of a patient or personnel to radiation. Post these procedures conspicuously in the console area.
Emergency and Safety: Owner Guidelines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the event that a person is thought to be exposed to an excess of radiation, the law in most localities of the United States requires the following steps:
5.7. Backup Interlocks
5.8. Emergency Beam Termination
!
Immediately notify the appropriate local, state, and federal authorities.
!
Request an investigation by a professional qualified in the detection of radiation.
!
Consult with medical experts in radiation treatment.
!
Check the following references: •
National Council on Radiation Protection and Measurement (NCRP) Reports Number 38 and 102
•
Appropriate state regulations
The Clinac is designed to terminate the exposure when the total dose displayed on the console monitor equals the set dose. Should this normal (DOS1) termination fail to occur, the following backup interlocks terminate the beam: !
When the total dose exceeds the dose set by the operator (DSFA interlock).
!
At a fixed percentage of monitor units beyond the value set by the operator or a fixed number of monitor units, whichever is less (DOS2 interlock).
!
When the numbers of monitor units (MU) from the primary and secondary dosimetry channels differ from each other by more than 5% or 2 MU, whichever is greater (DS12 interlock).
!
Upon coincidence between the displayed time and the time value set by the operator (TIME interlock).
!
In dynamic therapy, when there is a disagreement between the intended dose and position and the actual dose and position (DPSN interlock).
The operator must be aware of the progress of treatment at all times. If the beam does not terminate correctly, the operator should take immediate action (press the nearest Emergency Off button) and not wait for a backup system to activate. Beam termination by an interlock or means other than normal termination may be a sign of a significant equipment malfunction. Varian recommends that you suspend patient treatments after any abnormal termination until qualified personnel determine that it is safe to continue using the Clinac.
5.9. Emergency Plan
An emergency situation can arise at any time. The owner must establish procedures for handling emergencies. Personnel operating or working around the Clinac must be trained in these procedures. !
The emergency plan should include:
!
Emergency procedures (modified for local conditions) described in this manual
!
Periodic testing of emergency equipment, including the emergency pendant system, emergency lighting system, fire extinguishers, and emergency-off circuits
Emergency and Safety: Owner Guidelines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6. Appendix A: Venting Waveguide Gases
!
Evacuation routes in case of an emergency situation, with the routes posted near the control console
!
Scheduling and content of periodic drills and training
!
Identification of qualified personnel to be contacted in the event of a fire, medical emergency, or other situation requiring outside help
!
Procedures to restore operation of the Clinac following an emergency
!
Procedures for using a hazardous spill kit
!
Procedures for maintaining up-to-date copies of Clinac MSDSs in the event of a hazardous substance leak or spill
All Clinacs use either Freon 12 or SF6 as a dielectric gas in the RF waveguide system. Waveguide arcing causes Freon 12 to break down into hydrogen chloride gas as well as hydrochloric and hydrofluoric acids. With SF6, waveguide arcing produces hydrofluoric acids and other hazardous fluoride products. These gases could be toxic and should not be inhaled. The procedures for venting waveguide gases will reduce exposure to hazardous gases that can be present when the RF waveguide system is purged. Abnormal noises during BEAM ON often indicate RF arcing in the waveguide. Specifically, listen for: !
Distinct or repetitive metallic “dinging” noise.
!
Continuous racket or banging noises other than the normal high-volt pulsing sound.
If venting the system is needed, follow the procedures. Varian recommends reading the entire procedure (for the correct machine configuration) before beginning work. Varian can provide any of the service contained in this procedure. Call the Regional Varian Customer Support Office (see “Customer Support” on page 1-8) for details.
6.1. Testing for Waveguide Arcing
6.2. Prerequisites: Parts Ordering Procedure
Use an oscilloscope to confirm if the RF is arcing in the waveguide. Determine if the following waveforms are tearing-off or have abnormally high spikes. !
For Low Energy machines, observe Magnetron I, Forward Power, and Reflected Power.
!
For High Energy machines, observe Klystron I, Forward Power, Load Power 1, Load Power 2, and Reflected power.
If venting the Clinac is needed, some parts are required. Determine if there is a venting device on the Clinac. !
All high energy Clinacs have a built-in venting device at the stand.
!
All low energy Clinacs that use SF6 also have the venting device at the stand.
!
All low energy Clinacs that use Freon 12 do not have the venting device at the stand.
If the Clinac has a venting device: !
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Order the 872031-01 kit from Varian and perform the section “Venting a Clinac With a Venting System” on page 1-53.
Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
A plastic bag may be used instead of the Cubitainer (listed in the material list at the end of this procedure).
If the Clinac does not have the venting device and retrofit is not being performed: !
Neither kit needs to be ordered. Complete the section “Venting a Clinac Without a Venting System” on page 1-53.
!
For this procedure, provide a large plastic bag to contain the gas, and a rubber band to close the bag. A 30 – 50-gallon trash bag is ideal.
To retrofit the Clinac with the venting device, order the 872031-02 kit and complete the section “Retrofitting the Clinac With a New Venting System” on page 1-55.
6.3. Venting a Clinac With a Venting System
Clinacs with a venting device can be vented in two ways: If the room has an exhaust vent, use flexible tubing to route the discharged gas directly into the room exhaust vent. 1.
If there is no exhaust vent in the room:
2.
Order the 872031-01 kit to properly vent the gas.
3.
Capture the gas into the plastic Cubitainer or a 30–50 gallon trash bag.
4.
Be sure to close the bag securely with adhesive tape or a rubber band.
5.
Carry the bag to an exhaust vent or an unpopulated area outdoors, and open it.
WARNING: For your safety and the safety of others, be sure to discharge the gas away from people or animals. 6.
Discharge the container through at least 15 feet of flexible tubing to allow a safe upwind breathing zone. Allow 5–10 minutes to discharge all the gas.
6.4. Venting a Clinac Without a Venting System WARNING: Read this entire section before performing the steps. Before you begin: 1.
Decide where the gas will be vented: Into the ventilation system, or outdoors away from people or animals.
2.
Shut off the gas bottle on the Clinac.
Emergency and Safety: Appendix A: Venting Waveguide Gases
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6.4.1. Option 1 for Venting Gas
This option may be the easiest since the relief can be used to control the gas flow. 1.
Rotate the gantry to either 90° or 270°.
2.
Remove the top gantry cover.
3.
Locate the gas pressure relief valve at the RF system.
4.
Wrap the opening of the plastic bag over the relief valve so that the venting gases will fill the bag without escaping into the treatment room; cover the valve handle as well.
5.
Use one hand to secure the bag over the relief valve and the other hand to carefully pull the valve handle.
6.
Holding the bag shut, carefully remove it from the relief valve.
7.
Twist the bag shut and seal it with the rubber band or adhesive tape.
8.
Carry the bag to an exhaust vent or an unpopulated area outdoors.
WARNING: For your safety and the safety of others, be sure to discharge the gas away from other people or animals.
6.4.2. Option 2 for Venting Gas
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9.
Discharge the container through at least 15 feel of flexible tubing to allow safe, upwind breathing zone.
10.
Allow 5–10 minutes to discharge all the gas.
11.
If the gas venting system is being added to the Clinac, see “Retrofitting the Clinac With a New Venting System” on page 1-55.
1.
Use one of the hose connections on the stand instead of the relief valve.
2.
Determine which hose connection will be used to vent the gas. The connection chosen should allow enough room to comfortably hold the bag on the hose as well as allow for inflation of the bag.
3.
Test the hose connection selected:
4.
Try to loosen the hose connection slightly and immediately retighten it. This test helps ensure that full control over the hose connection can be turned on and off when venting the gas into the bag.
5.
Wrap the opening of the plastic bag over the hose so that the venting gases fill the bag without escaping into the treatment room.
6.
Discharge the container through at least 15 feel of flexible tubing to allow safe, upwind breathing zone.
7.
Allow 5–10 minutes to discharge all the gas.
Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6.5. Retrofitting the Clinac With a New Venting System
6.5.1. Inspecting the New Venting Device for Leaks
1.
Order the 872031-02 kit.
2.
Complete the venting procedure in “Venting a Clinac Without a Venting System” on page 1-53.
3.
Assemble the mounting bracket, clamps, and venting manifold as shown in drawing 1101708.
4.
Find the filter trap at the pressure regulator in the stand.
5.
On the Clinac, disconnect the yellow hose at the outlet of the filter trap that routes to the gantry.
6.
Connect the yellow hose to one end of the venting manifold.
7.
Use the new yellow hose (supplied with the kit) to connect the filter trap outlet to the open end of the venting manifold.
8.
Mount the manifold assembly next to the pressure regulator.
1.
Slowly refill the gas system to operating pressure, 39 psi, for Clinacs with 3-port circulators, and 32 psi for Clinacs with 4-port circulators.
2.
Perform a bubble test on the gas lines by applying the provided Snoop on all the line joints and fittings.
To seal a leak, tighten the joints and connections or apply new Teflon tape.
6.5.2. Verify the Gas Leak Rate
1.
Set the gas pressure to operating pressure, 39 psi, for Clinacs with 3-port circulator, or 21 psi for Clinacs with a 4-port circulator.
2.
Allow approximately ten minutes for the gas pressure to stabilize and warm up.
3.
Close the gas bottle main valve.
4.
Turn the gas pressure regulator all the way down.
5.
Write down the regulated pressure indicated on the regulator gauge.
6.
Observe the pressure and compare it with the previous day’s pressure.
If the leak rate exceeds 2 psi per day, call the regional Varian office for service.
6.6. Contents of 872031-01 and 872031-02 Kits
Order the 872031-01 kit if the Clinac already has the venting system. Order the 872031-02 kit if the Clinac is being upgraded with the venting device.
Emergency and Safety: Appendix A: Venting Waveguide Gases
1-55
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 1.2. Contents of 872031-01 and 872031-02 Kits -02
-01
Part Number
Description
1
1
88-301200-00
Cubitainer, 5 gallon*
1
1
88-301201-00
Faucet for the 5 gallon Cubitainer*
3ft
3ft
28-158988-00
Tygon tubing, 3/8 ID x 1/2 OD
1
28-628983-00
Cap, 1/4 flare
1
28-611673-00
Union, 1/4 flare x 1/4 MPT
1
27-109614-00
Valve, Shutoff
1
28-201311-00
Nipple, pipe, 1/4 inch
1
28-207010-00
Tee, 1/4 x 1/4 x 1/4 FPT
2
28-610922-00
Elbow, 1/4 Flare x 1/4 MPT
1
00-886892-02
Clamp, Tee Mounting
4
13-311160-00
Nut, KEPS, 10-32
1
00-834931-05
Hose Assembly, 20-inch
1
88-902021-00
Snoop
88-189795-00
Teflon Tape
00-886891-01
Bracket, Tee Mounting
Drawing
Documents included with kit
1
*The Cubitainer and faucet are optional: A 30—50 gallon trash bag can be used instead.
1-56
Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Two
Machine Physics
In this chapter, Accelerator Physics will be discussed to give the reader a greater understanding of how the Clinac treatment beam is produced.
Machine Physics
2-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Introduction:.................................................................................................................... 2-3 2. Definitions:: ..................................................................................................................... 2-3 3. Kinetic Energy Relationships:........................................................................................... 2-3 4. Rest Energy Relationships:............................................................................................... 2-4 5. Total Energy Relationships:.............................................................................................. 2-4 6. Conversion of Energy to Electron Volts: ............................................................................ 2-5 7. Measurement of Energy Change: ...................................................................................... 2-6 8. Example of Simple Acceleration:....................................................................................... 2-8 9. The Standing Wave Accelerator: ....................................................................................... 2-9 10. Impedance Matching: ................................................................................................... 2-11 11. Plotting: ....................................................................................................................... 2-12 12. Accelerator Equivalent Circuit: ..................................................................................... 2-13 13. Load Line Considerations: ............................................................................................ 2-16 14. Fill Time:...................................................................................................................... 2-16 15. Injection Timing: .......................................................................................................... 2-17 16. Electron Injection and Bunching: ................................................................................. 2-18 17. Advances in Linear Accelerator Design for Radiotherapy:.............................................. 2-19
Table of Illustrations Figure 2.1. The Basic Accelerator: ........................................................................................ 2-3 Figure 2.2. Electrostatic (DC): .............................................................................................. 2-8 Figure 2.3. Alternating Current (AC):.................................................................................... 2-8 Figure 2.4. Phase Velocity: ................................................................................................... 2-9 Figure 2.5. Forward Power Polarity:.................................................................................... 2-10 Figure 2.6. Reflected Power Polarity:................................................................................... 2-10 Figure 2.7. Summation of Forward and Reflected Power: .................................................... 2-10 Figure 2.8. Source vs. Load Impedance: ............................................................................. 2-11 Figure 2.9. Load Power vs. Load Resistance:....................................................................... 2-12 Figure 2.10. Power Calculation Parameters: ....................................................................... 2-12 Figure 2.11. Accelerator Equivalent Circuit: ....................................................................... 2-13 Figure 2.12. Equivalent Circuit Showing Resonant Components:........................................ 2-15 Figure 2.13. Load Line Showing Effect of Energy Slit: ......................................................... 2-16 Figure 2.14. Accelerator Equivalent Circuit: ....................................................................... 2-16 Figure 2.15. Fill Time: ........................................................................................................ 2-17 Figure 2.16. Fill Time Equivalent Circuit: ........................................................................... 2-17 Figure 2.17. Electron Injection Timing:............................................................................... 2-17 Figure 2.18. Effect of Energy Slit on Injection Timing: ........................................................ 2-18 Figure 2.19. Effect of Injection Timing on Target Current Waveform: .................................. 2-18 Figure 2.20. Velocity Vectors: ............................................................................................. 2-19
2-2
Machine Physics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
A basic electron accelerator consists of a vacuum chamber with two electrodes connected across a voltage source. Since electrons carry a negative charge, an electron entering the cavity between the plates will be attracted toward the plate with the positive charge and repelled by the negatively charged plate, as shown in Figure 2.1 below. In this example, with a 1-volt battery, the electron will be accelerated by 1 electron volt (eV).
e
–
e
–
1 eV
Vacuum
1 Volt
Figure 2.1. The Basic Accelerator
2. Definitions:
Before proceeding, it is important for the reader to understand the difference between the terms energy and intensity, used to describe treatment beam parameters. These can be defined in two ways: 1.
2.
Basic electron beam: Intensity:
The number of electrons passing a point within a given unit of time (beam current).
Energy:
The total energy possessed by each individual electron in the beam.
Treatment beam (electrons or photons): Intensity:
The amount of radiation delivered per unit of time to a point.
Energy:
The ability of the beam to penetrate.
The total energy possessed by an electron (or any other object) is the sum of its kinetic and rest energies. These will now be defined and discussed in some detail.
3. Kinetic Energy Relationships
Definition: Statement: Example 1:
Kinetic energy is that energy an object possesses by virtue of its motion. 1 2 The KE (kinetic energy) of an object in joules = --- mV at ev2 eryday velocities. Take a laboratory size object. (e.g., a baseball which weighs approximately 0.2 kilograms . 0.5 pounds moving at 30 meters/second . 60 miles/hour): 1 2 ∴ KE = --- ( 0.2kG ) × ( 30 m/sec ) 2 KE = 90 joules
Machine Physics: Introduction
2-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Example 2:
Take an electron (e–) which weighs 9.1 × 10–31 kG moving at 30 meters/second: 1 – 31 2 ∴ KE = --- ( 9.1 × 10 kG ) × ( 30m/sec ) 2 – 28 KE = 4.09 × 10 joules
Observation: Compare the kinetic energy of the baseball to that of the electron:
4.09 × 10-28 joules
90 joules vs. (Baseball)
(Electron)
Conversion:1 joule = 1 watt-second 1 kilowatt hour = 3.6 × 106 watt-seconds 2.5 × 10-5 kilowatt hours
1.14 × 10-34 kilowatt hours vs.
(Baseball)
4. Rest Energy Relationships
(Electron)
Definition:
Rest energy is that energy an object possesses by virtue of its mass.
Statement:
A. Einstein’s equation states the total energy: Et = mc2
Example 3:
where c is the speed of light (.3 × 108 meters/sec).
Take the baseball again and calculate the rest energy (RE): RE = (0.2 kG)(3 H 108 m/sec)2 = 1.8 H 1016 joules = 5 H 109 kilowatt hours
Example 4:
or
Now calculate the rest energy of the electron: RE = (9.1H 10–31 kG)(3 H 108 m/sec)2 = 81.9 H 10–15 joules = 2.275 H 10–20 kilowatt hours
or
Observation: Compare the rest energy of the baseball to that of the electron: 1.8 × 1016 joules 5 × 109 kilowatt hours
81.9 × 10–15 joules vs.
(Baseball)
5. Total Energy Relationships 2-4
Definition:
2.27 × 10–20 kilowatt hours (Electron)
Total energy is that energy an object possesses by virtue of its motion and its mass. Et (total energy) = KE + RE
Machine Physics: Rest Energy Relationships
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Example 5:
Take the baseball again and calculate its total energy (Et): Et = 90 joules + 1.8 H 1016 joules = 18,000,000,000,000,090 joules
Example 6:
Now calculate the total energy of the electron: Et = 4.09 H 10–28 joules + 81.9 H 10–15 joules Et . 81.9 H 10–15 joules
Observation: Almost all of the total energy of an object is due to its mass at these velocities.
6. Conversion of Energy to Electron Volts
Definition:
An electron volt is the kinetic energy an electron acquires by being accelerated in a vacuum through a potential difference of 1 volt (See Figure 2.1 on Page 2-3).
Statement:
The conversion factor to go from joules to electron volts (eV) is: 1.6 × 10–19 joules = 1 eV.
An electron has a rest energy of 81.9 × 10–15 joules; therefore, we must divide the rest energy by the conversion factor to obtain the same thing in eV.
Example 7:
Converting the rest energy of the electron: – 15
81.9 × 10 joules RE = -------------------------------------------------– 19 1.6 × 10 5
= 5.11 × 10 eV = 0.511MeV
Example 8:
Converting the rest energy of the baseball: 16
1.8 × 10 joules RE = -------------------------------------------– 19 1.6 × 10 35
= 1.12 × 10 eV 29
= 1.12 × 10 MeV
Observation: The difference of equivalent energies in eV between the electron and the baseball: 1.12 × 1029 MeV
0.511 MeV vs.
(Baseball)
Machine Physics: Conversion of Energy to Electron Volts
(Electron)
2-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Observation: In Example 2 the equivalent kinetic energy of the electron moving at 30 meters/second or 60 miles/hour in electron volts (eV) would have required a battery potential of 2 nanovolts (See Figure 2.1 on Page 2-3.) Example 9:
Now let us calculate the total energy an electron possesses by passing through a potential of 22 million volts: Et = KE + RE Et = 22 MeV + 0.511 MeV Et = 22.511 MeV
7. Measurement of Energy Change
Definition:
Gamma ( γ ) is the ratio of total energy (Et) to the rest energy (Er) of an electron. E γ = -----t Er
however,
Er = mo c
2
as stated before, where mo = original mass
2
and
E t = mc
therefore
mc γ = -------------2 mo c
and
m γ = ------mo
2
substituting mc2 for E canceling identical terms
Example 10: Calculating the effective increase in mass as a result of accelerating an electron to typical linear accelerator values: 4MeV + 0.511MeV γ = ----------------------------------------------------- = 8.828 0.511MeV + 0.511MeV- = 20.569 γ = 10MeV -------------------------------------------------------0.511MeV + 0.511MeV- = 36.225 γ = 18MeV -------------------------------------------------------0.511MeV 22MeV + 0.511MeV γ = --------------------------------------------------------- = 44.053 0.511MeV Observation: An acceleration of 22 MeV gives an equivalent increase in mass of approximately 44 times. The following equation is derived from Newton’s second law of motion (as applied to momentum): Statement:
γ =
1 --------------------v 2 1 – ⎛ ---⎞ ⎝ c⎠ 1
⎛ ⎞ --2⎜ 1 ⎟ γ = ⎜ --------------------2-⎟ ⎜1 – ⎛v ---⎞ ⎟ ⎝ ⎝ c⎠ ⎠ These equations are normally referred to as proof that an object with mass can never attain the speed of light.
2-6
Machine Physics: Measurement of Energy Change
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Needed:
A way to express velocity as a ratio of the speed of light.
Definition:
$ (beta) is the ratio of the velocity of an electron to the speed of light.
$ has other meanings different from this one and is not to be confused as it is defined here.
1
⎛ ⎞ --21⁄2 ⎜ 1 ⎟ 1 ⎜ --------------------2-⎟ = ⎛⎝ -------------------2-⎞⎠ ⎜1 – ⎛v 1 – (β) ---⎞ ⎟ ⎝ ⎝ c⎠ ⎠
v substituting $ for --c
1⁄2 1 Conclusion: γ = ⎛ -------------------2-⎞ ⎝ 1 – (β) ⎠ 2 1 γ = ------------------2 1 – (β)
1 1⁄2 β = ⎛ 1 – -----⎞ 2⎠ ⎝ γ
Exercise:
squaring both sides
solving for $
Calculate the percentage of the speed of light (c) as a function of kinetic energy.
Example 11: Ek + Er γ = -----------------Er 22MeV + 0.511MeV γ = -------------------------------------------------------0.511MeV γ = 44.053 1 1⁄2 β = ⎛ 1 – ----2-⎞ ⎝ ⎠ γ 1⁄2 1 β = 1 – ⎛ ----------------------2-⎞ ⎝ 44.053 ⎠
β = 0.9997
Conclusion: An electron accelerated to 22.511 MeV is traveling at approximately 0.9997 times, or 99.97% of, the speed of light.
Machine Physics: Measurement of Energy Change
2-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Observation: Accelerated Energy 17 KeV 100 KeV 0.511 MeV 4 MeV 6 MeV 10 MeV 18 MeV
8. Example of Simple Acceleration
Ratio of c .252 .548 .866 .993 .997 .998 .999
(X-ray tube) (Klystron) (Doubling rest energy)
In a simple accelerator, more energy can be achieved in two ways: increasing the accelerating voltage ( V a ), or increasing the number of accelerating cavities, as shown in Figure 2.2 below.
e–
e–
KE = (V1 + V2) eV
Vacuum
V2
V1
etc.
Figure 2.2. Electrostatic (DC) When high electron beam energies are required, batteries are not practical; however, since high-voltage AC power is easy to generate, it can be used to generate high-strength electrical fields in the cavities, as shown in Figure 2.3 below. Vacuum
e
–
e
V1
V2
V3
–
etc.
Figure 2.3. Alternating Current (AC)
2-8
Machine Physics: Example of Simple Acceleration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The best efficiency is achieved when the electron enters each cavity at the time when V a is at maximum strength. Therefore, the electrical field must propagate through the cavities at the same velocity as the electron. This is called the phase velocity as shown in Figure 2.4 below. Accelerators using this principle are called traveling wave accelerators.
Vacuum
d
–
e
e
φ1 = 0°
φ2
φ3
–
φ4
Figure 2.4. Phase Velocity
Definition:
v ϕ = Phase velocity which equals Distance -----------------------Time d v ϕ = --t
at 60 Hz
λ = 3000 miles
at 3 GHz*
λ = 10 centimeters
Conclusion: Thus a particle accelerator requires microwave frequency generators and distances. *Actually 2.856 GHz in high-energy and 2.998 GHz in low-energy Clinacs
9. The Standing Wave Accelerator
Statement:
E n = cos ( ωt – ϕ n )
If the cavities are one-quarter of the of the AC field wavelength, at the time when the first and fifth cavities are at maximum forward-acceleration potential, the third cavity is at maximum reverse acceleration potential. By the time the electron reaches the third cavity, the polarities will be opposite. Thus, the electron acquires additional energy in every other cavity. In a traveling wave accelerator, the forward power is absorbed in the final cavity. In a standing wave accelerator, the power in the final cavity is allowed to reflect back, creating a standing wave and doubling the field strength in each cavity. However, in order for the reflected power to match the phase of the forward power, one of the end cavities must be either half as long, or one and one-half times as long, as the others. If all cavities are the same size, the reflected power will cancel the forward power. Also, in order to allow the forward and reflected power to travel between the cavities, the connecting openings must be at least one-quarter of the wavelength.
Machine Physics: The Standing Wave Accelerator
2-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 λ 4
Electric Fields
+
0
-
0
+
0°
-90°
-180°
-270°
0°
Forward Power Figure 2.5. Forward Power Polarity
Reflected Power
λ 4
0°
-270°
-180°
-90°
0°
+
0
-
0
+
Electric Fields Figure 2.6. Reflected Power Polarity
λ 4
0°
-270°
-180°
-90°
0°
+2E
0
-2E
0
+2E
0°
-90°
-180°
-270°
0°
Electric Fields Figure 2.7. Summation of Forward and Reflected Power
2-10
Machine Physics: The Standing Wave Accelerator
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Impedance Matching
When power is being transferred from its source to its load, the most efficient transfer occurs when the impedance of the load (Rl) is equal to the impedance of the source (Ro). This explained in example 12, below:
Ro (Source)
Vo (Source)
Rl
(Load)
Il Figure 2.8. Source vs. Load Impedance Vo I l = ----------------Ro + Rl 2
Pl = Il Rl 2
Vo Rl P l = ------------------------2 ( Ro + Rl )
Vo - for I l substituting ----------------Ro + Rl
Example 12: Reference Figure 2.8 above. Case 1: Rl = Ro
Given:Ro = 1 ohm Rl = 1 ohm Vo is constant
1 - = 1 P l = ---------------------- = 0.25 watts 2 4 (1 + 1)
Case 2: Rl < Ro
Given:Ro = 1 ohm Rl = 0.5 ohm Vo is constant
0.5 0.5 P l = --------------------------2- = ------------- = 0.222 watts 2.25 ( 1 + 0.5 )
Case 3: Rl > Ro
Given:Ro = 1 ohm Rl = 2 ohms Vo is constant
2 2 P l = --------------------- = --- = 0.222 watts 2 9 (1 + 2)
Machine Physics: Impedance Matching
2-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11. Plotting
Figure 2.9, below, illustrates this effect.
.25 .222
P1
.5
1
2
R1
Figure 2.9. Load Power vs. Load Resistance Conclusion: For maximum transfer or power the load impedance must equal the source impedance. Needed:
Expression to find actual maximum power.
Ro
Vo
Rl Io Figure 2.10. Power Calculation Parameters
Statement:
Vo - if R o = R l I o = --------2R o 2
P max = I o R o VO ⎞ 2 2 - R I o R o = ⎛ --------⎝ 2R o⎠ o 2 Vo ⎞ 2 V o ⎛ --------- R o = --------⎝ 2R o⎠ 4R o
Vo - for I o substituting --------2R o and multiplying by R O
2
V ∴P max = ---------o4R o
2-12
Machine Physics: Plotting
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
12. Accelerator Equivalent Circuit
Figure 2.11, below, illustrates the relationships between the parameters.
Ro
1:n
Va n
Vo
I2
Ra
Ia
Va
Ira
Io
Figure 2.11. Accelerator Equivalent Circuit Statements: 1 V o = I o R o + ---- V a N I 2 = I ra + I a V I ra = ------aRa V substituting ------a- for I ra Ra
V I 2 = ------a- + I a Ra I o = nI2 V I o = n ⎛ ------a- + I a⎞ ⎝ Ra ⎠ V V ∴V o = n ⎛ ------a- + I a⎞ R o + ------a ⎝ Ra ⎠ n V a R o⎞ V V o = n ⎛ ------------+ nI a R o + ------a ⎝ Ra ⎠ n Ro 2 2 nV o = n V a ⎛ -------⎞ + n I a R o + V a ⎝ R a⎠ 2R 2 nV o = V a ⎛ n ------o- + 1⎞ + n I a R o ⎝ Ra ⎠
2
nV o n Ia Ro - – ----------------------V a = ----------------------R 2 Ro 2 o n ------- + 1 n ------- + 1 Ra Ra
Machine Physics: Accelerator Equivalent Circuit
Think about it!
rearranging terms
multiplying by n
combining and rearranging terms 2R dividing by n ------o- + 1 and solvRa ing for V a
2-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Definition: β (beta) is equal to the shunt impedance divided by the source resistance times the turns ratio squared. Ra β = -----------2 n Ro 2
n R 1 --- = ------------oβ Ra
inverting
R 2 n R o = ------aβ
rearranging 2
nV o n Ia Ro V a = ----------------------- – ----------------------2 Ro 2 Ro -----n -----+ 1 n +1 Ra Ra 2
nV o n Ia Ro V a = ------------------ – -----------------⎛1 --- + 1⎞ ⎛ 1 --- + 1⎞ ⎝β ⎠ ⎝β ⎠
Ro I a -----nV o β V a = ------------------- – ------------------1 1 ⎛ --- + 1⎞ ⎛ --- + 1⎞ ⎠ ⎝β ⎠ ⎝β βnV Ia Ra V a = -------------o- – -----------1+β 1+β
restating the equation for V a (see above) 2
n Ro substituting β for ------------Ra
R 2 substituting ------a- for n R o β
multiplying by β
2
Vo P max = --------4R o
restating the equation for P max (see above)
V o = ( 4P max R o )
1⁄2
solving for V o
P max will be denoted as P o . V o = ( 4P o R o )
1⁄2
nV o = n ( 4P o R o )
1⁄2
2
nV o = ( 4P o n R o ) Ra nV o = ⎛ 4P o -------⎞ ⎝ β⎠
1⁄2
βnV o = ( 4P o R a β )
2-14
1⁄2
1⁄2
multiplying by n combining terms Ra 2 substituting ------- for n R o β multiplying by β
Machine Physics: Accelerator Equivalent Circuit
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 βnV Ia Ra V a = -------------o- – -----------1+β 1+β
restating the accelerator voltage equation
2
Ia Ra ( 4P o R a β ) – -----------V a = ---------------------------1+β 1+β
Statement:
1⁄2
substituting ( 4P o R a β ) in accelerator equation
for βnV o
1⁄2
( 4R a β ) K 1 = --------------------------1+β Ra K 2 = ⎛ -------------⎞ ⎝ 1 + β⎠ Va = K1 ( Po ) Statement:
1⁄2
– K2 ( Ia )
substituting K1 and K2 in accelerator equation
Where Po equals the input microwave power to the accelerator and Ia equals the electron accelerated beam current.
This is not the same as target current!
Statements:
2
Va P cavity = -------Ra
P beam = V a I a The ratio of the accelerator impedance to the source impedance is given by $. At $ = 1, the accelerator is matched. All of the above calculations relate to an accelerator operating at resonance.
Ro
1:n
Va n
Vo
Io
I2
Ra
Ia
Va
Ira
Figure 2.12. Equivalent Circuit Showing Resonant Components
Machine Physics: Accelerator Equivalent Circuit
2-15
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ω–ω where δ = ---------------oωo
Ra Z = -------------------------------------------2 1⁄2 [ 1 + ( 2Q o δ ) ]
Figure 2.12 on Page 2-15 defines the off-resonant conditions.
13. Load Line Considerations
+5%
Operating Point
Va (Energy) -5%
±3% Energy Slit
Ia
(Beam Current)
Figure 2.13. Load Line Showing Effect of Energy Slit
14. Fill Time Ro
1:n
Va n
Vo
I2
Ra
Ia
Va
Ira
Io
Figure 2.14. Accelerator Equivalent Circuit
See reference Figure 2.15.
t
Statement:
2-16
--------------Ra ⎞ ⎛ R C⎞ - ⎜ 1 – e EQ ⎟ V a = ⎛ V o ------------------⎝ R o + R a⎠ ⎝ ⎠
Ro Ra where R EQ = ------------------Ro + Ra
Machine Physics: Load Line Considerations
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
RF Power Level
Va
t Figure 2.15. Fill Time
15. Injection Timing
Ro
S1
Vo
Ra
C
Ia
Ic
Figure 2.16. Fill Time Equivalent Circuit Vo β Ia Ra V a = ------------ – ------------1+β 1+β
for the steady state condition.
When capacitor current (Ic) equals the desired value of Ia, S1 is closed.
Desired RF Power Level
Va
Magic time to close S2 for gun pulse timing
t Figure 2.17. Electron Injection Timing
Machine Physics: Injection Timing
2-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figures 3.18 and 3.19 illustrate how of Gun timing, RF loading and the Energy Slit affect the target current.
±3% Slit
Injection too late
No Injection
Desired RF Power Level
Va
Injection too early
t Figure 2.18. Effect of Energy Slit on Injection Timing
4.5 – 5 µSec
Injection too early Optimal target current
Injection too late (or bad light pipe)
Figure 2.19. Effect of Injection Timing on Target Current Waveform
16. Electron Injection and Bunching
Statement:
The wave field is initially moving faster than the electron. so the electron appears to be moving backward with respect to the wave field.
Injection energy (velocity) must be correct to deposit the electron at the proper point on the wave front. The percentage of current captured is a function of the field. Speed of Light Injection Equation: 2
2πM o c ⎛ 1 – β⎞ 1 ⁄ 2 - ------------cos θ α = cos θ o – -------------------Eλ ⎝ 1 + β⎠
The cos2 approaches 1 if bunching parameters are correctly selected.
2-18
Machine Physics: Electron Injection and Bunching
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Wave field velocity vector
Resultant with reference to wave field velocity –
e
Electron velocity vector
RF power amplitude Region of bunched electrons
Figure 2.20. Velocity Vectors In the highest energy modes only about 1/3 of the electrons that enter the guide are captured. For lower energy modes, this number is even less.
17. Advances in Linear Accelerator Design for Radiotherapy
This section has been compiled from an article by Dr. C. J. Karzmark, published in 1984 by the Department of Radiology, Stanford University School of Medicine, Stanford, California 94305, and is included in this manual for reference only. In this section the paragraph numbering scheme of the original article is used and the bibliographic references are omitted.
I. Introduction
The microwave-powered electron linear accelerator, or linac, is becoming the dominant radiotherapy treatment unit. In the U. S., linacs now comprise over one-half of all megavoltage treatment units in service and about 90% of newly installed units. Several technical advances, combined with attention to how patients are most effectively set up and treated, have led to continuing improvements in linac radiotherapy during the three decades since their introduction in England and in the United States. A simplified exposition of linac theory and operation can be found in a contemporary reference by Karzmark and Morton. In it, the major modules of a medical linac are identified, their principles of operation are described individually, and then their collective functioning in providing a radiation treatment beam is discussed. Additional aspects are presented in an earlier technical review of the subject by Karzmark and Pering. The microwave accelerator structure (sometimes referred to as the “guide” or as the “waveguide”), the central component of linacs, consists of a linear array of microwave cavities. Such structures have undergone extensive de-
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
2-19
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 velopment and the result has been physically shorter accelerators having higher energy gradients along the accelerating axis. Most treatment units are isocentrically mounted, and many of these machines employ a horizontally mounted accelerator structure and a beam bending system. Here, improved beam transport magnet systems and treatment head designs help ensure flatness uniformity of treatment beams and their stability with time independent of gantry motion and position as well as of beam-limiting device orientation. Various methods have been developed to permit varying the electron beam energy over a wide range. For example, in single-pass (single traversal) linacs, the ratio of radio-frequency (rf) power to two portions of the accelerator structure is varied so as to keep a constant energy gradient in the first injection portion while providing a variable energy gradient in the second portion. In two-pass linacs, the phase of the microwave electric field encountered by electron beam is varied as it returns for the second pass. In microtrons, the beam extraction path is shifted from one orbit to another. By providing both high and low x-ray energies together with a broad range of electron beam energies, a wide variety of treatment plans can be implemented. The incorporation of digital electronics and computer techniques has led to improved reliability together with desirable monitoring, control and safety features. Human engineering improvements have involved aesthetics, function, patient safety, and comfort, as well as convenience for the radiotherapy treatment technologist. Several new research directions suggest opportunities for continued improvement in linear accelerators for radiotherapy.
II. Microwave Accelerator Structures
We can understand how linacs accelerate electrons by first examining how an electric E field wave pattern travels down a hollow cylindrical pipe, or waveguide as it is called. Such waveguides, and their hollow rectangular pipe counterparts, are used to transmit microwave power from an rf source to an accelerator structure.
A. Introduction
Waveguides replace conventional wires and cables which are inefficient for transmitting power at microwave frequencies. Figure 2.21(a) shows the E field pattern and charge distribution at one instant of time in a plane containing the axis of a cylindrical waveguide. The accompanying magnetic H field pattern, in this case circling around and orthogonal to the axis, is not shown here or in later illustrations since it is not directly involved in the accelerator process. Electrons injected along the axis would be accelerated by the moving E field. Unfortunately, however, the velocity of this E field pattern, the phase velocity of the wave exceeds that of light and is therefore unsuitable for continuing acceleration of charged particles. The wave is slowed by inserting washerlike discs into the waveguide, as shown in Figure 2.21(b), so that the wave stays in step with the accelerating electrons. These discs divide the waveguide into a series of cylindrical cavities, the basic structure of a linear accelerator. Virtually, all medical linacs operate at frequencies of approximately 3000 MHz in the so-called S band where typical accelerating cavities are about 10 cm in diameter and 2.5 to 5 cm in length. These cavities are arranged to serve two purposes: (1) to couple and distribute microwave power between adjacent cavities and hence, along the length of the structure; (2) to provide an E field with suitable axial distribution for accelerating electrons. The particular spatial configuration of E and H fields in a cavity is called the mode and denoted by abbreviations such as TEM (transverse electric and magnetic) field pattern as in a coaxial line or cavity or TM010 (fundamental transverse magnetic only field pattern), as in Figure 2.21(b).
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 By varying the aperture and length of the cavities initially traversed, the continuum of injected electrons is concentrated into discrete bunches as well as accelerated. This initial portion of the structure is called the buncher and its cavities are non-uniform. Their inner diameter, aperture diameter, and axial spacing vary to provide the increasing phase velocity E field to accommodate the accelerating electron bunches. Since the electron velocity is almost constant and near the speed of light beyond the buncher, subsequent cavities are made uniform for further acceleration of the bunches of electrons. Early buncher designs contained many cavities (see Figure 2.22); later only several cavities were used and more recently, a single half cavity. Improved understanding of bunchers has resulted in a reduction of electron injection voltage from the 100–200 kV region to the 1– 30 kV region. Typically, one-third of the injected beam is captured, bunched, and accelerated. (See Ref 5 for an elementary description of bunching action.) 8 8/2
(a)
+ +
–
–
–
–
+ +
+ +
–
–
+ +
–
–
–
–
+ +
+ +
–
–
8 8/2
(b)
+
–
+
–
+
–
+
–
Figure 2.21. (a) Spatial traveling wave electric E field pattern and charge distribution at one instant of time along the axis of a smooth cylindrical waveguide. (b) Spatial traveling wave electric E field pattern and charge distribution at one instant of time along the axis of a disk-loaded cylindrical waveguide. The direction of the electric E field is reversed every half wavelength 8/2. The pattern repeats every wavelength and there are four cavities per wavelength in the diskloaded structure. The direction of the E field also reverses every half cycle in time.
B. Traveling Wave Structure Linac
An early prototype traveling wave (TW) structure, cut in half along its cylindrical axis, is shown in Figure 2.22. The buncher section is on the left and the uniform section is on the right. Figure 2.23 shows an elementary TW accelerator with electrons and microwave power injected on the left. Accelerated electrons emerge on the right and the residual microwave power not transferred to the electron beam or structure walls is absorbed in a load at that end. The discs serve to separate the cylinder into a linear array of coupled cavities which both transport the microwave power and accelerate electrons down the structure.
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Figure 2.22. Cutaway traveling wave accelerator structure; the buncher section is on the left and the uniform section is on the right.
Figure 2.23. Short traveling wave accelerator structure. The rectangular waveguide on the left is the input coupler for conveying microwave power from a source to the structure. The acceleration of an electron bunch in a TW linac is similar to the way a boy on a surfboard, positioned just forward of the crest on a water wave of velocity vw, moves forward as shown in Figure 2.24(a). Similarly, Figure 2.24(b) shows how two electron bunches of velocity ve, are accelerated by the negative portion of the E wave moving at phase velocity vp. This E wave is created by the charge distribution shown in Figure 2.24(c). For clarity in Figure 2.24(c), the instantaneous axial E field patterns, such as depicted in Figure 2.21(b), have been omitted. The boy and the electron bunches move forward at the velocity of their respective wave motions, the phase velocity vw and vp, respectively. Traveling wave structures of 2B/4 design, in which one cavity in four contains an electron bunch at any one time, dominated early linac structure designs. Some later structure designs incorporated three cavities per wavelength 8, a phase shift of 2B/3 rad (120°) per cavity with one cavity in three containing an electron bunch at any one time. More recently, standing wave (SW) designs have appeared in which one axial cavity in two contains an electron bunch at any one time.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In general, TW structures have a larger frequency bandpass and hence, are less sensitive to frequency change than comparable SW structures for the same mode of operation. Since the buncher in TW linacs is adjacent to the power feed at the gun end, the magnitude of the electric field in the early cavities relatively independent of beam current so the buncher action is not greatly affected by beam loading. As a result, bunching in TW structures is efficient over a comparatively broad energy range, typically ±35%.
Figure 2.24. Traveling wave principle: (a) for a boy surfing on a water wave advancing to the right, (b) for electron bunches occupying a similar position on an advancing electric E field (for simplicity, E– is drawn upward), (c) the associated charge distribution which pushes (– charge) and pulls (+ charge) the electron bunches along the cylinder. There are four cavities per wavelength 8 and one electron bunch every four cavities.
C. Standing Wave Structure Linac
We can convert the traveling wave linac just described to a standing wave linac. We do so by arranging for both a forward moving (to the right) f and backward moving (to the left) b E wave, each of which is reflected at both ends of the structure. The two moving E field maxima and resultant wave are shown in Figure 2.25 at three sequential times one-quarter cycle apart. The resultant E field pattern is the sum of the forward and backward components and ideally, in the absence of the I2R copper losses and beam loading, is double the amplitude found in a compatible TW structure. Note that every other cavity (e.g., #2 and #4) in Figure 2.25 has zero field at all times, at times (e.g., t1, and t3), because both forward and backward waves are each zero, at other times (e.g., t2), because they are equal to magnitude but opposite in direction and hence completely cancel.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Such zero-field cavities couple microwave power between cavities but play no role in particle acceleration. They may be moved off axis to form a sidecoupled or bimodal SW structure, thus shortening the overall length for a given energy gain. Figure 2.26 illustrates a cutaway section of a side-coupled SW structure. The apertures between cavities of TW structures involve a design compromise. They should be large to transport microwave power efficiently between cavities but should be small enough to “squeeze down” the E field, thus maximizing its value along the beam axis. This design conflict is removed for the SW structure; axial acceleration cavities may be optimized for electron acceleration and coupling cavities for microwave power transport. The latter are made small for convenience, since to first order they contain zero electric field and hence, can be of much lower Q than on-axis cavities without entailing much I2R loss in their copper walls since almost no wall current flows.
TIME
PHASE
t1
ωt1
1
2
3
4
5
E
t2
ωt1 + π/2
E b
t3
f
b
f
ωt1 + π
E NEG.
POS. E FIELD MAXIMA
Figure 2.25. Standing wave electric E field patterns in an accelerator structure for combined forward and backward waves at three sequential instants of time. Two traveling waves moving in opposite directions (f forward, b backward) generate such a standing wave. At time t2, the E field is zero in all cavities. The pattern shown at time t1 will recur one-half cycle after time t3. Individual cavities are numbered across the top.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The axial apertures of SW accelerating cavities can be made small and equipped with extended “nose cones” which increase their axial length. They now function as zero-field drift tubes, and the electron bunch crosses the cavity gap as E is near its maximum value (see Figure 2.26). Figure 2.27 illustrates the evolution of such an SW structure from a TW structure and the associated E field pattern at one instant of time. The coupling cavities in Figure 2.27(d) are staggered, in contrast to Figure 2.27(c), to reduce asymmetries introduced by the coupling slots. Figure 2.28 shows the time variation of the E field in an SW structure for one complete microwave cycle. The pattern varies sinusoidally in magnitude but remains stationary in space just as a vibrating violin string. The actual field axial distribution resembles more that of Figure 2.27 than Figure 2.25. These somewhat rectangular shaped patterns can be represented by a sum of spatial harmonics comprising a sinusoidal fundamental as in Figure 2.25 and higher order sinusoids which do not produce net acceleration of synchronous electron bunches. The E field pattern recurs along the beam axis twice as frequently as in TW structures; that is, an accelerating electron bunch is contained in every second cavity instead of every fourth cavity. In SW structures, all the cavities tend to have the same electric field, since the rf power bounces back and forth from each end of the structure many times (typically 100 times) to fill the cavities with energy as expressed in E and H fields. Hence, beam loading of any cavity contributes to the reduction of the E field in all cavities, including the buncher cavities. As a result, bunching in SW structures is efficient over only a narrow energy range, typically ±5%. Thus, to maintain effective bunching action in SW structures, the input rf power must be increased with an increase in beam current.
Figure 2.26. Cutaway bimodal or side-coupled standing wave accelerator structure. The axial accelerating cavities are shaped for optimum efficiency and the off-axis coupling cavities are staggered to reduce asymmetries introduced by the coupling slots (courtesy of Los Alamos Scientific Laboratory, Los Alamos, New Mexico).
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(a)
(b)
(c)
(d)
NEG.
POS. E FIELD MAXIMA
Figure 2.27. Evolution of a standing wave accelerator structure from a traveling wave structure and related E wave patterns along the axis. The E field patterns below each structure show the spatial field along the axis at the same time in the microwave cycle.
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t1
0°
t2
45°
t3
90°
t4
135°
t5
180°
t6
225°
t7
270°
t8
315°
t9
360°
0
π/2
π
3π/2
2π
Figure 2.28. Axial E field pattern for one microwave cycle of time for the bimodal structure depicted in Figure 2.27(d). This cycle includes the sequential E field patterns shown. The corresponding phase angles in degrees and radians are shown on the right. The pattern at time t9 is a repetition of that at time t1.
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D. Structure Designs
Several new structure designs, in addition to those described, have been developed recently. Two side-coupled structures of the Los Alamos (LASL) design, such as illustrated in Figure 2.29(a) (and also Figure 2.26), may be interlaced in the Varian design as shown in Figure 2.29(b). A
(a)
A'
A
(b)
A'
Figure 2.29. Cross section and end views of two SW structures: (a) sidecoupled Los Alamos (LASL) design; (b) side-coupled interlaced Varian design. The sections A–A’, which include the cylindrical axes of symmetry, are shown on the left. By arranging the side cavity coupling to connect every other axial cavity as shown in Figure 2.29(b) instead of adjacent cavities and feeding each group of cavities with microwave power 90° out of phase, a remarkably short, high-gradient structure results. It withstands high average E field gradients without breakdown along its length, in part because the ratio of the peak E field at the cavity surface to average accelerating E field value on axis of almost four in the LASL design is reduced to about 1.3 for the Varian design. However, the latter is more difficult to manufacture and has higher I2R dissipative copper losses associated with its larger ratio of cavity wall area to cavity volume. Benguang et al. have recently studied the interlaced structure with the aid of a model and a computer program. They conclude that the structure is best suited for short straight-through design linacs. Schriber has provided a useful comparison of SW and TW structures.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 2.30 shows some structure variations which have been investigated or adopted for the relativistic (constant velocity), section of the linac structure. The on-axis coupling designs, (a), (b), (c) and (d), are smaller in diameter and entail simple machining and brazing operations than off-axis coupling designs (e), (f), (g), and (h). The constant impedance uniform TW structure [Figure 2.30(a)] characterized early structure designs. It provides a decreasing energy gain per meter along its length as the microwave power is attenuated by the structure and by transfer to the ever-more-energetic electron beam. The constant gradient non-uniform TW structure Figure 2.30(b)] provides a constant energy gain per meter for a specific loading, i.e., beam current.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 2.30. Linac structure variations showing E field maxima at one instant of time: (a) TW constant impedance; (b) TW constant gradient; (c) SW biperiodic with on-axis coupling cavities; (d) SW tri-periodic with on-axis coupling cavities; (g) disk and washer cross section; (h) disk and washer structure. The arrows represent the maximum E field values at one instant of time. Structures (e) and (f) are shown in more detail in Figures 3.29(a) and 3.29(b) respectively.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Here, the size of aperture varies regularly along the length, but the phase velocity is constant. The 10,000-ft. Stanford Linear Accelerator Center (SLAC) physics research accelerator is divided into 10-ft. sections of this design. The bi- and tri-periodic SW structures, Figures 3.30(c) and 3.30(d), respectively, are simple to manufacture and have smaller outside diameters than the side-coupled variations [Figures 3.30(e) and 3.30(f)] but their shunt impedance, a measure of efficiency, is usually lower and they have lower energy gains for a given power and beam loading. The on-axis coupling cavities of radiotherapy linac structures [Figures 3.30(c) and 3.30(d)] can be “thin” since their resonant frequency depends only on their diameter in the dominant TM010 mode. They contain only rf coupling fields and not accelerating fields and hence, can be of low-Q design. Power loss in these cavities is small since the E field and associated wall currents within them are small. Coupling between axial cavities is usually magnetic via peripheral slots [see Figures 3.26 and 3.27(b)] and axial apertures can be optimized for acceleration. The transverse magnetic modes (sometimes called E modes), having only circular H field lines, are suited for such magnetic coupling. Although most bi-periodic structures employ magnetic coupling using peripheral slots, an on-axis coupled structure has been built and tested at 3000 MHz. Schriber et al. have compared the performance of S band, standing wave linacs having either on-axis or off-axis coupling. They conclude that optimized on-axis coupled accelerating structures have about a 20% higher effective shunt impedance (hence, higher energy gradient) and are an attractive choice. A disc and washer test cavity [Figures 3.30(g) and 3.30(h)] has been built and tested at 1350 MHz for use on a proton linac where it compares favorably with the side-coupled design. However, for 3000-MHz electron linacs, its relatively large diameter, difficulties in supporting and cooling the inner washers, as well as the presence of higher order spatial modes due to the washer supports, lower the shunt impedance and mitigate against its use. Nevertheless, it is being studied for use in a high-power (300 W) 2856-MHz linac or 36-orbit racetrack microtron intended to produce negative pi-mesons (pions) for radiotherapy.
E. Structure Design Parameters and Operating Principles
Microwave cavities are efficient devices for accelerating charged particles. An accelerating potential of about 1 MV can be established across the gap of a 5-cm-long cavity with about 0.2 MW of pulse power loss in that cavity. We characterize this efficiency by a figure of merit or quality factor, Q, defined by Q = f 0 ⁄ ∆f ( Energy stored in cavity ) = 2πf ⋅ ----------------------------------------------------------------------( Energy loss per cycle )
(1)
where f 0 is the resonant frequency and ∆f the frequency difference between the half power points. The energy storage is in the E and H fields in the cavity volume. The energy loss is to the cavity interior surface from the currents which flow there, giving rise to the E and H fields. Microwave cavities for S-band accelerators may have unloaded values as large as 2 × 104 or larger, but this value decreases with beam loading and external coupling to adjacent cavities and back to the power source. Almost all medical linacs operate in the microwave S-band at a frequency of approximately 3000 MHz. If a beam-loaded cavity has a Q of 104, its half power ∆f is 300 kHz. For stable output with less than 1% reduction in energy, an automatic frequency control (AFC) system would be expected to limit frequency excursions to ±20 kHz. Using the same criterion, the dimensional tolerance for achieving compatible resonant frequencies for a group of S-band cavities is about 10–3 cm and precision machining is required. For copper, the temper-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ature coefficient of expansion is such that a change in resonant frequency f 0 of about 60 kHz/°C results and temperature control of the cavity inner surface of better than ½°C is desired for the structure. Alternatively, the AFC system can follow the resonant frequency f 0 of the structure as the temperature of the structure changes. Electron energy is denoted by V in this section to avoid confusion with the electric field E, and all powers are peak values during the pulse unless stated otherwise. The energy V, gained by an electron traversing a structure, is given by the integral of the first spatial harmonic of the axial electric field Ez, over the length of the structure, L in meters, multiplied by the electron charge e. Ez is equal to E z0 – sin θ , where E z0 is its input maximum value and θ is the phase factor, often near 90°, and described in connection with Figure 2.32. L
V = e ⋅ ∫ E z ⋅ dz
(Mev).
(2)
0
A useful figure of merit for structures and one that establishes the power needed for the requisite E field for a given energy gain is the shunt impedance r. 2
r = E z ⁄ ( dP ⁄ dz )
(MS/m).
(3)
Here, r is expressed as the square of the axial field divided by the power dissipated per unit length. Expressed in megohms per meter, shunt impedance values at S-band typically vary from 50 to 120 MS/m. Both Q and the power dissipation, dP/dz, depend on the precise shape of the conducting walls of the cavities and on the way current is distributed on them. For the general case, it is useful to consider the maximum energy V o gain for zero beam current and then subtract the effect of a beam current i in reducing Ez. V = Vo – iK 1
(MeV).
(4)
The constant K1 is a voltage attenuation coefficient and is characteristic of the particular structure. Strictly speaking, Eq. (4) defines the load line for a TW structure. When impedance matched, TW linacs look like a non-resonant, pure resistive load to the power source, and are characterized by a straight voltage-current load line. The SW load line is curved and falls slightly below that for the TW structure, except where it is tangent at the beam loading for the correct impedance match. SW linacs look like a resonant circuit with a reactive component absent only at one beam condition. The zero current energy V o may be expressed by V o = K 2 L ⋅ r ⋅ P Cu
(MeV).
(5)
Here, K 2 is a constant typically slightly less than unity, r is in megohms per meter, and P Cu is in megawatts copper loss to the total structure length L in meters. Microwave input power is expended as P o = P Cu + P l + P e
(6)
P e = V ⋅ i (MW),
(7)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 where P Cu represents copper losses associated with the intense cavity wall currents which give rise to the cavity E and H fields. The term P l represents residual or reflected power dissipated in a load, and P e , the power ultimately transferred to the electron beam. Linac structures are compared in terms of V o values, the zero beam condition. Thus, for L = 1 m, P o = 2 MW, and r = 50 MS/m, a V o value of 10 MeV is obtained. Electron energy is a square root function of each parameter in Eq. (5), and there are linear trade-offs between them. For example, we might halve the structure length and double the peak power P Cu , leaving the zero beam current electron energy unchanged. The effect of beam loading in a constant group velocity TW structure [Figure 2.30(a)] is described by Eq. (4), as a linear reduction in the rate of energy gain as the bunch proceeds down the length of the structure. We may compensate for this reduction by progressively reducing the aperture size which operates to reduce the group velocity and increase the stored energy in later cavities. The result is a constant gradient structure as depicted in Figure 2.30(b). The effect of beam loading in SW structures operates differently. Here, the effect is to reduce the average value of the E field in all cavities. A high shunt impedance denotes high efficiency in producing high-energy gain at zero beam current and is a significant parameter when comparing the performance of two structures under similar conditions of length, beam loading, and power. At times, we may choose to optimize one parameter at the expense of another. For example, the Varian structure depicted in Figure 2.29(b) has a peak to average axial E field ratio of 1.3 compared to 3.8 for the LASL structure of Figure 2.26(a), although the shunt impedance values do not differ greatly, for 83 MS/m for Varian versus 78 MS/m for LASL structures. Hence, the Varian structure allows much higher average field gradients without electrical breakdown, and a shorter linac structure can be constructed. A 10-cm-long 4-MeV linac using the Varian structure would require 1.8 MW of microwave power at zero beam current. Operationally, we require 0.8 MW of electron beam power; 4 MeV at 200 mA, during the pulse to provide an adequate x-ray dose rate. Hence, total power input to the structure would be 2.6 MW requiring about 3.2-MW magnetron power considering transmission losses, a value considerably in excess of that available from a typical 2-MW magnetron. At the present time, the Varian structure is not commercially employed. We can construct a more conservative 28-cm-long 4 MeV linac which requires only 0.75 MW of microwave power at zero beam current at 2-MW magnetron power at full beam current using either it or the LASL structure. The tradeoff here is between structure length and microwave power. Large amounts of power are expended in the normal metallic resistance of copper. Therefore, electron linacs are operated on a pulse basis with a typical duty factor of 0.001. Hence, the average power consumption in kilowatts is typically numerically equal to the peak value in megawatts. Individual microwave cavities are designed for operation in a particular resonant mode which connotes a specific geometrical pattern of E and H fields within the cavity. Most common for electron linacs is the TM010 mode which has only transverse H fields, as well as the requisite axial E2 fields. We design the structure for operation in this, the dominant mode, and find that other cavity resonant modes, such as the TM010 or TM011, are relatively far removed in frequency. However, the structure operating as a series of coupled cavities may also support additional modes at frequencies near the resonant mode desired, e.g., TM010. These coupled cavity, operational modes consume energy, and usually do not contribute to beam ac-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 celeration. This problem becomes more severe as the number of coupled cavities is increased; the number of coupled cavity modes increases and their frequency separation from the dominant resonant frequency TM010 decreases. This problem is known as “moding,” and design efforts are directed toward its reduction or suppression. For example, the tapered constant gradient TW structure depicted in Figure 2.30(b) is less subject to moding problems than the constant impedance structure of Figure 2.30(a). The periodicity found in constant impedance structure is interrupted since sequential constant gradient cavities differ slightly and coupled cavity mote resonances are not reinforced along the structure. A mid-structure feed point is one method of reducing moding in SW linacs. Before an injected beam of electrodes can gain maximum energy, the structure must be filled with electromagnetic energy. Each structure design has a characteristic fill time, typically about 1 Fs, which is determined by the structure design and the velocity of propagation of rf energy down the structure, the group velocity vg. TW linacs involve one-way propagation of waves, and their fields build up in space during the fill time. A typical group velocity in a TW linac is 0.01c where c is the velocity of light. SW linacs involved two-way propagation of waves. Their fields build up in time to a constant equilibrium value throughout the structure by the two waves bouncing back and forth many times between the two ends of the structure during the fill time. A typical group velocity for these waves is 0.05c in an SW structure and there are typically ten bunches in a 1 m structure. Electrons are injected into the structure with a delay approximating the fill time. The precise time is chosen such that the unloaded E field has built up to about the stable beam loaded value, thereby minimizing electron beam energy spread, and optimizing capture and bunching of injected electrons.
III. Widely Variable Energy Linacs
Clinical considerations suggest the need for a wide variety of beam energies, particularly for electron therapy where tumors of different sizes extend to different depths adjacent to or near a body surface. A range from 2 MeV to more than 30 MeV electron beams has been employed.
A. Clinical Need
Clinically, most tumors may be adequately irradiated with 4–6 MV x-ray beams and treatment units having these energies comprise the majority of linacs in use. However, thick body sections such as the lateral pelvis are advantageously treated with 10–25 MV x-rays. Marks has noted the advantage of treating certain tumors with combined low- and high-energy x-rays. Gale and Innes earlier cited the advantages of employing mixed high-energy x-ray and electron beams. Tapley and others have demonstrated an advantage of adding an electron boost irradiation to some specific lesions treated with megavoltage x-rays. It would appear from this perspective, that a single versatile linac design could advantageously provide a low and a high x-ray energy together with a wide variety of electron beam energies without the necessity of moving the patient. Such requirements pose difficult design problems but several units have been developed recently to solve them.
B. Technical Difficulties
The major problem arises because of the wide energy limits inherent in these specifications for both electrons and x-rays together with significant differences in beam current requirements for the two modalities. For x-ray dose rates comparable to those for electron therapy, the beam current re-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 quirement may be 100 or more times greater than for electron therapy. The overall energy gained by an electron is the integral of the axial E field encountered over the path length through the structure. The E field itself is proportional to P Cu where P Cu is the microwave power delivered to the copper walls of the structure; that is, after beam loading and other losses are considered. The major parameters in determining the range and magnitudes of electron energy are the type and length of the accelerator structure, the available microwave power, and beam loading as it depends on the beam current. Either a klystron or magnetron may be employed as the microwave power source. A klystron usually functions as an amplifier in conjunction with a low-power oscillator. This combination is more stable than a magnetron oscillator and its higher output can be varied more readily over a wider power range than can a magnetron. Although the klystron and associated electronics are more costly than a magnetron, the longer klystron life, sometimes ten times longer, may represent good value. Efficient capture of electrons injected into a linac structure, i.e., the bunching action performed by the first few cavity, is effective over a relatively narrow range of E field values and hence limited microwave power range. Outside this range, it may be difficult to capture and bunch sufficient numbers of electrons to satisfy the high-current requirement for low-energy x-ray therapy, the most demanding modality in this regard since x-ray production varies approximately as V 3 where V is the electron energy. The bunching action of the conventional TW structure is less susceptible to beam loading than the SW structure as explained in Sec. II.-B..
C. Early Designs
In view of these considerations, one early approach in covering a wide electron and x-ray energy range was to employ a relatively long TW structure whose bunching action encompasses the requisite beam currents and beam energy range. Beam energy is varied by changing the microwave power input but an increased energy spread of the beam also results because of poor electron bunching action as the energy range is increased. Beam energy is a square root function of input power and therefore changes slowly with changes in power. Klystrons function well down to half power and magnetrons over a much narrower range. A 50% reduction in klystron power achieves only a 30% reduction in beam energy. A typical energy requirement for electron therapy from 6 to 18 MeV would require a ninefold decrease in power and is beyond the capability of either klystrons or magnetrons themselves. Such an energy range can be accommodated by varying the linac input power through the use of an additional microwave attenuator and ancillary hardware. A very early method of reducing the electron beam energy was to detune the power source frequency from the structure resonant frequency f 0 . Since the resonance curve, Ez versus frequency, for these high-Q structures is very steep, the axial electric field Ez and beam energy change rapidly with tuning. However, this method of beam energy reduction proved unsatisfactory since operation on either of the resonance curve slopes is unstable. The attendant complications include reflections from the impedance mismatch, an increasingly non-uniform axial field Ez, and reduced capture and bunching of the injected beam from the gun. A detuned linac may give rise to excessive x-ray leakage radiation originating from unexpected “targets” where a tuned beam would not normally impinge. Another approach was to join two TW structures in cascade as shown in Figure 2.31. The microwave power is divided between them and the power
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 level and phase made variable for the section. The electron bunches accelerated in the first section are stably located just forward of the advancing wave crest as shown in Figure 2.24(b). By suitably adjusting the phase and power to the second section, they may be further accelerated or de-accelerated. Since the electron velocity is virtually independent of electron energy in this section, a stable position of the bunch on the traveling wave is easier to maintain. When de-accelerated in the second section, these bunches are less stable in position on the wave as explained below so that both an attenuator and phase shifter are used to optimize performance.
KLYSTRON MODULATOR
LOAD
CIRCULATOR
POWER DIVIDER
ELECTRON GUN
POWER SUPPLY
PHASE SHIFTER
ACCELERATOR TW SECTION I
ATTENUATOR EXIT ELECTRON BEAM
ACCELERATOR TW SECTION II LOAD
LOAD
Figure 2.31. Variable energy linac composed of two TW structures in cascade.
D. Phase Considerations
The operation of the two-section linac can be understood in terms of the position of an electron bunch with respect to the phase of the microwave E field being experienced. Figure 2.32 illustrates one cycle of the 3000-MHz E field and several positions where an electron, or bunch of electrons, traveling at a velocity ve, might be located and borne along at the phase velocity of the wave vp. Typically, electrons are accelerated at positions 1, 2, and 3; they are de-accelerated at positions 4, 5, and 6. In general, the phase, or phase angle, refers to a position along the wave here expressed in degrees. By convention, an electron at position 1 at 90° is said to be in phase and at position 5 at 270° as out of phase. The rate of energy gain or loss by an electron is determined by the electric field E it is experiencing and therefore by its position along the wave, i.e., the phase, and the amplitude of the wave. By shifting the phase of the wave, we move the electron bunch to a new position along the wave with a higher or lower value of E. By keeping the phase constant and varying the amplitude of the wave, the magnitude of the E field being experienced by the electron can be altered. The E field pattern recurs along the beam axis twice as frequently as in TW structures; that is, an accelerating electron bunch is contained in every second cavity instead of every fourth cavity. In SW structures, all the cavi-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ties tend to have the same electric field, since the rf power bounces back and forth from each end of the structure many times (typically 100 times) to fill the cavities with energy as expressed in E and H fields. Hence, beam loading of any cavity contributes to the reduction of the E field in all cavities, including the buncher cavities. As a result, bunching in SW structures is efficient over only a narrow energy range, typically ±5%. Thus, to maintain effective bunching action in SW structures, the input rf power must be increased with an increase in beam current.
MICROWAVE E FIELD CYCLE
E–
1 2
3
ve vp
0°
E+
180°
360° PHASE ANGLE
4
6 5
Figure 2.32. Electron bunches shown in representative positions on the microwave E field phase diagram. For simplicity, E– is drawn upward. The overall energy gained is determined by the integral of the electric field E over the path length of the electron through the structure. Electrons gain maximum energy if they are concentrated in a spatially narrow bunch along the axis at the wave crest position 1. They gain less energy but their position on the wave is more stable and axial spread minimized if they were located just forward of the crest at position 2, a position comparable to the idealized electron bunches of Figure 2.24(b). This axial phase stability of an electron bunch at position 2 is achieved as follows: An electron which leads the bunch at position 3 on the wave encounters a smaller E field slowing it down; if it lags the bunch at position 1 on the wave, it encounters a larger E field speeding it up. Both effects operate in sequential cavities to ensure a compact axial bunch and phase stable position as well as a constant terminal energy of the exit beam. This phase stable bunch is often called the synchronous bunch since it moves in synchrony with the wave. The two-section linac illustrated in Figure 2.31 depends on first bunching and accelerating a group of electrons located at or near position 2 in section I to a preset energy. Then, by adjusting the amplitude and phase of the E field in section II, we add to or subtract from this energy. In section II, the energy is added to electron bunches at positions 1, 2, and 3 or subtracted at positions 4, 5, and 6. The energy subtractive positions 4 and 5 are phase stable, but position 6 is unstable in phase and tends to spread a low-energy bunch spatially and, hence, energetically. However, for relativistic electrons over a few MeV in energy, the effect is not large. Beam current losses are more severe when operating linac section II in the energy subtractive mode. These lost electrons, which impinge on the structure disks and walls, are high energy and can give rise to significant anomalous leakage radiation levels. Typically, a relativistic electron bunch would occupy a 20°
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 phase angle interval; that is, somewhat longer than the idealized bunches illustrated in Figure 2.32. The feasible energy range of the two-section linac depicted in Figure 2.31 extends between the sum and difference values of the two individual structure energy gains. Representative electron differential energy spectra for operation with several differences between sections I and II have been reported by Lanzl. Since the first section is operated at constant power level, the electron bunching action performs well. However, the necessity of two microwave structures, two speed points, and additional high-power microwave hardware are complicating factors. Moreover, TW structures are inherently less efficient than SW structure in that residual power is dissipated in an external, or internal collinear load. The Varian Clinac 35 and the AECL Therac Sagittaire treatment units are of the TW two-section design as is an early 5–50 MeV linac described by Carpender et al. A hybrid variation consisting of a short backward wave TW structure followed by a long SW structure is shown in Figure 2.33. Here, the backward wave is magnetically coupled by slots placed peripherally in the disks between cavities. As in the side-coupled structure, the conflicting requirements of power transport versus electron acceleration are separated and optimized independently. The short TW structure is designed for low-power consumption and good buncher performance. Such backward wave structures have the unusual property that the phase velocity vp at which electrons are bunched and accelerated, is opposite in direction to the direction of power flow and group velocity vg, the velocity at which the structure fills with energy. Therefore, this TW structure has the electron gun and power feed on opposite ends of the structure. The klystron power is fed at the beam output end of the TW structure. The residual power appearing at the electron injection end is redirected to the center of the SW section. Electron bunches entering the SW section are traveling at an almost constant velocity near that of light and little energy spread ensues. Standing wave structures may be fed at any point along their length, thereby offering more flexibility in mechanical design. However, there is an advantage in feeding at the mid-region of SW structures, in that absorption of energy in coupled cavity modes is less troublesome.
KLYSTRON MODULATOR
ELECTRON GUN
TW SECTION (backward wave)
LOAD
POWER SUPPLY
SW SECTION (side cavity coupled)
EXIT ELECTRON BEAM
CIRCULATOR
PHASE SHIFTER
ATTENUATOR
Figure 2.33. Hybrid variable energy linac composed of a backward wave TW section followed by an SW section.
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E. Clinac 2500
A recent commercial entry, the Clinac 2500 illustrated schematically in Figure 2.34, features a 1.9-m side-coupled SW structure, wherein one of the side coupling cavities functions as an energy switch to provide two xray beam energies of 6 and 24 MV. This side cavity, shown inset in Figure 2.34, is changed mechanically to alter the x-ray energy by inserting a plunger with an attendant change of the E field amplitude or phase. The accelerating E fields in the buncher are unchanged when switching energy and hence, there is little change in energy spread of the beam. The Clinac 2500 can be viewed as a sophisticated two-section SW linac with an extremely compact power divider/phase shifter. A 5.5-MW klystron is the microwave power source. Six electron energies, 6–22 MeV, are provided. ENERGY SWITCHING SIDE CAVITY
DEMOUNTABLE GRIDDED ELECTRON GUN
DUAL PHOTON STANDING WAVE ACCELERATOR
270° ACHROMATIC BENDING MAGNET
COLLIMATOR ASSEMBLY
ISOCENTER AXIS
RF INPUT COUPLER
ENERGY SWITCHING SIDE CAVITY PLUNGER 2 1
ACCELERATING CAVITIES
Figure 2.34. A two x-ray energy linac using an SW structure and an energy switching side cavity (Clinac 2500, courtesy of Varian Associates).
Figure 2.35(a) illustrates two alternative methods of providing the high and low x-ray energy modalities in the Clinac 2500 side-coupled linac structure by employing two different locations of the energy switching side cavity. The high x-ray energy modality depicted in Figure 2.35(b) is provided by operating all of the axial accelerating cavities of sections I, II, and III of the structure approximately in phase (see electron bunch position 2 in Figure 2.32) with the energy switch plunger completely retreated. The average accelerating field E1 experienced is constant over the entire length of the structure and the terminal electron beam energy is proportional to the area under the average E1 rectangle of Figure 2.35(b).
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ENERGY SWITCHING SIDE CAVITY LOCATION 1
LOCATION 2
ELECTRON GUN I
(a)
II
III
EXIT ELECTRON BEAM
(b)
AVERAGE FIELD
SW SIDE-COUPLED STRUCTURE
(c)
AVERAGE FIELD
E1
E2
E1
(d) AVERAGE FIELD
E2
E1
Figure 2.35. Energy switching side-coupled SW structure and average E field distributions for operating conditions described in the text. In one method of providing the low-energy modality, the energy switching side cavity is placed at location 1, as in Figure 2.35(a), and its normal TM010 mode (E field configuration) is unchanged. The phase is held constant but the amplitude of the E field is reduced by inserting the plunger half way into the cavity into dashed position #1 of Figure 2.34. Here, the energy switch operates at location 1 in order to achieve the desired reduction in amplitude of the E field. Sections I, II, and III all operate in phase but the amplitude of the E field is reduced from E1, in section I to E2 in sections II and III as shown in Figure 2.35(c). The terminal energy is represented by the sum of areas under E1 and E2. Figure 2.35(d) illustrates an alternate method of providing a low-energy modality wherein the energy switch is placed at location 2 in Figure 2.35(a). Here, the normal TM010 E field configuration of the energy switching side cavity is changed to the TEM mode by inserting the plunger fully into the cavity in the dashed position #2 of Figure 2.34. The effect of this is to preserve the amplitude, but to reverse the phase in the axial accelerating cavities which follow it by 180°. The average electric E field now varies as shown in Figure 2.35(d). As a result, the terminal energy is now the energy gained in sections I and II, represented by the rectangle under E1 minus that lost in section III represented by the rectangle under E2.
F. Multiplepass Linacs; Therac 25
Recently, two linac designs have been introduced in which the high-energy requirement is achieved by passing the beam two or three times through the same accelerating structure. Such multiple-pass linacs could be con-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 strued as microtrons, but their geometry, construction, and isocentric mounting are similar to that of conventional linacs. Figure 2.36 illustrates one such unit described by Froelich. It employs either one or three passes through a 0.7-m side-coupled SW accelerating structure (2) and is magnetron powered. An energy gain of 2–8 MeV/pass provides readily adjustable electron energies from 2–24 MeV and x-rays of 6, 12, and 20 MV. One distinctive feature of this unit is the hollow cathode annular electron gun (1) which permits passage of the returning electron beam in the three-pass configuration. Such electron guns tend to have higher emittance than their axial counterparts; that is, beam angular divergence is greater and beam cross section is larger for a given beam current. Another feature is the unique beam return “butterfly” magnets (3) so-called because of the butterfly-shaped electron trajectories through them. These magnets are constructed in three sections to provide the 180° beam turnaround and have shaped pole edges to provide transverse focusing and isochrony so that the electron bunch is reproduced after traversing the magnet.
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1
2
3
3
0
(a)
20 cm 40
SCALE
0
0.5 m
1.0
SCALE
(b) Figure 2.36. (a) A multiple-pass linac or shuttle microtron which involves either one or three traversals of the electron beam through the SW structure; (b) multiple-pass linac mounted in an isocentric gantry (after Froelich) A somewhat similar commercial unit, the AECL Therac 25 shown in Figure 2.37(a), employs two passes of the beam, a hollow cathode gun and one return magnet. The 1.2-m SW structure is of the bi-periodic type shown in Figure 2.37(c) [and Figure 2.30(c)] and incorporates thin pancake-like, onaxis coupling cavities. It provides a 25-MV x-ray beam and eight electron energies between 5 and 25 MeV. The Therac 25 is shortened because of the double-pass design and fits into a shorter room than some high-energy units. The unit is powered by a 2.6-MW magnetron. The beam energy is varied by adjusting the distance of the 180° beam return magnet from the accelerating structure over a short interval of about 2.5 cm. This alters the phase of the accelerating E field encountered by the returning beam and hence, the energy gain of the beam during the second pass as described for Figure 2.32. The 2.5-cm adjustment, )x, of the four-sector, doubly achromatic, isochronous return magnet provides approximately a 0 –180° phase
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 angle range of the E field for the return passage of an electron bunch (see Sec. V.). REFLECTING MAGNET DOUBLE PASS ACCELERATING WAVE GUIDE ∆x
270° BENDING MAGNET
(a)
ELECTRON GUN/INJECTOR SYSTEM
PRIMARY COLLIMATOR
BEAM APERTURE ADJUSTABLE COLLIMATOR X-RAY
(b)
COUPLING SLOT
MACHINED SEGMENT PANCAKE COUPLING CAVITY ACCELERATING CAVITY
(c)
Figure 2.37. (a) A double-pass linac incorporating a single reflecting magnet; (b) half-cavity machined segment showing beam aperture and coupling slots; (c) cross section of biperiodic cavity structure (Therac 25, courtesy of Atomic Energy of Canada, Limited). The treatment head consists of a conventional four-jaw movable collimator suspended below a turntable that has three positions corresponding to the three modes of operation: photon therapy, electron therapy, and field illumination. The nominal 270° magnet system adjacent to the treatment head incorporates two dipole magnet sectors. After traversing this 270° magnet, the electron beam is scanned spirally at two fields per second on a scatterer using a quadrupole magnet. Although not providing a low energy x-ray mode, the Therac 25 provides high electron energies from a comparatively short structure using a magnetron, a relatively inexpensive microwave power source. The use of aluminum coated polyamide films (for example, Kapton, DuPont Company, Industrial Film Division, Wilmington, Delaware 19898) in the monitor ionization chamber is expected to significantly prolong its useful life since these films appear to have good electrical properties and outstanding resistance to radiation damage.
IV. Microtrons
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The microtron is an electron accelerator which combines the principles of the electron linac and the cyclotron.
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A. Introduction, Circular and Racetrack Designs
In the circular microtron, the electron gains energy from a microwave cavity (sometimes called a resonator) and describes circular orbits of increasing radius in a uniform magnet field. The cavity voltage, frequency, and magnet field are so adjusted that after each transit through the cavity, the electron gains sufficient energy so that its transit time in the magnet field increases by an integral number of microwave cycles. An increase in energy gain per orbit can be achieved by placing the cathode inside the cavity and allowing the electron beam to be pre-accelerated before reaching the entrance hole of the resonant cavity for the first time. The microtron exhibits axial phase stability characteristics similar to those of accelerating electrons in linacs as described in Sec. III.-D., but with a narrower energy spread. Splitting the magnet into two D-shaped pole pieces and separating them provide greater flexibility in achieving efficient electron injection and higher energy gain per orbit through the use of multi-cavity accelerating structures. This configuration, called a racetrack microtron, consists of two semicircular and two straight section orbits. One early racetrack microtron designed for radiotherapy provided 1.5 to 15 MeV electron beams through a common exit portal using from 1 to 6 accelerating orbits. The small beam emittance of microtrons (product of beam radius and divergence) and minimal energy spread simplifies the beam transport system and encourages the use of a single microtron for supplying several treatment rooms including, for example, an intraoperative electron beam for an operating suite. An additional advantage may be (depending on the energy and design parameters) attainment of high energies in a smaller system volume when compared with linear accelerators, due to the cubical geometry characteristic of microtron units. Depending upon their energy and design, microtrons may require large heavy iron magnet poles and a high degree of field uniformity. For the circular microtron, the required magnet volume, and hence cost, grows as the energy E3. In recent years, modifications of the conventional microwave accelerating structure and improvements in electron gun injection methods and magnet designs have opened up new possibilities for microtrons in radiation therapy.
B. Circular Microtron — 22 MeV
A novel 22-MeV circular microtron for radiation therapy, the Scanditronix MM 22, shown in Figure 2.38, has been described by Svensson et al. Extraction of the electron beam is made through a movable steel deflection tube which can be moved to select any of the orbits between number 10 and 42, with all the electron energies emerging along a single axis. The deflection tube displaces any selected orbit downward by a constant distance. As a result, the electron beam exits through the fixed horizontal extraction tube as shown in Figure 2.38, instead of continuing through the resonator cavity.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 MOVABLE DEFLECTION TUBE
ELECTRON BUNCHES
ELECTRON GUN
RESONATOR
WAVEGUIDE
EXTRACTED BEAM
Figure 2.38. Cross section of a 22-MeV microtron illustrating the path of electrons in the accelerator (left) and through an isocentric treatment unit (right). The range of motion of the deflection tube from orbit 10–42 is shown by arrows on the dashed line. The electron energy gain per turn is approximately 535 keV and the energy spread of the exit beam is about 35 keV [full width at half maximum (FWHM)]. The magnet poles for this circular microtron are 1.8 m in diameter, and a magnetic field uniformity of 1 part in 10,000 is required. Either a klystron or magnetron can be used as the microwave power source. Two x-ray energy options among 6 or 10 and 20 MV, are available and ten electron energies ranging from 2–22 MeV. A composite x-ray flattening filter is employed to improve depth dose characteristics; high atomic number Z in the center and low Z on the periphery. Two separated scattering foils are employed for electron therapy; a high-Z primary foil of constant thickness is used to spread the beam followed by a low-Z secondary foil of variable radial thickness to flatten the electron fields for sizes up to 40 cm in diameter at the isocenter. Capability of uncoupling one x-ray and electron collimator jaw along the beam axis permits easy abutting of x-ray, electrons, or x-ray to electron treatment fields without divergence. In one configuration, a Scanditronix MM 22 microtron feeds two isocentric treatment units and also provides a research beam. Being stationary, this microtron is more easily maintained, magnetron lifetime may be increased but electron gun replacement may be more frequent as compared to a linac.
C. Racetrack Microtron — 50 MeV
In one newly constructed unit (Figure 2.39), a six-cavity accelerating structure rather than a single cavity is used between the separated pole pieces to provide energy gains of 5 MeV per orbit. An exit beam energy ranging from 5–50 MeV for electrons and photons is provided. The beam energy is changed by moving an extraction magnet in or out to select the appropriate orbit for the desired energy as shown in Figure 2.39. The extracted beam exits along a common beam line independent of energy. A thorough description of the development of the 50-MeV microtron prototype is provided by Rosander et al. Attaining desirable radiation beam characteristics for electrons and x-rays becomes more difficult at higher energies. The microtron, by combining its inherent small beam emittance and energy spread with a scanned pencil beam such as described earlier, may provide one approach to improving beam characteristics. Here, the scanned pencil beam of electrons could fa-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 cilitate the provisions of large flat fields of electrons and x-rays. The 50 MeV as well as the MM 22 microtron designs involve moving either an extraction magnet or a steel deflection tube, respectively, within the evacuated system and this feature could present maintenance problems. movable extraction magnet MAGNET
MAGNET
linear accelerator electron bunches extracted beam
electron gun
Figure 2.39. Racetrack microtron in 5–50 MeV range for electron and x-ray therapy.
V. Beam Transport Magnet Systems
The beam transport magnet system includes solenoids and steering coils over the accelerator structure, together with focusing quadrupoles and bending magnets after the accelerator structure, as well as associated power supplies.
A. Introduction
The system confines, steers, and guides the electron beam from injection to the x-ray target or electron scatterer. For simplicity, magnet energizing coils and magnetic return paths, where used, have been omitted in many of the illustrations which follow. A fundamental design consideration for optimal performance is matching the beam transport system capabilities to the characteristics of the linac beam. Most treatment units are isocentric, a circumstance necessitating bending magnets in higher energy machines so that the correspondingly longer structure can lie horizontally in energy machines. However, the introduction of short SW structures has led to straight-through isocentric 4and 6MV units having a 100-cm source-axis distance (SAD) with moderate isocenter and room heights. No bending magnets are required, and the structure is so short that a solenoid need not be used to confine the beam. However, such straight-through designs let low-energy electrons strike the xray target, so the peak electron energy must be slightly higher than in a bent-beam machine to achieve the same x-ray penetration. The beam transport systems of bent-beam medical linacs must adequately respond to operating conditions which affect beam stability and symmetry. Asymmetries arise when the primary x-ray lobe does not strike the flatten-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ing filter symmetrically. This can be due to a change in the angle of incidence of the electron beam on the target or due to the lateral displacement of the position of the electron beam at the target (see Figure 2.40). These changes can result from changes in beam energy because of a change in microwave power or microwave frequency as well as from other sources such as mechanical strains associated with motions of the gantry or temperature variations as well as from the effects of stray magnetic fields. Such changes occur more often in the bending plane than in the transverse plane. Automatic frequency control and other feedback control devices, based on sampling ionization chambers, have become more sophisticated in limiting adverse response, (Sec. VI.-B.).
ELECTRON BEAM
∆φ
∆r
TARGET
FILTER
RESULTANT DOSE AT Dmax
(a)
(b)
(c)
Figure 2.40. Effect on resultant x-ray dose distribution at Dmax of misalignment between electron beam and flattening filter axis: (a) symmetrical flat field for a correctly aligned x-ray flattening filter, (b) asymmetrical field for an angular divergence )N, and (c) asymmetrical field for a radial displacement )r. At high energies, these effects can be pronounced because of the very peaked filter and high attenuation gradient as a function of its radius.
B. Definitions and Conventions
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Magnets bend (deflect) and separate (disperse) particles of differing momenta. These two actions are necessarily intermixed and we often wish to emphasize one action, for example, deflection in bending a small beam of electrons onto an x-ray target. Medical linacs may employ only one dipole magnet but at times two or more magnets are used to better separate the two actions. The electron beam which enters a beam transport system contains a spectrum of particles which must be restricted in momentum (or energy) range. Spatially, their trajectories may diverge from the central axis and be laterally displaced from it. The task of the transport system is to
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 bring this diversity of particles to a small and coaxially directed beam of particles on the central axis of the x-ray target or the scattering foil. The literature readily applicable to medical linac magnets is modest in extent. Some analyses make use of matrix methods and often differing conventions, notations, and terminology are used. Many phenomena involved with the transport of charged particles in magnetic fields can be described with first-order theory in which quantities involving the products of two or more small differentials of momentum or length are ignored. The theory is limited to particle beams with small energy spread and small angular and spatial spread. A general solution to the electron transport problem involves a second-order differential equation. Its two orthogonal solutions are sine- and cosine-like functions and all possible particle trajectories are some linear combination of them. A number of definitions follow which are useful in describing and understanding the performance of beam transport systems by these as well as less sophisticated methods. An electron of charge e and relativistic mass m moving with a velocity v at right angles to and through a uniform field of strength B experiences a magnetic force Bev perpendicular to its instantaneous direction of motion. It gains no energy from such a static field but is constrained to travel in the arc of a circle of radius D. The requisite centripetal force for such motion, mv2/D, is provided by the magnetic field such that: mv2/D = Bev or BD = mv/ e. The quantity BD measures the “stiffness” of the beam and is called the magnetic rigidity. For example, a 15,000-G dipole field will bend a 25-MeV electron beam with a radius of 5.66 cm. A 5-MeV electron beam can be bent with the same radius by a field of 3240 G. In addition to supplying the centripetal force to bend the beam through an angle, the bending magnet system provides forces to focus the beam. These focusing forces can be provided by tilting the pole faces with respect to each other to curve the field lines throughout the magnet or by tilting the pole edges to employ the curved fringe field lines at the magnet entrance and exit. Tilting the pole faces produces a radial gradient n of the magnetic field defined by ρ ∆B n = ---- ⋅ -------- . B ∆x
(8)
Some magnets provide such a non-uniform transverse field in which the magnetic field increases (n < 0) or decreases (n > 0) with bending radius of the particle. However, most dipole magnets provide a uniform field transverse to the bending plane and are characterized by a field gradient value of n = 0. In either case, the radial focusing (often analyzed in terms of the horizontal or axial motion) can be separated to first order from the transverse (vertical) focusing. The strong focusing properties of quadrupoles may also be employed. Electrons whose momenta differ from a central value P0 are deviated from the central orbit by a magnet. This phenomenon is termed energy dispersion. The term achromatic describes systems which bring particles of differing energies, which were originally paraxial, to the same focus. In a zero dispersion system (sometimes called singly achromatic), the particles diverge after reaching this focal point. In an achromatic system (sometimes called doubly achromatic), the particles do not diverge but remain parallel after reaching this focal point. Double focusing refers to an ability to focus both the transverse and radial image components at the same point along the central trajectory. A double-focused image is called stigmatic. The term energy focus is sometimes used for the momentum focus associated with
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dispersion and the term spatial focus is used for the radial and transverse focusing associated with the divergences and lateral displacement of particles with respect to the entering central ray. Divergence refers to the angular departure )n of the beam from a central axis or trajectory, displacement refers to the lateral positional departure )r from this axis. The latter two quantities are illustrated in Figure 2.40. The return magnets of multiplepass linacs or microtrons must be isochronous; that is, the length of time required to traverse the magnet is independent of electron energy. This ensures that the electrons comprising the bunch are not further separated in time and hence, in phase and energy on subsequent passes. A central orbit reference trajectory is defined herein by particles of momentum P0 which enter the transport system axially and are deflected along a central orbit of radius D0 passing through the median plane between poles [see Figure 2.43(b)]. It is convenient in examining beam transport to combine two main characteristics of a beam, its radius and its divergence, into a single parameter, the phase space area, which is termed emittance. For a constant velocity beam and with the reasonable proviso of no coupling between coordinate motions, the phase space is invariant in time as the beam moves through the transport system. Defining z along the central orbit reference trajectory, we can separate the transverse perturbation motion into small )x and )y displacements with respect to this coordinate. We define momentum coordinates Px and Py which are proportional to their divergences from the z axis. thus: x’ = dx/dz and y’ = dy/dz. The ensemble of particles moving through the system has a constant phase space for both x and y and can be defined at any position z in terms of the maximum excursions of their displacement and divergence coordinates at that position. This phase space can be expressed as an ellipse whose shape varies as the beam progresses through the system but whose area remains constant. The individual particles oscillate about the equilibrium orbit as representative points in phase space. We call the four-dimensional space the emittance g, with ε = xx′yy′ ⁄ π
2
(9)
An important corollary is that if we focus the beam to a smaller diameter, its divergence will increase and vice versa, but its emittance will remain constant. We can also reduce the diameter of a beam, and consequently the current and emittance, by intercepting a portion of it. Acceptance is a complementary term to emittance and refers to a characteristic of an equipment, a measure of its ability to accept and transport a beam rather than a property of the beam. For maximum efficiency, there must be a good match between the emittance of the entrant beam and the acceptance of the transport system, its counterpart. Unless the acceptance of the beam transport system is at least as large as the emittance of the entrant beam, some of the particles will not be transmitted. Beams characterized by small values of emittance are more readily transported over long distances. Most isocentric treatment units employ a nominal 90° or 270° beam-bending magnet with the accelerator guide structure mounted approximately horizontally in the gantry. The 270° magnet systems are usually achromatic and the resulting isocenter height is acceptably low. However, when a 90° magnet system is made achromatic, two or more magnets are involved. The beam path through it becomes long and the accelerator structure axis may lie well above the x-ray target. This can raise the isocenter height excessively in a high-energy machine. The sections which follow describe specific magnet systems.
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C. 90° Magnets
Figure 2.41 illustrates the effect of a simple 90° dipole magnet on the exit beam for entrant beams having an energy spread ± E about E0 or a radial displacement )x or a divergence )n. These aberrations, which are easiest to understand individually in the radial plane of simple 90° magnets can appear in combination and are also present in the transverse plane of magnets. They may, in part, be corrected in more complex magnet systems. Typically, the beam energy spread is restricted to ±10% or less about its central value. The trajectories of the low, central, and high-energy components in Figures 3.40(a) and 3.41(a) are denoted by 1, c, and h, respectively. MAGNET POLE ∆x
∆φ
ELECTRON BEAM c
c
c
∆x
l E0–∆E
c E0
(a)
∆φ
h E0+∆E
c
(b)
c
(c)
Figure 2.41. Effect of simple 90° dipole magnet deflection system on entrant beams which are (a) axial and nondivergent with E = E0 ±)E, (b) nondivergent with E constant but parallel and displaced )X above and below the entrant center line, and (c) axial with E constant but diverging )n above and below the equilibrium orbit. The electron beam entrant center line is denoted by c. The electron trajectories shown were constructed using straight-line entry and exit paths connected tangentially to arcs of constant radius when the electron is in the dipole magnet field. A single 90° bend magnet such as shown in Figure 2.42(a) is not achromatic. It can bend a mono-energetic beam on axis to a point at the x-ray target but the spread of energies of the actual beam results in a spread of such focal points at the x-ray target as shown. This spreading effect can be minimized by reducing the bending radius of the magnet, restricting the emittance and the energy spread of the beam, stabilizing the operation of components which affect beam energy or by incorporating a second magnet to provide focusing. However, even with these precautions, changes of energy as well as variation of the angle and position of the entrant electron beam will produce detrimental asymmetries in the exit beam and treatment field in the non-achromatic 90° systems more readily than in the achromatic 270° systems. The principal effect of an energy change of the entrant beam in a non-achromatic 90° magnet system is a lateral displacement at the target although a secondary effect could be the appearance of new leakage foci. To be effective, energy controlling slits must be located near the output of a 90° magnet, but here they tend to become enlarging the effective focal spot size. A uniform field 90° achromatic bending magnet system suggested by K. L. Brown is described by Penner. It consists of a single quadruple magnet positioned in the plane of symmetry between two 45° deflection magnets. The magnet system is achromatic and double focusing but its design requires a high isocenter. The performance of the 90° bent-beam Clinac 6 treatment unit has been reported by Horsley and its magnet described by Avery.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
D. 270° Magnets
Figure 2.42(b) illustrates a 270° triple-focus, zero dispersion (singly achromatic) uniform field magnet wherein a ±10% beam energy spread is brought to a single focal point on the target, in part, by choice of the angle of the entrant and exit pole faces. The higher energy component is deflected through a circle of larger radius, the lower energy component l through a circle of smaller radius, but both converge on the target at the same point on the central energy trajectory c. However, a change in mean energy of the beam from the accelerator structure will result in a change in mean angle of the beam at the target and hence to asymmetry in the x-ray field. ELECTRON BEAM
h c l
MAGNET POLE
MAGNET POLE
ELECTRON BEAM
TARGET
l c h
TARGET
(a)
(b)
d
2
MAGNET SECTORS
d
1
MAGNET POLE a2 SECTION d1-d2 ELECTRON BEAM MAGNET POLES
a1
(c)
(d) d
2
TARGET MAGNET POLE Y Z X
S2
SLITS Z
MAGNET POLES
S1 l c h
S1 a2
X S2
d
1
ELECTRON BEAM a1
BX = BY = 0 BZ = GXn
TARGET
(e)
SECTION d1-d2
(f)
Figure 2.42. Nominal (a) 90° and (b) 270° bending dipole magnets with representative electron orbits. Trajectory c corresponds to the central energy E0, trajectories l and h correspond to E0 –10% and E0 +10%, respectively. One type of 270° achromatic magnet is shown in (c) and (d) and another type in (e) and (f).
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Magnet edges contribute important focusing properties. In one approximation, the transverse field can be assumed to begin and end abruptly at an edge near where the beam enters and where it leaves the magnet. A beam of particles entering normal to such an edge will be radially converged. The edge may be angled other than at 90° with respect to the particle beam and particles will be more rapidly converged (focused) or less rapidly converged (defocused) in the radial plane. The fringing magnetic field which exists beyond the pole edges, provides additional focusing for an angled pole edge. Here, a radial field component exists which interacts with the azimuthal particle velocity to produce a transverse (vertical) force. This force will be focusing (toward the central orbit plane) or defocusing (away from it) depending on whether the pole edge angle, with respect to the beam, is greater or less than 90°. If the edge tilt produces focusing in the transverse plane, then it produces defocusing in the radial direction, and vice versa. As in the case of magnetic quadrupoles, transverse focusing and radial defocusing go together and vice versa. The pole edge angles chosen in Figures 3.42(b) and 3.42(c) provide this edge focusing effect. Cross has shown that some judicious choices of angles can result in two-directional focusing in both the radial and transverse directions. In order to minimize distortions in radiation field flatness due to changes in beam energy, it is desirable to employ an achromatic magnet system. Figure 2.42(c) illustrates one way to build a 270° achromatic magnet system by combining uniform and non-uniform field regions in the same magnet. It focuses a range of entrant momenta, and an input configuration of lateral displacements and angular divergences of the electron beam, to an optically similar configuration at the output focal plane. Its entrant pole face angles, a1, and a2, can be adjusted for optimal radial and transverse focusing of each mono-energetic bundle of rays at the d1 – d2 plane. In addition, two adjustable pole sections of the magnet, shown in section view d1 – d2 [Figure 2.42(d)] provide an adjustable radial field gradient with n > 0. This radial gradient is controlled by adjusting angle a3 [Figure 2.42(d)] to re-converge the different energy bundles of rays into a single spot at the x-ray target, the distribution of rays in this spot being the same as in the beam cross section which enters the magnet. The Siemens Mevatron treatment units employ this type of bending magnet. Establishing and adequately preserving the electron beam position and direction at the x-ray target or scattering foil to maintain a symmetrical radiation field become more difficult for higher energy linacs which operate over a wide range of energies. High-energy treatment units incorporate as many as six discrete magnets in their beam transport system. Because of iron hysteresis and saturation effects, a programmed sequence of magnet current changes may be employed to reliably and accurately establish or change a given beam energy or modality. A three element, double focusing, 270° achromatic magnet system has been described by Hutcheon and Heighway. It employs an input quadrupole doublet and a 180° uniform field magnet separated by a short drift space from a 90° uniform field output magnet to provide a nominal 270° system. The system minimizes the magnet dimensions in the direction opposite to the exit trajectory, facilitating a lower isocenter height. It is suitable for electron beam energies from 5 to 50 MeV, an overall energy spread of 20% and a total bending angle between 230° and 290°. The AECL Therac 25 treatment unit employs this type of bending magnet.
E. Mirror 270° Magnet
The 270° magnet illustrated in Figures 3.42(e) and 3.42(f) is a design of Enge. It is sometimes called a “loop” or “pretzel” magnet but more often an achromatic magnetic mirror since particles which traverse it appear to be
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 reflected from the y–z surface at the entry point. It is employed in the Brown-Boveri Dynaray treatment units. Typically, its non-uniform field increases with x having an n = 1 exponent value near the edge and n = 0.8 elsewhere and angle a1 = a2 = 45°. The x–y proportions of the trajectory loops are constant independent of momentum, i.e., the loops have the same shape, and all particles exit at the origin independent of momentum, i.e., there is no dispersion and the deflection is achromatic. This magnet is capable of faithfully focusing a wide range of momenta as limited by slits S1 and S2 placed on the plane of symmetry [see section d1 – d2 in Figure 2.42(f)]. The Varian Clinac 2500 employs a modified loop magnet wherein a simple two-step gap replaces the smoothly varying gap along the x axis [Figures 3.42(e) and 3.42(f)]. The beam trajectory in the modified loop magnet has been described by Tronc. The Dynaray-4, employing a 270° “mirror” magnet is described by Sutherland.
F. Three-sector 270° Magnet
The illustrations comprising Figure 2.43 describe the nominal 270° magnet system used in the Varian Clinac 18 treatment unit and are shown mounted in the treatment head in Figure 2.45. Figure 2.43(a) is a crosssectional view of this magnet in the median bending plane of the central orbit. It incorporates three uniform field magnet sectors, or pole sets, M1, M2, and M3, with short drift tubes connecting them. A magnetic shunt between poles (not shown) provides a relatively magnetic field free region for passage of the particles between sectors. The emittance of the beam entering the magnet system is typical of the output beam of an electron linac in terms of cross-sectional are, divergence, and energy spread. The performance of the system is analyzed with respect to a particle which enters along the central axis with reference momentum P0 and whose central orbit reference trajectory is shown as a heavy dashed line in Figures 3.43(a) and 3.43(b). It has been shown by Brown that the properties of such a system are completely determined to second order by specifying five representative trajectories or paths relative to the reference trajectory. Spatial departures from the reference trajectory by particles of reference momentum P0 are separated into orthogonal radial and transverse components for initially axial but divergent trajectories Sx,y and for initially parallel but displaced trajectories Cx,y. The momentum trajectories Dx dispersed from the reference trajectory in the median plane are for particles initially axial and with momenta within ±)P of reference momentum P0. Two of these trajectories, shown in Figure 2.43(a), depict divergence from the central orbit (Sx) and lateral displacement from the central orbit (Cx) in the median plane. Figure 2.43(b) is a simplified view similar to Figure 2.43(a) depicting trajectory Dx, of momentum dispersed particles initially on the central axis. The trajectories of all particles through the system are symmetrical about a plane of asymmetry located at 135 midway along and normal to the reference trajectory. As shown, the energy selection slits S1 and S2 which are placed at ± )P about radius D0 of the reference trajectory intercept all particles except a narrow momentum band ±)P centered about P0. An end view of this magnet system in the transverse plane is shown in Figure 2.43(c). The entering angular divergence Sx and lateral displacement Cx in the beam cross-section from the accelerator are reproduced at the target plane as shown in Figure 2.43(a) with no significant increase in their magnitude. Figure 2.43(d) illustrates focusing properties in the transverse plane due to the fringing fields of the shaped pole faces along and near the reference trajectory and depict angular divergence Sy and lateral displacement Cy. Again, these transverse divergences and displacements are repro-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 duced at the target with no significant increase in their magnitude. The entering radial and transverse slop and displacement perturbations are equal in magnitude and remain uncoupled in the transport system to first order. Since their dimension in the transverse plane is equal to that in the bending plane, the focal spot is circularly symmetric. This system is achromatic since both the spatial dispersion and its derivative are zero at the output plane. The energy determining slits shown in Figures 3.43(a) and 3.43(b) are sources of leakage radiation from stopped electrons. However, this bremsstrahlung is directed away from the isocenter and not contributory to patient exposure. A magnetic analysis of the electron treatment beams from a Clinac 18 gave energy dispersion values of ±0.4 to ±0.7 MeV FWHM over the range 6–18 MeV, respectively. These dispersion values include the effect of scatter from the linac thin window and 1 m of air. A description of magnetic and threshold techniques for energy calibration of high-energy radiations has been given by Lanzl.
MAGNET MAGNET COIL SECTOR
PLANE OF SYMMETRY PLANE OF SYMMETRY S2
M
ENTRANT TRAJECTOR Y
M1
TARGET
CX
(a)
S
M
SLITS
S1 M2
M3
SLITS
S
SX
CENTRAL ORBIT REFERENCE TRAJECTORY
D
M
CENTRAL ORBIT REFERENCE TRAJECTORY
n
MAGNET COIL VACUUM CHAMBER
–∆ρ
+∆ρ ρ0
ENERGY SELECTION SLITS
n M3
BEAM APERTURES
–∆ρ
(b)
ρ0
PLANE OF SYMMETRY
M1 CY SY
(c)
+∆ρ
CENTRAL ORBIT REFERENCE TRAJECTORY
(d)
N
N
SY
N
S
S
CY
S
M1
M2
M3
Figure 2.43. Nominal 270° achromatic bending magnet system: (a: cross-section view in the radial (bending) plane of the orbit; (b) simplified view in same plane showing momentum dispersion and the use of energy selection slits to limit the momentum to a narrow band ± P about the central reference momentum P0; (c) transverse section end view of the magnet system; (d) central orbit trajectories (simplified) along transverse section.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Typically in this magnet system, the energy defining slits are set for ±3% transmission and are sometimes placed between dipoles M1 and M2 rather than at the plane of symmetry as shown in Figure 2.43(a). The energy servo system, fed by signals from the ionization chamber monitor system described in Sec. VI.-B., controls the input microwave power level so as to maintain constancy of beam energy. The inherent open-loop stability of the system confines the energy spread to within ±1% during a typical treatment, and to within ±5% during a treatment day. The closed-loop gain confines the energy spread to within ±0.1% during a treatment, and to within ±1% during a treatment day.
G. Other Magnets
Panofsky and McIntyre describe an achromatic beam translation system involving two sector magnets. Their objective is to translate the beam without energy dispersion, in order to dispose of unwanted low-energy electrons. More recently, a system involving two magnetic quadrupoles acting as magnetic mirrors have been combined with two microwave cavities to translate, chop, and bunch an electron beam. Here, use is made of an earlier discovery that a magnetic quadrupole with a rectangular aperture can be provided by current sheets bounded by iron rather than the shaped iron pole faces of more conventional designs with their angle aperture limitations. The multiple-pass linacs described earlier incorporate unusual 180° turnaround magnet designs which were described there. Two early radiotherapy beam transport systems not cited in the preceding sections merit attention. Both are isocentric in design and involve rotation of the magnet system about a fixed horizontal linac axis. In one system, the electron beam from a single-section 45-MeV TW linac is first momentum analyzed by a 45° magnet followed by energy defining slits. The beam is then redirected by a 135° magnet so as to be perpendicular to the axis of rotation. The magnet system can rotate ±45°, with respect to the zenith, about the horizontal linac and patient axis. The second magnet system is incorporated in the AECL Therac Sagittaire, a two-section TW linac. It involves a similar but more sophisticated magnet system incorporating additional magnets and permitting 360 rotation around the recumbent patient.
H. Scanned Pencil Beams
Most beam transport systems provide as output a small pencil beam of electrons which are incident on an x-ray target or scattering foil in a fixed position and direction. In an alternative design, the nominal 90° bending magnet is preceded and followed by a scanning magnet as shown in Figure 2.44. The two scanning magnets are placed orthogonal to each other. The electrons still impinge on the x-ray target at a fixed point but their angle of arrival is varied by the two scanning magnets in order to sweep the direction of the x-ray lobe. By suitably varying their magnetic fields, a raster scan of the x-ray lobe direction can be generated similar to that used in a television picture tube. Alternatively, a spiral scan can be generated by appropriate variation of scan magnet currents. An x-ray scanning system incorporating a quadrupole magnet has been described. By retracting the x-ray target, a scanned electron therapy beam is obtained. Such scanned electron beams provide electron treatment fields of good uniformity, low x-ray contamination and could be “programmed” for the arbitrarily shaped fields characteristic of electron therapy. The x-ray lobe scanning feature can largely compensate for the unflatness arising from the angular distribution of bremsstrahlung production and the variation of energy with angle across the field. A thin flattening filter can then be used thereby avoiding significant changes in the photon energy spectrum in the middle of the field rela-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 tive to the edges of the field. This makes it easier to provide large uniform xray fields at high energies. The scanning feature, however, adds complexity and cost and dosimetry is more difficult. For example, monitor ionization chamber collection efficiency may be significantly impaired with attendant non-linearities of the dose monitor. Isodose measurements in a phantom are more complex because of the scanned nature of the beam. BENDING MAGNET ELECTRON BEAM
SCANNING MAGNETS
(a)
(b)
Figure 2.44. Scanned pencil beam provided by a three-magnet system (adapted from Brahme)
Scanned beam systems are interlocked so that if scanning becomes inoperative, no significant hazard arises from the stationary air-scattered, highenergy beams. This “pencil beam” or electrons is minimally scattered by the vacuum window of the structure and the intervening air. For low electron energies, it may be as broad as 10 cm at l00-cm distance, but narrows with increasing energy. A study of the scanned electron beam from a Therac 20 accelerator has been carried out by Pfalzner and Clarke. This unit employs a scanning quadrupole with sawtooth modulated currents of 0.615 Hz and 4 Hz, respectively, in the x and y directions. They conclude that the electron beams of the Therac 20 do not differ appreciably from those of the nearly mono-energetic 22-MeV MM-22 microtron beams, as reported by Svensson, Brahme et al. A high-energy scanned pencil beam of electrons was used earlier by Carpender et al. on a 5–50 MeV linac. This two-section linac was equipped with a novel magnet system for scanning the pencil beam of electron.
VI. Treatment Head
The characteristics of electron and x-ray treatment beams are strongly influenced by the design of the x-ray treatment head.
A. Introduction
A representative design is illustrated in Figure 2.45. In addition to a bending magnet, fixed shielding, and large movable collimators, the head contains the x-ray target, flattening filter, and in some cases, dual electron scattering foils, often mounted on a large carousel. Included also is a field
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 light with a sizable 45° mirror for illumination of large field sizes and optical distance indicator optics together with an increasingly large diameter and complex ionization chamber assemblies for monitoring and control. Accessibility for service often becomes difficult, but a recent “swing away” collimator design may significantly improve access. An earlier Therac 20 head design allows one of the magnet half yokes and associated shielding to be hinged for easy access. X-RAY TARGET (RETRACTABLE) BENDING MAGNET ASSEMBLY ELECTRON ORBIT
FLATTENING FILTER SCATTERING FOILS
DUAL IONIZATION CHAMBER FIELD DEFINING LIGHT RANGE FINDER COLLIMATORS
Figure 2.45. A representative linac treatment head (Clinac 18) with major components identified. The desire for larger field sizes, improved beam characteristics and convenient, functional accessories has led to a number of studies and improvements in treatment head design. The trend from 4–6 MeV to higher energy x-ray treatment beams, larger field sizes and the increasing use of accessories have resulted in an increase in treatment head size in terms of its height above the accelerator structure axis (radially away from isocenter) and its diameter below it. The former stems from the incorporation of the larger 270° bending magnets, thereby increasing the height of both the treatment head and the isocenter. The increase in diameter stems primarily from the requirement for large fields. The large treatment head diameter may interfere with optimal placement of the treatment beam in some anatomical regions such as lateral breast. Increased length of the treatment head toward the isocenter may limit the use of outboard accessories for short SAD units, such as the Clinac 4, as well as increase the amount of
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 scatter radiation reaching the patient from the head. Large, heavy, movable collimator jaws are needed for megavoltage x-ray beams with field sizes variable from zero area to as much as 40 × 40 cm at 1 m from the source. High-energy isocentric machines incorporate large bending magnets. These must be shielded extensively to attenuate radiation arising from their energy selection slits and elsewhere. A high-density, high atomic number material such as uranium (usable up to about 6 MeV), tungsten, or lead are frequently used to conserve space and to rapidly attenuate x-rays. When tungsten or lead are employed for collimators, head shielding, x-ray targets or flattening filters above 10 MV, photoneutron production increases rapidly. In some machines, low-density hydrogenous shielding materials have been incorporated in the head for neutron shielding, further increasing its size. Some 4–6 MeV units employ short accelerating structures in straightthrough beam designs in both isocentric and non-isocentric treatment units. However, at 8 MeV and above, longer accelerating structures are required and are usually incorporated into bent-beam, isocentric units. At such higher energies, x-ray beam flatness becomes increasingly sensitive to angular and spatial misalignment of the x-ray lobe with respect to the axis of the flattening filter. The effects on flatness can be described in terms of displacement )r and divergence )n of the electron beam with respect to the axis of the filter as illustrated in Figure 2.40. Such misalignments can result from small changes in beam energy but are less likely to do so for achromatic magnet systems which may incorporate narrow energy defining slits. They are more likely to result from mechanical strains which occur from the stress of gantry rotation, from beam stopper extension or retraction, from temperature changes of mechanical and electronic components, as well as from the presence of nearby ferromagnetic materials. The effect of such small-energy, displacement and divergence changes on an electron beam traversing a simple 90° magnet system is illustrated in Figures 3.41(a), 3.41(b), and 3.41(c), respectively. Achromatic magnet systems tend to limit the effect of such changes in entrant )E, )r and )n changes on the output beam image and hence, field symmetry and stability. Their output phase space image is a faithful reproduction of their input phase space image with unity magnification. Beam energy stability of a few percent is required to ensure satisfactory constancy of symmetry in bent-beam linacs. Under normal operation, the energy interval selected by the bending magnet slits is centered about the current maximum as illustrated inFigure 2.43(b). If the energy of the beam entering the bending magnet from the accelerator structure shifts, the primary effect is a reduction in output. A secondary effect is a change in beam symmetry in the radial plane, the sense of the change depending on which side of the current maximum the new distribution is centered. Similar, but far less severe, stability problems apply to electron therapy. Naylor and Chiveralls have examined the variations in x-ray beam flatness and calibration with gantry angulation and with time for an 8-MV unit equipped with a 90° bending magnet. They conclude that such variations are confined to a few percent when averaged over four-week periods, an interval pertinent in treatment. Padikal et al. describe a method for assessing the stability of symmetry with gantry angle rotation. In an early study, Naylor and Williams call attention to the need for frequent symmetry, dose monitor, and beam energy checks of linac treatment units. Characteristics of the Mevatron 15-MV photon beam have been described by Paul et al. The x-ray and electron beam dosimetry characteristics, as well as neutron measurements for a Mevatron 80, are described in a series of recent reports. The treatment
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 head design for this 20-MeV unit has been described earlier. Similarly, the x-ray beam characteristic of the Clinac 18 has been reported by Connor et al., and those of the Therac 20 by Patterson and Shragge Design considerations involving the treatment head and pertinent to electron and x-ray therapy, neutron production considerations, together with ionization chamber, dosimetry, and beam steering aspects, follow.
B. Ionization Chamber, Dosimetry, and Beam Steering System
The monitor ionization chamber of a contemporary, high-energy linac is constructed of several plates or electrodes, whose areas may be divided into sectors so as to serve two different monitoring purposes: (1) dosimetry of the x-ray and electron treatment beams, and (2) monitoring of the intensity distribution of the radiation field. The resulting signals can be used in automatic feedback circuits to steer the beam through the accelerator, bending magnet, and onto the target (or scatterer) in order to ensure beam flatness and symmetry. How these needs are satisfied in one representative treatment unit, the Varian Clinac 18, is described below and illustrated in Figure 2.46, a simplified diagram of the ion chamber, dosimetry, and beam steering system. The transmission ionization chamber, shown in the treatment head diagram of Figure 2.45, subtends the entire useful beam and provides two independent outputs for the dual dosimetry monitor. Located just below the flattening filter or scattering foil, it samples the flattened xray or scattered electron beam. This ionization chamber, shown in more detail in Figure 2.46, consists of three polarizing plates and two collecting plates, the latter divided into four collecting sectors, with the sectors defining four distinct laminar collecting volumes. A 500-V polarizing voltage is used with a plate spacing of 1 mm. The two inner D-like sectors provide signals for both dosimetry and steering and the two outer arclike sectors for steering only. The dosimetry system monitors and displays readings related to the quantity and uniformity of the useful beam of radiation. Sutherland early recommended that integrated dose be based on monitoring only the central portion of the field. Most treatment fields are small, and he found that monitoring the entire beam profile can result in axial calibration errors as large as ±3%. In one construction technique, the collecting plate sectors are formed by vacuum deposition of a thin metallic coating on defined areas of an insulating lamina of mica Additional grounded metallic coatings, not shown in Figure 2.46, surround the collecting areas and serve as guard rings to minimize leakage currents over the insulation. Ionization chambers may be sealed to the outside air making them free of the need for temperature and pressure corrections. The electron beam center line through the linac structure and 270° bending magnet is shown in Figure 2.46. This center line establishes the angular orientation of the semicircular collecting plates with respect to the radial and transverse coordinate planes of the beam through the bending magnet as shown. In general, the upper collecting electrode of Figure 2.46 is concerned with signals pertinent to the radial plane, the lower collecting electrode to the transverse plane signals. Semicircular plates A and B are oriented to provide signals related to the radial plane. Their signals are first amplified via A1, and A2, then summed via A3 to provide a console indication of dose rate and of integrated dose via integrator #1 for MU1 channel as shown. Similarly, semicircular plates C and D are oriented to provide signals related to the transverse plane. They feed MU2 channel via amplifiers A5 and A6 summing amplifier A7 and integrator #2. The two dose channels are completely independent, either can terminate the preset exposure with the second channel lagging the first by a constant 40 monitor units.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Accelerator Guide Angle R Steering Coils
Position R Steering Coils
Buncher R Steering Coils
Buncher T Steering Coils
Position T Steering Coils
n Beam Electro
Angle T Steering Coils Target Dose Rate Meter A A 1 B A 2
A3 A4
F A 9 E A 10
A13
G A 11 H A 12 D A 5 C A 6 -500V P.S.
A14 A8
A+B
MU1 Integrator
A-B
(A - B) + (E - F)
SYM1
E-F
G-H
(C - D) + (G - H)
SYM2
C-D
A7
C+D
MU2 Integrator
Figure 2.46. Five electrode ionization chamber with simplified block diagram of dosimetry and beam steering system. The radial and transverse coordinate planes of the bending magnet orbit are identified in the upper left. As shown in Figure 2.46, two groups of four steering coils are used in a servo-feedback system using signals from the ion chamber sectors to control and limit the lateral displacement (position) and angular divergence (angle) of the electron beam in the radial and transverse directions. Both groups of steering coils provide small angular deflections of the electron beam. The angle steering coils shown on the left in Figure 2.46 are located immediately adjacent to the 270° bending magnet entrant aperture. The position steering coils, shown on the right in Figure 2.46, are located at a distance from this aperture and at the end of the accelerating structure. The principal effect of their small angular deflection is to provide a lateral displacement or position correction at the bending magnet entrant aperture. These coil groups are energized by error signals generated if the electron beam strikes the target, or scattering foil at an angle or at a position that produces an asymmetrical x-ray or electron beam, as shown in Figures 3.40(b) and 3.40(c), respectively.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Two of the beam position (lateral displacement) steering coils are connected to limit the radial displacement, and two the transverse displacement of the electron beam from the field flattening filter axis as shown in Figure 2.40(c). The beam angle steering coils are located near entrant dipole M1 of the 270° magnet system shown in Figure 2.43(a). Two of them are connected to limit the radial angular divergence, and two the transverse angular divergence of the electron beam from the field flattening filter axis as shown in Figure 2.40(b). Not shown is a third group of four coils positioned around the beam at the buncher end of the linac structure. These buncher coils also control motion in the radial and transverse planes, steering the electron beam leaving the gun onto the center line of the structure. Signals from peripheral plates E and F are amplified by amplifiers A9 and A10. Their difference signal (E–F) from A13 feeds the radial position steering coils. The radial position steering coils control the radial component of lateral displacement of the electron beam with respect to the flattening filter axis as shown in Figure 2.40(c). Amplified signals from semicircular plates A and B are subtracted in difference amplifier A4 which feeds the radial angle steering coils. The radial angle steering coils control the radial component of angular divergence of the electron beam with respect to the field flattening filter axis shown in Figure 2.40(b). The angle and position asymmetry signals from amplifiers A4 and A13, respectively, are combined to provide a visual display of radial plane beam asymmetry at the console. They are set to provide an operational radial asymmetry limit beyond which the beam is turned off. In a similar manner as shown in Figure 2.46, but not described here, the transverse position and transverse steering coils are connected to amplifiers and ion chamber sectors so as to set servo-control, limit, and display the electron beam asymmetry in the transverse plane. The symmetry meter can be switched to indicate either radial or transverse symmetry. It and the associated steering coils are connected to use all signals from the collecting electrodes. The buncher, beam position, and beam angle steering amplifiers are each provided with six programmable gain and balance controls corresponding to the one x-ray and five electron modes of operation.
17.1. Electron Therapy
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Since the requisite beam current for electron therapy is typically several hundred times less than for x-ray therapy, most shielding problems, including neutron attenuation, are comparatively insignificant even at the higher energies. One is concerned clinically with providing wide, flat electron fields with modest penumbra, a relatively low surface dose, deep penetration to 80% depth dose for each selected energy, rapid falloff of dose with depth on the distal side of the depth dose curve as well as a low contaminating x-ray background. Achieving and assessing these electron beam characteristics have been the subject of a number of studies. Usually, electron applicator cones are attached to the treatment head with the x-ray collimator jaws set to provide fields a few centimeters wider. There is accompanying improvement in electron field flatness from this procedure, preferentially at shallower depths. A too wide setting may create exposure problems outside the treatment volume as in the case of pregnant patients for one particular linac. Schneider has evaluated such leakage radiation for another model of linac, but measurements by Jones suggests that the leakage may be a machine-specific characteristic which should be evaluated by individual users. Often, a lead or Lipowitz’s alloy (for example, Cerrobend, an alloy of Cerro Copper and Brass Company, 16,000 St. Clair Avenue, Cleveland, Ohio 44110) cutout defines the final field size and shape and is placed in the end of the applicator at or near the patient’s skin surface. The
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dosimetry of such shaped small and irregular fields has been described for electrons from 4 to 10 MeV. Electrons scattered off the applicator walls improve flatness at the periphery of the field at a shallow depth with less penetrating electrons and hence, poorer flatness at greater depth. A continuously variable electron beam collimator has been described by Robinson and McDougall, and at least one manufacturer makes such an option available. Most studies find that a single, high atomic number scatterer will adequately flatten small fields up to about 10 cm in diameter for low electron energies up to about 10 MeV. Additional steps are taken at higher energies and for larger fields. Providing several different scatterer thicknesses, often in a carousel, facilitates optimization of beam characteristics for different energies. A dual-foil scattering system, with a few centimeters or more between the two foils, significantly improves electron beam flatness characteristics, particularly above 15 MeV and for fields 15 cm in diameter and larger. The first high atomic number (Z) foil is selected to minimize energy loss for a given scattering distribution and the second foil made of a low-Z composite, thicker on axis, functions more as a field flattening absorber preferentially scattering electrons peripherally. Hence, the electron applicator in such systems primarily serves to define the field size and only affects flatness secondarily. A thorough analysis of the dual-foil scattering system has been provided by Mandour and Harder. Scanning the treatment field with a pencil beam of electrons is an alternate approach to electron therapy. It offers advantages at higher energies of 25– 50 MeV where bremsstrahlung contamination from scattering foils becomes significant. Earlier, Rozenfeld et al. developed a scanned pencil beam system for arbitrarily shaped fields defined by a full size template, in lieu of using scattering foils. The isocentrically directed beam, up to 50 MeV in energy, employs a complete, non-achromatic magnet system. It combines a linear translation of the magnet up to 21 cm along the direction of the gantry axis with an indexed rotation of the gantry to provide 5-mm spacing between scan lines at the skin surface. A treatment is completed in one scan series covering the field, and different electron energies can be used in different portions of the field. This scanned beam system employs a large, non-achromatic beam transport system with attendant stability problems of the treatment beam. Earlier, Briot et al. investigated the scanned electron beam of a Sagittaire 35-MeV treatment unit. They conclude that an adjustable outboard metallic collimator, which could be attached to the x-ray jaws, improved the depth dose and dose gradient at the edges of the field. Others compared the Siemens betatron and Sagittaire linear accelerator electron beams. Bell and Waggener have described a method for rapid determination of the energy of electron treatment beams from medical linacs. Microtrons, having a wide energy range, have come to be employed for electron therapy. A controversial aspect of linac versus microtron treatment beams concerns dependence of the characteristics of the electron beam depth dose on the energy spread of the electron beam. Brahme and Svensson contend that the microtron beam has a depth dose significantly improved (sharper buildup and steeper falloff regions) over that of the linear accelerator and attributed, in part, to the narrower energy spread of the microtron beam. Fregene suggests that a small difference in energy spread, which is of the same order of magnitude in the microtron and linear accelerator beams, should not lead to appreciable differences in depth dose at 10 MeV. Some of the disagreement between observers might be explained on the basis that measurements were performed on different accelerators having different treatment heads (different collimators, scatterers, geome-
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 try, etc.) using different phantom materials and dosimetry techniques. Recent measurements by Johnsen et al. of electron beams for a single linac operated at nominal peak energies of 6 and 12 MeV appear to have resolved this controversy. Based on magnetic analysis of the beams, they compare narrow energy spread beams (0.1 and 0.2 MeV FWHM) with broad (0.8 and 1.2 MeV FWHM) for 6 and 12 MeV, respectively. They conclude that, when consistent measurement techniques are used for the beams tested on the same beam-collimator system, the electron depth dose characteristics are not significantly affected by these relatively large changes in the width of the accelerator’s energy spectrum. Earlier, Bjarngard et al. found no significant difference between the Mevatron XII electron depth dose curves and those from a microtron.
C. X-ray Therapy
A number of studies have focused on x-ray target selection, beam penetration (e.g., percent depth dose at 10 cm or depth of 50% depth dose), and flatness aspects. Podgorsak et al. examined the effects of different atomic number targets and flattening filters on small 10-cm-diam fields 100 cm from the target for energies of 25 MeV. They found that x-ray output on the central axis does not depend significantly on the Z of the targets. However, an Al target gives a more penetrating beam, although high-Z targets emit more radiation at large angles. They also found that an Al filter hardens the beam and a high-Z filter softens it. They recommend a thick Al target and flattening filter above 15 MeV for the most penetrating beam. Below 15 MeV, a high-Z target and low-Z filter are recommended. At 25 MeV, an Al flattening filter is 25 cm in length, an impractical size to incorporate in most linac treatment heads. Ideally, one wishes flattened fields for all field sizes, at all depths, an impossible requirement because of energy changes and scattering in the phantom. One early 4-MV filter design provided satisfactory flatness at 10-cm depth but resulted in excessive dose at shallow depths near the edges of large fields. Invariably, flattening filter choice involves a compromise in order to achieve uniform small and large fields over a range of depths. Two flattening filters are provided in some treatment units to optimize beam flattening with the changeover at about 10 × 10 cm fields, or an additional filter may be attached to the accessory ring for assuring large field flatness. McCall et al. examined linac depth doses using a semi-empirical analytic depth dose model correlated with experimental measurements. They find that an Al filter at 25 MeV produces a great deal more beam hardening on axis than do Ni and W and consequently, there is a larger variation of E with production angle for Al, a significant effect for large field sizes. Al is a good flattener, but the penalty is significant energy spread across large flattened fields. For the Clinac 35, operating at 25-MV x-ray energy, they recommend a Cu target with an Fe filter containing a W conical insert for an optimum combination so as to minimize energy variation with angle and restrict beam hardening on the centerline. Other important aspects of a good target-flattener system are that the flattener must not become too radioactive in operation, and neutron production must be minimized. Thus, other good properties notwithstanding, copper filters are not commonly used above 10 MeV since the gamma-ray dose rate from 9.76 min Cu-62 activation becomes very high. Iron, on the other hand, has essentially the same absorption properties as copper, and the induced activity is much weaker.
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Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Flattening filters may absorb 50%–90% of the central axis photon intensity. Brahme et al. recommended scanned photon beams at high energies pointing out that the flatness and intensity problems are greatly alleviated. Hence, some radiation shielding problems (including those for neutrons) may be as much as 2–5 times less for high-energy scanned photon beams than for a heavily filtered unscanned beam. Methods of measurement and characteristics of leakage radiation for a Clinac 18 treatment unit have been described by Lane et al. The spectral change problem associated with flattening filters was cited earlier by Hansen et al. who identified its importance for a 4-MV linac beam. Later, Larsen et al. developed a calculative program for filter design and applied it to a 4-MV beam. Their program sums the primary and scattered components in an iterative manner to fit the dose profile. Jones points out that the dominant effect is selective hardening of the beam by the flattening filter, and that for thin targets the energy increases with distance off axis. More recently, Hanson and Berkley measured the off-axis quality change for 4 to 10 MV beams and suggested a technique to correct for the effect in treatment planning calculations. Megavoltage x-ray beams exhibit an increased depth dose in the buildup region and a shift of dmax toward the surface as the field size is increased. Recent investigations indicate that these effects are largely due to low-energy electron contaminants originating in components of the treatment head, primarily the flattening filter. Treatment heads can be equipped with electron filters to attenuate this component. Reductions of 10%–20% in surface dose and an increased depth of dmax from 2.5 to 4.5 cm at 25 MV have been observed. Moyer has identified systematic patient x-ray dose errors for 4 and 10-MeV linacs associated with elongated rectangular collimator settings. This collimator-exchange effect, which depends on which movable collimator pair forms the larger or smaller field dimension, may be as large as several percent for highly elongated rectangular fields.
D. Neutron Leakage and Contamination
A significant number of neutrons are produced by high-energy x-ray beams. For most relevant materials, the neutron production threshold occurs at 8–10 MeV, rises rapidly and then plateaus above 20-MeV photon energy. Neutrons which originate in the primary collimator, target, and flattening filter contaminate the useful beam. Others are filtered through the treatment head, some are generated in the patient and most are multiply scattered by barriers comprising the room. These neutrons, together with x-ray leakage, affect the patient as well as those outside the treatment room. McCall and Swanson provide a thorough description of neutron sources and their characteristics originating in linac treatment heads. They find that high-Z materials do not significantly alter the neutron fluence but do substantially reduce the average energy of the transmitted spectrum. The principal source at 25 MeV is the primary collimator which contributes one-half or more of the fluence, often followed by the target and flattening filter, in that order. Depending on the material, energetic neutrons can induce radioactivity in the treatment head and patient support components of the linac as well as the treatment room barriers. Such radioactivity could constitute an appreciable source of exposure, particularly for the technologist (see Sec. VI.-C.). With respect to neutron production, a recent measurement showed that a tungsten flattening filter five radiation lengths thick produced two and one-half times as many neutrons as an equivalent
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 steel filter at 25 MV, although ratios as high as seven times have been predicted. Recent examination of neutron biological effectiveness has led to a scrutiny of neutron protection requirements by regulatory agencies. This scrutiny and the associated need to verify neutron production data, as well as to establish appropriate measurement, calculation, and dose reduction techniques were the subjects of a recent conference. A consensus appears to be developing wherein proposed neutron leakage requirements might reasonably be met in contemporary high-energy linacs. Often, the shielding provided for x-ray leakage is sufficient for neutron leakage as well, and satisfies the total leakage limitation of 0.1% measured in rads. Rawlinson and Johns note that the energy imparted outside the useful beam due to scatter can be more than 20 times greater than the energy due to leakage. Some simplifications in assessment have resulted from defining two pertinent measurement surfaces for protection purposes. One is a plane circular “patient” surface of radius 2 m centered on and perpendicular to the axis of the beam at the normal treatment distance and relates to patient protection. A second cylinder-like complex surface is defined by all points at 1 m from the path of the electrons between the electron gun and the target or electron window and relates to room shielding. A recent report of AAPM Task Group No. 21 on neutrons from high-energy x-ray medical accelerators provides a careful reasoned, quantitative analysis of the problem together with recommendations regarding risk to the radiotherapy patient. The report concludes that the principal risk of carcinogenesis is extremely small, and the implementation of more restrictive regulation is unnecessary and would be counterproductive.
Acknowledgments
2-64
Several commercial treatment units are cited herein to illustrate design principles and variations of them. Their selection in no way constitutes an endorsement of a particular equipment. The choice, at times, depended on the availability of detailed information and numerical data relating to the design. I am grateful to those who have responded to my requests. Many individuals have contributed to my understanding of medical linacs and to the preparation of this review. Their generous efforts have often facilitated my exposition of specific topics. Several have critiqued portions or all of this manuscript. I am grateful to all of these individuals: L. Atherton, A. Brahme, K. Brown, E. Dally, P. Fessenden, D. Goer, R. Jean, S. Johnsen, P. La Riviere, R. Levy, R. McCall, A. McEuen, G. Meddaugh, N. Pering, T. Smith, E. Tanabe, L. Tauman, J. Ting, and W. Turnbull. I am particularly grateful to Craig Nunan whose broad background and critical appraisal have added immensely to my perspective. Pieter Huisman’s talents have provided many of the illustrations. Juanita Clack’s outstanding secretarial skills, patience, and untiring efforts are much appreciated and have greatly simplified my task.
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Three
Modulator Theory
This chapter will discuss the theory of non-resonant series line type modulators used in Varian C-series Clinacs.
Modulator Theory
3-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Introduction:.................................................................................................................... 3-5 2. Basic Concepts: ............................................................................................................... 3-5 3. DeQing Principles:............................................................................................................ 3-7 4. Non-resonant Transmission Line Principles: ..................................................................... 3-8 5. Pulse Shape Definitions: ................................................................................................ 3-16 6. Line Type Modulator Load Element Principles: ............................................................... 3-17 7. Fault Conditions: ........................................................................................................... 3-18 8. Thyratron Theory: .......................................................................................................... 3-19 8.1. Introduction: .......................................................................................................... 3-19 8.2. Operating Notes on Hydrogen-filled Tubes: ............................................................. 3-23
3-2
Modulator Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Illustrations Figure 3.1. Resonant Charging Circuit:................................................................................. 3-5 Figure 3.2. Voltage and Current Waveforms of Capacitor C1: ................................................ 3-6 Figure 3.3. Resonant Charging Circuit with Series Charging Diode: ...................................... 3-6 Figure 3.4. Voltage and Current Waveforms on C1 with Series Charging Diode: .................... 3-7 Figure 3.5. Addition of DeQing Switch Circuit:...................................................................... 3-7 Figure 3.6. DeQing Waveforms: ............................................................................................ 3-8 Figure 3.7. Distributive Transmission Line: .......................................................................... 3-8 Figure 3.8. Finite (Lumped) Network: .................................................................................... 3-9 Figure 3.9. Resonant Charging with PFN: ............................................................................. 3-9 Figure 3.10. The Pulse Discharge Circuit: ........................................................................... 3-10 Figure 3.11. PFN Discharge Waveform Formation: .............................................................. 3-11 Figure 3.12. PFN Discharge Waveforms vs. Load Impedance: .............................................. 3-13 Figure 3.14. Typical Line Type Modulator for Linear Accelerator Applications:..................... 3-14 Figure 3.13. Basic Line Type Modulator Circuit: ................................................................. 3-14 Figure 3.15. Pulse Shape Definitions, Theoretical: .............................................................. 3-16 Figure 3.16. Pulse Shape Definitions, Practical: .................................................................. 3-16 Figure 3.17. Equivalent Circuit of Magnetron as Load:........................................................ 3-17 Figure 3.18. Magnetron Voltage-Current Characteristics:.................................................... 3-17
Modulator Theory
3-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 3.19. Conventional Industrial Thyratron. (a) Anode. (g) Control Grid, (k) source of electron emission: .............................................................................................................. 3-19 Figure 3.20. Critical Grid Potential in Relation to Anode Voltage:........................................ 3-20 Figure 3.21. Reference Points Associated with the Interpretation of Pulse Shapes: .............. 3-22 Figure 3.22. Schematic Diagram of Line Discharge Circuit: ................................................ 3-23 Figure 3.23. Charging Voltage Waveforms: ......................................................................... 3-24 Figure 3.24. Circuit for Recovery Time Measurement:......................................................... 3-27 Figure 3.25. Methods of Providing Negative Grid Voltage Swing: ......................................... 3-28 Figure 3.26. Waveforms with Pulse Transformer Trigger Drive: ........................................... 3-30 Figure 3.27. Inductive Overswing with Bias: ....................................................................... 3-30 Figure 3.28. Inductive Overswing without Bias:.................................................................. 3-30 Figure 3.29. Waveforms Normally Seen During Modulator Adjustments:............................. 3-31 Figure 3.30. Schematic Diagram of Tetrode: ....................................................................... 3-33 Figure 3.31. Single Pulse Drive for Tetrode: ........................................................................ 3-33 Figure 3.32. Simplified Single Pulse Drive: ......................................................................... 3-34 Figure 3.33. Double Pulse Drive for Parallel Operation: ...................................................... 3-35 Figure 3.34. Single Pulse Drive for Parallel Operation: ........................................................ 3-35 Figure 3.35. Anode Circuit Using Separate PFN’s: .............................................................. 3-36 Figure 3.36. Arrangement for Adjustment of Current Sharing:............................................ 3-36 Figure 3.38. Thyratrons in Series: ...................................................................................... 3-37
3-4
Modulator Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
The material in this chapter should be read and thoroughly understood by the student before proceeding to the specific modulator descriptions in the High and Low Energy Clinac Beam Delivery System course manuals.
2. Basic Concepts
Consider the circuit of Figure 3.1 below.
S1
L1
B1 C1
Edc
Figure 3.1. Resonant Charging Circuit When switch S1 is closed, current will begin to flow through inductor L1 to charge capacitor C1. Initially the reactance of L1 will limit current flow resulting in a voltage drop equal to the battery voltage appearing across L1. As time passes, current does begin to flow. This results in the buildup of a magnetic flux or field in the core of L1 (storage of energy). Eventually, the charge across C1 approaches a value equal to the battery voltage Edc. The current through L1 at this point is maximum and both L1 and C1 have stored energy. Current in the circuit begins to decrease because there no longer exists a voltage difference between C1 and the battery. This causes the magnetic field in L1 to collapse. The collapsing field produces a continuation of current flow in L1 that creates a voltage source that adds to the battery voltage. C1 now begins to charge higher than the battery voltage Edc, eventually approaching 2Edc when all the stored energy in L1 has been transferred to C1. There now exists a voltage difference between the battery voltage Edc and the capacitor voltage 2Edc. This causes current flow to reverse in the circuit resulting in the extra stored energy in C1 now causing a reverse current to flow in L1. A magnetic field is again developed in L1 in the reverse direction and L1 now has an excess of stored energy. This cycle of events will continue until all the original stored energy in L1 is dissipated in the I²R losses of the circuit. As a result of this cyclic reaction, a damped oscillation of current will flow between L1 and C1. The oscillation has a resonant time period determined by the value of L1 and C1 defined by the relationship: 1 TC = -----------------2π LC
Modulator Theory: Introduction
3-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π
2π
2Edc Edc 0
t
E (Voltage) I (Current) Resonant Time Period
2π LC
Figure 3.2. Voltage and Current Waveforms of Capacitor C1 It can be seen by the waveform of Figure 3.2 that the capacitor C1 will eventually have a charge voltage equal to the battery voltage Edc after several cycles have occurred. This resonant charging condition can be put to use to allow C1 to maintain the stored energy in L1 by adding a diode in series between L1 and C1 as indicated in Figure 3.3.
S1
L1
B1 Edc
C1
Figure 3.3. Resonant Charging Circuit with Series Charging Diode The sequence of events in the circuit of Figure 3.3 is exactly the same as the circuit of Figure 3.1 except that when the capacitor C1 reaches a voltage equal to 2Edc the diode CR1 will block the reverse flow of current back through L1. The waveforms of Figure 3.4 indicate these events. There are three major advantages to the use of the circuit in Figure 3.3:
3-6
a.
The power source voltage required is only half that of the capacitor stored value.
b.
The efficiency of the transfer of energy is raised from approximately 50% to almost 100%.
c.
It is possible to regulate the voltage charge on the capacitor.
Modulator Theory: Basic Concepts
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π 2Edc E (Voltage) Edc
I (Current)
0
t π LC Time Period Figure 3.4. Voltage and Current Waveforms on C1 with Series Charging Diode
3. DeQing Principles
Regulation of the charge voltage on Capacitor C1 can be accomplished by the circuit of Figure 3.5. Consider switch S2 and resistor R1 shown in parallel with L1. It was stated that the current through L1 was maximum when C1 was charged to Edc and zero when charged to 2Edc (refer to Figure 3.4). If the switch S2 is closed at some time after the current in L1 has reached maximum value and the charge voltage on C1 has reached Edc, the stored energy in L1 will be routed through S2 and be dissipated in resistor R1.
S1
L1
B1 Edc
S2
R1
C1
Figure 3.5. Addition of DeQing Switch Circuit By controlling the exact time when S2 is closed, any charge voltage level from Edc to approximately 2Edc can be placed on C1 resulting in regulation of the stored energy in C1. The shorting circuit consisting of S2 and R1 has been identified by the term “DeQing” circuit. The term “DeQing” has been created to identify, in a simple manner, the action of the circuit composed of S2 and R1 across L1. The word “Q” or “Q Factor” is defined as the ratio of the reactance to resistance of an inductor, capacitor, or resonant circuit. With reference to Figure 3.5, it can be seen that when switch S2 is closed, R1 is placed in parallel with L1. This lowers the “Q” of the inductor. Thus, the circuit is called a “DeQing” circuit, etc.
Modulator Theory: DeQing Principles
3-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π 2Edc E (Charged Voltage Level of C1) Edc I (Current through R1) 0
I (Charging Current to C1)
t
π LC Time Period Figure 3.6. DeQing Waveforms Observe the current and voltage waveforms of Figure 3.6. The current waveform indicates an abrupt drop to zero when switch S2 is closed. The energy still remaining in the inductor L1 is the area under the curve indicated by the shaded area under the dashed line. This energy is dissipated by resistor R1.
4. Non-resonant Transmission Line Principles
Non-resonant transmission lines are lines that are either infinitely long or terminated in some impedance. A uniform transmission line has what is called a “characteristic impedance”. This is the impedance that would be measured at the end of such a line if it were infinitely long. The importance of this characteristic impedance lies in the fact that if any length of line is terminated in an impedance of this value, then all the energy flowing along the line will be absorbed at the termination and none is reflected back along the line. A result of this is that the input impedance of any length of transmission line terminated in its characteristic impedance is equal to that characteristic impedance. Transmission lines such as coaxial cables can be represented as a network of distributive series inductive and parallel capacitive elements as diagramed in Figure 3.7. This distributive network can be further simplified or simulated by constructing a finite number of series and parallel elements in the form of the network diagramed in Figure 3.8. This network is commonly called a PFN (Pulse-Forming Network) when used for energy storage purposes in a line type modulator.
OPEN CIRCUIT
IN
Figure 3.7. Distributive Transmission Line
3-8
Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The development of a pulse-forming network that simulates a transmission line is a problem of network mathematics. No network having a finite number of elements can exactly simulate a transmission line which in reality has distributed rather than lumped parameters. The pulse-forming network serves a dual purpose of storing exactly the amount of energy required for a single pulse and of discharging this energy into a load in the form of a pulse of specified shape. The required energy may be stored either in the capacitances or in the inductances, or in combinations of these circuit elements. Networks in which the energy is stored in the electrostatic field of the capacitors are referred to as voltage-fed networks. Networks in which the energy is stored in the magnetic field of the inductors are referred to as current-fed networks. Networks of the voltage-fed type are generally used in practice because only with this type of network can gaseous switches such as thyratrons be used to switch the energy.
OPEN CIRCUIT
IN
Figure 3.8. Finite (Lumped) Network The network of Figure 3.8 can be defined as a voltage-fed network where the stored energy is in the capacitors. If the capacitor C1 of Figure 3.5 is replaced by the network of Figure 3.8 to form Figure 3.9 and the circuit switch S1 is closed, the same sequence of events occur as originally happened in Figure 3.5. Now the current begins to charge each capacitor of the network through the series inductance of each section of the network. The result is that each capacitor will charge to 2Edc at a charge time that is increasingly longer for each capacitor down the network. However, the difference in charging time of each network section is relatively insignificant to the total charge time of the entire circuit. This is due to the large charge time defined by the charging choke L1 and the total value of all the capacitance within the network. The PFN inductors have no significant effect during the charge cycle because of their small inductance value compared to the large inductance of L1.
S1
CR1
L1
B1
L2
C1 Edc
S2
L3
C2
L4
C3
C4
R1
PFN
Figure 3.9. Resonant Charging with PFN
Modulator Theory: Non-resonant Transmission Line Principles
3-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Since the application for which these modulators are usually designed requires the application of an essentially rectangular pulse of energy, the open-ended transmission line (PFN) considered in the foregoing example may be taken as the starting point in a discussion of the discharge circuit. Figure 3.10 represents a simple discharging circuit consisting of a PFN, which is in a charged state of 2Edc, a switch S1, and a load resistance R1 which has the same impedance as the PFN characteristic impedance.
L1
S1
L2
C1 Edc
L3
C2
L4
C3
C4
PFN
R1
Figure 3.10. The Pulse Discharge Circuit When S1 is closed the following events occur: 1.
C1 begins to discharge through L1 into the load resistor R1. Initially, all the charge voltage across C1 will be developed across L1. As time passes, current flows through L1 and begins to dissipate in load resistor R1. The time required is dependent on the values of L1–C1 and R1. Eventually the voltage across R1 will reach Edc because the impedance of the load resistance R1 is equal to the impedance of L1 and C1.
2.
At this point C1 will stop discharging and C2 will begin to discharge through L2 and L1 into R1. Similarly, C2 will stop discharging at Edc and C3 will begin to discharge. This sequence of events continues until the last capacitor C4 begins to discharge. When C4 reaches Edc, it will continue to discharge to zero. Then C3 will discharge to zero. Eventually all capacitors will discharge to zero and all the energy will have been dissipated in R1.
The above sequence of events results in a rectangular current pulse of energy being supplied to R1 with a duration which is twice the transmission time (aggregate discharge times) of the network. Reference Figure 3.11. From the foregoing discussion, as illustrated by Figure 3.11, it can be seen that if the load impedance is equal to the line impedance (Rl=Zo), that is if the line is matched to the load, the current out of the line consists of a single rectangular pulse of 2 time periods (The time to discharge each capacitor down the line and then back). These conditions satisfy Ohm's law where half the voltage appears across the load but for twice as long thus dissipating all the stored energy in the network.
3-10
Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 EC1
EC3
EC2
EC4 EC3
EC1
EC2
Voltage t
IC2 IC1
IC4 IC3
IC2 IC3
IC1
Current t
IL2 IL1
IL4 IL3
IL2 IL3
IL1
Current t
T1
T2
Current
2T
t
Discharge Figure 3.11. PFN Discharge Waveform Formation
Modulator Theory: Non-resonant Transmission Line Principles
3-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The effect of mismatching the load is to introduce a series of steps into the transient (rectangular) discharge. As can be seen in Figure 3.12, these steps will all be of the same sign when the mismatch is Positive (the load resistance is greater than the line impedance) Rl>Zo, and of alternate sign when the mismatch is negative (the load resistance is less than the line impedance) Rl
3-12
Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
(+)EL RL = ZO Matched
0
2T
(+)EL RL = 2ZO Positive Mismatch
0
2T
4T
6T
8T
(+)EL RL = ½ZO Negative Mismatch
0 (–)
2T
4T
6T
8T
Figure 3.12. PFN Discharge Waveforms vs. Load Impedance
Modulator Theory: Non-resonant Transmission Line Principles
3-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
S1
L1
CR1
B1
L2
C1 Edc
S2
L3
C2
L4
C3
C4
R1
PFN S3
R2 (Load)
Figure 3.13. Basic Line Type Modulator Circuit In the following discussion refer to Figure 3.14.
3-Phase High Voltage Power Supply
Clipper Current Fault Monitor System
Compensated Voltage Divider Circuit 3000:1 Ratio
˜ 10 KV L1
CR1
V1 R1
˜ 20 KV
PFN
T2
CR2 C5 R4
DeQing Switch Trigger Gen. System
˜ 6.7V R5
HVPS Current Monitor Circuit
Current Toroid
C1
V2
C6
C3
C2
CR3
C4
R3
End Clipper Circuit
Main Switch Trigger Gen. System Pulse Transformer Turns Ratio = 1:11
R2
Klystron Equivalent Circuit De-Spiking Network
HVPS O/C Fault Monitor System
C5 R4
T1 RL CRL
CL
Figure 3.14. Typical Line Type Modulator for Linear Accelerator Applications
3-14
1.
A 10 KV DC high voltage power supply has been substituted for the battery.
2.
Switches S2 and S3 have been replaced with high voltage high current type thyratron tubes.
Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.
The load resistor R2 has been replaced by a pulse transformer T1 and the equivalent circuit of klystron (magnetron in low energy Clinacs), Rl, CRl and Cl.
From the foregoing discussion on impedance matching, it is clear that the PFN must be terminated with a load impedance equal to its characteristic impedance. The impedance associated with high power line type modulator used for klystron-driven accelerators will be typically around 12.5 ohms and for magnetron-driven accelerators, 25 ohms. (The klystron will have an impedance of 1400 ohms and the magnetron 400 ohms.). The pulse transformer will require a step up turns ratio of approximately 1:11 for klystrons and 1:4 for magnetrons because the impedance transformation of a pulse transformer changes as the square of the turns ratio of its windings. Note the new components: 1.
A high voltage power supply current monitor resistor R2 and a fault detector system has been placed in the return path of the power supply. The voltage across the resistor will be proportional to current flow from the power supply. If the current exceeds a specific limit the monitor system will turn off the high voltage power supply. Thus, any failure of components in the charging circuit will be detected.
2.
The DeQing thyratron is controlled by a DeQing trigger system which receives input information on the DC charge level of the PFN through a compensated voltage divider circuit R4,C5–R5,C6 that has a voltage division ratio of 3000 to 1.
3.
The main switch thyratron is controlled by a trigger generator system. This will enable repetitive operation of the modulator circuit at any desired PRF (pulse repetition frequency) within the design limitations of the components of the charge circuit.
4.
A diode CR3, resistor R3 and a current toroid T2 have been added across the end of the PFN. The diode is polarized so current will only flow through the resistor when the charge on the PFN goes negative. The toroid will detect any current flow and the clipper current fault monitor system will turn off the high voltage power supply thus protecting the components in the discharge circuit.
5.
A resistor R4 and capacitor C5 have been placed in series across (in parallel with) the primary of the pulse transformer to form a despiking network. In a practical modulator the pulse output of the PFN is of the order of 1 to 5 microseconds duration. The rise and fall time of the pulse will usually be in tenths of microseconds. The pulse transformer must be designed to transmit these very short pulses. However, during the rise and fall time of the pulse, the transformer will tend to appear as a very high impedance which will cause overshooting of the pulse voltage due to mismatched conditions. This can cause serious problems with the klystron or magnetron as well as the pulse transformer. The de-spiking network will appear as a matched impedance during the fast rise and fall time of the pulse due to the low reactance of C5 effectively placing resistor R4 across the PFN during these times. R4 will be equal in value to the characteristic impedance of the PFN.
Modulator Theory: Non-resonant Transmission Line Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Pulse Shape Definitions
A pulse can be considered as a trapezoidal shape with a finite rise and fall time and a defined width and peak amplitude as indicated by Figure 3.15.
100%
50%
Pulse Width
0% Flat Top Width
Rise Time
Fall Time
Figure 3.15. Pulse Shape Definitions, Theoretical Generally the rise time is defined as the time between 10% and 90% of the rising portion of the waveform and the fall time is likewise defined as 90% to 10% of the fall portion of the waveform. The pulse width is considered to be defined as the width at the 50% amplitude point. Additionally, pulses of a practical nature have definitive values of overshoot and droop as well as reverse tails as indicated in Figure 3.16. % Overshoot 100% 90% % Droop
50%
Pulse Width
10% 0% Reverse Tail Rise Time
Fall Time
Figure 3.16. Pulse Shape Definitions, Practical
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Modulator Theory: Pulse Shape Definitions
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6. Line Type Modulator Load Element Principles
The output pulse of a line type modulator can be applied to either a passive element such as a resistance that will dissipate the pulse energy in the form of heat, or an active device such as a klystron or magnetron that converts the pulse energy into a form of high frequency electromagnetic energy. A magnetron will be utilized as the active load element for our discussions. The circuit elements of Figure 3.17 define the equivalent circuit of a magnetron as a load element of the modulator.
Magnetron Oscillator
RL
Line Type Modulator
Pulse Output
CRL +
CL
VS
–
Figure 3.17. Equivalent Circuit of Magnetron as Load The behavior of a magnetron as a load element of the modulator is equivalent to that of an ideal diode CRL, that is a diode that has a linear E–I (voltage–current) characteristic, in series with a battery of voltage Vs whose polarity is in opposition to the applied voltage pulse of the modulator. For circuit analysis it is possible to represent such a load as a resistance RL in series with the diode CRL and battery of voltage (Vs). The stray capacity of the magnetron can be represented as a capacitor CL in parallel across the network. The effect of the equivalent circuit of Figure 3.17 can be defined by the voltage–current characteristic of Figure 3.18.
Current Actual Theoretical 0 Voltage (VS) Figure 3.18. Magnetron Voltage-Current Characteristics
Modulator Theory: Line Type Modulator Load Element Principles
3-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The magnetron does not begin to conduct current until the applied pulse voltage approaches the effective value of the bias voltage Vs. At this point the magnetron begins to conduct heavily with only the small additional rise in the applied voltage. During the rise time of the pulse the stray capacitance of the magnetron CL tends to limit the rate of rise of the applied voltage to a realistic value. When the modulator pulse begins to fall the stray capacitance CL tends to prevent the rapid fall of the pulse.
7. Fault Conditions
Whenever a magnetron arcs its internal impedance drops toward zero, causing a reflected low impedance to appear across the PFN. This results in the circuit action as defined under negative mismatch conditions. The PFN discharges negative and at this point the thyratron will stop conducting. This results in a large amount of energy being left in the PFN capacitors stored as a reverse charge. Referring to Figure 3.14, the end clipper diode CR3 conducts this energy through resistor R3 where it is all dissipated as heat. This condition must not be allowed to continue. Some form of monitor system will be used to detect this current flow and turn off the high voltage power supply, thus preventing serious damage to any components. Whenever the magnetron misfires, that is, does not conduct during a pulse, the result is a reflected high impedance mismatch at the PFN, resulting in a slow rate of discharge of the PFN. The charge circuit normally has a charge time several orders of magnitude longer than the discharge time. Typical values would be charging time 1 millisecond and discharge time of 4 to 6 microseconds. The charging choke L1 appears as an open circuit during the short discharge time while the thyratron is conducting thus isolating the power supply from the thyratron which would look like a short to the power supply. Whenever the PFN is mismatched positive it tends to discharge very slowly, resulting in possible continuous firing of the main thyratron. This results in excessive current being monitored by the power supply current monitor resistor R2. The HVOC (High Voltage Over-Current) fault monitor circuit will turn off the high voltage power supply, thus protecting the components from a major failure. The preceding discussion has dealt with only the key factors in the operation of a line type modulator driving a magnetron. Most of the principles discussed also apply to klystron systems, except that the klystron is basically an amplifier tube and behaves accordingly, e.g., there are no problems associated with misfiring and internal arcing within the tube is abnormal and cannot be tolerated. There are many other aspects of its operation that must be dealt with to fully understand any given system or accelerator machine performance. This is only a preliminary exercise.
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Modulator Theory: Fault Conditions
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8. Thyratron Theory
The following information is compiled from The Operation and Use of Hydrogen-Filled Tubes by R. E. Lake, B.Sc. and H. Menown, M.Sc., and edited for this manual.
8.1. Introduction
The conventional thyratron with mercury vapor or rare gas filling was first introduced in the early 1920’s, and the basic geometry of the industrial type of tube has changed very little since then. The tube consists essentially of an anode (a), control grid (g), and source of electron emission (k), as shown in Figure 3.19. With a positive voltage on the anode, the tube will remain in a nonconducting state if a suitable voltage (usually negative) is applied to the grid. This voltage depends on the anode voltage, and for every value of anode voltage there is a critical grid potential (Figure 3.20). The electrons leaving the cathode are prevented from reaching the grid/anode space by the potential barrier at the grid. As the grid voltage is made less negative, or anode voltage more positive, the field due to the anode attracts an increasing number of electrons from the cathode. These collide with gas atoms and when the resultant electrons receive sufficient energy and sufficient collisions occur, cumulative ionization takes place and the tube fires through. The voltage across the tube then drops to a value (10-15V for mercury and rare gases), which is dependent on the application and the nature and pressure of the gas filling. The current passed is then largely determined by the external circuitry.
Figure 3.19. Conventional Industrial Thyratron. (a) Anode. (g) Control Grid, (k) source of electron emission
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 3.20. Critical Grid Potential in Relation to Anode Voltage During breakdown the negative potential on the grid attracts positive ions and these form a sheath through which the grid potential cannot penetrate. Thus, any increase in the negative value of the grid supply voltage has no effect on the current passing. The tube will return to its non-conducting state after removal of anode voltage for a sufficient time (known as the recovery time) to allow the charged particles to disappear. The voltage on the grid then returns to its original value, and a positive voltage can be reapplied to the anode without conduction taking place. The tube, therefore, acts as an electronic switch which may be closed by the application of a positive signal to the grid but which can only be opened by the removal of anode voltage for a minimum time. The grid is almost always a far more massive affair than is found in vacuum tubes since it has to withstand recombination heating and bombardment without allowing a rise of temperature sufficient to cause primary grid emission. The apertures in the grid may consist of perforations of various shapes and sizes or may even be a single large hole or an annulus. Sometimes, a baffle in the form of a disc is attached to the cathode side of the grid with a small spacing between the baffle and the grid proper. Such baffling modifies the characteristic of the tube so that the potential of the grid in the non-conducting state may be either zero or positive. At the same time, this baffle helps to prevent deposition of cathode material on the grid proper. In this case ionization must occur between grid and cathode near the grid apertures before breakdown to the anode can take place. The first reported use of hydrogen as a filling for gas tubes was in 1936. Nothing further was done until the 1940’s when the rapid growth of radar required the development of a switch tube Able to operate at higher frequencies and with more reliable triggering than the mercury tubes then in use. (Pulse generators — Radiation Laboratory Series p. 335 et seq. K. J. Germeshausen. Published by McGraw Hill.)
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 With mercury, the maximum voltage that can be developed across the tube is about 30V. At a higher voltage, positive ion bombardment of the cathode becomes very intense and cathode destruction rapidly occurs. In pulse operation this is invariably about 100V so that destruction of the cathode is inevitable. With hydrogen this critical voltage is at least twenty times greater due to the much lower mass of the hydrogen ion. The recovery time of the hydrogen tube is an order of magnitude less than a similar mercury or rare gas tube because of the higher mobility of the proton. The chemical activity of hydrogen immediately introduces the problem of gas clean-up. To overcome this, all the materials used for the tube structure have to be of a very high degree of purity which is closely controlled. The processing of all the components used and of the tube itself during the exhaust and filling schedule is also very critical. In addition, tube geometry has a considerable influence on the rate of gas clean-up for given operating conditions. The common method of improving the life of hydrogen-filled tubes is by means of a hydrogen replenisher or reservoir. This consists essentially of a controlled weight of (usually) titanium hydride which is maintained at a relatively constant temperature within the tube envelope. Under these conditions titanium, hydrogen and the hydride are in thermal equilibrium and any variation of the amount of hydrogen within the tube envelope causes an adjustment of the equilibrium conditions. By this means the pressure within the envelope is maintained relatively constant. The hydride is usually contained in a metal cylinder which may be closed at one or both ends depending on the cylinder material. The capsule is heated by placing it in series or parallel with the cathode heater or by means of a separate supply. The latter arrangement is sometimes convenient if the thyratron is required to operate over a wide power range, and allows for a less critical design of the reservoir. It is obviously less convenient from the user's view point. Hydrogen thyratrons have been manufactured in the United Kingdom since about 1951. Development in recent years has concentrated on the multielectrode structure, which offers advantages over the conventional triode form. This is discussed in more detail in a later section. The particular nature and application of hydrogen tubes has resulted in the use of terms and definitions peculiar to these devices. The more important and less familiar of these are given below.
8.1.1. Peak Forward Grid Drive
The peak positive value of the unloaded grid pulse with respect to the cathode. This must be added to the bias voltage (if any), to determine the amplitude of the output pulse from the trigger generator.
8.1.2. Recovery Impedance
The ratio of the potential difference between the instantaneous potentials appearing at the thyratron grid and the on-load grid bias voltage to the grid current, at the same instant during the recovery period. This can be expressed as: Inst. ( grid potential – bias voltage )--------------------------------------------------------------------------------current
8.1.3. Recovery Time
The time interval between the cessation of anode current and the instant when the grid regains control under specified anode and grid circuit conditions.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.1.4. Grid 2 Pulse Delay
The time interval between the voltage pulses on the terminals of grid 1 and grid 2 with the thyratron removed, measured at the 26% level on the leading edge of each pulse. It is essential that the grid pulses overlap.
8.1.5. Anode Delay Time
The time interval between the 26% point on the leading edge of the unloaded grid pulse and the instant when anode conduction occurs. (In tetrodes the grid 2 pulse is used as reference.)
8.1.6. Anode Delay Time Drift
The change in anode delay time over a specified period of time as a result of continued operation of the thyratron under specified conditions.
8.1.7. Time Jitter
The pulse-to-pulse variation of the instant when anode conduction occurs referred to the 26% point on the leading edge of the unloaded grid pulse. (In tetrodes the grid 2 pulse is used as reference.)
8.1.8. R.M.S. Current
Normally computed as: ( peak current ) × ( mean current )
The reference points associated with the interpretation of pulse shapes are shown in Figure 3.21.
Spike Amplitude
Spike Duration Amplitude
70.7%
26%
Pulse Duration Time of Rise
Time of Fall
Figure 3.21. Reference Points Associated with the Interpretation of Pulse Shapes
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.2. Operating Notes on Hydrogenfilled Tubes
The most important application for hydrogen thyratrons is in line-type pulse generators for radar transmitters and particle accelerators where high powers have to be switched rapidly and with precise timing and where small size, high efficiency and tolerance of ambient temperature variations are obvious advantages. A growing use is to be found for thyratrons as protection devices for “Single Shot” use, e.g., to discharge condenser banks in thermonuclear research.
8.2.1. Thyratrons in Line Type Modulators
The basic circuit for the DC charging line type modulator is shown in Figure 3.22, together with the simpler relationships most often used.
LC
LN CN
E
ZL
Figure 3.22. Schematic Diagram of Line Discharge Circuit E
= DC power supply voltage
LC
= charging choke inductance
LN
= total network inductance
CN
= total network capacitance
ZN
= network impedance
f
= pulse repetition frequency
epy i b = -----------------ZN + ZL
tp
= pulse width
epy = 2E + epx
epy
= thyratron peak anode voltage
epx = thyratron peak inverse voltage
ZN =
LN -----CN
t p = 2 L N C N = 2C N Z N
2 epy 2 epy P L = ⎛ ----------------⎞ × Z L = ----------⎝ Z N + L⎠ 4Z L
I
= thyratron mean anode current
ib
= thyratron peak anode current for matched load
ZL
= load impedance
PL
= peak load power
f
= π ( L C )1 ⁄ 2 C N
1 ------------------------------
I = i b ft p ZN × ZL epx = ------------------- × epy ZN + ZL
Although simple in diagrammatic form, this circuit has many components which are distributive in nature and interact with each other in different ways which depend upon their relative values. The modulator circuit must be able to cope with the situations that arise from switching on high voltage at a low value through normal full load operation to mismatch or short circuits produced by the RF load tube. (For a fuller treatment see Pulse Generators — Radiation Laboratory Series.) Those features of the circuit which most affect the performance and reliability of hydrogen thyratrons will now be discussed.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.2.2. The Charging Circuit
The pulse forming network (PFN) is charged to approximately twice the DC supply voltage in a time which equals half the period of the resonant circuit formed by the charging choke inductance and network capacitance. After the thyratron has fired and the voltage has collapsed, sufficient time must be made available for the thyratron to recover. This means that the anode voltage must be kept below cathode potential until the thyratron has recovered.
Supply Voltage
epy
T (a)
TIME
Supply epy Voltage
epy
T
TIME
(b)
Supply Voltage
T
TIME
(c) T = charging period epy = thyratron peak voltage
Figure 3.23. Charging Voltage Waveforms If the thyratron is triggered at the instant the network voltage reaches a maximum then the condition of resonant charging is achieved, see Figure 3.23(a) If this triggering is delayed then the peak voltage will commence to fall unless a charging diode is present, see Figure 3.23(b). (The same effect is produced by a low value of charging choke inductance for a given triggering frequency.) If triggering frequency is increased or inductance increased then a condition of linear charging is approached, Figure 3.23(c). Therefore, in order to make most time available for recovery of the switch tube, the charging choke inductance should be calculated for the highest repetition frequency that will be used. Where the repetition frequency is high (greater than 5000 p.p.s.) or the duty ratio is high (of the order of 0.01), the time available for recovery becomes a very important factor. (See section on Recovery.) In these cases, to enable the switch tube to recover, it may be necessary to delay the charging of the PFN by using a triggered charging diode or a saturable reactor. The power supply should have good regulation to avoid any increase in anode voltage under conditions of interrupted triggering, and should also be free from appreciable overshoot when ‘'snap start” conditions are required.
8.2.3. Charging Diodes
The charging diode must be rated to withstand a peak inverse voltage (p.i.v.) of not less than the full network voltage since at the end of its charging period the anode may swing down to near earth potential. The mean current requirements of high-power modulators may preclude the use of vacuum tubes. In such cases, a gas tube may be used which often takes the form of a “triggered diode” such as the FX297. When it is necessary for the hold-off period to be long, the grid of the charging “diode” may be triggered through a suitably insulated pulse transformer but for conditions not far from resonant a high resistance potential divider will be satisfactory. The inclusion of an anode inductor helps to protect the charging diode from the initial spike of charging current produced by the self capacity of the charging reactor, and also from the full effect of the inverse voltage swing which occurs after the pulse forming network is
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 charged. When delayed triggering of the charging diode is required in order to effect an increase of time available for recovery of the modulator switch tube, care must be taken in the design of the trigger circuit. It is possible for triggering to be caused by RF generated in the modulator when the switch tube fires, due to the ease with which the charging “diode” is triggered. This will give prefiring before the delayed trigger voltage appears. A low impedance drive circuit will help to overcome this.
8.2.4. Thyratron Anode Circuit
Associated with the thyratron anode in conventional arrangements are the pulse forming network and inverse diode, and the most important parameters to be considered are rate of rise of current through the thyratron (di/dt) and inverse voltage at the anode (epx) after the main pulse. A high rate of rise of current will increase leading-edge heating and also the rate of gas clean up in the tube. The latter is particularly important in nonreservoir tubes. The mean rate of rise of current as shown on an oscilloscope trace is not necessarily a good indication, since the peak value may be considerably higher than this due to stray capacitance from the network to ground, and from charging and inverse diode heater transformers. For this reason the three-terminal network shown in Figure 3.22 is preferred since the bulk of these stray capacitances are removed from the thyratron anode. The end inductive section of the network should be mounted close to the epy - . If di/dt is in thyratron anode, and will have a minimum value L A = ------------di ⁄ dt amperes per microsecond, then LA is calculated directly in microhenries. It must be realized that the mean current through the thyratron is the sum of the DC power supply current and the inverse diode current (if any).
8.2.5. Inverse Diode Circuit
The inverse diode or clipper tube is normally found in high-power equipment to protect both the load and thyratron. In the event of a short circuit in the load the circuit impedance is halved and the peak current through the thyratron is doubled. With no inverse diode in the circuit the thyratron anode voltage swings negative to a value approaching the peak positive value and the succeeding charging cycle starts from there. This cycle will provide a positive voltage tending to double the previous value and, if the short in the load persists, this amplification will continue until limited by circuit losses or component failure. The inverse diode circuit must dissipate the energy involved. Depending on the modulator conditions, the resistance of this circuit may lie between 4Zn and about 40Zn. The lower value is to be preferred for fast removal of inverse voltage but higher values may be necessary to permit recovery of the switch tube. A value of around 40Zn will limit the increase in peak positive voltage to approximately 1% following a short circuit where the modulator duty ratio is 0.001. An inductive overswing circuit usually results in the inverse voltage rating of the thyratron being exceeded, due to the time of application being longer than in the resistive case. The primary inductance of the pulse transformer may be sufficient to produce this effect, particularly If the anode of the inverse diode is connected directly to earth. Any excess of inverse voltage in amplitude or time may result in arc-back and overheating of the grid structure, with the possibility of distortion in extreme cases. The rise of negative voltage appearing on the thyratron may be very rapid and the amplitude of the spike is approximately the same whether a diode is used or not. However, as soon as the diode conducts, the voltage rapidly
Modulator Theory: Thyratron Theory
3-25
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 collapses to a value determined by the inverse diode circuit impedance. When pulse transformer loads are used the effect is to delay the rise of the inverse voltage and thus tend to reduce the possibility of arc-back in the switch tube. Vacuum diodes are able to withstand higher voltages than gas tubes but present a higher impedance and are more likely to suffer failure in the event of a prolonged period of reduced load impedance. For this reason the FX297 is widely used, preferably triggered by a simple pulse transformer fed from the main current pulse, or alternatively by a capacity divider.
8.2.6. The Trigger Circuit
The values of jitter and anode delay time drift quoted in tube data sheets are extreme values to cover variation during life and are measured under conditions of minimum trigger amplitude, drive current and rate of rise of voltage. In practice, the trigger signal applied to the thyratron grid should be from a low impedance source and should have a high rate of rise of voltage and a pulse amplitude sufficient to cause rapid ionization of the gas in the grid-cathode space. This will minimize jitter and anode delay time drift. A trigger pulse duration very much in excess of the minimum value specified on the tube data sheets is wasteful and may, in certain circumstances, inhibit recovery of the thyratron after the main current pulse has been switched. In tetrodes, excessive current to grid 1 or excessive delay in the application of the grid 2 pulse may lead to a deterioration in tube performance. Any modulation superimposed on these parameters or on the bias supply will show up as jitter on the main current pulse. The minimum trigger pulse amplitudes quoted in data sheets refer to cathode potential, and the value of any negative bias used should be added to this figure to give the required minimum unloaded pulse amplitude from the trigger generator. This amplitude must always be checked at the thyratron socket with the tube removed. At the instant of firing, the grid potential rises rapidly. A voltage spike of the order of 20ns duration and an amplitude which is an appreciable fraction of the thyratron anode voltage, may be observed on the oscilloscope trace of the grid waveform. This spike can cause breakdown in the trigger unit. The amplitude of this spike increases as the rate of rise of current in the tube is increased. A series resistor, placed adjacent to the grid terminal and aided by the stray capacity, may provide a filter against the spike. A filter network is sometimes necessary, but should be kept as small as possible since it will degrade the grid firing pulse front and thereby increase anode delay time drift. Alternatively a nonlinear resistor may be used in parallel with the trigger unit output.
8.2.7. Recovery
3-26
At the end of the current pulse, a plasma exists throughout the tube which presents a short circuit to positive anode voltages. Therefore, the circuit is arranged so that the thyratron anode is held at a slight negative potential until recovery is complete. The tube has recovered when re-application of a positive anode voltage does not cause further conduction. During the recovery period the plasma decays with time as recombination of ions and electrons mainly on electrode surfaces. Since there are relatively close spacings in the tube between anode and grid, and also in the grid, the plasma density drops rapidly in this region with a time constant of the order of a microsecond. The grid-cathode plasma decays much more slowly because of the wider gaps involved and retains the inherent nature of a plasma by having approximately equal numbers of electrons and ions. Recovery is complete when the grid cathode plasma has shrunk away from the grid apertures so as not to come under the influence of any applied anode voltage,
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 but deionization is not complete until all the ions have disappeared from the tube. It should he noted that a tube has recovered long before it has deionized. Negative bias applied to the grid decreases recovery time. The bias voltage does not pull ions out of the discharge but when the grid potential goes negative with respect to the cathode, the anode field penetration is reduced. Grid bias may be applied via a resistor or an inductor. Both of these must inherently retard the application of negative bias but they are necessary to enable a forward drive pulse to be applied economically. The trigger unit output stage may be a cathode follower, a blocking oscillator or a pulse transformer. Care is needed in the design of the associated circuitry since not only must it apply a positive pulse to the grid but it must also be capable of passing several amperes of deionization current which follow the pulse. A paper by Malter and Johnson (RCA. Rev., June 1950) shows the change in plateau length and exponential decay of positive ion current to the grid of a small thyratron. The wave shape is shown in Figure 3.24.
g
rRg
rs
Grid Voltage
–150V
Es
Time
Cathode Potential
Bias Potential
Ecc
2µF
Exponential Decay
Grid Waveform after Current Pulse Figure 3.24. Circuit for Recovery Time Measurement
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Grid Voltage
(a)
Inductive Swing Time
Cathode Potential Grid Voltage
(b)
Inductive Swing Time
2µF
Grid Voltage
Cathode Potential
(c)
Bias persisting because of capacitor Inductive Swing Time
2µF
Grid Voltage
Cathode Potential
(d)
Bias persisting because of capacitor Inductive Swing Time
Cathode Potential Grid Voltage
(e)
Time
Cathode Potential
Exponential decay
Figure 3.25. Methods of Providing Negative Grid Voltage Swing
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The published data shows recovery curves for various values of bias and recovery resistance. All these measurements have been made in a specially developed circuit which presents a purely resistive impedance between grid and bias (if any) during the recovery period (Figure 3.24). It will be noted that the zero bias recovery time is independent of recovery resistance. These recovery times are much longer than those achieved from a pulse transformer drive since the energy stored in the pulse transformer causes an inductive swing of the grid which produces an effective bias in the grid and facilitates recovery Figure 3.25(a). This improvement may also be achieved by the insertion of an inductance of around 6mH directly between grid and cathode Figure 3.25(b). Figure 3.25(c) and Figure 3.25(d) show similar effects when a capacitor is included as a virtual bias source. Some advantage may be gained from the longer persistence of the bias and control of positive overshoot. Cathode follower outputs can also produce appreciable bias, especially at high repetition frequencies (e.g., 50kHz), from the energy stored in the coupling capacitor (Figure 3.25e). When a bias is applied via an inductor, a voltage (e = L di/dt) builds up due to the deionization current more negative than the bias and providing the subsequent swing back to the bias level is slow enough, a reduction in recovery time will be obtained. In the case of bias applied through the secondary of a pulse transformer excessive damping may inhibit recovery (Figure 3.26). A saturable inductance provides a relatively high impedance during the forward grid pulse but by suitable design this will saturate at the end of the main anode current pulse and present a low impedance to the grid and facilitate recovery (Figure 3.27). Again in the absence of any bias supply the forward pulse drive may be used to produce the bias source (Figure 3.28). e –E
g cc - where eg is the value of the grid Recovery impedance is defined as -----------------i c
voltage at any instant during the recovery period, ic is the value of the grid current at the same instant and Ecc is the grid bias supply voltage. The published recovery characteristics of thyratrons show that decreasing the resistance in series with the bias supply or increasing the bias voltage reduces the recovery time. From inspection of these a value of voltage and resistance may be found which will guarantee recovery in a particular circuit for which the time spent negative by the anode is known. The bias supply must of course be capable of passing a large current without appreciable voltage drop for the duration of the recovery period and should, therefore, be shunted by a suitable capacitor (between 0.1 and 10 µF depending on requirements). It must also be capable of recharging the capacitor rapidly and should be free from ripple which would cause jitter of the output pulse.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Time
R
Bias Voltage
Grid Voltage
Plateau as deionization current is limited by circuit impedance If R is too great, exponential decay
Steady bias level
2µF Exponential decay + inductive bias
Normal pulse transformer driven circuit
Figure 3.26. Waveforms with Pulse Transformer Trigger Drive
Bias Voltage
Grid Voltage
Time
Inductive swing + switching action of saturation
2µF
Figure 3.27. Inductive Overswing with Bias
Grid Voltage
Time
Bias persisting because of capacitance
2 µF
Figure 3.28. Inductive Overswing without Bias The modulator engineer will, however, always have a simple check on his design by using an oscilloscope to display the grid wave shape See Figures 4.29(a) and 4.29(b). A long “plateau” indicates that either the thyratron is still conducting because its anode is being held slightly positive or the grid bias is being applied through a very high impedance. The appearance of instabilities on the grid wave shape as the high voltage is increased is also indicative of insufficient time allowed for recovery of the switch tube. Further increase of high voltage invariably results in trip outs under these circumstances.
3-30
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 AMPS
CATHODE CURRENT PULSE
VOLTAGE
Arc level
Start of current pulse
Plateau
TIME
CATHODE CURRENT PULSE
GRID VOLTAGE WAVEFORM Arc level Thyratron anode ceases to conduct Plateau
Bias level
Figure 3.29. Waveforms Normally Seen During Modulator Adjustments
8.2.8. Thyratron Heater Voltage
Although not strictly a circuit element, the effects of heater voltage variations are worthy of mention. A tolerance of ±7.5V is normally allowed on heater voltage but every endeavor should be made to keep within closer limits. This applies particularly to tubes which include a hydrogen reser-
Modulator Theory: Thyratron Theory
3-31
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 voir in series or parallel with the cathode heater. Any variation of cathode heater voltage thus affects gas pressure as well as cathode temperature. Anode delay-time drift will be affected by changes in heater voltage. A large proportion of the observed jitter is caused by the field from the cathode heater and in cases where jitter requirements are to be reliably less than 1.0ns. DC heater supplies should be used.
8.2.9. Cooling
The current passing through the tube and in particular the rate of rise of current are dependent on a sufficient availability of ionized particles. The discharge path between cathode and anode represents the region of high temperature and thus there will normally be a distribution of gas density within the envelope, the density being highest at the coolest spot. If the envelope is cooled artificially, then a higher than normal gas density will occur at the envelope giving a low density in the discharge region This may limit the rate of rise of current demanded by the circuit. thereby increasing the dissipation within the tube, and accelerating the rate of gas clean-up.
8.2.10. Mounting
Most thyratrons may be mounted in any position although the size of the larger tubes generally dictates a base-down position. Care should be taken that strong RF or magnetic fields are kept away from the thyratrons. These could cause ionization within the tube envelope seriously affecting the hold-off capabilities of the tube and increasing the recovery time. Some gas clean-up may also occur. For medium- and high-power tubes the anode connector should preferably be of large surface area.
8.2.11. Warmup Time
The time quoted on tube data sheets is the minimum necessary for the cathode to reach operating temperature and for the gas pressure to reach a minimum value where reservoirs are used. If trigger pulses are applied before the expiry of the warm-up time then grid/cathode breakdown may be observed, but this does not mean that the cathode temperature or gas pressure are high enough for full power operation. If the ambient temperature before warm up is very low (say below 20° C) then some increase in warm-up time may be necessary.
8.2.12. The Tetrode Thyratron
The advantages offered by the tetrode over the triode in pulse modulator use are sufficiently important to warrant special mention. The reduction of firing time variations in thyratron circuits is always foremost in the minds of both tube designer and users and, while much can be done by careful circuit design, the variations obtained with triode thyratrons may still be excessive for certain applications. Firing time variations fall into two main classes: (a) long-term variations, both repetitive and cumulative. The first of these is largely due to the warming up of tubes and components each time an equipment is used. The second, a much slower variation is due to gradual reduction of gas pressure during the life of the thyratron; (b) short-term or transient variations due to various causes including interference from the A.C. heater field.
3-32
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Anode
Figure 3.30. Schematic Diagram of Tetrode Variations due to all these causes are reduced when tetrode thyratrons are employed. Tetrode thyratrons have two grids which are pulsed successively with a delay of about 1.0 µs. Grid 1 has a positive characteristic with respect to the cathode as in the triode, and grid 2 is given a negative characteristic with respect to grid 1 (Figure 3.30). Grid 1 performs its “priming” function very much as in the triode while grid 2 performs what may be termed a “gating” function when ionization by the grid 1 pulse is well under way. This results in very rapid and precise takeover by the anode. The negative bias applied to grid 2 assists recovery in the inter-pulse period and thus makes the tetrode thyratron eminently suitable for use in high p.r.r. precision radar equipment. In order to switch pulses which are successively accurate in time over a long period the two grids should be pulsed from separate sources with the grid 2 drive delayed by about 1.0 µs. When only one grid drive pulse is available, this may be made use of as in Figure 3.31. 1kΩ
Pulse input –100V bias
0.1µF
G2
0.001µF 1kΩ
G1 5kΩ
Figure 3.31. Single Pulse Drive for Tetrode
Modulator Theory: Thyratron Theory
3-33
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A more simple drive circuit is shown in Figure 3.32. It must be stressed, however, that some of the inherent advantages of the tetrode may be lost by the use of these circuits but they will be satisfactory for many applications. An increase in time jitter up to about 5.0ns may result and anode delay time drift may be as much as 0.1 µs as against 0.02 µs. 1kΩ
Pulse input –100V bias
G2
0.1µF 1kΩ
G1 5kΩ
Figure 3.32. Simplified Single Pulse Drive These three circuits all depend on transferring the discharge from the grid 1 electrode to grid 2 and thence to the anode. The grids should not be simply strapped together, since this may result in a discharge to the grid 1 electrode which would not necessarily transfer to grid 2 and would result in erratic firing.
8.2.13. The Parallel Operation of Hydrogen Thyratrons
In common with all gas-filled devices, thyratrons cannot be connected directly in parallel without some form of impedance in series with each tube. In repetitive pulse applications, the triode thyratron does not provide a sufficiently precise pulse-to-pulse triggering facility for parallel operation to be successful. The tetrode tube does not have this defect. In theory, any number of tetrodes may be parallel connected, although in practice six is a convenient maximum. Separate pulse-forming networks, each with its own charging diode, are recommended, since fault conditions will result in each thyratron discharging its own network and this will not involve as much energy as in the case of one large network. Further, the triggering requirements are less complicated as will be shown below. The following circuits show suitable arrangements for two tubes, but they may be extended for additional tubes as required. Figure 3.33 shows a suitable circuit where two driving pulses are available each pulse being split between the two thyratrons. Figure 3.34 shows an arrangement using a single driving pulse which automatically provides a delay on grid 2 to each tube. Figure 3.35 shows the general arrangement of the anode circuit.
3-34
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 R2
Pulse 2 I/P –100V bias
G2
1kΩ
V2 R1
G1
1kΩ
R2
G2
1kΩ
V1 R1
Pulse 1 I/P
G1
1kΩ
Figure 3.33. Double Pulse Drive for Parallel Operation Where the use of a single network cannot be avoided, it is essential that each tube has an anode inductor of the same value and arrangements should be made for adjustment of the current sharing between tubes. This is most conveniently achieved using the circuit of Figure 3.34, and making R2 variable. This provides adjustment of the delay to grid 2 on each tube. Figure 3.36 shows suitable component values. This arrangement has been successful in the parallel operation of six CX1140 tubes to provide a pulse output power of 75MW. R2
Pulse I/P 0.1µF
G2 0.001µF
R1
V2 G1
R3 R2 0.1µF
G2 0.001µF
R1
V1 G1
R3
Figure 3.34. Single Pulse Drive for Parallel Operation When several thyratrons are being used, a check should be made on the triggering supplies by removing one of the thyratrons from its socket to ensure that the amplitude of the available voltage pulse at that socket is greater than the minimum specification requirements when all the other tubes are conducting between grids and cathode.
Modulator Theory: Thyratron Theory
3-35
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Charging diode
Anode Inductor
PFN
I/P Charging choke
G2 G1
V2
Anode Inductor
PFN
G2 G1
V1
RL
Figure 3.35. Anode Circuit Using Separate PFN’s The rate of rise of current rating of the assembly is the sum of the individual tube ratings, and this often provides a capability in excess of a single larger tube.
5kΩ
Pulse input –100V bias
G2
0.001µF
0.1µF 1kΩ
G1 5kΩ
Figure 3.36. Arrangement for Adjustment of Current Sharing The driving pulse should have a fast rising front and be of near maximum rated amplitude to minimize anode delay time and jitter. This applies particularly to the circuit of Figure 3.33, due to the effect of capacitive loading on the leading edge. This effect may be considerably reduced as shown in Figure 3.37 by the use of a variable delay line in series with R2 instead of the 0.001 F capacitor for applications requiring minimum delay time and jitter. The values of series resistors given will, in general, provide satisfactory performance, but if other values are used, the grid 1 current must be kept within the specified limits. With separate networks and separate identical anode inductances, current sharing between tubes is automatic, and small differences in delay time between tubes do not affect the performance.
3-36
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1 µS Delay Line
R2
Hydrogen thyratron (tetrode)
R4 0.1µF
R1
R3
2µF Bias –60 to 100V
Figure 3.37. Using a Delay Line to Minimize Time Jitter
8.2.14. The Series Operation of Hydrogen Thyratrons
Where extremely high voltages have to be switched, thyratrons may be operated in series without the necessity of insulated trigger supplies. The general arrangement is shown in Figure 3.38 with suggested component values. Tetrodes are preferred since they generally have a lower anode delay time than triodes. Furthermore, in the arrangement shown, it is essential that the switch tube should fire with low current triggering. a property not normally found in triodes. The operation is as follows. Tube (1) at the lower voltage is triggered normally, using separate pulses to each grid, or a single pulse and the conventional RC arrangement. When this tube fires, the voltage at its anode falls and thus the voltage at the cathode of tube (2) falls. Capacitor (C1) between grid 1 and grid 2 of tube (2) helps to maintain the resulting high potential between grid 1 and cathode and thus the tube fires to grid 1, and thence to grid 2 resulting in complete breakdown.
HV
C1 100µµF
C3
10 M Tube 2
10 M
C2 Tube 1
Figure 3.38. Thyratrons in Series
Modulator Theory: Thyratron Theory
3-37
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The insulated heater transformer for tube (2) will have capacitance to ground (C2) which, if not minimized, will require an equal compensating capacitor (C3) to be placed in parallel with tube (2). This divider, assisted by the tube interelectrode capacitance, will automatically provide equal voltage distribution between tubes. Added capacitance should be minimized since this will discharge through the tube, and must be connected to the network side of the anode inductors. To avoid the presence of isolated elements, a resistive divider is preferable. The value of this should not be so low as to cause a serious drain from the power supply, and should not be so high as to introduce (with the various capacitances) time constants at variance with the PRF required.
8.2.15. Hydrogen and Deuterium
The advantages of hydrogen over mercury and the rare gases have already been mentioned in Part 1 of this article. However, an increasing use is made of deuterium as the gas filling in place of hydrogen. Chemically the gas is similar to hydrogen, so that conventional hydrogen reservoirs operate equally well with deuterium. The Paschen breakdown curve, however, shows that deuterium is capable of higher hold-off voltages than hydrogen for the same pressure. The greater mass of the ion, however, means less mobility and so recovery time is increased (by a factor of 2 for the same geometry). At the same time, surface recombination effects are reduced and the arc loss is lower. For this reason, deuterium is mostly used in high-power tubes, where recovery time is usually of less importance than hold off voltage and dissipation.
8.2.16. Single Shot and Crowbar Applications
The increasing use of high-power klystrons, traveling wave tubes and other such devices has demanded that some protection should be given in the event of an internal flash-over or similar fault. This protection may be provided by a gas-filled spark-gap or thyratron across the main high voltage supply, which, when triggered by the fault current, effectively short circuits the high voltage until the main circuit breakers operate. Since the “crowbar” device may have to remain quiescent for many thousands of hours until suddenly required, it is desirable that it should consume no power, and essential that it should function with complete reliability when needed. The spark gap has an advantage over the thyratron in its zero power consumption on standby. However, for both spark gap and triode thyratron, there is no simple method of ensuring that the crowbar circuit is in a state of readiness, e.g., the device may have become “leaky” during a long standby period. With tetrode thyratrons the switching action may be controlled completely by the second grid and it is thus possible to run a discharge continuously to grid 1 without affecting the peak hold-off voltage under DC conditions provided bias is applied to the grid. It has been found that a current of about 100mA continuously to grid 1 in no way affects the life or performance of the tube, and that this grid 1 current may then be used as a simple monitor, e.g., by lighting an indicator bulb, to show that the crowbar circuit is in a state of readiness. It is possible for a reversal of high voltage to take place following crowbar action and before circuit isolating elements have responded. If the trigger voltage to the crowbar tube has been removed, then high voltage will be reapplied to the load as this voltage becomes positive again. It is thus desirable that the trigger voltage waveform should be a train of pulses rather than a single pulse. The peak forward voltage given in the data sheets for these thyratrons is the rating for repetitive pulse work, and is normally reduced for DC holdoff applications. At the same time the peak current may be considerably increased provided the repetition frequency is not greater than 1 per 10 seconds.
3-38
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Four RF Theory
This chapter will discuss microwave RF system theory, including microwave technology, in order to familiarize the student with the application of this technology to the functional operation of the Clinac RF System.
RF Theory
4-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents
1. Introduction:.................................................................................................................... 4-5 2. Traveling Waves on Transmission Lines:........................................................................... 4-5 3. Waveforms: ...................................................................................................................... 4-8 4. Waveguides: ................................................................................................................... 4-11 5. Resonant Circuits: ......................................................................................................... 4-14 6. RF Transmission Theory: ............................................................................................... 4-16 7. RF Waveguide Design: .................................................................................................... 4-17 7.1. Modes:.................................................................................................................... 4-17 7.2. The TE10 Mode:...................................................................................................... 4-18 7.3. Coupling:................................................................................................................ 4-18 7.4. Determining the TE10 Dominant Mode of a Waveguide: .......................................... 4-19 8. Transmission Lines: ....................................................................................................... 4-20 8.1. VSWR: .................................................................................................................... 4-21 9. Vector Analysis – 3dB Quadrature Hybrid: ..................................................................... 4-21 10. Circulators: .................................................................................................................. 4-22 11. Klystron Theory:........................................................................................................... 4-24 11.1. Theory of Klystron Operation: ............................................................................... 4-24 11.2. Associated Equipment: ......................................................................................... 4-34 11.3. Power Supplies: .................................................................................................... 4-35 11.4. Cooling: ................................................................................................................ 4-39 11.5. RF Circuits: .......................................................................................................... 4-45 11.6. Tuning:................................................................................................................. 4-49 11.7. Noise in Klystron Amplifiers: ................................................................................. 4-51 11.8. Summary:............................................................................................................. 4-52
4-2
RF Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Illustrations Figure 4.1. Propagation of a Step Wavefront on a Transmission Line:.................................... 4-5 Figure 4.2. Infinitely Long Line: ............................................................................................ 4-6 Figure 4.3. Finite Line Terminated by its Characteristic Impedance: ..................................... 4-6 Figure 4.4. Traveling Waves on an Open-end Line:................................................................ 4-7 Figure 4.5. Traveling Waves on a Shorted-end Line: .............................................................. 4-9 Figure 4.6. Voltage and Current Waves on a Shorted-end Line:........................................... 4-10 Figure 4.7. Parallel Strip Lines:........................................................................................... 4-11 Figure 4.8. Waves on a Parallel Strip Transmission Line: .................................................... 4-12 Figure 4.9. Electric Field Distribution: ................................................................................ 4-13 Figure 4.10. Electric and Magnetic Fields in a Closed Rectangular Waveguide: ................... 4-13 Figure 4.11. Electric and Magnetic Fields of the Dominant TE10 Mode in a Rectangular Waveguide:......................................................................................................................... 4-14 Figure 4.12. Tuned Resonant Circuit: ................................................................................. 4-15 Figure 4.13. Resonant Circuit Having Two Natural Frequencies: ......................................... 4-15 Figure 4.14. WR 284 RF Waveguide: ................................................................................... 4-16 Figure 4.15. RF Waveform Divided into ¼-Wavelength Intervals:......................................... 4-16 Figure 4.16. RF Rectangular Waveguide Dimensions: ......................................................... 4-17 Figure 4.17. The Transverse Electric (TE) Mode: ................................................................. 4-18 Figure 4.18. The Transverse Magnetic (TM) Mode:............................................................... 4-18 Figure 4.19. Loop Coupling:................................................................................................ 4-19 Figure 4.20. Probe Coupling: .............................................................................................. 4-19
RF Theory
4-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 4.21. Frequency Determining Parameters: ............................................................... 4-19 Figure 4.22. Characteristic (Surge) Impedance: .................................................................. 4-20 Figure 4.23. PFN Parameters:............................................................................................. 4-20 Figure 4.24. 3dB Quadrature Hybrid Vector Analysis: ........................................................ 4-21 Figure 4.25. 3-Port Circulator: ........................................................................................... 4-22 Figure 4.26. Low Energy Clinac 4-Port Circulator and Phase Diagram: ............................... 4-22 Figure 4.27. High Energy Clinac 4-Port Circulator and Phase Diagram (Used in Shunt Tee Clinacs):............................................................................................................................. 4-23 Figure 4.28. High Energy Clinac 4-Port Circulator and Phase Diagram: .............................. 4-23 Figure 4.29. Triode Vacuum Tube Amplifier: .......................................................................... 4-25 Figure 4.30. Sectional View of a Klystron: .............................................................................. 4-26 Figure 4.31. Generation of Alternating Current in a Cavity:.................................................... 4-28 Figure 4.32. High-Power Four-Cavity Klystron: ...................................................................... 4-30 Figure 4.33. High-Power Four-Cavity Klystron, Simplified: ..................................................... 4-31 Figure 4.34. Effect of Tuning on Klystron Performance:.......................................................... 4-32 Figure 4.34. Effect of Tuning on Klystron Performance:.......................................................... 4-32 Figure 4.35. Klystron Power Supply Connections: .................................................................. 4-35 Figure 4.36. Typical Liquid Cooling System for a Klystron Amplifier: ...................................... 4-42 Figure 4.37. Typical RF Circuitry for a Klystron Amplifier: ..................................................... 4-46
4-4
RF Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
The material in this chapter should be read and thoroughly understood by the student before proceeding to the specific RF system descriptions in the High and Low Energy Clinac Beam Delivery System course manuals.
2. Traveling Waves on Transmission Lines
Figure 4.1 has been drawn to show the mechanism by which a step wavefront (pulse) travels along a real transmission line. q
i=I
S1 Edc
i=0
e=0
e=E
i=I
e
q
E
0
S
i I 0
S
Figure 4.1. Propagation of a Step Wavefront on a Transmission Line When switch S1 is closed, the battery with a terminal voltage of Edc is applied to the input of the transmission line. The voltage Edc does not appear instantly at all points along the line. Instead, a wave of voltage progresses along the line. The farther from the battery a given point on the line is, the later the time at which the line voltage at that point jumps from 0 to Edc. A current wave also travels along the line exactly in step (in phase) with the voltage wave. Current flows away from the battery in the top conductor and returns to the battery in the bottom conductor. Plus and minus signs are placed on the conductors at points where voltage exists between the conductors, and magnetic flux lines are shown encircling the conductors wherever current is flowing. The reason that the line cannot charge all at once is due to the existence of series inductance along each conductor as well as shunt capacitance across the conductors. The voltage wave can progress only as fast as the line current can carry a charge to the wavefront to produce the change in voltage. The current wave can progress only as fast as the voltage can develop across each short section of conductor at the wavefront, thus starting current in that corresponding section of line inductance. The voltage and current waves must move in phase with each other along the line. The rate of travel (propagation) along the transmission line is proportional to the square root of the inductive and capacitive reactance per unit length of line. T =
RF Theory: Introduction
LC
4-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The characteristic impedance of the line is proportional to the square root of the quotient of the inductive and capacitive reactance per unit length of line. Rz =
L--C
Thus far, only the start of a wave from the input terminals of a line and the propagation along a line of infinite length have been discussed. The effects that occur when a wave reaches the end of a finite length of line must also be considered. In Figure 4.2 a 100-volt battery has been connected through a 1K series resistance to an infinitely long transmission line with the same characteristic impedance as the series resistance.
1K
id
i
ed
e
S1
100V
RC = 1K
Figure 4.2. Infinitely Long Line The circuit of Figure 4.3 is the same except that the line has been shortened to a length equal to 1 microsecond of propagation time and terminated in its characteristic impedance of 1K ohms. In both circuits, a 50-volt wave, and 50-mA wave travel along the line from the input terminals after the switch is closed.
1K
100V
S1
id
i
ed
e
RC = 1K
1 µS Travel Time
Figure 4.3. Finite Line Terminated by its Characteristic Impedance For the first microsecond of time both lines have the same propagation characteristics, because the remainder of the infinitely long time is equivalent to a 1K resistance also. However, the circuit of Figure 4.3 will develop a steady-state condition as soon as the wave reaches the 1K termination resistance. Once in the steady state, 50 volts appears across the conductors of the line, and a 50-mA current will continue to flow through the conductors and terminating resistance, as expected when two 1K ohm resistors are connected in series across the 100 volt battery. The circuit of Figure 4.4 (A) uses the same 1 microsecond line segment with the end left unterminated. When the switch is closed, the line first appears as a 1K impedance connected in series with the 1K resistors. This is because the end terminal conditions do not affect the wavefront before it reaches the end of the line. A 50-volt, 50-mA wave starts along the line from the input terminals. The current must be zero at the end of the line at all times due to the open circuit. To maintain this zero current condition when the wavefront reaches the end of the line, a second 50-mA current wave must start back along the line toward the battery. There must also be a re-
4-6
RF Theory: Traveling Waves on Transmission Lines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 flected voltage wave associated with the reflected current wave. This is because the characteristic impedance of the line also relates to the quotient of the voltage and current wave. ( across line -) R c also = E --------------------------------I ( across line )
The voltage-wave amplitude is 50 volts thus causing the total line voltage to rise to 100 volts. These traveling waves are indicated in Figure 4.4 (B) and corresponding time variations of e and i are shown by Figure 4.4 (C). 1K
id
i
ed
e
S1
100V
RC = 1K
A. Open End Transmission Line
e 50V
t = ½ µS
s
0
i 50mA
s
0
ed
100V 50V
t = 1½ µS
s
0
id 50mA
0
s
B. Traveling Wave
ed
100V 50V
0
2 µS
t
2 µS
t
id 50mA
0
C. Time Waveforms Figure 4.4. Traveling Waves on an Open-end Line
RF Theory: Traveling Waves on Transmission Lines
4-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
3. Waveforms
The incident (forward) wave of Figure 4.4 (A), in traveling from the battery to the open end of the line, charges the line capacitance from 0 to 50 volts because of the 50-mA current supplied by the battery. When the wave reaches the open end, no capacitance remains to be charged. The 50-mA current is, however, maintained by the line inductance and thus charges the capacitance near the open end of the line to twice the battery voltage. This voltage is produced across the line near the open-end in a direction to reduce the line current toward zero. This increase of voltage and decrease of current near the open end of the line corresponds to a reflected wave traveling back toward the battery. As the reflected wave progresses back to the battery, the entire line is charged to 100 volts and the total current on the line is reduced to zero. Thus, a static condition is reached with the line at 100 volts, and no further changes occur in the circuit. In the circuit of Figure 4.5 on Page 4-9 a short circuit is placed at the end of the section. As a result, the voltage at the short circuit must always be zero, and at the instant the 50-volt, 50-mA incident wave reaches the short circuit, a reflected voltage wave of 50 volts amplitude and of a reversed polarity starts toward the battery. A current wave of 50 mA amplitude is also associated with the reflected voltage wave, and the current in this wave flows in the same direction as the incident current wave. After the reflection is over, a current of 50 mA flows from the battery into the moving wavefront, and a current of 100 mA flows from the wavefront to the short circuit. Thus, the line capacitance at the wavefront is discharged by the net current of 50 MA, and the voltage across the conductors drops to zero. When the reflected wave reaches the battery, the line voltage is zero at all points and the current is 100 mA. No further changes occur because 100 mA is exactly the current drawn by the 1K resistor connected in series with the shorted line section across the 100 volt battery. A fact of fundamental importance is that a wave of any shape can be propagated along a transmission line without any change of shape or magnitude. The voltage wave is always accompanied by a current wave of similar shape. Thus for RF frequencies a sine wave generator rather than a battery can be connected to the transmission line, and accordingly the same functions occur with the sine wave as occurred with the battery. When a line is terminated in an impedance that is different from the characteristic impedance (mismatched) the voltage and current on the line are no longer the result of a single wave traveling from generator to load. Instead, the total voltage and current are the algebraic sums of two waves traveling in opposite directions. As an example, consider the diagram of Figure 4.6 on Page 4-10 in which a sine wave generator with an internal resistance of RL is connected through a switch S to a transmission line of impedance RZ and terminated in a short circuit. The behavior of the line after the switch is closed is similar to that of a shorted line with a battery as the source (reference Figure 4.5), except that sinusoidal waves of voltage and current travel along the line. In order that the voltage at the short circuit may be zero at all times, the voltage reflected back at the short circuit must be equal in magnitude to the incident voltage, but reversed in polarity. The generator has an internal resistance which is equal to the line impedance RZ. This causes the line to behave with respect to the reflected waves exactly as if the line were terminated in its characteristic impedance resulting in steady state conditions when the reflected wave reaches the input terminals.
4-8
RF Theory: Waveforms
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 1K
id
i
ed
e
S1
100V
RC = 1K
A. Shorted Transmission Line 1 µS Travel Time
e 50V
t = ½ µS
s
0
i 50mA
s
0
ed t = 1½ µS
50V
s
0
id 100mA 50mA
0
ed
s
B. Traveling Wave
50V
0
2 µS
t
id 100mA 50mA
0
2 µS
t
C. Time Waveforms Figure 4.5. Traveling Waves on a Shorted-end Line
The incident and reflected voltage and current waves are shown by Figure 4.6 on Page 4-10 (B) and (C) respectively. The resultant sum of these waves is indicated by Figure 4.6 (D). The total voltage at any point on the line varies as the individual waves move in opposite directions along the line. At time (t1), an instant slightly later than that for which Figure 4.6 (B) and (C) are drawn, the peak of the incident voltage wave is at the short circuit end, and the component voltages have equal magnitude and opposite polarity at each point on the line. Thus, the voltage would be zero everywhere on the line as indicated by the line marked t1 in Figure 4.6 (D).
RF Theory: Waveforms
4-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 At other instants, cancellation of the individual waves does not occur. For example, at time (t2) a quarter period later than time (t1) a zero value of the component waves in Figure 4.6 (B) and (C) is at the short circuit end, and the total voltage marked by the curve (t2) in Figure 4.6 (D) is a sine wave of twice the amplitude of either component wave. Looking at additional instants of time (t3) and (t4) a quarter period later, etc., is indicated by the curves (t3) and (t4) respectively in Figure 4.6 (D).
S RS
i e
A. Circuit Diagram
+
–
e wave
e wave
e
B. Traveling Waves of Voltage
+
–
i wave
i wave
i
C. Traveling Waves of Current
t2
+
2em
e
t1,t3 t4 +
2i m
t1 t2,t4 i t3
D. Standing Waves
Figure 4.6. Voltage and Current Waves on a Shorted-end Line The total voltage and current patterns of Figure 4.6 (D) are defined as standing waves. By performing additions at other intermediate times to the quarter period instants, it may be shown that the total voltage and current have a sine wave distribution along the line with zeroes at the short circuit and half-wave intervals from the short circuit. The amplitude and polarity of this sine wave varies as the voltage at each point changes sinusoidally with time. The points of zero voltage are called voltage nodes, and the points of maximum voltage are called antinodes.
4-10
RF Theory: Waveforms
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Referring back to Figure 4.1 on Page 4-5, it should be noted that a magnetic flux or field is associated with the inductance of a transmission line and that an electric field is associated with the capacitance of the line. Thus, the energy in terms of volts and amperes can be translated into units of electric field flux and magnetic field flux. The latter two conditions are to be found within an RF field that is propagated through free space.
4. Waveguides
A waveguide performs the same function at microwave frequencies as a two-wire transmission line does at lower RF frequencies. The waveguide can be considered to possess all the same principles of operation discussed in the preceding sections. However, the transmission of energy is dealt with in terms of the electric and magnetic fields rather than in terms of voltage and current. A special form of transmission line, called a parallel strip line and pictured in Figure 4.7, can be used to define the electric and magnetic field intensities related to the voltage and current waves on a transmission line. The line of Figure 4.7 (A) is composed of two parallel flat-plate conductors of width (w) and spacing (h). The line is considered perfect with no losses with zero resistance in the conductor plates and zero conductance in the dielectric (air) between the plates. Incident and reflected waves of voltage and current can be propagated along the line in a like manner to the two wire transmission line of Figure 4.1 on Page 4-5. Consider that sinusoidal waves are traveling along the line and let the voltage be defined between the plates and the current along the plates be denoted by e and i respectively. i (out) w
i e
h
e i
Direction of Propagation
(A) Pictorial Representation
h
i (in)
w
(B) Cross Section
Figure 4.7. Parallel Strip Lines The two conductors can be considered as plates of a capacitor. The charge on the capacitor appears on the inner surfaces of the plates, and an electric field is developed between the plates because of the voltage (e). The charge and field flux at any cross section point increases and decreases in phase with the voltage wave at that section. The pattern of the electric field is indicated by the solid arrows of Figure 4.7 (B). If (w) is much larger than (h), the electric field will be uniform except at the edges of the strip plates. The current in the conductor plates develops a magnetic field that encircles each conductor. The direction of the field is related to the direction of the instantaneous current on the conductor plates by the right-hand rule and is indicated by the dashed line arrows of Figure 4.7 (B). Because of the plane geometry of the conductors, the magnetic field is uniform in most of the region between the conductors. Because the conductors have no resistance, the current will flow entirely on the surface of the conductors. Thus, the magnetic field as well as the electric field is prevented from entering the conductors. Figure 4.8 shows the resultant waves in the strip line.
RF Theory: Waveguides
4-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
i e i
Direction of Propagation
(A) Pictorial Representation
e i
(B) Voltage and Current Waves
E H
(C) Electric and Magnetic Field Waves Figure 4.8. Waves on a Parallel Strip Transmission Line The waves in the parallel-strip line are a very simple example of many forms of electromagnetic waves that can take place. When the conducting boundaries have a more complex shape, a great variety of more complex field patterns can be generated. The waves of the parallel strip line may be described as fundamental or simple transverse linearly polarized waves. A wave of this type for which both the electric and magnetic field vectors lie in the transverse plane of the conductors (the transverse plane is the plane that is tangent to the conductor surface and perpendicular to the direction of propagation of the wave on the conductor) is defined as the TEM wave or mode. If only one field of vectors lies in the transverse plane the wave is either a transverse electric (TE) or transverse magnetic (TM) wave. It is possible to take the parallel strip line and develop a waveguide structure from it. If the frequency of the wave to be propagated along the strip line is high enough so that it has a half wavelength λ ⁄ 2 value that is equal to or less than the width of the plates, there will exist across the plates an electric field distribution as shown in Figure 4.9 on Page 4-13.
4-12
RF Theory: Waveguides
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
λ 2
λ 2
(A)
(B) Figure 4.9. Electric Field Distribution
The magnetic field will be as indicated before at right angles to the electric field as referenced in Figure 4.7 (B). If there is no voltage or electric field present at the sides of the plates, we could connect them as shown in Figure 4.10.
Figure 4.10. Electric and Magnetic Fields in a Closed Rectangular Waveguide Now the magnetic field will be contained within the walls of the structure. We now have a waveguide transmission line that has all the same characteristics and follows the same laws of propagation as a plane transmission line. It is possible to propagate several different types of electromagnetic waves within a waveguide. Each wave is characterized by a different electric and magnetic field configuration. Associated with each wave type is a cutoff frequency below which, for a particular size guide, the propagation becomes impossible. In general the larger the guide, the lower the cutoff frequency of each type of wave. The wave with the lowest cutoff frequency is defined as the dominant wave or mode. To transmit the dominant mode, a rectangular guide must have a width of at least half the free space wave length of that wave. The height is not critical and is usually made about one half the width. In waveguides, the electric and magnetic fields are confined to the space within the guide walls. Thus, all power is transmitted along the guide with no radiation nor dielectric losses of any practical importance. The electric field (voltage) is defined across the height or short dimension of the guide as in the parallel strip line and the magnetic field current is defined parallel to the opposite walls of the waveguide as identified by Figure 4.11 on Page 4-14.
RF Theory: Waveguides
4-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 8 2
A
Direction of propagation
A
(A) TOP VIEW
B
B (B) END VIEW SECTION B-B
(C) SIDE VIEW SECTION A-A
Figure 4.11. Electric and Magnetic Fields of the Dominant TE10 Mode in a Rectangular Waveguide Note that the letters “TE” define the electric field as transverse and the subscripts “1” and “0” indicate the number of half cycle variations of the electric field along the width and height dimensions of the guide in accordance with industry convention of identification.
5. Resonant Circuits
Circuits composed of lumped inductive and capacitive elements can be made to resonate at any desired frequency by selection of the values of inductance and capacitance used. To increase the frequency, the size of the inductance and capacitance must be made smaller. At extremely high frequencies the electrical size as well as the physical size of the components becomes so small that the stray inductance and capacitance of the circuit begins to affect the resonant frequency of the circuit. This results in different construction techniques being employed to raise the frequency higher. In the UHF region parallel wire or coaxial cable transmission lines are used in place of the lumped components. At microwave frequencies resonant cavities are used. All forms of resonant circuits, whether lumped components, transmission lines or cavities, have certain natural frequencies of oscillation. A natural frequency of oscillation is that which can be sustained by the circuit (assuming it has no losses) once started that will continue indefinitely even though there is no internal connection to a power source. Each natural frequency of oscillation is called a natural mode or resonant mode. The term “mode” indicates the manner of oscillation. Various modes may occur depending on the nature of the design and excitation parameters. Figure 4.12 on Page 4-15 shows a simple parallel resonant circuit consisting of a single inductor L1 and capacitor C1. Assume the capacitor C1 is charged before switch S1 is closed. When S1 is closed, the circuit will oscillate at some discrete frequency dependent on the fact that both L1 and C1 must have equal reactance. The frequency can be determined by: 1 f 0 = -----------------2π LC
4-14
RF Theory: Resonant Circuits
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
S1
L1
C1
Figure 4.12. Tuned Resonant Circuit Both voltage and current vary sinusoidally with time, and the mode pattern may, therefore, be described by specifying the phase and magnitude of the voltage and current during the oscillation. Because the reactance of L and C are equal at only one frequency, the circuit of Figure 4.12 has only one mode of oscillation. The circuit of Figure 4.13 has two natural modes of oscillation. (Note the circuit is drawn to resemble a two-section transmission line shorted at one end and open-circuited at the other).
L1
C1
L2
C2
Figure 4.13. Resonant Circuit Having Two Natural Frequencies Below the frequency (f0) at which L1 and C1 and also L2 and C2 resonate, the reactance of the parallel combination of L2 and C2 is inductive. The L2 and C2 combination in series with L1 may, therefore, be considered an inductance that resonates with C1 at some frequency less than (f0). Similarly, the L2 and C2 combination and C1 in series is less than C1 and resonates with L1 at a frequency greater than (f0). From the foregoing discussion a shorted transmission line can be considered to have many frequencies at which it could oscillate dependent on how it is excited. Resonant cavities can be considered as an extension of the transmission line into a single physical metal walled chamber fitted with suitable devices for admitting and extracting electromagnetic energy. One example of a resonator cavity is a rectangular box that may be thought of as a section of rectangular waveguide closed at both ends by conducting plates. Because the end plates appear as short circuits for a wave traveling along the waveguide, the cavity is analogous to a transmission line section with a short at both ends. Resonant modes will occur at frequencies for which the distance between the end plates is a multiple of half the guide wavelength. Higher order waves as well as the dominant wave may give rise to several resonant frequencies, and thus a great variety of resonant modes of operation are possible.
RF Theory: Resonant Circuits
4-15
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The cavity formed by closing the ends of a section of waveguide is only one of many cavity configurations that can be used to generate microwave energy. By appropriate choice of cavity shape, advantages such as ease of tuning, compactness, simple mode spectrum, and high Q may be obtained.
6. RF Transmission Theory
The RF waveguide used on our machines is called WR 284 or JAN RG 48/U. It has a usable frequency range (S Band) of 2.6 to 3.95 GHz in the TE10 mode and its dominant TE10 mode is 2.5 GHz.
2.875" I.D.
1.312" I.D.
Figure 4.14. WR 284 RF Waveguide RF waves travel 186,000 miles/sec or 300 × 106 meters/sec in air. Therefore, the wavelength (8) of one cycle of a particular frequency, measured crest to crest, is equal to: 6
Where:
300 × 10 λ = ----------------------f (in Hz) 300 = ------------------------f (in MHz)
8 = the wavelength of one cycle in meters. 300 × 106 = the distance in meters covered at a speed 186,000 miles/sec.
λ
λ 4
λ 2
λ 4
Figure 4.15. RF Waveform Divided into ¼-Wavelength Intervals To convert from length in meters to inches divide by 0.0254. To convert from length in inches to meters multiply by 0.0254.
4-16
RF Theory: RF Transmission Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF waves travel somewhat slower in waveguides or parallel lines, depending on material and conditions, as much as 15% slower. Therefore, the distance measured crest-to-crest will be shorter compared with the same frequency in air. Example: A quarter-wavelength ( λ ⁄ 4 ) of a particular frequency on any transmission line is calculated using the formula: × KL = 246 -----------------f
Where: L
= ( λ ⁄ 4 ) in feet.
K = Transmission line constant (1.000 for air, usually 0.975 for parallel line to 0.85 for air dielectric coax). f
7. RF Waveguide Design
= Frequency in MHz.
Figure 4.16 illustrates the design of a typical rectangular RF waveguide.
b
a Figure 4.16. RF Rectangular Waveguide Dimensions Dimension (a) is usually 1.3 × λ ⁄ 2 of the frequency to be transmitted, in air. The low frequency cutoff of the guide is roughly twice its width converted to wavelength (TE10 dominant). Dimension (b) is determined by the power to be generated and is usually 0.2 to 0.5 times the wavelength in air. Bends required in waveguides should be made so that the radius is no shorter than 2 wavelengths.
7.1. Modes
There are basically two methods of transmitting energy through a waveguide, the TE and TM mode. The TE mode is most commonly used because: !
It is easy to excite.
!
It is plane polarized.
!
It is each to match to a radiator.
!
Its cutoff frequency is dependent on only one guide dimension (a) therefore easy to design.
RF Theory: RF Waveguide Design
4-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the TE mode, the electric field lies in transverse planes that contain the X and Y axes, and the E lines are parallel to the Y axis and perpendicular to the Z axis of the guide, as shown in Figure 4.17.
Magnetic lines E lines Figure 4.17. The Transverse Electric (TE) Mode The magnetic field is composed of closed loops that lie in the transverse plane that contains the X and Y axes and are wholly transverse to the guide's Z axis, as shown in Figure 4.18.
E lines
M lines Figure 4.18. The Transverse Magnetic (TM) Mode
7.2. The TE10 Mode
7.3. Coupling
The digits after the mode specify the following: 1.
The first digit states the number of λ ⁄ 2 variations in the wide or transverse part of the waveguide.
2.
The second digit states the number of λ ⁄ 2 variations in the narrow section of the waveguide.
There are two methods of extracting energy from a waveguide, loop and probe coupling. In loop coupling, a small loop is inserted into the guide that cuts the magnetic lines (H lines) and therefore acts as a transformer, as shown in Figure 4.19 on Page 4-19. In probe coupling, a small antenna is inserted into the guide parallel with the electric field (E lines), as shown in Figure 4.20 on Page 4-19.
4-18
RF Theory: RF Waveguide Design
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.19. Loop Coupling
Figure 4.20. Probe Coupling
7.4. Determining the TE10 Dominant Mode of a Waveguide
a
a = 2.875"
Figure 4.21. Frequency Determining Parameters
RF Theory: RF Waveguide Design
4-19
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 λ --- = 2.875 inches 2 λ = 2 × 2.875 inches λ = 5.750 inches Λ (meters) = 0.0254 × 10
–2
× 5.750
–2
λ = 14.6 × 10 300 f (MHz) = -----------------------λ (meters) 2
3 × 10 f = --------------------------–1 1.46 × 10 3
f = 2.05 × 10 f = 2.05 GHz
The TE10 dominant frequency is 2.05 GHz.
8. Transmission Lines
Transmission lines can be classified as resonant or non-resonant. Resonant lines are used primarily for impedance matching, phase shifters, inverters, wave filters and chokes. Non-resonant lines are lines that are either infinitely long or terminated in its characteristic impedance. The voltage and current waves move in the same phase with each other and all of the energy is absorbed by the load.
Figure 4.22. Characteristic (Surge) Impedance Z 0 = characteristic impedance Z 1 = total inductive reactance of line = 2πfL 1 Z 2 = total capacitive reactance of line = ------------2πfC Z1 2 Z 0 = Z 1 Z 2 + ⎛ -----⎞ ⎝ 2⎠ 2
Z Where the term ⎛⎝ -----1⎞⎠ represents the number of sections; as this number apZ2
proaches infinity then this term will approach zero. The PFN in the Clinac is a transmission line, so let us calculate its impedance.
Ltotal = 36.5 µH
Ctotal = 18 µF Figure 4.23. PFN Parameters
4-20
RF Theory: Transmission Lines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 As explained in Chapter 3 of this manual, the load resistor(s) on the despiking network can be used as a temporary load while troubleshooting the modulator. The load resistance, 12.5 ohms, would match the impedance of the generator (PFN line).
8.1. VSWR
VSWR is the ratio of the effective voltage at a loop to the effective voltage at a node. It is also equal to the ratio of the characteristic impedance of the line to the impedance of the load, or vice versa. To measure VSWR, a probe can be inserted in a slot cut in the waveguide that acts as an antenna and is excited by the E lines that flow parallel to it. Since the line current flows parallel to the slot, the effective resistance of the waveguide is not appreciably reduced by the presence of the slot.
9. Vector Analysis – 3dB Quadrature Hybrid
4.24 shows a vector analysis of the input and output signals of the 3dB quadrature hybrid. Port 1 Output Vi 2
Port 2 Output Vr 2
V1
Phase Shift
V1 – V2
0°
0V
90°
–2V
270°
+2V
180°
0V
V2
Vr (–90°)
Vi (–90°)
2
2
Vi 2
Vr (–90°) 2 V2
Vr (–180°)
Vi (–90°)
2
2
V1
Vi 2 V1
Vr 2
Vr (–270°)
Vi (–90°)
2
2
Vi 2 Vi (–90°)
Vr (–270°) 2
2 Vr (–180°)
V2
2
Vi = incident signal Vr = reflected signal
Figure 4.24. 3dB Quadrature Hybrid Vector Analysis
RF Theory: Vector Analysis – 3dB Quadrature Hybrid
4-21
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 When the RF Power first reaches the accelerator structure all of it is initially reflected back toward the source. Once its amplitude has stabilized, if its frequency is equal to the resonant frequency of the accelerator it begins to flow into and resonate within the accelerator. At the end of the pulse, as the amplitude begins to decrease, the power is again totally reflected. In order to separate the forward and reflected power, a circulator is employed.
10. Circulators
There are two types of circulators used in Varian Clinacs, 3-port and 4port. All Clinacs being manufactured as of this writing use 4-port circulators. Many older low-energy Clinacs use 3-port circulators, which are simpler in design. Figures 5.25 and 5.26 show the Low Energy Clinac 3-port circulator and 4-port circulator respectively.
To Water Load
From Water Load
RF Power Path
Magnet
Ferrite Beads
To Magnetron
From Accelerator
From Magnetron
To Accelerator
Top View
Cutaway View
Figure 4.25. 3-Port Circulator Magnets
Blocked off
1
4
3 2
From Magnetron
1 3
3
2 4
1
4 2
To Accelerator To Water Load
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
90° Phase Rotation
1
2
3
4 90° Phase Rotation
4
1
2
180° Phase Rotation NOTE: Circles indicate where phase rotation occurs
Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
Figure 4.26. Low Energy Clinac 4-Port Circulator and Phase Diagram (Configured as 3-Port)
4-22
RF Theory: Circulators
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 4.27 shows the High Energy Clinac 4-port circulator used with the shunt tee power attenuator. In 1996, Varian eliminated the shunt tee and began operating the klystron in its linear mode, thus removing the need for the fourth port. On these machines, port 3 of the Magic Tee is blocked, and the RF power from the Klystron is applied to port 4 of the 3dB Quadrature Hybrid, as shown in Figure 4.28. Magnets
From Klystron
1
4
3
To Shunt Tee
1 3
2
3
2 4
1
4 2
To Accelerator To Water Load #2
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
4
90° Phase Rotation
1
2
3
90° Phase Rotation
4
1
2
180° Phase Rotation Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
NOTE: Circles indicate where phase rotation occurs
Figure 4.27. High Energy Clinac 4-Port Circulator and Phase Diagram (Used in Shunt Tee Clinacs) Magnets
Blocked off
1
4
3 2
From Klystron
1 3
3
2 4
1
4 2
To Accelerator To Water Load
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
90° Phase Rotation
1
2
3
4 90° Phase Rotation
4
1
2
180° Phase Rotation NOTE: Circles indicate where phase rotation occurs
Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
Figure 4.28. High Energy Clinac 4-Port Circulator and Phase Diagram (Configured as 3-Port for KLM Clinacs)
RF Theory: Circulators
4-23
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11. Klystron Theory
This discussion is primarily for those engineers and technicians with little or no knowledge of microwaves who may find themselves suddenly confronted with operating and maintaining a high power microwave transmitter using klystron amplifier tubes. It is assumed that the reader is familiar with the theory and operation of triode RF amplifiers and general electronic circuit theory. We will discuss a simplified theory of operation for klystron amplifier tubes, the associated equipment required to make a klystron amplifier tube operate properly, some of the things that must be protected from malfunctions, and some generalized operating procedures associated with this type of equipment. After reading this information, the reader probably could not design a klystron amplifier; however, he should be able to understand better what is going on in the equipment and the reasons behind some of the operating instructions presented in a typical klystron amplifier Instruction Manual. Knowing and understanding this simple theory should help the operator to realize “why” certain functions are built into a klystron amplifier, and the “why” of certain operating instructions that he may receive. We feel that this understanding is important to a man who must operate or service a fairly complicated and expensive piece of electronic equipment. A little history: The klystron amplifier was in its infancy even at the end of World War II, although reflex klystrons had been developed (for use as local oscillators in radar receivers) to a fairly high degree of sophistication by the end of the war. Since World War II the klystron amplifier has undergone a spectacular evolution. It has become one of the most widely used devices for the amplification of microwave signals, particularly for high power applications. Klystron amplifiers currently in production cover microwave frequency ranges from UHF to 100 GHz, or higher; outputs range from a few milliwatts to many megawatts peak and more than 100 kilowatts average; power gains vary from 3 to 90 dB; and sizes vary from extremely small tubes that can be held easily in the palm of the hand to tubes that are more than 12 feet long. The uses of klystron amplifiers cover almost every microwave application, from low level signal generators, to giant radar equipment and huge transmitters for deep space communication and command. Some of the equipment is complicated and quite expensive. A serious shortage of engineers and technicians trained to operate and maintain this equipment has resulted from the rapidity with which this equipment has been developed and produced. Many engineers and technicians, experienced on lower frequency equipment using conventional vacuum tubes, have required retraining to operate this microwave equipment. We hope this information will help slightly in the training process.
11.1. Theory of Klystron Operation
The basic theory of klystron amplification is quite simple. In fact, the klystron amplification principle can be readily explained by an analogy with a simple triode RF amplifier. Obviously there are some differences (which will be explained), and these differences are what make a klystron amplify at microwave frequencies whereas a triode will not. First, let us consider the basic theory of operation of a simple triode amplifier. Figure 4.29 shows a simplified diagram of a triode amplifier with resonant circuits at both the input and the output. Such resonant circuits restrict the bandwidth of the amplifier and increase the gain. Such an amplifier might be part of an intermediate frequency amplifier circuit typically used at frequencies from 10 to 100 megacycles.
4-24
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF Output
Resonant grid circuit
RF Input Resonant Plate Circuit
Figure 4.29. Triode Vacuum Tube Amplifier A triode radio tube consists of three elements: a cathode that emits a stream of electrons, a grid that stands in the path of the stream, and a plate that attracts the electrons and catches them after they pass through the grid. The grid acts as a valve, opening or closing the passage of electrons according to the voltage on it. The RF input signal comes to the grid as a weak alternating current, oscillating at the RF frequency. The oscillating voltage thus applied to the grid modulates the flow of electrons across the tube at the RF frequency. The electron stream then delivers, at the plate, an alternating current that reproduces the weak signal on the grid with amplification. This alternating current at the plate flows through the resonant plate circuit and excites alternating voltages across it; these voltages constitute the RF output from the amplifier. Now the time it takes an electron to cross the tube is in the order of a billionth of a second. This transit time is short compared with the cycle of a long radio wave (around a millionth of a second); therefore the electron is slowed or speeded by the voltage on the grid at a given moment of the RF cycle. The flow of electrons, therefore, can “follow” the voltage fluctuations on the grid. In microwaves, however, the oscillations are so rapid (i.e., the cycle is so short) that the voltage on the grid may go through several complete oscillations while an electron travels across the tube. In other words, the grid voltage changes too fast and produces only chaos among the electrons. The grid voltage can no longer impose its signal pattern on the electron flow. There are other reasons why the conventional triode tube fails in the microwave range, but this is the most fundamental one the simple fact that the transit time of an electron from cathode to plate is long compared with the time of one cycle of the microwave signal. The klystron tube makes a virtue of the very thing that defeats the triode the transit time of the electrons. What it does is to “modulate” the velocity of electrons so that, as they travel through the tube, they sort themselves into groups and arrive at their destination in bunches. These bunches deliver an oscillating current to the output resonant circuit of the klystron. Figure 4.30 on Page 4-26 shows a cutaway representation of a typical klystron amplifier. Schematically it is very similar to a triode amplifier in that it includes an electron gun, resonant circuits, and a collector (which is roughly equivalent to the plate of a triode). In fact, the klystron amplifier consists of three separate sections the electron gun, the RF section and the collector section.
RF Theory: Klystron Theory
4-25
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Control Grid
Buncher Gap
RF Input
Bunch of Electrons
RF Output
Catcher Gap
Electrons
Collector
Heater
Cathode
Electron Gun
Anode
First Cavity (Buncher)
Drift Tube
RF Section
Intermediate Cavity
Last Cavity (Catcher)
Collector
Figure 4.30. Sectional View of a Klystron Let us consider, first, the electron gun structure: As in the triode, the electron gun consists of heater and cathode, a control grid (sometimes), and an anode. Electrons are emitted by the hot cathode surface and are drawn toward the anode that is operated at a positive potential with respect to the cathode. The electrons are formed into a small, dense beam by either electrostatic or magnetic focusing techniques, similar to the techniques used for beam formation in a cathode ray tube. In some klystron amplifiers a control grid is used to permit adjustment of the number of electrons that reach the anode region; this control grid may be used to turn the tube completely on or completely off in certain pulsed-amplifier applications. The electron beam is well formed by the time it reaches the anode. It passes through a hole in the anode, passes on to the RF section of the tube, and eventually the electrons are intercepted by the collector. The electrons are returned to the cathode through an external power supply (not shown on Figure 4.30). It is evident that the collector in the klystron acts much like the plate of a triode as far as collecting of the electrons is concerned. However, there is one important difference; the plate of a triode is normally connected in some fashion to the output RF circuit, whereas, in a klystron amplifier, the collector has no connection to the RF circuitry at all. From the above discussion it is apparent that the klystron amplifier, as far as the electron flow is concerned, is quite analogous to a “stretched-out” triode tube in that electrons are emitted by the cathode, controlled in number by the control grid, and collected eventually by the collector. Now let us consider the RF section of a klystron amplifier. This part of the tube is physically quite different from a triode amplifier. One of the major differences is in the physical configuration of the resonant circuit used in a klystron amplifier. The resonant circuit used with a triode oscillator, at lower frequencies, is generally composed of an inductance and a capacitor, while the resonant circuit used in a microwave tube is almost invariably a metal-enclosed chamber, known as a cavity resonator.
4-26
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A very crude analogy can be made between the resonant cavity and a conventional L-C resonant circuit. The gap in the cavity (see Figure 4.30) is roughly analogous to the capacitor in a conventional low frequency resonant circuit in that alternating voltages, at the RF frequencies, can be made to appear across the cavity gap. Circulating currents will flow between the two sides of the gap through the metal walls of the cavity, roughly analogous to the flow of RF current in the inductance of an L-C resonant circuit. Since RF voltages appear across the sides of the cavity gap it is apparent that an electric field will be present, oscillating at the RF frequency, between the two surfaces of the cavity gap. When a cavity is the correct size, it will resonate to microwaves of a certain frequency. The cavity can be tuned to various microwave frequencies by adjusting its size by some mechanical means. A crude analogy to the cavity resonator would be a glass goblet that resonates at a certain pitch depending on the level of the water in it, i.e., the size of the air cavity in the goblet. As shown in Figure 4.30, electrons pass through the cavity gaps in each resonator, and pass through cylindrical metal tubes between the various gaps. These metal tubes are called “drift tubes.” In a klystron amplifier the low-level RF input signal is coupled to the first resonator, which is called the “buncher” cavity. The signal may be coupled in through either a waveguide or a coaxial connection. The RF input signal will excite oscillating currents in the cavity walls, if the cavity is the correct size (that is, tuned to the right frequency). These oscillating currents will cause the alternate sides of the buncher gap to become first positive, and then negative, in potential at a frequency equal to the frequency of the RF input signal. Therefore, an electric field will appear across the buncher gap, alternating at the RF frequency. This electric field will, for half a cycle, be in a direction that will tend to speed up the electrons flowing through the gap; on the other half of the cycle the electric field will be in a direction that will tend to slow the electrons as they cross the buncher gap. This effect is called “velocity modulation,” and it is the mechanism that permits the klystron amplifier to operate at frequencies higher than the triode. After leaving the buncher gap, the electrons continue toward the collector in the drift tube region. Ignore for the moment the intermediate resonator shown in Figure 4.30, and let us consider the simple case of a two-cavity klystron amplifier. In the drift tube region the electrons that have been speeded up by the electric field in the buncher gap will tend to overtake those electrons that have previously been slowed (by the preceding half of the RF wave across the buncher gap). It is apparent that, since some electrons are tending to overtake other electrons, clumps or “bunches” of electrons will be formed in the drift tube region. If the average velocity of the electron stream is correct, as determined by the original voltage between anode and cathode, and if the length of the drift tube is proper, these “bunches” of electrons will be quite completely formed by the time they reach the catcher gap of the last cavity (which is called the “catcher”). This results in bunches of electrons flowing through the catcher gap periodically, and during the time between these bunches relatively fewer electrons flow through the catcher gap. The time between arrival of bunches of electrons is equal to the time of one cycle of the RF input signal. These bunches of electrons will induce alternating current flow in the metal walls of the catcher cavity as they pass through the catcher gap. If the catcher cavity is of correct size (tuned to the proper frequency) large oscillating currents will be generated in its walls. These currents cause electric
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 fields to exist, at the RF frequency, within the catcher cavity. These electric fields can be coupled from the cavity (to the output waveguide or coaxial transmission lines) resulting in the RF output from the tube. It is not particularly obvious why a bunch of electrons, passing through the catcher gap, should generate an oscillating RF current in the walls of the catcher cavity. Fortunately, a qualitative explanation is easy to understand. Refer to Figure 4.31 which shows the catcher cavity at three instants of time as a bunch of electrons flow across the catcher gap. The electrons are shown passing the catcher gap, traveling from left to right. To simplify the explanation, we have shown grid wires across the gaps; the grid on the left side of the gap is labeled No. 1, while the grid on the right is labeled No. 2. Since the grid wires, and the cavity walls, are made of high-conductivity metal, such as copper, a large number of free electrons will be present in the metal. In Figure 4.31, as the bunch of electrons approaches Grid No. 1 the free electrons in Grid No. 1 will be repelled since negative charges repel each other. This will tend to cause these electrons to flow from the grid wires into the cavity walls and around the cavity walls toward Grid No. 2. This is shown by the current flow path in Figure 4.31A. The result is that Grid No. 2 will tend to accumulate a surplus of negative charges, whereas Grid No. 1 will have a scarcity of negative charges present. Figure 4.31B shows the instant when the bunch of electrons is between Grids 1 and 2. At this instant the electrons in the bunch are repelling free electrons in both grids equally, and the net current flow around the cavity walls is essentially zero. Direction of Electron Flow
Direction of Electron Flow Electron Bunch
Grid 1
Grid 2
A
Grid 1
Grid 2
B
Grid 1
Grid 2
C
Figure 4.31. Generation of Alternating Current in a Cavity Figure 4.31C shows the instant just after the electron bunch has passed to the right of Grid No. 2. Remember that Grid No. 2 has accumulated an excess of free electrons already and these free electrons would tend to redistribute themselves back toward Grid No. 1 even if the electron bunch was not present. However, the electron bunch further repels the excess free electrons in Grid No. 2 and tends to “push” these free electrons back toward Grid No. 1. As the electron bunch moves further to the right, the electrons will redistribute themselves to essentially an equilibrium condition during the time “between bunches.” The process of course repeats every cycle of the RF wave because a bunch of electrons comes past the catcher gap in a time equal to the interval of one cycle of the RF wave. Since the resonant cavity is a high-Q circuit the oscillating currents tend to be essentially sinusoidal although the bunches of electrons arrive in short bursts. The situation is quite analogous to striking a pendulum one blow for each cycle of its oscillation; this will cause the pendulum oscillation to build up although the driving force is not continuously applied. Another analogy is a Class C triode amplifier where bursts of
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 current generate essentially sinusoidal voltages in the plate resonant circuit. RF power can be taken from the output (catcher) cavity by coupling to the oscillating current flowing in the cavity walls (or to the electric fields inside the cavity which are generated by these oscillating currents). If the amplifier is functioning properly, the oscillating current in the catcher cavity will be considerably larger than the oscillating currents in the buncher cavity; consequently, amplification has taken place. When the bunches of electrons pass through the output gap in the catcher cavity, they deliver energy to this cavity which causes currents to flow in the cavity walls. Since the electron beam is delivering energy to the cavity, it is slowed in velocity; therefore the beam arrives at the collector with less total energy than it had when it passed through the input cavity. This difference in electron beam energy is approximately equal to the RF energy delivered from the output of the cavity. It is appropriate to mention here that the velocity modulation effect does not form “perfect” bunches of electrons. There are some electrons which come through “out-of-phase.” These electrons show up at the last gap between the bunches. The electric field, at the time these out-of-phase electrons come through, is in a direction to accelerate them; so some few electrons will actually have their velocity increased as they come through the output gap. The electrons reaching the collector therefore have a wide spread of energy. Some of them (the out-of-phase electrons) may have velocities almost twice as high as the average electron velocity; other electrons (the “in-phase,” useful electrons) will be materially slowed and will arrive at the collector with a velocity much less than they started with. In the previous discussion we have considered only a two-cavity klystron amplifier, having neglected the intermediate cavity shown on Figure 4.30. Klystron amplifiers have been built (to our knowledge) with as many as seven cavities, i.e., with five intermediate cavities. The effect of the Intermediate cavities is to improve the bunching process; the result is to increase amplifier gain, and to a lesser extent, the amplifier efficiency. Adding more intermediate cavities is roughly analogous to adding more stages to an I-f amplifier, i.e., the gain of the overall amplifier is increased, and the overall bandwidth is reduced, if all stages are tuned to the same frequency. The same effect occurs with the klystron amplifier. However, it is well known that the bandwidth can be increased, and the gain reduced, by staggertuning an I-f amplifier. This analogy carries over to the klystron amplifier. A given klystron amplifier tube will deliver high gain and narrow bandwidth if all the cavities are tuned to the same frequency; this is called “synchronous-tuning.” If the cavities are tuned to different frequencies the gain of the klystron amplifier will be reduced and the bandwidth may be appreciably increased; this is called “stagger-tuning.” Most klystrons which feature relatively wide bandwidth are stagger-tuned. The appropriate method of accomplishing stagger tuning is discussed in more detail later.
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.32. High-Power Four-Cavity Klystron
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Collector Output Window Water Circuit
Collector Pole Piece Output Iris Output Cavity (Catcher)
Tuning Diaphragm
Electron Bunch
Water Circuit
Third Cavity Magnetic Circuit
Second Cavity
Drift Tube
Input Cavity (Buncher)
Focus Coils
Input Loop
Anode Pole Piece Anode Heater
Figure 4.33. High-Power Four-Cavity Klystron, Simplified
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The klystron is not a “perfect” linear amplifier; that is, the RF power output is not linearly related to the RF power input at all operating levels. Another way of stating this is that the klystron amplifier will “saturate,” just as a triode amplifier will “limit” if the input signal becomes too large. In fact, if the RF input is increased to levels above saturation, the RF power output will actually decrease. Figure 4.34 shows the plot of typical klystron amplifier performance for various tuning conditions. The RF output is plotted as a function of the RF input. Curve A of Figure 4.34 shows typical performance for synchronous tuning. Under these conditions the tube has maximum gain. The power output is almost perfectly linear, with respect to the power input, up to about 70 per cent of saturation. However, as the RF input is increased beyond that point, the gain decreases and the tube saturates. As the RF input is increased beyond saturation, the RF output decreases. The reason for this decrease in output is quite interesting. Remember, in our previous discussion, that the electron bunches were formed by the action of the RF voltage across the buncher cavity gap. This RF voltage speeded up some electrons and slowed other electrons, resulting in formation of bunches in the drift tube region. Obviously this speeding up and slowing effect will be increased as the RF drive power is increased. The saturation point on Figure 4.34 is reached when the bunches are most perfectly formed at the instant they reach the output (catcher) gap. This results in the maximum power output condition. When the RF input is increased beyond this point, the bunches are most perfectly formed before they reach the output gap, i.e., they form too soon in the drift tube region. By the time the bunches have reached the output gap they tend to “debunch” because of the mutual repulsion of the electrons, and because the faster electrons have overtaken and passed the slower electrons. This causes the power output to decrease.
Envelope of Saturation Peaks Saturation Point
RF Output
A
B C D
RF Input
Figure 4.34. Effect of Tuning on Klystron Performance
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Curves B, C, and D illustrate a phenomenon of klystron amplifiers which is difficult to explain theoretically, but which should be recognized by personnel operating these amplifiers. It turns out that, if we start with a multicavity klystron which is synchronously tuned, and then tune the next-tothe-last cavity to a higher frequency, we find that the gain of the amplifier is reduced but that the saturation power output level may be increased. This effect is shown by curves B and C. Curve B represents a small amount of detuning of the next-to-the-last cavity, and curve C represents even more detuning of that cavity. Note that the gain of the tube has been reduced (it takes more RF input to obtain a given RF output), and that the saturation output power is higher than obtained with synchronous-tuning (curve A). As stated previously, this stagger-tuning also results in a wider bandwidth for the amplifier. Many klystron amplifiers are operated in this fashion because it enables one to obtain more power output, with the same beam power input, and therefore increases the efficiency of the tube; of course this can only be done if enough RF drive power is available to operate under the stagger-tuned condition. As one might intuitively expect, we can go too far with this stagger-tuning, and the saturation output will eventually drop. This is illustrated by curve D of Figure 4.34. Figure 4.30 does not show one very important item which is usually required for high power klystron amplifier operation. This is an axial magnetic field, i.e., one which is parallel to the center line of the klystron. In klystron amplifiers which are physically “long” it is quite difficult to keep the electron beam formed properly during its travel through the RF section. Since electrons are negatively charged particles, they tend to repel each other; this causes the beam to “spread” in a direction perpendicular to the axis of the tube. If this occurs, the electrons will strike the drift tubes and be collected there, rather than passing through the drift tubes to the collector. To overcome this beam spreading an axial magnetic field is used. The action in the magnetic field is to exert a force on the electrons to keep them going in the correct direction during their transit through the RF section. The magnetic field may be developed by a permanent magnet or by one, or more, electromagnet coils. A permanent magnet is generally used on tubes which are physically small or of medium power rating. Unfortunately, the size and weight of a permanent magnet become excessive for long or high power tubes, making it necessary to use electromagnets. In some large tubes several, separate, magnet coils are used; the current in each coil is individually adjustable to optimize the magnetic field shape. The magnetic field is normally terminated as quickly as possible after the catcher cavity so that the beam can spread before it hits the collector. This tends to spread the electron beam interception over a large surface on the collector; this minimizes collector-cooling problems which would result if the beam remained concentrated at the time of interception. Even with an axial magnetic field some electrons will go “astray” and not remain in the main electron beam. These electrons will be intercepted by the anode or the klystron drift tubes. In high power tubes is it particularly important to minimize the number of these stray electrons because they generate heat when they strike the drift tubes. In high-power klystrons this heating can be a very severe problem because drift tubes are difficult to cool. Temperatures can become high enough to melt the metal in the drift tubes and destroy the tube. The collector is normally insulated from the RF section of large klystron amplifiers to permit separate metering of the electrons intercepted by the drift tubes, and those intercepted by the collector. The e1ectrons intercepted by the RF section are normally called “body current,” while those elec-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 trons intercepted by the collector are normally called “collector current.” Obviously, the sum of the body current and the collector current is equal to the total current in the electron beam which is normally called “beam current.” Klystron amplifier specifications will quite often place a maximum limit on allowable body current. The previous discussion (describing the general theory of klystron operation) implied that klystron amplifiers normally have actual metal grid structures across the gaps in the resonant cavities. Many low power klystrons do indeed have wire-mesh grids. However, most high-power klystrons do not have actual grids across the gaps, because such grids would intercept sizable amounts of the electron beam. It is very difficult to cool grid structures, and large beam interception would cause the grids to melt, destroying the tube. Fortunately, by proper design, the klystron can be made to work efficiently without actual grid wires across the gaps. The absence of these grids does not change the operating principles discussed previously, but it does have a secondary effect on the klystron performance. It turns out that, if the electron beam has a very small diameter compared with the size of the drift tubes, the beam does not “couple” strongly to the gaps and therefore it does not react as strongly with the klystron cavity. Therefore, the performance of a klystron amplifier, which does not have gridded gaps, can sometimes be improved by permitting the electron beam to be as large as possible (while keeping the body current down to the maximum specified for the tube). The size of the beam can be somewhat controlled by the magnetic field strength. We therefore find that the klystron performance can sometimes be improved by adjusting the magnetic field in a way which does not result in the minimum possible body current condition, i.e., by adjusting the field so that the beam shape is somewhat larger than the minimum obtainable. In gridless-gap klystrons therefore, best operation may be obtained with a body current which is not the minimum obtainable; however, body current must be kept within the maximum specified for the tube. Body current usually increases with RF input level which might be expected since RF causes electron bunches to form. The dense electron concentration in the bunch causes the electrons to repel each other, and the diameter of the bunch may become larger than the diameter of the beam without the bunches. Consequently, some of the electrons in the bunch may be lost to the drift tubes, and the body current may increase.
11.2. Associated Equipment
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In the preceding sections, we have discussed the basic theory of operation of the klystron amplifier tube. Considerable additional equipment is required for a complete amplifier system. First, we will need power supplies to deliver the voltages and currents required for the klystron and for the electromagnets. In high-power systems we will need various types of cooling to get rid of the power supply energy which is not converted into RF output power. We will need various RF circuit components to control and measure the RF input to the klystron tube, and to measure the RF output from the tube. For testing we may need a dummy load to dissipate RF output when it may be inconvenient, or impossible, to radiate. We will need a large collection of meters and protective devices to monitor performance and to protect operating personnel (and the equipment itself) in case of equipment malfunction or operator error. This associated equipment will be discussed in this section.
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11.3. Power Supplies
Figure 4.35 is a simplified diagram showing the power supplies used in a typical klystron amplifier. In most klystron tubes the anode and RF section of the tube are connected inside the vacuum envelope. These parts are normally called the tube “body,” and they are generally operated at ground potential as shown in Figure 4.35. Operating the tube body at ground is convenient because the input and output connections (either waveguide or coaxial) are then at ground potential; this makes it easy to connect into the rest of the system. Also, this keeps the cavity tuners at ground potential, eliminating any danger to personnel who are tuning the tube. The beam power supply, shown in Figure 4.35, generates the voltage required to accelerate the electrons and form the electron beam. It must also deliver the beam current required for the klystron tube itself. As shown, the positive end of the beam supply operates at (nearly) ground potential, whereas the negative output from the supply is the high potential point in the system. RF Input
RF Output
Cathode
Collector
HEATER SUPPLY
Current Limiting Resistors
Crowbar
GRID SUPPLY OR PULSER
ELECTROMAGNET SUPPLIES
Body Current Meter
BEAM SUPPLY Beam Current Meter
Beam Current Overload
Body Current Overload
Collector Current Meter
Direction of Electron Flow
Figure 4.35. Klystron Power Supply Connections The design details for beam power supplies vary widely depending upon the application of the power amplifier. However, in general, they employ fairly conventional circuits. They usually include means of adjusting the ac voltage to the primary of the power transformer, either an auto-transformer (such as a Variac), an Inductrol, or perhaps an ac generator whose output is varied by adjusting the dc field control. Beam supplies incorporate a step-up transformer, a rectifier circuit, and an LC filter. Either solid-state, hardtube, or gaseous diodes are used in the rectifier circuit. Tube rectifiers normally have a lower initial cost; however, they require periodic replacement. Solid-state rectifiers, particularly for high voltage and high current, are usually more expensive initially; however, their reliability is excellent after the initial design and debugging. Filter design is quite conventional; the amount of filtering depends upon the allowable ripple for the system. In some special cases, extremely low ripple and extremely good beam voltage regulation is required. For low- and medium-power systems this can often be achieved by electronic regulation of the dc output voltage. The circuits are, technically, fairly conventional, but they may become quite complicated and expensive for the mediumpower systems.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 For extremely high-power systems electronic regulation has not proven practical to date. For these systems it is normal to obtain the primary power from a motor generator set. The inertia of the large MG set effectively “smooths out” variations in the incoming line voltage; and the ac output from the generator can be quite easily regulated by a feedback system to the generator field. Brute-force filtering is used to achieve the allowable ripple. Beam voltage and beam current metering are always provided, as well as beam current overload protection. Some systems have beam over-voltage protection to protect the tube against the possibility of power-supply runaway (rare), and against operator error (more common). Since the beam supply is the source of most of the energy in the system, it is usually turned off when any malfunction occurs in the system; this will be discussed in more detail later. A variable voltage beam supply is usually provided so the tube can be operated at whatever power level is desired (within maximum ratings). It is also desirable to have low beam voltage capability; this is useful when a new tube is installed and initial adjustments are being made. Some systems have a feature that automatically starts the beam voltage at a low value when the supply is first turned on; the voltage then slowly increases until it reaches a preset level; and it may regulate to that level for changes in ac line voltage. The voltage automatically runs down to a low level when the supply is turned off. For high power systems it is normal to have some series resistance between the beam supply and the klystron cathode; this limits the tube current to some finite value in case the tube should arc from cathode to ground. Without some limiting resistance the peak current during an arc could be very high and might destroy the cathode surface; with current limiting resistance a tube can often be “cleaned up” even if it is somewhat gassy or if it arcs on initial turn-on. Most klystron amplifiers include a “getter,” and some include a VacIon vacuum pump. A getter will absorb a limited amount of gas that may accumulate after long storage periods, and therefore may permit a slightly-gassy tube to clean up and be perfectly serviceable (if the tube is not damaged during initial start-up). The VacIon pump operates continuously and will absorb a tremendous amount of gas and may allow a tube to continue in service for its normal life even if it has a small vacuum “leak.” Some high power amplifiers use a “crowbar” system to discharge the beam supply very quickly in case of an internal klystron arc, or other high-voltage fault condition. Most crowbar systems consist of a triggered spark gap connected across the power supply. Circuits are provided which will trigger the gap in case of excessive body current, arcs in the output waveguide (to be discussed later), and (sometimes) loss of magnetic field. When the trigger gap is fired, the main spark gap breaks down. This discharges the energy stored in the beam power supply very quickly and prevents damage to the klystron or associated equipment. Crowbars are normally used only on very high voltage, or very high-power systems where the amount of stored energy could cause serious damage. They are normally not used in equipment operating at less than 20-kilowatts average-power; those systems can be adequately protected by simple current-limiting resistors (which are much less complicated and much less expensive than a crowbar system). A crowbar system will normally operate in a few microseconds and therefore will limit the amount of destructive energy delivered to the tube to a very low value. Conversely, it may take several seconds to turn off and discharge a
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 large beam power supply because of the long time required for overload relays and ac contactors and filter capacitors to discharge. The heater supply furnishes power for the klystron heater, which heats the cathode that emits the electrons for the electron beam. Most klystrons have an “indirectly-heated” cathode, i.e., the cathode is heated simply by being close to the heater windings. A few klystrons have “bombarded” cathodes. In tubes of this type a voltage is applied between the heater and the cathode (with the cathode positive). The heater and cathode then function as a conventional diode. The heater becomes hot enough to emit electrons. These electrons are drawn to the cathode by the “bombarder” voltage. When they strike the cathode they liberate their energy (to the cathode) as heat, just as the plate of a diode is heated by the electrons striking it. Bombarded cathodes have been largely displaced by recently developed “impregnated” cathodes, although a few bombarded-cathode tubes are still in production. The heater supply is normally a rather simple unit. It may deliver either alternating or direct current. For many applications, an ac supply is adequate; it consists simply of a variable auto-transformer, a step-down transformer, and appropriate voltage and current metering. In a few systems that require extremely low-noise performance, dc supplies are necessary. The heater supply must be insulated to withstand the full (negative) beam voltage potential. Meters are normally used to show the heater voltage or current or both. Since these meters must operate at a high negative potential, they may sometimes give incorrect readings due to the large electrostatic fields present; special care must be taken in the design of metering circuits to prevent these false indications. Heater voltages are normally adjustable to take care of individual tube-to-tube variations and compensate for variation of incoming ac line voltage. A normal klystron heater presents very nearly a short-circuit to the power supply when the heater is cold (first turned on). Therefore, it is normal to use some type of current limiting in the heater power supply. Many klystron specifications require that this initial “surge” current be limited to 150 per cent of normal operating current. Protective circuits are often used to turn off the beam power supply if the heater supply fails, since some tubes will be damaged if the beam voltage is “on” while the heater and cathode are cooling after a heater supply failure. Some high power klystrons require that the cathode assembly be cooled by an air blower. An airflow protective interlock is normally included to turn off the heater and beam voltage supplies if the klystron blower ceases to operate. The entire “electron gun” section of some very-high-voltage tubes is immersed in oil, for insulation and cooling. Some klystron amplifiers have a grid (or modulating-anode, which performs the same function) to control the number of electrons in the electron beam. Such grids are often used in pulsed systems to turn the tube either full-on or full-off; a few systems employ grid modulation for transmission of intelligence. In most gridded klystron tubes the grid is never allowed to go positive with respect to the cathode, as this might cause undue grid interception and result in burnout of the grid element. A grid power supply is required in those tubes that have grids. These power supplies and pulsers may take many forms depending upon the system application and will not be discussed in detail. It is important to note, however, that the grid power supply must be insulated for the full beam voltage. Fortunately, most klystron amplifiers designed for communication service do not use grids. The collector of most high power klystrons is insulated from the body of the tube. This allows separate metering and overload protection for the body
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 current and for the collector current which would be impossible if the collector and the body were connected internally. In most systems the collector and body operate at very nearly the same potential; any potential difference is normally only the difference in voltage drop across the various metering circuits. Figure 4.35 shows three electromagnet coils, apparently wrapped around the body of the klystron. Some klystrons are indeed made with the electromagnet coils physically a part of the tube itself. However, in most systems the electromagnet coils are separate from the tube, and the klystron is inserted into the electromagnet structure. In Varian klystrons the electromagnet is designed physically to support and center the klystron tube in the correct position; no physical adjustment of the electromagnet coils is provided or required for correct operation. Many modern klystron amplifiers have only one electromagnet coil and therefore require only one power supply; others may have as many as six separate coils, requiring one power supply for each coil. Electromagnet power supplies are usually simple. They are dc supplies using conventional rectifying techniques. Voltage variation is normally accomplished with an auto-transformer on the input. The supplies are usually well filtered so that the output current contains relatively small ripple components. Ripple on the electromagnet current may cause the electron beam in the klystron to “wander” slightly, at the ripple frequency; this can cause undesirable amplitude and phase modulation of the RF output signal. Voltage and current metering is normally supplied for each of the electromagnet power supplies. If an electromagnet power supply should fail, the electron beam would almost certainly spread, and the total beam current would be intercepted on a small section of the drift tube. In most cases, this would cause the drift tube to melt and permanently destroy the tube. Therefore, klystron amplifier equipment normally has under-current protection in each of the electromagnet coil circuits. When the magnet current falls below a predetermined level, the beam supply is turned off to prevent damage to the klystron. Redundant protection is provided by the bodycurrent overload circuits, which also turn off the beam supply in case of magnet current failure or misadjustment. Figure 4.35 shows the method normally used to monitor body current, collector current, and beam current separately; the diagram shows the most frequent arrangement where the klystron body operates at ground potential. In many systems separate monitoring of collector current is not done since the collector current and total beam current are normally almost equal. It is quite unusual, in a relatively high-power klystron amplifier system, to allow the body current to exceed 10 per cent of the beam current, because high body current usually means low efficiency and increases the danger of burning out drift tubes in the klystron. In very-high-power klystrons the body current is often limited to 1 or 2 per cent of the total beam current. Over-current protection is almost always supplied both for body current and beam current. If a tube arcs internally, the arc will always occur between cathode and anode. When this occurs the body current immediately becomes excessive, tripping out the body current overload relay. If an arc occurs, the beam current is also much higher than normal, and the beam current overload will also trip out. In fact, almost any high-voltage system fault (such as an insulation breakdown from high voltage to ground) will cause excessive current through the body current meter and overload relay.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Because of the possibility of extremely high currents flowing under fault conditions, the protection of the body current and beam current meters (from burnout) presents a somewhat difficult problem. This problem is normally solved by using very high-current solid-state rectifiers, connected back-to-back, across the meters. Sometimes adding a small resistance or inductance in series with the meter is necessary. Surge capacitors are normally placed across the combination. Connecting the rectifiers back-toback is necessary because fault conditions often cause oscillating currents to flow through the meters.
11.4. Cooling
Most low power klystron amplifiers are air cooled, while all high power k1ystron amplifiers are liquid cooled. At the present state-of-the-art, air cooling can be used up to RF output levels of about one kilowatt, CW. However, we find a few special cases where liquid cooling is employed with tubes having a power output as low as 10 watts; these tubes are used in special applications that are beyond the scope of this bulletin. Remember that the main source of power (and therefore heat) in a klystron amplifier package is the beam power supply. The power generated by the beam supply must go somewhere; part of it is converted to RF power; the remainder eventually shows up as heating somewhere in the klystron. The klystron cooling must be adequate to handle the entire beam power because, if no RF output is being generated (either due to low RF input power, or detuning of the klystron tube) then all of the beam power is dissipated in heat somewhere within the tube. As discussed previously, most of the electrons in the beam eventually end in the collector. When they strike the collector, their energy is dissipated and turned into heat. The small fraction of the beam lost to the drift tubes also generates heat. Klystron amplifiers are normally somewhere between 30 and 50 per cent efficient. It is obvious, therefore, that a tube rated at 10 kilowatts output must be designed to dissipate between 20 and 33 kilowatts depending upon its efficiency. A tube rated at 100 kilowatts must be able to get rid of about 250 kilowatts as heat. It is obvious therefore that very advanced cooling techniques are necessary. The power levels involved can melt a hole in the drift tube, or in the collector, in a small fraction of a second if the cooling system fails and adequate protective devices are not provided. There are other but smaller sources of heat in a klystron amplifier system. The heater must be hot to heat the cathode for electron emission. This heat will be conducted and radiated to the exterior surfaces of the electron gun assembly, and must be dissipated. Large tubes require a blower on the electron gun assembly to get rid of this heat. The electromagnet will generate a considerable amount of heat; the power generated by the focus coil power supply is all dissipated in the electromagnet. Large electromagnets are almost always liquid cooled. If the cooling liquid for the electromagnet fails for any reason, the focus coil power supply must be shut off quite soon or the magnet will burn out; the beam voltage must also be removed (preferably before turning off focus coil supply) to protect the tube against excessive body current, as discussed previously. Earlier in this discussion we described how electron currents oscillate back and forth in the metal walls of the resonant cavities. Although these cavities are made with very high-conductivity metal (usually copper), the metal does present a finite resistance to these oscillating currents; therefore, heat will be generated in the cavity walls. The amount of heat generated can be quite sizable in high-power, high-frequency tubes. For instance, consider the case of a 20-kilowatt CW, X-band klystron amplifier. In this tube, ap-
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 proximately 1 kilowatt of heat is generated by the circulating RF currents in the output cavity. Since the cavity is approximately a 1-inch cube, it is apparent that removing this kilowatt of heat is a formidable problem. Cooling the cavity tuners is particularly difficult. Tuners normally incorporate some type of metal bellows arrangement, to permit changing the cavity size and still maintain the vacuum envelope of the tube. Metal bellows are thin structures, normally more lossy than the remainder of the cavity walls; therefore, a large amount of heat is often generated in the tuner assembly. Removal of this heat is a serious problem in high power tubes, and water cooling is invariably necessary. Another problem associated with cavity heating is not immediately apparent. Remember that the resonant frequency of the cavity depends upon its physical size. The cavities are made of metal that expands as it gets hot; this effect tends to change the resonant frequency of the cavity and to detune the tube. As the tube detunes, the power output will drop; then the RF heating decreases and the tube will tend to come back “in tune.” If this problem was not considered in the initial tube design, it would be quite possible to design a tube that would never “settle down”; it would be continually unstable in its operation. This situation indeed exists in some tubes which use “external cavities.” These external cavities are cooled by air rather than by liquid, and the cavity tuning is seriously affected by the ambient air temperature. All high-power Varian klystrons are liquidcooled, including the cavities and the tuners. The cavities are maintained at a stable temperature by controlling the temperature of the cooling liquid, and “thermal-detuning” is no problem. Drift tube heating is a serious problem in very high-power klystrons, and in medium-power, high-frequency, klystrons. The drift tubes that are inside the vacuum envelope are physically quite small, and it is difficult to remove the heat by conduction to the region outside the vacuum envelope. In some high-power tubes, it is actually necessary to bring the cooling liquid inside the vacuum envelope, and around the drift tubes, to remove the heat from the drift tubes. In recent years it has become necessary to cool the waveguide in some high-power, high-frequency systems. RF currents circulate in a waveguide that is carrying power, just as in the cavity walls of the klystron. An X-band waveguide carrying 5 kilowatts CW becomes too hot to touch in normal ambient air. Fortunately, the waveguide can be cooled easily by soldering copper tubing along the sides of the waveguide and running cooling liquid through the tubing. Most klystron amplifiers have a dummy load to dissipate the RF power during adjustment and test (when it may be undesirable to radiate). All highpower dummy loads are cooled, usually by liquid. In many loads the RF energy is dissipated in the cooling liquid itself, since water, oil, and ethyleneglycol (the normal cooling liquids) are quite lossy at microwave frequencies. In other types of dummy loads the RF energy may be dissipated in a solid, lossy material. Some of these lossy-material loads can be cooled by air blasts (for low- and medium-power applications); higher power versions are liquid cooled. We have discussed the various sources of heat in a klystron amplifier system, to impress the reader with the fact that an expensive klystron can be destroyed in a matter of seconds if the cooling system fails. A well-designed system uses many protective devices to prevent this from happening. The moral is: Check the operation of these protective devices periodically, and never “short-circuit” the protective interlocks.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Systems that use blowers for cooling will usually have an airflow switch. If the blower fails, the switch will open and remove power from the appropriate power supplies. Systems employing liquid cooling normally distribute the liquid into several paths, since the flow requirements are quite dissimilar. A well-designed amplifier system will have a low-flow interlock in each of the various paths. If one liquid-cooling circuit becomes plugged, the flow interlock will open and remove power from the system. Liquid-cooling systems also include pressure gauges and pressure switches, temperature gauges and over-temperature switches. Many systems have pressure or flow regulators. Some systems include devices that will sound an alarm before trouble actually occurs; sometimes the situation can be corrected without shutting down the equipment. In a liquid-cooled system, it is obviously necessary to pump the cooling liquid through the various parts of the tube, and the other equipment that is generating heat. The liquid becomes hotter as it is pumped through these channels, and it is then necessary to get rid of the heat that has gone into the liquid. Some type of heat exchanger is required. Most systems use a liquid-to-air heat exchanger, which consists of a radiator, and a blower which blows air through the radiator this system is very similar to that on an automobile. The hot liquid passes through the radiator and heats the radiator surface. The air blows across the radiator surface and removes the heat from the radiator. Therefore, the liquid that exits from the radiator is cooler than the liquid that entered the radiator. In some other systems, primarily those used on shipboard, a liquid-to-liquid heat exchanger may be used. In this device, two liquid cooling paths are involved. One path carries the coolant pumped through the klystron amplifier. The other path may carry sea water. Heat is transferred from the klystron cooling liquid to the sea water, and the sea water is dumped back into the ocean. Liquid-to-liquid heat exchangers are smaller than liquid-to-air heat exchangers, and they are also quieter, since no blower is required. Refer now to Figure 4.36, a diagram of a typical klystron amplifier liquidcooling system. The right half shows the method of distributing the cooling liquid to each of the individual channels. The cool liquid enters the “highpressure manifold” at the top of the drawing. From the high-pressure manifold, the liquid passes through valves used to adjust the flow (in each individual channel) to the desired level. The liquid then passes through the component of the system that requires cooling, such as the klystron collector, body, etc. Flow meters are normally incorporated in each of the individual channels to monitor the flow and to make it easy to adjust the flow to the desired level. Liquid flow-switches are placed in each individual channel, so that the appropriate power supplies will be turned off if the flow in that channel falls below the minimum required level. The liquid then goes through shutoff valves into the “low-pressure manifold.” Both manifolds are often fitted with temperature gauges and pressure gauges, over-temperature interlocks, and over-pressure interlocks.
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De-ionizer
By-pass Line
Heat Exchanger (Radiator)
P
Pump
M
Filter
Air-flow Alarm
M
Blower
Temperature Operated By-pass
Liquid Level Alarm
Nitrogen Tank
Power Supply
Misc. Cooling
Temperature Gauge
Shut-off Valves
Low-flow Interlocks
Flow Meters
Pressure Regulator
Pressure Gauge
Klystron Body
Over-temp Interlock
Pressure Gauge
Low-pressure Manifold
Klystron Collector
High-pressure Manifold
Over-temp Interlock
Flow Adjusting Valves
Temperature Gauge
Over-press Interlock
Waveguide
Magnet
Over-press Interlock
Dummy Load
Drain
Drain
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.36. Typical Liquid Cooling System for a Klystron Amplifier
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In Figure 4.36 we have shown the waveguide cooling in series with the magnet cooling. Individual system arrangements vary considerably. For instance, in some systems it is possible to put the klystron body cooling in series with the magnet cooling; in other systems the waveguide cooling may be in series with the RF dummy load, etc. The important thing to note is that each of the individual channels must have provision for adjustment and measurement of the amount of liquid flowing, and must be provided with low-flow interlock switches for protection. A pressure regulator is often installed somewhere in the system. In Figure 4.36 it is shown between the high-pressure and low-pressure manifolds. The middle of Figure 4.36 shows provisions for cooling the power supply, and any number of other things that may be liquid-cooled in a typical system. The hot liquid passes from the low pressure manifold, usually through a filter, and into the heat exchanger.Figure 4.36 shows a liquid-to-air heat exchanger consisting of a radiator and a blower. After the liquid has been cooled in the heat exchanger, it goes into the “coolant-storage and de-aerator tank.” The de-aerator tank deserves some discussion. Bubbles have a tendency to form in this type of liquid cooling system. Cool liquid will tend to pick up air bubbles if the system is open to the air at any point. As the liquid is heated, these bubbles tend to come out of solution. They will tend to collect in “high” parts of the system and may cause difficulty in filling the system in the first place. A fairly small bubble-content in the cooling liquid can seriously diminish the cooling efficiency, and may even cause damage to the equipment. Furthermore, air bubbles cause undesirable oxidation of the metal parts of the system. Fortunately, it is quite easy to remove the bubbles. The de-aerator tank is fitted with baffles. The liquid enters the tank and passes (rather slowly) around and through the baffles in the tank. The bubbles will be released and rise to the surface, rather than remaining in the liquid system. With a de-aerator of this type, it is quite easy to remove bubbles from the system and keep the liquid bubble-free. The air can be bled from the tank, and the tank refilled with liquid if necessary. A low-liquid level switch is normally used in the coolant storage tank. This switch can be connected to ring an alarm that alerts operating personnel to the situation; it may sometimes be connected to shut down the equipment in the event the liquid falls below the safe level. The liquid passes from the de-aerator tank into a de-ionizer. If free ions are allowed to build up in the cooling liquid, they may eventually cause damage to the collector and body channels in the klystron. The seriousness of this problem varies widely from system to system, and from tube to tube. Some systems require that ions be kept to a very low level. The de-ionizer, normally built with a replaceable cartridge, will remove ions from the liquid and prevent this problem from occurring. The liquid passes from the deionizer into the pump, where it is pumped into the high-pressure manifold. One or more filters are normally included in the cooling system to remove any accumulation of foreign material. In some klystrons the cooling passages are very small and can be plugged easily by dirt or sludge in the cooling system. Figure 4.36 shows a nitrogen tank that can be connected to “pressurize” the de-aerator tank. This requires some explanation. Some klystron amplifiers may be at a higher elevation than the heat exchanger and pump portion of the cooling system; a typical case would be a klystron amplifier on a large parabolic antenna, with the heat exchanger on the ground. These systems are quite hard to fill because it is difficult to remove the air from the
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 system initially. The problem can be solved with the pressurizing arrangement shown in Figure 4.36. The storage tank is first filled as full as possible. The drains on the manifolds are opened, and pressure is applied to the storage tank from the nitrogen bottle. The nitrogen pressure forces the liquid out of the tank and up to the manifolds. The air in the lines escapes through the drain valves. When the lines and manifolds are full, the drains are closed. The nitrogen tank is valved off and the storage tank may be filled to the top. The pump can then be started and the remaining air trapped in the system will normally be picked up by the coolant and delivered to the de-aerator tank. Figure 4.36 shows a bypass line around the heat exchanger radiator, and a temperature operated bypass valve. This is the system that is often used to control the temperature of the coolant liquid. In the previous discussion, we pointed out that klystron tuning may be changed somewhat by the temperature of the coolant, since this can change the physical size of the cavities. Fortunately, temperature control is done easily by bypassing some coolant around the radiator. In systems where the heat exchanger is fairly close to the klystron tube, the temperature valve may be simply a bimetal mechanism that senses the temperature of the liquid as it leaves the radiator. If the liquid is too hot, the valve closes partially and causes more of the total liquid to flow through the radiator. Conversely, if the temperature is too cold, the valve readjusts itself to cause more of the liquid to go around the radiator via the bypass line. If the klystron amplifier is a long distance from the radiator, a temperature sensor is normally placed at the high-pressure manifold. This temperature sensor operates the bypass valve. Since the temperature sensor is a long way from the temperature controller, and it takes an appreciable length of time for the liquid to travel this distance, a proportional controller may be necessary to keep the system from “hunting.” Some systems use motor-operated louvers (in the air stream between the blower and the radiator) for temperature control, rather than the bypass arrangement. Either system, of course, can only control the temperature to some point above ambient since the liquid leaving the radiator will always be somewhat hotter than the temperature of the air blowing through the radiator. Most systems are designed to control the liquid to a temperature between 10 and 20°F higher than the maximum-expected ambient temperature at the particular location. It is fairly easy to hold the liquid temperature within +5 °F. This close temperature control results in very stable klystron tuning. Some discussion of cooling liquids is appropriate here. Distilled water is the best all-round liquid for cooling klystron amplifiers. Some very-high-power amplifiers specify that only water can be used. Normal tap water usually has a large mineral content, and causes scaling of the klystron cooling surfaces. Scaling reduces heat transfer and may eventually completely close the cooling channels. If this occurs, the tube will be seriously damaged. Unfortunately, water freezes at an inconveniently high temperature. Many low- and medium-power klystrons permit the use of ethylene-glycol and water as the cooling liquid. The cooling efficiency of ethylene-glycol and water is not as good as pure water. Furthermore, ethylene-glycol reacts with certain types of metals and hoses that might be used in the system; therefore, special care must be taken in designing a system that is to use ethylene-glycol. Only nonferrous metals should be used in a cooling system for a klystron amplifier.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Some very large tubes permit only water to be used for the coolant. This complicates the design of the cooling system, since care must be taken to protect it from freezing. This is no problem if the system is continually operated, but becomes a very serious problem if the system is shut down in cold weather. Some large systems are designed with immersion heaters in the coolant tank. If the klystron is shut down for any reason, these immersion heaters are turned on, and the pump is left running to keep the coolant circulating and prevent freezing. Additional information on klystron cooling is contained in Varian Application Engineering Bulletin No. 17.
11.5. RF Circuits
We have discussed the theory of klystron amplifier operation, power supply requirements, and cooling requirements. Now let us consider what is necessary to get RF into, and out of, the klystron amplifier tube. Figure 4.37 on Page 4-46 shows the RF components typically associated with a klystron power amplifier. We will not consider the RF exciter (or “driver”), since we are only discussing the amplifier portion of a complete transmitter. The RF input signal from the amplifier is derived, of course, from an RF exciter. This signal is shown on the left of Figure 4.37. The RF input signal normally goes through a ferrite isolator so that a constant RF load impedance is presented to the exciter. The input cavity of a well-designed klystron amplifier normally presents a low VSWR, to the input signal, at the resonant frequency of the cavity; but the VSWR increases very rapidly for frequencies slightly off the resonant frequency of the cavity. It is desirable to isolate these high VSWR's from the exciter; the ferrite isolator accomplishes this function. After the ferrite isolator, the input signal is normally applied to a variable attenuator. The attenuator is used to adjust the input signal level so that the amplifier may operate at saturation, or at lower-than-saturation levels if this should be desired. It may be desirable to monitor the amount of RF input power being applied to the tube. This is normally done with a directional coupler and some sort of RF power monitor. This monitor may be a simple crystal detector and meter, or it may be a thermistor and RF power bridge arrangement. The input coupler can also be used to help tune the first cavity of the klystron to resonance. Many klystrons have a coarse-tuning indicator that allows them to be set approximately to frequency. However, this is not true for all tubes, and it may be quite difficult to get them on frequency when they are first put into a system. The first cavity tuning can be done easily by using the input coupler as a “reflected-power” coupler. To do this, put the input monitor on the opposite arm from that shown in Figure 4.37. When this is done, the monitor will show the power that is “reflected” from the first cavity of the tube, back toward the RF exciter. We stated earlier that the cavity presents a low VSWR at its resonant frequency; but if it is not tuned to the frequency of the RF input signal, the input power will be mostly reflected from the first cavity. If we now set up to monitor this reflected power, and then tune the cavity, the reflected power will decrease and go through a null when the cavity is tuned to the frequency of the input signal. This simple procedure allows one to “find” the first cavity and tune it to resonance. After this is done, the RF input monitor is connected to again monitor “forward” input power.
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RF Input
Ferrite Isolator
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RF Input Monitor
Variable Attenuator Input Coupler Crystal Switch Klystron
To Beam Supply
Control Unit
Waveguide Arc Sensor
Back-Power Coupler
Back-Power Meter
Low-pass Filter
Harmonic Filter
RF Output Monitor
Thermistor
Low-pass Filter
Forward-Power Coupler
RF Sample
Sampling Coupler
Dummy Load
RF Switch
To Antenna
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.37. Typical RF Circuitry for a Klystron Amplifier
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Between the input coupler and the first cavity, the diagram shows a crystal switch. This is a very fast-acting device that will insert between 20 and 30 dB attenuation in the RF input line. It is used primarily to remove RF input from the tube quickly in case of arcs in the output waveguide; this will be discussed in more detail later. The crystal switch is normally biased to have low RF attenuation. This completes the discussion of the components associated with the RF input to the tube. Let us now consider the RF output components. The first item normally found in the RF output circuit is a waveguide-arc sensor. Waveguide arcs are a troublesome problem in high power CW systems, particularly those at the higher frequencies where waveguide sizes are quite small. Typically, waveguide arcs will occur at power levels above 5 kilowatts at S-band, and above 1 kilowatt at X-band. Although the cause of this arcing is not completely understood, one of the most plausible theories is that ions build up due to thermal ionization from heating of contaminants within the waveguide, or from local area heating at small discontinuities within the waveguide. This ionization builds up in a CW system until the dielectric strength of the gas in the guide is sufficiently reduced to cause a sustained arc to form. Apparently, this buildup of ionization does not occur as readily in pulse systems because the ions have time to disperse during the interval between the pulses. In any event, once the arc is formed in a CW system, it almost invariably travels toward the source of RF power. If the arc is allowed to reach the output window of the klystron, local heating will occur and the window may be destroyed very quickly. Since the arc presents an effective short-circuit to the waveguide, a very high VSWR exists, and it is quite common to start a secondary arc in other points in the waveguide feedline or at the output window of the klystron. Experience with very high-power CW amplifiers at X-band indicates that the arc should be quenched in a few microseconds to prevent damaging the tube. This short time precludes removing the RF power by de-energizing the power supplies due to the long time lag of the relays and contactors involved in power supply shutoff. Removing RF drive is the fastest method of quenching the waveguide arc. This removes the RF output power and the arc disappears. Since the arc causes a bright light in the normally dark waveguide interior, a good way to sense the arc is to “look” into the waveguide with some light sensitive device, such as a photoelectric cell or a solar cell. Solar cells have proven superior to photo cells for this service, because they respond more quickly to the presence of light, and because they are less affected by temperature. When an arc occurs, the sensor will develop a voltage, which can be used (with follow-up control circuitry) to change the bias on the crystal switch. This inserts a large amount of attenuation in the RF input to the klystron; the RF output falls 20 or 30 dB, and the arc is extinguished. Additional circuits may be used to remove the beam voltage from the klystron if desired. The next component shown in the RF output circuitry is a “backpower” directional coupler. This coupler is connected to monitor power reflected from a mismatch in the antenna or the dummy load. Klystron amplifier specifications typically require the load VSWR to be below 1.2. Excessive reflected power causes high voltage gradients in the waveguide and excess heating in the tube. It may also tend to detune the last cavity and lower the output. Backpower monitoring is normally done with a directional coupler and some type of RF power meter. As shown in Figure 4.37, this may be a
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 simple crystal detector and meter, or it may be a thermistor and an RF power bridge. The backpower coupler can be arranged for redundant waveguide-arc protection. If an arc occurs between the coupler and the antenna, a very high VSWR will be present, and the reflected (back) power will rise suddenly. This can be sensed by the crystal detector, which can trigger the control unit of the waveguide arc-detector system. The control unit then changes the bias on the crystal switch, increases its attenuation, and removes the RF output. In this fashion, waveguide arcs occurring far from the light sensor can be detected. Again the beam power supply may be turned off if desired. A variable attenuator is often inserted between the backpower coupler and the detector to permit convenient adjustment to the desired operating level. The backpower meter is often of the type that includes upper-limit contacts. If the backpower slowly increases to an excessive level, these contacts will close and can be used to turn off the beam power supply. The next component in the RF output circuit is the “forward power” directional coupler. This coupler monitors the power being delivered to the antenna (or to the dummy load). The power indicating device can, again, be either a simple crystal detector and meter, or it may be a thermistor and an RF power bridge. A variable attenuator is normally included between the coupler and the power monitoring device to permit convenient adjustments. In some systems the power meter has both upper- and lower-limit contacts. These contacts can be arranged to ring alarms if the power output varies excessively. Many power amplifier systems use a third directional coupler to sample the RF output. Such a sample may be used to monitor noise performance of the equipment, to check distortion in the output signal, etc. The RF output is normally applied to an antenna. However, a dummy load is very useful for absorbing, and accurately measuring the power being generated. A dummy load is also handy for initial adjustment and tune-up when it may be undesirable to radiate. An RF switch is incorporated in some amplifiers to permit connecting the klystron easily either to the antenna or to the dummy load. This should be done only when the drive has been removed from the tube. A harmonic filter is sometimes included in the RF output circuit. Depending upon the type of tube and the operating conditions, the output of the klystron may be rich in harmonics. It is common for the second and third harmonics to be only 20 dB below the fundamental. In some situations radiation of this harmonic power causes objectionable interference. The harmonic energy can be removed by a harmonic filter in the system. This is simply a low-pass filter that absorbs the harmonic power, while passing the fundamental. Figure 4.37 shows low-pass filters in each of the directional coupler output arms. Directional couplers have the “unhappy” characteristic that they usually couple harmonics more strongly than the fundamental. An appreciable amount of harmonic energy in the RF output may cause incorrect readings in the RF power meters. Simple low power, low-pass filters remove this harmonic energy from the RF power monitors and prevent this inaccuracy. It is particularly important to have a low-pass filter in the forward power coupler, because the ratio of harmonic energy to fundamental energy is quite dependent upon the way the klystron tube is tuned. If one is watch-
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ing the forward power meter while tuning the tube, and the harmonics are not suppressed by a filter, it may be difficult to tune the tube correctly. This discussion has covered the microwave components usually used with a high-power klystron amplifier. The components discussed allow all of the important parameters to be monitored. In addition, some of these components provide features necessary to protect the tube and operating personnel.
11.6. Tuning
Tuning a klystron amplifier is very simple, when one understands the principles. Let us consider first the steps to tune the klystron to the synchronous-tuned condition. This is the simplest tuning adjustment. Remember that it results in highest-gain and narrowest-bandwidth operation of the tube. Many klystrons have a dial arrangement that allows adjustment of the cavities to approximately the right frequency before applying power to the tube. However, some tubes have no integral tuning indicators. Most tubes, when delivered, are tuned to some frequency, indicated by test data accompanying the tube. So, at least, one knows where the tube is tuned when new, and which direction to go for the desired frequency. The instructions also give the “direction” of tuner rotation to raise, or lower, the cavity frequency. All these things help tune the tube the first time. Let us consider the most difficult case, a tube with no built-in tuning indicators and no indication of the present tuning. In addition, the driver is fixed-frequency so that its frequency cannot be matched to that of the tube; one must tune the tube to the driver. The tube is installed in the transmitter; the exciter is operating and delivering power; the cooling has been turned on, and voltages are applied to the tube. Everything is working, but there is no power output, because the tube is not tuned to the frequency of the exciter. How can you get power from the tube? It is simple. First, you must adjust the first cavity frequency. When we were discussing Figure 4.37, we mentioned that you could find the first cavity tuning by reconnecting the “RF input” directional coupler to read the power “reflected” from the tube. This is done by moving the RF input power monitor to the “reflectedpower” arm of the coupler. (Don't forget to put the termination on the “forward power” arm of the coupler). You are now set up to monitor the power reflected from the first cavity of the amplifier. This power will be minimum when the first cavity is tuned to the driver frequency. When you start tuning a tube, it is a good idea to keep some mental notes on how far you've gone, so you can return to the starting point. A simple way to do this is “count turns” as you rotate the tuning tool. So, suppose you begin tuning the first cavity, rotating the tuner in a clockwise direction, and counting turns as you go. Look for a significant “dip” in the RF input monitor that is measuring the reflected power. You may find some small dips on the way, but look for the major one, which will be the correct tuning point. Most tubes are equipped with tuner stops to prevent damage to the tuner mechanism. Suppose you rotate the tuner clockwise and do not find any major dips in the reflected power all the way to the tuner stop. Then, return to the starting point, counting turns as you go. Then continue counterclockwise (again counting turns from the original position) until you find a significant dip in the reflected power. Minimize the reflected power by tuning. Now you can leave the first cavity alone for a while, since it will be almost on resonance. Reconnect the RF input monitor to read forward power.
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Next, tune the output cavity. Ignore any intermediate cavities for the moment. You may still have no measurable reading on the output meter. Increase the RF drive to the highest level you can get from the exciter. Now you are ready to tune the output cavity. Although you may have tuned the first cavity counterclockwise, it does not necessarily follow that the other tuners should be turned the same direction. Refer carefully to Operating Instructions or to markings on the tube itself. Start tuning the output cavity, counting turns as you go. With luck, you will soon bring the output cavity to resonance, and will see an indication on the RF output meter. Maximize this reading by tuning the last cavity of the tube. Once you see any reading on the RF output meter, the rest is simple. You know you have the first and last cavities in tune (or almost). If you have a three-cavity tube, now adjust the middle cavity. Determine the tuning direction from the instructions or tube markings, or carefully try one direction and then the other. Simply tune the cavity to maximize the power output reading. However, once you approach a sizable output, it is a good idea to reduce the RF drive to be sure that you do not inadvertently saturate the tube. Tune the middle cavity to maximize the power output reading. Now reduce the drive to a low level, so that the power output meter is far below full power (less than 30 per cent of full-power). Retune the input cavity to resonance, then retune the output cavity to resonance, then retune the middle cavity to resonance. The tube is now synchronouslytuned. Synchronous-tuning is always done with low RF input power. Now increase the RF drive until the tube saturates (or to whatever power level you may wish to use). The procedure for a four-cavity tube is very nearly the same. First, tune the input and output cavities to resonance, then the intermediate cavities one at a time. Cavities are often numbered, number 1 being the input cavity and the number 4 the output. For synchronously-tuning a four-cavity tube (after you have some power output), reduce the drive to a low level and tune in the sequence 1-4-3-2; i.e., first tune the input cavity, then the output cavity, then the next-to-the-output cavity, and then the next-to-the-input cavity. The tuning of the fourth cavity is normally quite “broad,” whereas the tuning of the first, second, and third cavities is quite “sharp.” The reason is that the output cavity is fairly “low-Q” compared with the other cavities. You have now learned to tune a klystron to the synchronous-tuned condition. This is always the first tuning condition even if you want to staggertune the tube later. There are several methods of stagger-tuning, two of which will be discussed briefly. Remember, in Theory of Operation we stated that stagger tuning could be used to obtain more power than synchronous-tuning. This is done simply by adjusting the third cavity to a higher frequency. The detailed steps are approximately as follows: First, tune the tube synchronously at low RF input; then increase the RF drive until the tube is saturated. Now, leave the drive alone and detune the third cavity in the high-frequency direction. The Operating Instructions for the tube will tell you whether this is clockwise or counterclockwise. As you detune the third cavity, the output will decrease, because more drive is needed for saturation. Continue detuning until the power output has dropped approximately 6 to 10 dB. Now increase the RF drive power until the tube again is operating at saturation. You will find that this “new” saturated output is higher than the output obtained with the tube synchronously-tuned. You may be able to “squeeze a little more out” of the tube, but probably not
4-50
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 much. You can detune the third cavity still farther and then increase the drive to see if you get more power output than before. The power output maximum is normally quite broad; you will be able to detune the third cavity considerably either side of this point without making an appreciable change in output. Eventually you may become limited by the amount of power available from the exciter. A second type of stagger-tuning should be mentioned. This is stagger-tuning to achieve a desired bandwidth characteristic over the passband. The problem is similar to broadbanding an IF amplifier by stagger-tuning. You will need an RF driver that can be swept in frequency rapidly (electronically-swept), and whose power output is constant during sweeping. You will need to sample the RF output with a crystal detector. Apply the crystal detector output to an oscilloscope so that you can see the passband of the tube. The X-axis of the oscilloscope sweep must be synchronized with the RF input sweep voltage. Again you will start with the synchronous-tuned condition and probably with the tube operating at saturation. To broadband the tube, it is usually best to detune the third cavity to the high frequency side, and to detune the second cavity to the low frequency side (assuming a four-cavity klystron). The first and fourth cavities are normally left tuned to the center of the passband. You may wish to adjust the RF input power periodically, as you detune the klystron, to keep the tube operating near saturation. You will find that the bandwidth of the tube is larger when you are operating at saturation than below saturation. The details of the broadband tuning that you may wish to accomplish are beyond the scope of this note. We have only indicated the equipment that is necessary and the general procedures to be followed.
11.7. Noise in Klystron Amplifiers
Volumes have been written about noise in microwave systems; obviously, we can only touch the very high points in this discussion. Noise is anything that causes the RF output signal to be different from the RF input signal. We have already mentioned that the output may contain harmonics. This is primarily because the RF output cavity is excited by “bunches” of electrons that come through the output gap once every cycle. These bunches essentially “kick” the output cavity and cause oscillating currents to flow in it. Since the driving force on the output cavity is not continuous, but rather occurs in quick kicks, it is intuitively evident that the output current may not be purely sinusoidal; therefore, it will contain harmonic components. This situation is quite analogous to a class-C triode amplifier in which the plate current flows in bursts, and sets up oscillating currents in the resonant plate circuit. Class-C amplifiers are also rich in harmonics for the same reason. In general, the harmonic output from klystron amplifiers is largest (percentage-wise with respect to the fundamental carrier power) when the tube is operating at saturation, or is being over-driven beyond saturation. Harmonic content decreases (percentage-wise) when the tube is operated below saturation. As discussed previously, harmonics can be reduced in the output by using harmonic filters. Another source of distortion is non-linearity of the klystron. If the RF input signal is amplitude-modulated, the RF output may not “perfectly” follow the RF input. This can result in distortion, becoming worse as the tube is driven closer to saturation on the peaks of the RF input signal. In general, klystron amplifiers should not be used to amplify amplitudemodulated signals if the RF output is driven higher than about 0.7 of the
RF Theory: Klystron Theory
4-51
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 saturated level. Between 70 and 100 per cent of saturation, considerable distortion can occur. A klystron amplifier will generate a certain amount of “white noise,” just as any other electron tube. White noise occurs primarily because an electron beam is never “perfectly” homogeneous. The number of electrons will vary slightly with time, primarily due to shot noise at the cathode surface; this variation shows up as random noise in the RF output. A certain amount of noise may also be generated by electrons striking the drift tubes. These are the electrons that create the body current. The body-current interception may be slightly random; this again will perturb the electron beam and cause a small amount of random noise to appear in the output. You should understand one interesting effect about klystron amplifiers. Intuitively, one would think that the output of an amplifier cannot possibly be “quieter” than the input signal. In certain cases, the klystron amplifier can, indeed, have an output that is quieter than its input. Consider a tube operated at saturation; this is the normal situation when the intelligence is being transmitted by frequency-modulation of the carrier. And suppose that the output of the RF exciter is fairly “noisy” with amplitude-modulation. The klystron amplifier has the desirable property that it will suppress amplitude-modulation of the input signal, if the tube is being operated at saturation. An examination of the output-vs.-input curves shown in Figure 4.34 on Page 4-32 will explain how this happens. It is obvious that, with the amplifier operating at saturation, rather large changes in the amplitude of the RF input signal will cause no change in the amplitude of the RF output signal. In some systems this effect is very noticeable, and it is not uncommon to find that the AM noise from the exciter can be suppressed by 10 to 20 dB, simply by operating the amplifier at saturation. Additional information on noise characteristics of klystrons is given in Varian Application Engineering Bulletins Numbers 11 and 18. Definitions of AM and FM noise and methods of measurement are discussed. Equations for computation of noise caused by power supply ripple are derived and examples are given for typical conditions.
11.8. Summary
4-52
This bulletin has attempted to familiarize you with the basic principles of klystron amplifiers and the equipment usually associated with these tubes. Precautionary measures and safety devices have been described in considerable detail in order to explain their importance, and to convince you that “cheating” them for expediency will very likely result in expensive damage to tubes, equipment, or personnel. Tuning procedures described are those used for Varian klystrons but are generally applicable to similar tubes. Too much cannot be said in favor of studying Operating Instructions for equipment and tubes thoroughly before applying power to your transmitter.
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Five
Ion Chamber Theory
This chapter covers the rudimentary concepts of ion chamber characteristics and is written for an intended audience of engineers, test personnel, and manufacturing personnel, wishing to learn more about Varian's ion chamber.
Ion Chamber Theory
5-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents
1. Introduction:.................................................................................................................... 5-3 2. Present Configuration: ..................................................................................................... 5-3 3. Efficiency: ........................................................................................................................ 5-5 4. Upper Limit of Dose Range (Saturation): ........................................................................... 5-6 5. Applied Electric Field: ...................................................................................................... 5-7 6. Effects of Temperature and Pressure: ............................................................................... 5-7 7. Beam Opacity: ................................................................................................................. 5-8 8. Inverse Square Law: ......................................................................................................... 5-8 9. Insulation Materials: ........................................................................................................ 5-9 10. Pulse Shape: ................................................................................................................ 5-10 11. Concluding Remarks: ................................................................................................... 5-11 12. References: .................................................................................................................. 5-12
Table of Illustrations Figure 5.1. Basic Ion Chamber Components and Characteristics:......................................... 5-3 Figure 5.2. Simplified Dosimetry/Steering System Block Diagram: ....................................... 5-4 Figure 5.3. Geometric Layout of Electrode and Signal Plates:................................................ 5-4 Figure 5.4. Fraction of Ions Collected as a Function of Dimensionless Variable 1/Up: ........... 5-6 Figure 5.5. Upper Limit of Dose Range: ................................................................................ 5-6 Figure 5.6. The Different Regions of Operation of Gas-filled Detectors:.................................. 5-7 Figure 5.7. Diagram Illustrating the Inverse Square Law: ..................................................... 5-8 Figure 5.8. Diagram showing the Derivation of the Pulse Shape VR (t): ............................... 5-10 Figure 5.9. Output Pulse Shape: ........................................................................................ 5-11
5-2
Ion Chamber Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
Gas filled ion chambers are a type of radiation detector. The normal operation mode is based on collection of the charges through the application of an electric field. These charged particles are created by direct ionization of the gas within the ion chamber. After a neutral molecule is ionized, the resulting positive ion and free electron produce the basic electrical signal developed by the ion chamber. Figure 5.1 illustrates the basic principles of an elementary ion chamber. A volume of gas is enclosed within an electric field. At equilibrium, the current flowing in the external circuit will be equal to the ionization current collected at the electrodes, and a sensitive ammeter can be used to measure the ionization current. Notice on Figure 5.1 that current output is constant after the knee of the curve, especially at low dose rates.
Gas Enclosure I Electrodes V
I
High Irradiation Rate
Low Irradiation Rate
V Figure 5.1. Basic Ion Chamber Components and Characteristics
2. Present Configuration
The present monitoring ionization chamber of the high energy Clinac 1800 and Clinac 2100C is constructed of several plates or electrodes. The purpose of the ion chamber is twofold: 1. to monitor the beam position, and 2. to monitor beam intensity. These needs are satisfied as illustrated in Figure 5.2, a simplified diagram of the ion chamber, dosimetry, and beam steering systems. This ionization chamber consists of two collecting plates sandwiched between three polarizing plates. The two collecting plates are oriented 90° in relation to each other to allow inplane and crossplane beam symmetry monitoring. The collecting plates are divided into four sectors, each defining a distinct laminar collecting volume.
Ion Chamber Theory: Introduction
5-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Accelerator Guide Angle R Steering Coils
Position R Steering Coils
Buncher R Steering Coils
Buncher T Steering Coils
Position T Steering Coils
n Beam Electro
Angle T Steering Coils Target Dose Rate Meter A A 1 B A 2
A3 A4
F A 9 E A 10
A13
G A 11 H A 12 D A 5 C A 6 -500V P.S.
A14 A8
A+B
MU1 Integrator
A-B
(A - B) + (E - F)
SYM1
E-F
G-H
(C - D) + (G - H)
SYM2
C-D
A7
C+D
MU2 Integrator
Figure 5.2. Simplified Dosimetry/Steering System Block Diagram
Electrode Plate Signal Plate Figure 5.3. Geometric Layout of Electrode and Signal Plates
5-4
Ion Chamber Theory: Present Configuration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The two inner D-like sectors provide signals for both dosimetry and beam angle monitoring. The outer two arc-like sectors are for beam position monitoring only. The collecting plate closest to the target monitors the radial plane, the collecting plate closest to the patient monitors the transverse plane. A polarizing voltage of 500 volts is used with a plate spacing of 1mm. A good rule of thumb for dielectric breakdown is 24,000 V per in. So at 0.040" (mm) we should be able to hold off 1000 V.
3. Efficiency
Assume the plates to be a distance d apart and held at a potential difference v. The positive plate is on the left and the negative plate is on the right. The positive ions will occupy the space to the right as they are pulled to the negative plate, while the negative ions will occupy the space to the left as they are attracted to the positive plate. If no recombination occurs, the charge collected per second, i.e., the current, is: I = Qc × d × A
where:
(Equation 117)
Qc is a charge per unit volume per second. d is the plate separation. A is the area.
Recombination occurs when + and – ions meet and recombine before the charged ions make it to the collecting electrode. When recombination is taken into consideration, an efficiency factor must be multiplied to Equation 117. For pulsed radiation, whose duration is short compared with the collection time, the collection efficiency or fraction of charge collected for pulsed radiation, fp, may be expressed by: f p = 100 ⁄ U p × ln ( 1 + U p )
where:
Up is the dimensionless parameter: k 2 U p = -- × d × q′ v
where:
(Equation 118)
(Equation 119)
k is a gas constant. q is the charge in esu produced per cm3 per second. v is the voltage potential between the plates in volts. d is the separation of the plates.
The actual current or “charge collected per second” with recombination taken into consideration is Equation 117 multiplied by the efficiency (Equation 118). I = Qc × d × A × fp
(Equation 120)
From Equation 120 it can be seen that if the distance d is moving because of unstable signal plates, the effect on the current signal is enormous. Equation 120 also suggests that we want: a. small plate separation, and b. high voltage.
Ion Chamber Theory: Efficiency
5-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 10.0
10.0
Pulsed Radiation Fraction Collected fP
0.96
0.8
d V
P
0.7
0.6
0.5
±
±
N ±
+
+
+
–
–
–
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
0.4
X+
X
V+ 0.2 0.1
1.0
10
0.94
0.92
0.90
Fraction Collected – fP
0.98
0.9
0.88
X– V–
0.86 1000
100
Dimensionless Variable 1/UP
Figure 5.4. Fraction of Ions Collected as a Function of Dimensionless Variable 1/Up
4. Upper Limit of Dose Range (Saturation)
A constant dose sensitivity throughout the dose range provides a linear response (i.e., reading vs. dose, r vs. D). Saturation is manifested by a decrease in the dose sensitivity. If pushed to the limit, this function will become zero and then become a negative value. Figure 5.5 illustrates a double-valued dose response function resulting from a decrease in dosimeter sensitivity at high doses. One factor that can add to saturation in an ion chamber is recombination. An ion chamber is said to be saturated to the degree that ionic recombination is present. Saturation is minimized by ensuring that a large electric field exists everywhere within the ion chamber. Increasing the ion collecting potential generally helps, but is limited by electrical breakdown of insulators. For a more detailed explanation, refer to Attix1 page 281.
Reading
dr dD g = 0
Negative slope
Double-valued Function
r
dr Slope = dDg = sensitivity
Dg
Dose
Figure 5.5. Upper Limit of Dose Range
5-6
Ion Chamber Theory: Upper Limit of Dose Range (Saturation)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Applied Electric Field
The different regions of applied electric fields (volts per meter) are shown in Figure 5.6. Region I has very low values of applied voltage, 0 to 100 V for a plate spacing of 0.040". The electric field is insufficient to prevent recombination of the original ion pairs. Region II shows the normal mode of operation for ionization chambers. Region III is the region of true proportionality, and represents the mode of operation for proportional counters. Increasing the applied electric field even further introduces nonlinear effects as shown in region IV. If the applied electric field is increased even further, we enter the Geiger-Mueller region. The present ion chamber configuration uses an Acopian (or equivalent) power supply of 500V dc. Experimental data shows that the ion chamber output is constant from 300 – 600 volts. Good voltage regulation is essential, since HV fluctuations induce current to flow in the electrometer input circuit from the capacitive coupling. Also, we previously mentioned that reducing the plate separation increases the collection efficiency and signal to noise ratio.
Pulse Amplitude (log scale)
V GeigerMueller Region
IV Limited Proportional Region
2 MeV 1 MeV
II I
Ion Saturation
III Proportional Region
Applied Voltage Figure 5.6. The Different Regions of Operation of Gas-filled Detectors
6. Effects of Temperature and Pressure
Whenever absolute ionization measurements are made, corrections must be applied to account for the change in density of the gas (with pressure and temperature). The mass of a given volume of air at temperature T and pressure P is related to its mass at 0°C and 760mm Hg by: ( 273.2°K ) ( P mm ) m(T,P) = m(0°C, 760mm) ----------------------------------------------------------( 273.2°K + T ) ( 760mm )
Ion Chamber Theory: Applied Electric Field
(Equation 121)
5-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The first bracketed term corrects for the expansion of the gas with increased temperature. The second term corrects for changes due to pressure. If the ion chamber was calibrated at another temperature, the appropriate (absolute) temperature must be used. This correction formula assumes that the ion chamber cavity is not sealed and that pressure inside the chamber is atmospheric. This is not the case with any Clinac ion chamber; it is hermetically sealed with a weld. The changes in mass in a sealed ion chamber should be nearly zero. A worst case scenario would be an unsealed ion chamber. A 5°F change in temperature (2.7°C) represents a 0.9% change in mass. A 10°F change in temperature (5.6°C) represents a 2.0% change in mass. The effects of pressure changes are not as dramatic.
7. Beam Opacity
In dealing with a compound or a mixture of molecules, it is sometimes convenient to describe the mixture by an effective atomic number, Z. The concept is useful in dealing with ion chambers. The effective atomic number, Z, of a mixture may be defined by: Z =
where:
m
m
m
a 1 Z 1 + a 2 Z 2 + …a n Z n
m
(Equation 122)
a1 to an are the fractional numbers of electrons per gram belonging to materials of atomic numbers Z1 to Zn respectively. m has the experimental value of 3.5 for air.
8. Inverse Square Law
The ion chamber is not located at isocenter where one wishes to measure dose. The actual photon fluence, φ , (number of photons per unit area) seen by the ion chamber is quite high as illustrated below.
φ2
φ1
P
a
b
a b
f1
f2 Figure 5.7. Diagram Illustrating the Inverse Square Law φ φB
f
2
2 In general it follows that -----A- = φ B -----2
(Equation 123)
f1
This is a simple statement of the inverse square law. In Varian's High Energy Clinacs f2 = 100cm, f1 = 25cm. The calculated resulting ratio of these two distances squared is 16; which means that the photon fluence is 16
5-8
Ion Chamber Theory: Beam Opacity
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 times higher at the ion chamber than at isocenter. This is only approximate and does not take back scatter and secondary electron fluence into consideration. Experimental data shows that the photon fluence is 20 to 50 times higher at the ion chamber than at isocenter.
9. Insulation Materials
Polystyrene, polyethylene are excellent insulators for ion chambers. Teflon is more readily damaged by radiation and should be avoided. Charged-particle beams incident on a thick insulator will build up charge wherever the particle stops at the end of their path. When large insulating plastics are irradiated to high doses by electron beams, the charge buildup due to stopped electrons may cause electric fields strong enough to influence the paths of the primary electrons in the ion chamber.
9.1. Mica
Mica is the name given to a group of minerals of related similar physical properties characterized chiefly by perfect basal cleavage (they can split readily in one direction). The ASTM visual quality classifications for mica are as follows: V-1
clear
V-2
clear and slightly stained
V-3
fair stained
V-4
good stained
V-5
stained, A quality
V-6
stained, B quality
V-7
heavy stained
V-8
black dotted
V-9
black spotted
V-10 black stained In practice, first quality is equivalent to V-3 and second quality to V-4. Varian presently purchases V4 mica through Spruce Pine Mica Co. The micas are complex silicates of aluminum with potassium, magnesium, iron, sodium, lithium, fluorine and traces of other elements. The mica we use in our application is Muscovite, H2KAl3(SiO4)3. Other types of micas are: Phlogopite, Biotite, Lepidolite, Paragonite, Zinnwaldite. The mica thickness specification was dropped from 0.010" to 0.007" +0.003" in January 1988 because of the dwindling mica supply. The 0.003" reduction in thickness constitutes a 50% stiffness change. The unstable mica assemblies resulted in changes in symmetry and dose calibration. There is also suspicion that other changes occurred since we had instabilities even in chambers with selected thick mica. This phenomenon is pretty well understood now and engineering changes have been implemented to minimize the mica ion chamber problem, allowing manufacturing yields to go up from 40% to 90%. This is only a short term solution because the mica supply is still limited. The gold is silkscreened on with thicknesses ranging from 0.0001" to 0.0007". Shortly after this document was published, Varian switched to DuPont Kapton™ as the insulating material for all High Energy Clinacs except for the 2500 and 2500C.
Ion Chamber Theory: Insulation Materials
5-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Pulse Shape
The pulse shape depends on the configuration of the electric field and the position at which the ion pairs are formed. Assume a parallel plate ion chamber with electric field intensity Ef, voltage V across the electrodes and separated by a distance d. (Equation 124)
Ef = V ⁄ d
A further simplification assumes that all ion pairs are formed at an equal distance x from the positive electrode. This situation is sketched in Figure 5.8.
+
vt –
Vch
d
vt
C
R
VR
x
VO
Figure 5.8. Diagram showing the Derivation of the Pulse Shape VR (t) for the Ion Chamber Signal The pulse shape is most easily derived on arguments involving the conservation of energy. The energy required to move the charges from their origin must come from the energy originally stored across the capacitance C. This 2 energy is 1 ⁄ CV 0 where V0 is the applied voltage. After a time t, the ions will have drifted a distance v+ t toward the cathode, where v+ is the ion drift velocity. Similarly, the electrons will have moved a distance v– t toward the anode. Both of these motions represent the movement of charge to a region of lower potential (dV). This energy is equal to Q(dV) for both ions and electrons, where Q is the total charge and dV is the change in electric potential. The charge Q = noe, where no is the number of original ion pairs and e is the electron charge. Conservation of energy can be written: Original stored energy 1 ⁄ 2 CV o
2
Energy Energy Remaining = absorbed + absorbed + stored by ions by electrons energy =
–
n o eEv t
+
+
n o eEv t
+
1 ⁄ 2 CV ch
2
(Equation 125)
The signal voltage is measured across R in Figure 5.8 and will be denoted as VR. The following equation describes the initial portion of the signal pulse and predicts a linear rise with time: no e + – V R = -------- ( v + v )t dC
5-10
rising portion
(Equation 126)
Ion Chamber Theory: Pulse Shape
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The decaying portion of the signal pulse is dependent on the position “x” at which the electrons were originally formed within the chamber. The pulse reflects only the drift of the electrons and will have an amplitude given by: n o ex V D = ---------ed
decaying portion
(Equation 127)
If the collection circuit time constant were very large (RC>>t+) the maximum amplitude of the signal pulse would be: no e V max = ------c
maximum amplitude
(Equation 128)
VR
Vmax = no e c Velec
t–
t+
t
Figure 5.9. Output Pulse Shape Many of the details which are omitted in the preceding discussion can be found in the theoretical books on ionization chambers by Attix2 and Knoll3.
11. Concluding Remarks
The ion chamber converts flux (MU’s/minute) through the ion chamber into a more easily measurable quantity: a current signal. Factors affecting this conversion process include: !
collector and polarizing plate spacing
!
chamber saturation
!
voltage regulation of the polarizing plates
!
density of the plate substrate
!
temperature and pressure (although to a lesser degree than the above-mentioned factors).
Ion Chamber Theory: Concluding Remarks
5-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In Varian's application, the current signal generated is a pulsed signal. The basic equations for the pulse shape were presented (in terms of voltage instead of current, Thevenin's theorem was used). Discussions of manufacturing concerns include the effects of the inverse square law governing radiation attenuation. The photon fluence level is 20 to 50 times more intense at the Ion chamber versus isocenter (because of scatter and secondary electron fluence). Also, use of Teflon as a high voltage insulating material should be avoided. Again, the primary objective of this report is to inform. If you require more detailed information on any of the topics discussed, please consult the References list below, as well as the endnotes referenced in the text of this chapter.
12. References
H. Johns and J. Cunningham, The Physics of Radiology, Thomas, Illinois (1983). B. B. Rossi and H. H. Staub, Ionization Chambers and Counters, McGrawHill, New York (1949). D. H. Wilkinson, Ionization Chambers and Counters, Cambridge Univ. Press, Cambridge (1950).
1. F. Attix, Introduction to Radiological Physics and Radiation Dosimetry, John Wiley & Sons, New York (1949) 2. Ibid. 3. G. Knoll, Radiation Detection and Measurement, John Wiley & Sons, New York (1979)
5-12
Ion Chamber Theory: References
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Six
Vacuum Theory
In order to understand the Clinac vacuum systems, a basic knowledge of vacuum theory is required. This chapter will provide the reader with sufficient information to be able to perform service and maintenance on these vacuum systems.
Vacuum Theory
6-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Introduction:.................................................................................................................... 6-3 2. The Nature of Vacuum: .................................................................................................... 6-3 2.1. What Is Vacuum?: .................................................................................................... 6-3 2.2. What About Pressure?: ............................................................................................. 6-4 2.3. How Is a Vacuum Produced?: ................................................................................... 6-4 2.4. Different Types of Vacuum:....................................................................................... 6-4 2.5. Where Is Vacuum Used?: .......................................................................................... 6-5 2.6. Why Is Vacuum Needed?: ......................................................................................... 6-5 3. Temperature: ................................................................................................................... 6-6 4. Pressure:.......................................................................................................................... 6-7 4.1. What is Gas?: ........................................................................................................... 6-7 4.2. Atmospheric Pressure: .............................................................................................. 6-7 4.3. Pressure Measurement: ............................................................................................ 6-8 4.4. Partial Pressure: ....................................................................................................... 6-9 4.5. Vapor Pressure: ...................................................................................................... 6-10 4.6. Effects of Pressure: ................................................................................................. 6-12 4.7. Pressure Ranges: .................................................................................................... 6-12 5. Gas Particles: ................................................................................................................. 6-13 6. Gas Laws: ...................................................................................................................... 6-13 6.1. Avogadro’s Law: ...................................................................................................... 6-13 6.2. Boyle’s Law:............................................................................................................ 6-14 6.3. Gas Expansion: ...................................................................................................... 6-14 6.4. Charles’ Law: .......................................................................................................... 6-15 6.5. Gay-Lussac’s Law: .................................................................................................. 6-16 6.6. General Gas Law: ................................................................................................... 6-16 7. Gas Flow: ....................................................................................................................... 6-17 7.1. Viscous Flow: ......................................................................................................... 6-17 7.2. Molecular Flow: ...................................................................................................... 6-17 7.3. Mean Free Path: ..................................................................................................... 6-18 8. Conductance:................................................................................................................. 6-18 8.1. Conductance in Viscous Flow: ................................................................................ 6-19 8.2. Conductance in Molecular Flow: ............................................................................. 6-20 9. Review of the Nature of Gases: ....................................................................................... 6-20 10. Ion Pump: .................................................................................................................... 6-21 10.1. Components: ........................................................................................................ 6-22 10.2. How the Pump Works: .......................................................................................... 6-22 10.3. Vacuum System Use: ............................................................................................ 6-26 10.4. Summary:............................................................................................................. 6-26 11. Vacuum Gauges:.......................................................................................................... 6-26 11.1. Thermocouple Gauge: ........................................................................................... 6-26
6-2
Vacuum Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
In the first part of this chapter, we will introduce you to vacuum: !
What it is
!
How it relates to pressure
!
How it is produced
!
The different types of vacuum
!
Where it is used
!
Why we need it
You will also learn about temperature as a factor in vacuum work and the types of pressure and how it is measured. Finally, we will discuss some basic concepts used in vacuum work. These are: !
The effects of pressure
!
Pressure ranges in vacuum systems
!
Some basic laws about the behavior of gases
!
Some types of gas flow
!
How we measure the work done by vacuum systems
2. The Nature of Vacuum
The word vacuum comes from the Latin “vacua,” which means “empty.”
2.1. What Is Vacuum?
Actually, vacuum is only partially empty space. In a vacuum, some air and other gases have been removed from a contained volume. This volume is usually called the work chamber. It separates the vacuum from the outside world.
Vacuum Theory: Introduction
6-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A more practical definition for vacuum is what exists in any contained volume where there is less gas than there is in the surrounding atmosphere. We will see that these gases exert a force on the surface area of the container. This force is called pressure. We can measure the pressure in the chamber by comparing it with the atmospheric pressure on the outside. In this way, we can find out how much gas is left in the vacuum.
2.2. What About Pressure?
Pressure is defined as force per unit area. Gases are composed of small particles. These gas particles are in constant motion. As these particles move around in space, they hit objects. When they hit something, they exert a force, or pressure. We can take a unit of area and measure the number and intensity of particle impacts on that surface. The result is a pressure measurement.
2.3. How Is a Vacuum Produced?
A vacuum is made by removing air and other gases from the work chamber. We remove the air and other gases by using special pumps, called vacuum pumps.
There are many, and very different, kinds of vacuum pumps. Some of them actually remove the gases. Other pumps trap the gases or change their form. In any case, the pump’s job is to take as many gases out of circulation as necessary.
2.4. Different Types of Vacuum
6-4
There are different degrees of vacuum, called rough vacuum, high vacuum, and ultrahigh vacuum. Which one is used depends on the application. As the chambers below show, the better (or higher) the vacuum is, the less air and gas are present.
Vacuum Theory: The Nature of Vacuum
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
2.5. Where Is Vacuum Used?
GOOD
BETTER
BEST
ROUGH VACUUM
HIGH VACUUM
ULTRA-HIGH VACUUM
Vacuum is used for many products and processes. Some of them are: Table 6.1. Uses of Vacuum Rough Vacuum
High Vacuum
Ultrahigh Vacuum
Food processing
Tube processing
Space research
Evaporation
Heat treating
Materials research
Freeze drying
Integrated circuit manufacture
Metallurgy
Distillation
Decorative coating
Physics research
Sputtering
Particle acceleration*
Surface analysis
Electrical conduction (neon lights)
Chemistry research
Molecular beam epitaxy
E-beam welding Vapor deposition Ion implantation Insulation (thermal)
*as in Varian Clinac® Linear Accelerators
2.6. Why Is Vacuum Needed?
We use a vacuum when we need a space that is very clean. It must be free of gases that can interfere with what we want to do. Let us take iron, for example. When iron is left out in air, it reacts with the gases in the air, and the result is rust. This would not happen in a vacuum. VACUUM SYSTEM WALL
RUSTY
Vacuum Theory: The Nature of Vacuum
CLEAN
6-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Another example is television. If gases are not removed from a TV tube, the electrons are blocked from reaching the screen — no picture! The easiest way to define “clean” is to say that everything is contaminated, or dirty, to some degree. It is a matter of how much contamination is present. The less contamination, the “cleaner” something is. Let us look at some of this contamination that we are trying to remove. Atmospheric air is a mixture of gases. Over 99% of atmosphere is nitrogen and oxygen. All other gases make up less than 1%. Table 6.2 Gas
Percent by Volume
Nitrogen
78.08
Oxygen
20.95
Argon
0.93
Carbon Dioxide
0.03
Neon
0.0018
Helium
0.0005
Krypton
0.0001
Hydrogen
0.00005
Xenon
0.0000087
Water vapor, another common gas, is not listed above because the amount changes with atmospheric pressure and temperature. Water vapor, which varies from 0.6% to 6% by volume, is one of the biggest sources of vacuum contamination, or “dirt.”
3. Temperature
We have mentioned temperature already in our discussion. Most of us are familiar with the Fahrenheit (°F) and the Celsius or Centigrade (°C) scales of temperature measurement. In the world of vacuum, we are also concerned with the absolute temperature as well. Temperature is a qualitative measurement of energy. The hotter something is, the more energy it contains. Or, if we want to get rid of gases, we could pump the energy out of them until they become frozen. That is, we have lowered the temperature of the gases. Calculations of heat and energy do not work well in the Celsius and Fahrenheit scales because of the negative numbers. This is where the absolute or Kelvin scale comes in. Let us compare some temperatures and conversion factors. Table 6.3. Temperature Scales
6-6
°F
°C
°K
Reference
212
100
373
Boiling point of water
32
0
273
Freezing point of water
321
196
77
LN2 temperature
437
261
12
Cold head temperature
459
273
0
Absolute zero
Vacuum Theory: Temperature
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Conversion factors: 5 °C = --- ( °F – 32 ) 9
°K = °C + 273
9 °F = ⎛⎝ --- °C⎞⎠ + 32 5
5 °K = --- ( °F – 32 ) + 273 9
Now let us discuss some information about gases.
4. Pressure
Earlier we defined pressure. Now, we will explain the kinds of pressure vacuum is concerned with. We will also describe how we measure pressure. First, let us look at what a gas is.
4.1. What is Gas?
What is a gas? It is a state of matter where the individual particles are free to move in any direction and tend to expand uniformly to fill the confines of a container. The gas particles are very small and freely moving. Some, like hydrogen and oxygen, are very reactive and easily form stable chemical compounds with other gases or elements. Other gases, such as helium and argon, are inert. These are sometimes known as the noble (inert) gases. They do not tend to form compounds. All gases have mass and are thus attracted to the earth by the force of gravity. This “ocean” of gas we call “air” has weight. This weight pushing on the earth’s surface is called atmospheric pressure. By definition, pressure (P) is the force (F) exerted on some particular area (A), such as a square inch, square foot, or square centimeter. Put into mathematical terms,
P = F --- (Pressure = Force per Unit Area) A
At 45° N latitude and at sea level, the average pressure exerted on the earth’s surface is 14.69 pounds per square inch (absolute), or 14.69 psia. When the temperature is 0°C, this 14.69 psia is called a standard atmosphere (1 std atm). Gas behavior is usually described with reference to “standard conditions” of temperature and pressure (stp).
4.2. Atmospheric Pressure
We use several different pressure scales. Here are four readings, all at standard conditions: 14.7 psia = 760 torr = 1 std atm = 101,325 pascal The average atmospheric pressure at sea level (45° N latitude) is 14.7 psia, 760 torr, or 101,325 Pa. Vacuum processes are usually done at pressures much lower than atmospheric pressure. Atmospheric pressure changes with distance above sea level (altitude) and changes in our weather.
Vacuum Theory: Pressure
6-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 6.4. Average Pressure at Various Altitudes Altitude (Ft)
Pressure (Torr)
Altitude (Ft)
Pressure (Torr)
–1,000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000
787.87 760.00 732.93 706.66 681.15 656.40 632.38 609.09 586.49 564.58 543.34 522.75
11,000 12,000 15,000 20,000 25,000 30,000 40,000 50,000 60,000 100,000 120,000 140,000
502.80 483.48 429.08 314.51 282.40 226.13 141.18 87.497 54.236 8.356 3.446 1.508
Source: U.S. Standard Atmosphere, 1962 (NASA)
A way to measure the force exerted by the atmosphere was developed in the mid-1600s by Evangelista Torricelli. It consisted of balancing a fluid of known weight against the weight of air. The first fluid used was water. Later, mercury was used. The measurement was made using an instrument called a barometer. We have named a pressure unit, torr, in Torricelli’s honor. Vacuum
3
1 in of water = 0.36 lb. Weight of water = 406.8 in.3 × 0.36 = 14.69 lb. Note: 33.9 ft 406.8 in
Mercury is 13.56 times heavier than water, so the mercury barometer will be 13.56 times shorter; i. e., 406.8 in/13.56 = 30 in.
Vacuum
1 Atm 760 mm. 30 in.
Water
1 Atm
Mercury
The Barometer
4.3. Pressure Measurement
There are several different scales for pressure measurement. Millimeters of mercury, torr, and microns are all commonly used. Pascal (Pa) is the metric unit for pressure measurement and is the international standard. The following table shows some of the common scales. The values for these scales are all listed at the same pressure one standard atmosphere (1 std atm).
6-8
Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 6.5. Pressure Equivalents Atmospheric Pressure (Standard) 0 14.7 760 760 760,000 101,325 1.013 1013
psig (gauge pressure) pounds per square inch (psia) mm of mercury torr millitorr or microns pascal bar millibar
Here is a table for the equivalent values for one torr and one millitorr (mtorr). Table 6.6 One Torr =
One Millitorr =
1/760 atmosphere
1/1000 TORR
1 mm of mercury
1/1000 mm of mercury
1000 microns or millitorr
10-3 torr or 1 millitor
103 microns or millitorr
0.001 torr
133 Pascal
0.133 Pascal
A conversion table and equivalents for the different measurement scales are provided in the Appendix.
4.4. Partial Pressure
The total pressure of a mixture of gases is the sum of each of the individual gas pressures in the mixture. This is known as Dalton’s Law of Partial Pressure. Each individual gas pressure in a mixture is called a partial pressure. At standard conditions (760 torr, 0°C), each gas exerts a pressure relative to its percent of the total volume: for example: N2 = 78% = 0.78 × 760 = 593 torr. Table 6.7. Partial Pressures of Gases Corresponding to Their Relative Volumes Gas (Air)
Symbol
Nitrogen
N2
Oxygen
O2
Argon
Ar
Carbon Dioxide
CO2
Neon Helium
Vacuum Theory: Pressure
Ne He
Percent by Volume
Partial Pressure Torr
Pascal
78
593
79,000
21
159
21,000
0.93
7.1
940
0.03
0.25
0.0018 0.0005
33 –2
1.8
–3
5.3 × 10
1.4 × 10 4.0 × 10
10–4
1.1 × 10–1
Krypton
Kr
0.0001
8.7 ×
Hydrogen
H2
0.00005
4.0 × 10–4
5.1 × 10–2
Xenon
Xe
0.0000087
6.6 × 10–5
8.8 × 10–3
Water
H 2O
(5 to 50 torr typically)
Variable
Variable
6-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.5. Vapor Pressure
When a liquid or solid becomes a gas, we call that process evaporation. The gas produced, we call a vapor. It, of course, exerts a pressure. This pressure we refer to as the vapor pressure for that particular material. The act of turning the gas back into a liquid, we call condensation. When a solid evaporates to a gas directly, we call that process sublimation. In general usage, vapors are gases that tend to condense back to the liquid state at moderate temperatures and pressures. All substances have a characteristic saturation vapor pressure that varies directly with temperature. The lower the temperature, the lower the vapor pressure. This is true for all substances. Water deserves special attention because of its behavior in the vacuum system. It is present in air as a gas in relatively large quantities. In the vacuum system, it is hard to remove condensed water vapor from surfaces at room temperatures. Table 6.8. Vapor Pressure of Water at Various Temperatures Temperature in °C
Pressure in Torr
100.0 (Boiling)
760
50.0
93
25.0
24
0.0 (Freezing)
4.8
40.0
0.1
78.5 (Dry Ice)
5.0 × 10–4
196.0 (LN2)
1.0 × 10–24
Table 6.9. Vapor Pressures of Some Liquids Liquid
Vapor Pressure in Torr @ 20°C (68°F)
Benzene
74.6
Ethyl Alcohol
43.9
Methyl Alcohol
96.0
Acetone Turpentine
184.8 4.4
Water
17.5
Carbon Tetrachloride
91.0
High Vacuum Pump Oil
1.0 × 0–7
Acetone has the highest vapor pressure of the liquids on this list. It evaporates the fastest of those substances on the list. It releases the most gas into the chamber in a given length of time. High vacuum pump oil is the least volatile liquid on the list. It will take the longest time to evaporate. When gases become cooled sufficiently, they liquefy and/or freeze. These curves give the vapor pressure for selected gases when they are liquids or solids. In the illustration below, curves to the right of the vertical dotted line (77°K, 196°C) indicate low vapor pressures at this temperature. Curves to the left show high vapor pressures at this temperature, which is the boiling point of liquid nitrogen.
6-10
Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 (BOILING POINT)
TEMPERATURE (°C) –260 –250 –200
–270
–272
–100
0 100
VAPOR PRESSURE (TORR)
3
10 2 10 1 10 100 10–1 –2 10 –3 10 10–4 –5 10 –6 10 –7 10 –8 10 –9 10 –10 10 10–11
He H2
Ne N2 O2 H2O CO2 NO
1
2
3 4 5
10 20 30 40 50 TEMPERATURE E (°K)
100
200
400
VAPOR PRESSURES OF COMMON GASES
Gases at the left side of the chart have high vapor pressures at extremely low temperatures. Note: Vapor pressure of all gases is the same at the boiling point in atmosphere (760 torr) even though they boil at different temperatures. All materials have a vapor pressure, even though it may be very small. Note that, for some of these materials, their vapor pressure may be high enough to be a problem in some vacuum systems. 3
101 Zn
10–1
Cd Mo
Pb
–3
10
Ti Cu
–5
10
W Fe
–7
10
–9
5000
4000
3000
2000
600
400
200
10–11
1500
10
800 1000
VAPOR PRESSURE (TORR)
10
ABSOLUTE TEMPERATURE (°K)
Vacuum Theory: Pressure
6-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.6. Effects of Pressure
Before the air and other gases are pumped from the work chamber, constant, high-speed motion makes the particles bump into each other and into the chamber walls. This activity develops a total actual (absolute) pressure of 14.7 pounds per square inch (psia). As we have already seen, 14.7 psia is the average atmospheric pressure at sea level. Therefore, the pressure is the same inside and outside the chamber.
As air is pumped out of the chamber, pressure drops. However, we can never remove all particles from the chamber.
After most of the free-moving gas (sometimes called the volume gas) is removed, there are still other sources of gas entering the system. Gases come off of surfaces in the vacuum system or out of the materials inside the work chamber. This is called desorption or outgassing. Vacuum systems can implode because of the external atmospheric pressure, causing the walls to collapse inward.
4.7. Pressure Ranges
These are the pressure ranges generally used in vacuum work: Rough (low) vacuum759 to 1 × 10–3 torr (approx.) High vacuum1 × 10–3 torr to 1 × 10–8 torr (approx.) Ultrahigh vacuumLess than 1 × 10–8 torr
6-12
Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Gas Particles
Let’s talk about the nature of the gases that exert this pressure. They are made from naturally occurring chemical elements. These elements are the building blocks of earthly matter. The smallest identifiable part of an element is one of its atoms. An atom has a dense center portion known as the nucleus. This nucleus has particles called protons and neutrons. The protons have a positive electrical charge. Neutrons are neutral. The number of protons — and therefore the electrical charge in the nucleus — is different for each element. If the atom has more or less than its normal number of neutrons, it is called an isotope of the element and is unstable. Under normal conditions, the nucleus is surrounded by a number of electrons. Electrons have a negative electrical charge. The number of electrons balances the positive charge and this makes the atom electrically neutral. Neutrons and protons weigh approximately the same and make up the bulk of the atom. The atoms of the different elements have different numbers of protons and neutrons. They thus have different masses. This means they have different weights (masses). They are classified by their atomic mass or weight. We call this atomic mass units or amu. Molecules simply consist of one or more atoms joined together. with definite chemical and physical characteristics. Molecules are likewise classified by their molecular weight (or mass). This is simply the sum total of the individual atomic weights that make up the molecule. Some of the elements usually exist as gases. Some of these, like hydrogen, nitrogen and oxygen, travel as molecules with two or three atoms bound together. Some gases are composed of more than one element, such as water (H2O). For instance, the atomic weight of hydrogen (H) is 1 amu. Its molecule is made up of two hydrogen atoms (H2) so its Molecular weight or mass is 2 amu. The atomic weight of oxygen is 16 amu. Thus, the molecular weight of water (H2O) is 18 amu. That is the mass of two hydrogen atoms plus the mass of one oxygen atom (1 + 1 + 16). Under certain conditions, an atom or a molecule can become electrically charged. It is then referred to as an ion. This process will be considered in more detail in the discussion on Ionization.
6. Gas Laws
Let’s look at what happens to gases as we use them in our vacuum system. We first assume that gases are perfect — and in general, they are. So we can apply some “laws” to their behavior Let’s look at some of these laws.
6.1. Avogadro’s Law
Under the same conditions of pressure and temperature, equal volumes of all gases have the same number of particles (molecules, actually). We call this a mole. One mole of any gas has 6.023 × 1023 particles, under standard conditions (760 torr, 273°K), occupies 22.4 liters, and weighs one molecular weight. We know this as Avogadro’s Law. 1. How many particles would be in a standard liter? 23
6.023 × 10 particles- = 2.69 × 10 22 particles ⁄ l -------------------------------------------------22.4l
2. How many in a standard cubic centimeter? 23
–3
6.023 × 10 particles 10 -l = 2.69 × 10 25 particles/cc --------------------------------------------------- = -----------22.4l 1cc
Vacuum Theory: Gas Particles
6-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6.2. Boyle’s Law
Boyle’s Law, P1V1 = P2V2, or original pressure times original volume equals new pressure times new volume. Reduce the volume by half, the pressure is doubled. This equation predicts new pressure or new volume whenever the other is changed by any amount, providing that the temperature remains the same.
BOYLE’S LAW P1V1 = P2V2
VOLUME = 10 LITERS
VOLUME = 5 LITERS
PRESSURE = 50 TORR
PRESSURE = 100 TORR SAME NUMBER OF GAS MOLECULES
NOTE: TEMPERATURE HELD CONSTANT 6.3. Gas Expansion
6-14
Gas expands tremendously under vacuum (from Boyle’s Law). This happens to gas absorbed in fingerprints and dirt in general.
Vacuum Theory: Gas Laws
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Water and solvents are also sources of large gas loads. The large volumes these materials produce are a major part of “outgassing.” Suppose you have a chamber which has a volume of 100l at a pressure of 1 × 10–4 torr. If 1 std cc of gas is suddenly added, what will be the pressure? Let’s use Boyle’s Law. Note that we are really calculating a new pressure, not a new volume. Also, the partial pressure of the gas we are adding will add to the gas pressure already there. P 1 V1 = P 2 V2 For the gas we are adding to the chamber: 760 torr × 1 cc = P2 × 100l Solving for P2 and converting cubic centimeters to liters: –3
760 torr × 1cc 10 l P 2 = ---------------------------------- × ------------100l 1cc –3
P 2 = 7.6 × 10 torr
Now the total pressure in the container is the sum of the pressure there (1 × 10–4 torr) plus the pressure from the gas we added (7.6 × 10–3 torr). P total = P chamber + P 2 –4
–3
= 1 × 10 torr + 7.6 × 10 torr –3
= 7.7 × 10 torr
We see that the 1 cc of gas at atmospheric pressure contributed much more to the pressure in the chamber than the gas already there!
6.4. Charles’ Law
Let’s look at what happens to the volume of gas as we change the temperature. As we cool a gas, its volume gets smaller. If we heat the gas, its volume increases. We call this Charles’ Law. The equation looks like this:
V V1 ------ = -----2T1 T2
Charles’ Law states that if the absolute temperature is doubled, the volume of gas is doubled providing that the pressure is unchanged.
Vacuum Theory: Gas Laws
6-15
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 CHARLES’ LAW V1 = V2 T1 = T2 1 ATM
1 ATM
TEMPERATURE = 100° K
TEMPERATURE = 200° K
VOLUME = 10 LITERS
VOLUME = 20 LITERS
SAME NUMBER OF GAS MOLECULES
HEAT
AMOUNT OF GAS DOES NOT CHANGE
NOTE: PRESSURE HELD CONSTANT
6.5. Gay-Lussac’s Law
If Charles’ Law is examined carefully, a more specific relationship develops: If the temperature of a volume of gas at 0°C is changed by 1°C, the volume will change (plus or minus) by 1/273 of its original value. This is Gay-Lussac’s Law. Thus: °C V = V o + ⎛ ---------⎞ × V o ⎝ 273⎠
Rearranging this equation gives us: °C V = V o ⎛ 1 + ---------⎞ ⎝ 273⎠
Lord Kelvin used this relationship to develop the absolute temperature scale.
6.6. General Gas Law
We can combine these laws to get a general gas law (Boyle’s and Charles’ combined): P2 V2 P1 V1 ------------ = -----------T1 T2
The general gas law combines pressure, volume, and temperature in a single equation. The temperature in Charles’ Law and the general gas law is expressed in the absolute scale, or degrees Kelvin; to convert from °C to °K add 273 to °C. Thus: 100°C + 273 = 373°K.
6-16
Vacuum Theory: Gas Laws
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
7. Gas Flow
Since we want to move gas molecules out of the vacuum chamber, we should know how gas flows. Of the many types of gas flow, we will discuss two kinds: viscous flow and molecular flow. Both types of flow have to do with how tightly molecules fill a space.
7.1. Viscous Flow
Generally, gas molecules occupying a space at a pressure greater than 1 × 10–2 torr act very much like a fluid, so this is called viscous flow. In the viscous flow range, the molecules are constantly bumping into each other. The molecules are so closely packed together that as our vacuum pump moves some of them out of the chamber. others will rush to fill up that empty space. In viscous flow conditions, molecular movement is predictable. When a molecule is hit or hits a surface, we can predict its movement after impact with reasonable accuracy. Because the molecules are tightly packed and move predictably, we can use smaller diameter hoses and tubulations for rough pumping operations. Viscous flow conditions will generally allow us to move great quantities of molecules per unit time from one place to another.
7.2. Molecular Flow
Molecular flow occurs when the molecules are so far apart that they no longer have any influence on each other. Their motion is strictly random. This occurs at low pressures where fewer molecules are present. Depending on the pressure, a gas molecule might travel inches, feet, or even miles before it strikes another molecule. This means we can’t depend on molecular interaction to push or start a flow pattern. In the molecular flow range, molecular movement is unpredictable. This is why we have such large inlets in high vacuum pumps. The use of large inlets increases the probability that one of these randomly moving molecules will move into the pump.
Vacuum Theory: Gas Flow
6-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In molecular flow, the molecular motion is explained by kinetic theory, which uses statistics (chance) to describe the condition. The difference between viscous flow and molecular flow does not depend upon the pressure alone. It also depends upon the dimensions of the vacuum container (pipes, chamber, etc.). Basically, it depends upon the mean free path and whether it is longer or shorter than the container dimensions. Let’s take a look at what is meant by “mean free path.”
7.3. Mean Free Path
As we lower the pressure in the vacuum chamber, the amount of space between the gas particles increases. The particles bump into each other less frequently. The average distance a particle moves before it bumps another particle is the mean free path. Table 6.10. Molecular Density and Mean Free Path 7.6 × 102 Torr (atm)
1 × 10–3 Torr
1 × 10–9 Torr
# mol/cm3
3 × 1019 (30 million trillion)
4 × 1013 (40 trillion)
4 × 107 (40 million)
MFP
2 × 10–6 in.
2 in.
30 mi.
At atmosphere, the mean free path is extremely short, about two millionths of an inch. Under vacuum, fewer molecules remain, and the mean free path is longer. Its length depends on the number of molecules present, and therefore on the pressure. The mean free path for air can be estimated from the relationship:
–3
(----------------------------------------------------------------------------------------Mean Free Path = 5 × 10 torr cm )P torr
From this, we can see that as the pressure gets lower, the mean free path gets longer. Likewise, as the pressure gets lower, there are fewer molecules of gas present, so there is less chance of them running into each other. In 1 cc of gas at standard conditions (760 torr at 0°C), there are about 3 × 1019 gas molecules and the mean free path is about 2 × 10–6 cm (a few millionths of an inch). At 1 × 10–9 torr, there are about 4 × 107 molecules/cc, and the mean free path is about 30 miles or 50 kilometers. The number of molecules per unit volume (in this example cubic centimeters) is called the gas density.
8. Conductance
When we talk about moving a gas through a vacuum system, we use the term conductance. Conductance is the ability of an opening or pipe to allow a given volume of gas to pass through in a given time. It is expressed in such units as liters per second, cubic feet per minute or cubic meters per hour. In molecular flow, a good conductance path is wide and short. It has few turns, thus allowing free gas flow. In viscous flow, these conditions are not so important. This is because the molecules tend to push one another along under the influence of a pressure difference.
6-18
Vacuum Theory: Conductance
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
In the molecular flow range, a 1-in2 opening has a 75 l/sec conductance. The pump speed, in this case 400 l/sec, is really 75 l/sec as far as the chamber is concerned because the molecules must go through the hole before they can be pumped. To improve system performance, the conductance must first be improved. (Make the hole bigger!)
CHAMBER
2
1 IN. OPENING (75 l/SEC) 2 OR 11.6 l/SEC CM
400 l/SEC PUMP
MOLECULAR FLOW
To repeat: In the molecular flow range, a pump works only when molecules migrate into the pump by chance.
8.1. Conductance in Viscous Flow
The volume of gas that can flow per unit of time through a pipe under viscous flow conditions is related to the fourth power of the pipe diameter and is inversely related to the length of the pipe. For example, if you use a pipe with a diameter twice that of the pipe presently being used, it will allow 24 or sixteen times as much gas to flow through it, assuming that the length of the pipe is the same. Now let’s compare this to molecular flow conditions.
Vacuum Theory: Conductance
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8.2. Conductance in Molecular Flow
The volume of gas that can flow per unit of time through a pipe under molecular flow conditions is related to the cube of the diameter and inversely to the length of the pipe. Using the same pipe as in the viscous flow example, doubling the diameter of the pipe will, at most, allow 23 or eight times the flow for the same length of pipe. In either case of viscous flow or molecular flow, making the pipe shorter will increase the flow of gas through the pipe. Please note that these are gross statements that are subject to all kinds of qualifications. There is another region where we approach molecular flow, but the flow is not really viscous either. This region is called the transition range. There is another set of calculations to be used for the transition range, but we will not discuss them in this text.
9. Review of the Nature of Gases
Before continuing, a few basic facts about the atomic and molecular nature of gases should be reviewed. These facts will be useful for understanding how ion pumps operate. Let us review them briefly here. An atom is the smallest particle of matter that can exist and still retain the basic characteristics of the material or element from which it came. Molecules are simply one atom or two or more atoms joined together; many gases exist as molecules. Atoms and molecules normally have an equal number of protons (positively charged particles) and electrons (negatively charged particles). The neutrons in the nucleus contribute to the weight (mass) of the atom, but not to the charge. The atoms are thus neutral, or electrically balanced. If this balance is upset, useful work can be produced. If we remove electrons from the atom, we have made a positively charged atom or molecule we call an ion. This process of creating ions is called ionization. We can put these otherwise useless charged particles to work because we can direct their motion using a magnetic or electrical field.
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Vacuum Theory: Review of the Nature of Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Ion Pump
Let us now make our ion pump by connecting two electrodes to a high-voltage supply. Electron flow will be from cathode to anode as in this drawing. Ions will carry current from anode to cathode. Fewer ions than electrons will be produced so that we can say that the current through the pump is the “ion current.”
ELECTRON SOURCE
(–)
(+)
CATHODE
ANODE
(–)
POWER SUPPLY
(+)
In this drawing, a free electron is attracted to a positively charged anode. On the way to the anode, it collides with a neutral atom, ionizing it. Now two electrons are free to continue toward the anode, increasing the probability of still further ionization. The positively charged ions are then accelerated toward the negatively charged cathode. They may strike the cathode with such force that they stick to the cathode material, and are thereby pumped. As one gas molecule is driven into the cathode, one or more molecules of the cathode is usually released from this surface. This process is called sputtering. The ion pump is also a gas capture pump. It is not designed to pump heavy gas loads. For this reason, it is not generally used alone in high-production applications. Instead, it is more often used in research and analytical applications where there is no need to repeatedly and rapidly cycle the work chamber to atmosphere. When combined with a Titanium Sublimation Pump (TSP), it also provides adequate pumping for these applications. Ion pumps are clean operating devices. They are electronic devices which use no moving parts or oils. It is possible to achieve pressures in the 10–11 torr range, with overnight bakeout of the system. The bakeout process drives residual gas off walls. This gas is then pumped by the ion pumps. In research and analytical applications, the ion pump’s cleanliness, bakeability, low power consumption, vibration-free operation and long life make it the pump of choice for most ultra-high vacuum uses. Ion pumps come in various sizes. A small appendage ion pump is used not for pumping down, but for maintaining vacuum conditions in operating devices such as transmitting tubes. Larger pumps can be used to evacuate small chambers, or several can be connected in parallel with other ion pumps to pump down larger chambers.
Vacuum Theory: Ion Pump
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10.1. Components
10.2. How the Pump Works
A basic ion pump cell consists of two titanium cathodes and an anode. All are placed between the poles of a strong permanent magnet.
NEUTRAL GAS MOLECULE ANODE
CATHODE MAGNET ELECTRON
AN ELECTRON IS “BROKEN FREE” FROM A GAS MOLECULE. THE GAS MOLECULE THEN BECOMES A POSITIVE ION.
10.2.1. Pump Operation
The magnetic field forces the free electrons to travel in long helical paths instead of straight lines. This increases the probability of collision with molecules on their way to the positively charged anode. This, in turn, increases the ionization probability, and therefore the amount of useful pumping action that can be performed by the pump. Because of the action of the magnetic field, the electrons do not easily come in contact with the anode. As a result, a “cloud” of electrons is formed in the anode area. This electron cloud becomes fairly stable during pump operation. The electron density is high enough for efficient ionization of gas molecules. Therefore, a hot filament electron source is not needed. So, the name for this process is cold cathode discharge. The positively charged ions, which are relatively heavy particles, are accelerated into the negatively charged titanium cathodes. This impact causes sputtering, or chipping away of the titanium cathode material. Sputtered titanium deposits onto the internal structure of the pump. There it is available for chemical combination with gas molecules to convert them to solids. Thus we have the needed pumping action.
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Vacuum Theory: Ion Pump
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 TITANIUM CATHODES
POSITIVE ION ANODE
SPUTTERED ATOM FROM CATHODE
MAGNET In addition, a second pumping action takes place. Some of the ionized molecules strike the cathodes with enough force to become buried in them. This burial prevents them from recombining and becoming a free gas again.
(+) (–)
(–)
MAGNET Still another pumping process occurs in the case of hydrogen, which diffuses directly into and reacts with the cathode plate. Also, neutral molecules in the anode regions can literally be buried or “plastered over” by the sputtered cathode material. Complex molecules may also be split in the discharge to smaller, more readily pumped molecules.
Vacuum Theory: Ion Pump
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 There is a problem with the pump design we have described (also called a diode configuration). Some of the buried molecules can be released again into the vacuum system. This re-release can be caused by heating of the cathodes or reduction of cathode material due to sputtering. It can also be caused by a molecule or atom being physically separated from the sputtered film.
10.2.2. Pumping Characteristics of Different Configurations
Ion pumps are available in different design configurations. Each design has its own special pumping characteristics. In the diode pump, as we have seen, the ions strike the cathode plate and react with the sputtered titanium. The triode pump, which is a variation on the diode pump, improves inert or noble gas pumping. Titanium cathodes are in the form of grids. Ions sputter titanium onto the pump walls. This angled impact sputters more titanium than in the diode model and thus furnishes more material for argon or noble gas burial. Because of the electrical arrangement of the pump components, the glow discharge that happens in “starting” the diode pump is typically confined in the triode pump. As a result, the triode pump can be started at slightly higher pressure. TITANIUM CATHODE PLATES
CONTROL UNIT
CONTROL UNIT MAGNET N
S
N
S
MAGNET
MULTICELL ANODE PUMP WALL FORMS THIRD ELECTRODE IN TRIODE PUMP SPUTTER CATHODES TITANIUM VANES
MULTICELL ANODE
ION PUMP SCHEMATICS (A) DIODE ION PUMP
(B) TRIODE ION PUMP
VB
V+ ANODE + ARGON
SPUTTERING
MAGNETIC FIELD
TITANIUM ATOMS ENTRAPMENT OF BURIED ARGON IONS
MAGNETIC FIELD
ANODE +
SPUTTERED TITANIUM, ARGON ATOMS BURIED HERE
+
PUMP WALL TI SPUTTER CATHODE
OBLIQUE IMPACT CAUSES MAXIMUM SCATTERING
ARGON IONS
PUMPING MECHANISMS (A) DIODE ION PUMP
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(B) TRIODE ION PUMP
Vacuum Theory: Ion Pump
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10.2.3. Other Characteristics
The ion pump is self-regulating. At the higher pressures, where much ionization takes place, more current flows. At low pressures, less current flows. This characteristic current drain can be used to measure the pressure, or degree of vacuum achieved with the pump. This feature eliminates the need for an ion gauge on the system.
10–2
10–3
PRESSURE (TORR)
10–4
10–5
10–6
–7
10
10–8
10–9 1µA
10µA
100µA
1mA
10mA 100mA 1Amp
PUMP CURRENT
Ion pumps are long-lived; the lower the pressure, the longer the life. Once they begin pumping, they quickly lower the pressure to the long-life region. As long as they are not pumping against a leak, they will last for years. Ideally, ion pumps should be started at pressures approaching 10–5 torr. At higher pressures, the plasma discharge that is generated minimizes pumping speed and reduces cathode life. A more common and practical approach is to sorption rough the pump to less than 10–2 torr before applying the ion pump power. At very low pressures, the time taken to begin the ionization process may be excessively long. Table 6.11. Typical Diode Pump Service Life Pressure (Torr) –3
Life (Hours)
10
20
10–4
200
–5
2,000
–7
200,000 (over 20 years of constant operation)
10 10
Table 6.12. Life (Pumping N2 at 10–4 Torr)
Vacuum Theory: Ion Pump
Type
Life
Triode
35,000 hours — approx. 4 years
Diode
50,000 hours — approx. 6 years
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10.3. Vacuum System Use
Ion pumps are typically used in systems which demand ultra-clean, ultrahigh vacuum. This type of vacuum system is pumped to high vacuum or lower pressure and then kept in that condition for long periods of time. A load-lock chamber is often built on the system to allow access to the chamber without bringing the chamber back to air. Typical uses are for electron microscopes, mass spectrometers, and surface analysis, to mention a few. Very little maintenance can be performed on ion pumps other than an occasional bakeout. When pumping eventually deteriorates to the point where operating pressures can no longer be attained, pump replacement or sometimes anode/cathode assembly replacement is necessary.
10.4. Summary
We have discussed the pressure ranges of vacuum pumps and the major types of pumps in each range. By now, you should be familiar with the different types of vacuum pumps what their major components are and how they work. You have also learned how they are placed in vacuum systems and some general maintenance information. Let’s go on now to gauges. These are major vacuum components that tell you what is going on inside your vacuum system.
11. Vacuum Gauges
To transfer heat by convection, we need massive numbers of molecules flowing. Your hot-air furnace heats by convection. Some gauges use this principle between 760 torr and 2 torr but are generally less accurate in this pressure range. To transfer heat by radiation, we need light energy. Not the kind of light that you see, but typically infrared light. The heat you feel when standing in front of a fireplace is mostly the radiated heat. No gas molecules need be involved; that is, radiation is independent of the number of gas molecules present. Radiated heat is the only way to transfer heat inside of a vacuum system at high vacuum. There are insufficient molecules present to provide heat transfer by either conduction or convection. Now, let’s go on to discuss gauges that depend on heat transfer to work.
11.1. Thermocouple Gauge
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Vacuum Theory: Vacuum Gauges
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The thermocouple, or TC, gauge is another rugged, simple instrument. It is used to measure pressures in the rough vacuum range. It does its work well under less than ideal conditions. The TC gauge measures temperature and converts it to a pressure reading. Many modern thermocouple gauges have been modified to use convection as well as conduction principles. This effectively extends their useful range to atmosphere. It is typically considered as a very approximate device. Let’s take a look at how it works.
11.1.1. How the Gauge Works
A thermocouple gauge consists of a gauge tube and control unit. Within the gauge tube is a heated filament. Spot welded to the filament is a thermocouple that measures the temperature of the hot wire. The meter is calibrated in pressure units, not in temperature. TO VACUUM SYSTEM TC GAUGE TUBE FILAMENT
CONTROL UNIT THERMOCOUPLE
At atmospheric pressure, there will be many molecular collisions with the heated filament. The gas molecules conduct heat away from the filament. The amount of heat removal can be related to the amount of gas in the chamber. At higher pressures, with lots of molecules, much heat will be conducted away from the wire. Therefore, the wire will be at a lower temperature (cooler). When we pump away the gas, there are fewer molecules to collide with the wire. The wire is therefore at a higher temperature (hotter). THERMOCOUPLE GAUGE PRINCIPLE FILAMENT
(+)
(–) THERMOCOUPLE
METER
Vacuum Theory: Vacuum Gauges
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 There is not a linear relationship between wire temperature and pressure, so the pressure scale on your TC gauge is not linear. The gauge stops responding at about 1 millitorr (10–3 torr) because the heat loss through radiation is now the largest factor. The heat lost through radiation is also constant. Therefore, the gauge reads “zero.” Compared to other gauges, the TC gauge has a slow response time. This is because the wire must have time to heat up or cool down as the pressure changes. Some newer gauges speed up the response time by operating the gauge at constant temperature and measuring the change in current required to hold the temperature constant.
11.1.2. Maintenance
If the sensing unit, or gauge head, gets dirty, it may be cleaned with an appropriate solvent. Most people will simply discard the TC gauge and install a new one in its place. Whenever you clean or replace a TC gauge, it should be adjusted to read the proper values. To do this, you expose the gauge head to a pressure of 10–4 torr or less and adjust the control unit to read zero on the pressure gauge. If for some reason you cannot obtain a pressure below 10–4 torr, then install a “good” gauge and set the system gauge to read the same pressure. Please check the operation manual for your particular unit for adjustment instructions, because they do vary in detail.
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Vacuum Theory: Vacuum Gauges
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Seven Glossary
This chapter is a glossary of terms commonly used in radiotherapy, and is intended for reference use only.
Glossary
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Table of Contents A:......................................................................................................................................... 7-3 B: ........................................................................................................................................ 7-5 C: ........................................................................................................................................ 7-8 D: ...................................................................................................................................... 7-12 E:....................................................................................................................................... 7-15 F: ....................................................................................................................................... 7-18 G: ...................................................................................................................................... 7-19 H: ...................................................................................................................................... 7-21 I:........................................................................................................................................ 7-22 J: ....................................................................................................................................... 7-24 K:....................................................................................................................................... 7-24 L: ....................................................................................................................................... 7-25 M: ...................................................................................................................................... 7-25 N: ...................................................................................................................................... 7-27 O: ...................................................................................................................................... 7-28 P: ....................................................................................................................................... 7-29 Q: ...................................................................................................................................... 7-31 R:....................................................................................................................................... 7-32 S:....................................................................................................................................... 7-35 T: ....................................................................................................................................... 7-38 U: ...................................................................................................................................... 7-40 V:....................................................................................................................................... 7-40 W:...................................................................................................................................... 7-41 X:....................................................................................................................................... 7-42
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Glossary
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Absorbed Dose
See Dose, Absorbed Dose.
Absorbed Fraction
A term used in internal dosimetry. It is that fraction of the photon energy (emitted within a specified volume of material) which is absorbed by the volume. The absorbed fraction depends on the source distribution, the photon energy, and the size, shape, and composition of the volume.
Absorption
The process by which radiation imparts some or all of its energy to any material through which it passes. (See also Compton Effect, Pair Production and Photoelectric Effect.) Absorption Coefficient: Fractional decrease in the intensity of a beam of x or gamma radiation per unit thickness (linear absorption coefficient), per unit mass (mass absorption coefficient), or per atom (atomic absorption coefficient) of absorber, due to deposition of energy in the absorber. The total absorption coefficient is the sum of the individual energy absorption process (Compton effect, photoelectric effect, and pair production). Linear Absorption Coefficient: A factor expressing the fraction of a beam of x or gamma radiation absorbed in unit thickness of material. In the ex– µx pression I = I 0 e , I0 is the initial intensity, I the intensity of the material x, and µ is the linear absorption coefficient. Mass Absorption Coefficient: The linear absorption coefficient per cm. divided by the density of the absorber in grams per cu. cm. It is frequently expressed as µ/p, where µ is the linear absorption coefficient and p the absorber density. Self-Absorption: Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located; in particular, the absorption of radiation within a sample being assayed.
Accelerator
A machine that accelerates electrically charged atomic particles to high velocities. Electrons, protons, deuterons, and alpha particles can be accelerated to nearly the speed of light for use in nuclear research. Types of accelerators include the betatron, cyclotron, linear accelerator, and synchrotron.
Achromatic
A type of bend magnet in which particles of differing energies are brought to the same focus.
Actinic
A type of radiation that is capable of producing a chemical change.
Activation
The process of inducing radioactivity by irradiation.
Activity
The number of nuclear transformations occurring in a given quantity of material per unit time.
Adsorption
The adhesion of one substance to the surface of another.
Alpha Particle
A charged particle emitted from the nucleus of an atom having a mass and charge equal in magnitude to those of a helium nucleus, i.e., two protons and two neutrons. Aluminum Equivalent: The thickness of aluminum affording the same attenuation, under specified conditions, as the material in question.
Glossary: Absorbed Dose
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Alternating Current (A.C.)
Electric current that flows for a given length of time in one direction and immediately flows in the opposite direction for the same length of time. It usually consists of 60 complete cycles per second.
Alveoli
The terminal air sacs of the lung.
Ampere (A)
A unit of electrical current or rate of flow of electrons.
Amplification
As related to radiation detection instruments, the process (gas, electronic, or both) by which ionization effects are magnified to a degree suitable for their measurement.
Amplifier, Linear
A pulse amplifier in which the output pulse height is proportional to an input pulse height for a given pulse shape up to a point at which the amplifier overloads.
Amplifier, Pulse
An amplifier, designed specifically to amplify the intermittent signals of a nuclear detector, incorporating appropriate pulse-shaping characteristics.
Analyzer, Pulse Height
An electronic circuit which sorts and stores the pulses according to height.
Angstrom Unit (Å)
One angstrom unit equals 10–8 cm.
Anion
Negatively charged ion.
Annihilation (Electron)
An interaction between a positive and a negative electron in which they both disappear; their energy, including rest energy, being converted into electromagnetic radiation (called annihilation radiation).
Anode
Positive electrode; electrode to which negative ions are attracted.
Arc Therapy
Radiation therapy in which the source of radiation is moved through a limited arc about the patient during treatment. In this way, a larger dose is built up at the center of rotation within the patient’s body than on any area of the skin. Multiple arcs may be used. Synonymous with arc treatment and rotation therapy.
Ataxia
The inability to coordinate muscular movements.
Atom
Smallest particle of an element that is capable of entering into a chemical reaction.
Atomic Mass (u)
The mass of a neutral atom of a nuclide, usually expressed in terms of “atomic mass units.” The “atomic mass unit” is one twelfth the mass of one neutral atom of carbon 12; equivalent to 1.6604 × 10–24 gm.
Atomic Number (Z)
The number of protons in the numbers of a neutral atom of a nuclide. The “effective atomic number” is calculated from the composition and atomic numbers of a compound or mixture. An element of this atomic number would interact with photons in the same way as the compound or mixture.
Atomic Weight
The weighted mean of the masses of the neutral atoms of an element expressed in atomic mass units.
7-4
Glossary: Alternating Current (A.C.)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Attenuation
The process by which a beam of radiation is reduced in intensity when passing through some material. It is the combination of absorption and scattering processes and leads to a decrease in flux density of the beam when projected through matter.
Attenuation Coefficient
A general term used to describe quantitatively the reduction in intensity of a beam of radiation as it passes through a particular material. Attenuation Coefficient, Compton: The fractional number of photons removed from a beam of radiation per unit thickness of a material through which it is passing as a result of Compton effect interactions. Attenuation Coefficient, Linear: The fractional number of photons removed from a beam of radiation per unit thickness of a material through which it is passing due to all absorption and scattering processes. Attenuation Coefficient, Pair Production: That fractional decrease in the intensity of a beam of ionizing radiation due to pair production in a medium through which it passes. Attenuation Coefficient, Photoelectric Effect: That fractional decrease in the intensity of a beam of ionizing radiation due to photoelectric effect in a medium through which it is passing.
Attenuation Factor
A measure of the opacity of a layer of material for radiation traversing it; the ratio of the incident intensity to the transmitted intensity. It is equal to I0/I, where I0 and I are the intensities of the incident and emergent radiation, respectively. In the usual sense of exponential absorption (I = I0e–µt), the attenuation factor is e–t where t is the thickness of the material and µ is the absorption coefficient.
Auger Effect
The emission of an electron from the extranuclear portion of an excited atom when the atom undergoes a transition to a less excited state.
Autoradiography
Record of radiation from radioactive material in an object, made by placing the object in close proximity to a photographic emulsion.
Autotransformer
A transformer with a single wrapping or winding of wire, with both ends of the wire attached to the primary alternating current.
Avalanche
The multiplicative process in which a single charged particle accelerated by a strong electric field produces additional charged particles through collision with neutral gas molecules. This cumulative increase of ions is also known as “Townsend ionization” or “Townsend avalanche.”
Average Life (Mean Life)
The average of the individual lives of all the atoms of a particular radioactive substance. It is 1.443 times the radioactive half-life.
Avogadro’s Number (Avogadro’s Constant) (NA)
Number of atoms in a gram atomic weight of any element; also the number of molecules in a gram molecular weight of any substance. It is numerically equal to 6.023 × 1023 on the unified mass scale.
Backplane
A printed circuit board with connectors for inserting other printed circuit boards.
Back Pointer
A linear accelerator accessory used to identify the central axis of the radiation beam.
Glossary: Attenuation
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Backscatter
The deflection of radiation by scattering processes through angles greater than 90 degrees with respect to the original direction of motion.
Barriers, Protective
Barriers of radiation-absorbing material, such as lead, concrete, and plaster, used to reduce radiation exposure. Barriers, Primary Protective: Barriers sufficient to attenuate the useful beam to the required degree. Barriers, Secondary Protective: Barriers sufficient to attenuate stray radiation to the required degree.
Beam
A unidirectional or approximately unidirectional flow of electromagnetic radiation or of particles. Useful Beam (Radiology): Radiation that passes through the aperture, cone, or other collimating device of the source housing; sometimes called “primary beam.”
Beam Axis
A geometric line from the target (or source) outward along the geometric center of the beam.
Beam Hardening
The process of eliminating the low-energy photons from a beam of x-rays. This process changes the quality of the beam in such a manner that the average energy of the beam increases.
Beam Quality
The spectral energy distribution of the radiation beam. Beam quality affects the penetration of the beam through tissue and the relative absorption of the energy in different types of tissue.
Beam Shaping
The use of special blocks, wedges, compensators, and other devices to create a treatment beam of the geometric proportions required for a treatment plan beyond the capabilities of the collimator.
Becquerel (Bq)
The new special unit of activity. One becquerel equals one nuclear disintegration per second.
Bend Magnet Assembly
A beam transport system for guiding the electron beam from the linear accelerator structure to the x-ray target or electron scattering foil.
Beta Particle
Charged particle emitted from the nucleus of an atom, with a mass and charge equal in magnitude to that of the electron.
Betatron
A magnetic induction accelerator which makes use of a varying magnetic field to accelerate electrons. Electrons are injected into a toroidal vacuum chamber which is between the poles of an iron-core magnet. The rate of change of the magnet flux and magnetic field at the orbit radius are related to maintain a constant radius for the accelerating electrons.
Biologic Effectiveness of Radiation
(See Relative Biologic Effectiveness.)
Blood Dyscrasia
Any persistent change from normal of one or more of the blood components.
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Glossary: Backscatter
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Bone Marrow
Soft material which fills the cavity in most bones; it manufactures most of the formed elements of the blood.
Bone Seeker
Any compound or ion which migrates in the body preferentially into bone.
Brachytherapy
Therapy at short distances with beta or gamma radiation. Implantation or placement therapy with needles, inserts, or other such applications containing radioactive materials. Useful in the treatment of various diseases.
Bragg-Gray Principle
The relationship between energy absorbed in a small gas-filled cavity in a medium to energy absorbed (in the medium) from ionizing radiation. The relationship is expressed as Em = W × Jg × Sgm, where Em = energy/mass absorbed in the medium, W = average energy needed to produce an ion pair in the gas, Jg = number of ion pairs/mass formed in the gas, and S = ratio of the stopping power for secondary particles in the medium to that in the gas.
Branching
The occurrence of two or more modes by which a radionuclide can undergo radioactive decay. For example, RaC can undergo α or ß– decay, 64Cu can undergo ß–, ß+, or electron capture decay. An individual atom of a nuclide exhibiting branching disintegrates by one mode only. The fraction disintegrating by a particular mode is the “branching fraction” for that mode. The “branching ratio” is the ratio of two specified branching fractions (also called multiple disintegration).
Bremsstrahlung
Secondary photon radiation produced by deceleration of charged particles passing through matter.
Buildup
The increase in absorbed dose with depth below the surface in a material irradiated by a beam of photons or particulate radiation. Buildup may be of two kinds: Electron Buildup: This is due to the production by the incident radiation of increasing numbers of forward-moving high-energy electrons increasing with depth until a maximum electron fluence rate has been reached. This effect gives rise to the phenomenon of “skin sparing” and is most marked for photon energies greater than about 400 keV. The effect is not noticeable for x-ray photons generated by potentials of less than 400 kV. For high-energy beams, this process is more important. Photon Buildup: Multiple photon scattering in the superficial layers of the phantom, which may lead to an increase in absorbed dose for a short distance. This effect is observed particularly with photons generated by potentials of 50 to 150 kV and large field sizes.
Buildup Factor
The ratio of the intensity of x or gamma radiation (both primary and scattered) at a point in an absorbing medium to the intensity of only the primary radiation. This factor has particular application for “broad beam” attenuation. “Intensity” may refer to energy flux, dose, or energy absorption.
Buncher
The input resonant cavity in a klystron or linear accelerator.
Burial Ground (Graveyard)
A place for burying unwanted radioactive objects to prevent escape of their radiations, the earth or water acting as a shield. Such objects must be placed in watertight, non-corrodible, containers so the radioactive material cannot leach out and invade underground water supplies.
Glossary: Bone Marrow
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Calibration
Determination of variation from standard, or accuracy, of a measuring instrument to ascertain necessary correction factors.
Calorie (cal)
Amount of heat necessary to raise the temperature of one gram of water 1°C (from 14.5 to 15.5°C).
Cancer
Any malignant neoplasm (Popular usage).
Capillary
A small, thin-walled blood vessel connecting an artery with a vein.
Capture, Electron
A mode of radioactive decay involving the capture of an orbital electron by its nucleus. Capture from a particular electron shell is designated as Kelectron capture, L-electron capture, etc.
Capture, K-Electron
Electron capture from the K shell by the nucleus of the atom. Also loosely used to designate any orbital electron process.
Capture, Radiative
The process by which a nucleus captures an incident particle and loses its excitation energy immediately by the emission of gamma radiation.
Capture, Resonance
An inelastic nuclear collision occurring when the nucleus exhibits a strong tendency to capture incident particles or photons of particular energies.
Carcinogenic
Capable of producing cancer
Carcinoma
Malignant neoplasm composed of epithelial cells, regardless of their derivation.
Card Cage
A chassis or frame that holds printed circuit boards.
Carrousel
An rotating assembly in the treatment head that places various elements, such as flattening filters and scattering foils, into the beam path.
Catalyst
A substance which alters the velocity of a chemical reaction (positive catalysts increase velocity) yet may be recovering practically unchanged after the reaction has occurred.
Cataract
A clouding of the crystalline lens of the eye which obstructs the passage of light.
Cathode
Negative electrode; electrode to which positive ions are attracted.
Cation
Positively charged ion.
Cell
(Biological) The fundamental unit of structure and function in organisms.
Cells, Somatic
Body cells, usually with two sets of chromosomes, as opposed to germ cells, which have only one set.
Central Axis
The straight line passing through the center of the target (or source) and the center of the final collimator. Concentric with the beam axis.
Chamber, Cloud
A device for observing the paths of ionizing particles. It is based on the principle that supersaturated vapor condenses more readily on ions than on neutral molecules.
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Glossary: Calibration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Chamber, Ionization
An instrument designed to measure a quantity of ionizing radiation in terms of the charge of electricity associated with ions produced within a defined volume. (See also Condenser r-Meter.) Air-Wall Ionization Chamber: Ionization chamber in which the materials of the wall and electrodes are so selected as to produce ionization essentially equivalent to that in a free-air ionization chamber. This ionization is possible only over limited ranges of photon energies. Such a chamber is more appropriately termed an air-equivalent ionization chamber. Extrapolation Ionization Chamber: An ionization chamber with electrodes whose spacing can be adjusted and accurately determined to permit extrapolation of its reading to zero chamber volume. Free-Air Ionization Chamber: An ionization chamber in which a delimited beam of radiation passes between the electrodes without striking them or other internal parts of the equipment. The electric field is maintained perpendicular to the electrodes in the collecting region. As a result, the ionized volume can be accurately determined from the dimensions of the collecting electrode and the limiting diaphragm. This ionization chamber is the basic standard instrument for x-ray dosimetry within the range of 5 to 1400 kVp. Standard Ionization Chamber: A specially constructed ionization chamber from which other ionization chambers can be calibrated. Thimble Ionization Chamber: A small cylindrical or spherical ionization chamber, usually with walls of organic material. Tissue Equivalent Ionization Chamber: An ionization chamber in which the material of the walls, electrodes, and gas are so selected as to produce ionization essentially equivalent to that characteristic of the tissue under consideration. In some cases it is sufficient to have only tissue equivalent walls, and the gas may be air, provided the air volume is negligible. The essential point in this case is that the contribution to the ionization in the air made by ionizing particles originating in the air is negligible, compared to that produced by ionizing particles characteristic of the wall material.
Chamber, Pocket
A small, pocket-sized ionization chamber used for monitoring radiation exposure of personnel. Before use, it is given a charge, and the amount of discharge is a measure of the radiation exposure.
Charger-Reader
An auxiliary device used for establishing a particular voltage level in an ionization chamber and subsequently for evaluating that voltage level.
Charge, Space
The electric charge carried by a cloud or stream of electrons or ions in a vacuum or a region of low gas pressure, when the charge is sufficient to produce local changes in the potential distribution. It is of importance in thermionic tubes, photoelectric cells, ion accelerators, etc.
Cerenkov Radiation
Blue light emitted when a charged particle moves in a transparent medium with a speed greater than that of light in the same medium.
Circuit, Anticoincidence
A circuit with two input terminals which delivers an output pulse if one input terminal receives a pulse, but delivers no output pulse if pulses are received by both input terminals simultaneously or within an assignable time interval.
Circuit Breaker
A switch that automatically interrupts an electrical circuit upon sensing an abnormal flow of electrical current.
Glossary: Chamber, Ionization
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Circuit, Coincidence
An electron circuit that produces a usable output pulse only when each of two or more input circuits receives pulses simultaneously or within an assignable time interval.
Circuit, Integrating
An electron circuit which records the total number of ions or events collected for a given time from which an average value for the number of ions or events per unit time can be found.
Cladding (Clad)
An external layer of material applied directly to nuclear fuel or other material to provide protection from a chemically reactive environment, to provide containment of radioactive products produced during the irradiation of the composite, or to provide structural support.
Clinical
Pertaining to the observed symptoms and cause of disease.
Coincidence
The occurrence of counts in two or more detectors simultaneously or within an assignable time interval. A true coincidence is one that is due to the incidence of a single particle or of several genetically related particles. An accidental, chance, or random coincidence is one that is due to the accidental occurrence of unrelated counts in the separate detectors. An anticoincidence is the occurrence of a count in a specified detector unaccompanied simultaneously or within an assignable time interval by a count in other specified detectors. A delayed coincidence is the occurrence of a count in one detector at a short, but measurable, time after a count in another detector. The two counts are due to genetically related occurrence, such as successive events in the same nucleus.
Collimator
A device for confining the elements of a beam within an assigned solid angle. Sets of metal blocks, fixed and movable, in the treatment head that limit the treatment field to the desired size.
Collimator Rotation Readout
A display that indicates the degrees of rotation of the collimator about the central axis.
Collision
Encounter between two subatomic particles (including photons) which changes the existing momentum and energy conditions. The products of the collision need not be the same as the initial systems. Elastic Collision: A collision in which no change occurs either in the internal energy of each participating system or in the sum of their kinetic energies of translation. Inelastic Collision: A collision in which changes occur both in the internal energy of one or more of the colliding systems and in the sums of the kinetic energies of translation before and after the collision.
Compensator
A slab of material placed in the treatment beam to compensate for unevenness of machine output or body contour.
Compton Effect
An attenuation process observed for x or gamma radiation in which an incident photon interacts with an orbital electron of an atom to produce a recoil electron and a scattered photon of energy less than the incident photon. (See also Absorption, Pair Production, and Photoelectric Effect.)
Condenser rMeter
An instrument consisting of an “air-wall” ionization chamber together with auxiliary equipment for charging and measuring its voltage. It is used as an
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Glossary: Circuit, Coincidence
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 integrating instrument for measuring the exposure of x or gamma radiation in roentgens (R). (See also Chamber, Ionization) Console Group
The Clinac operator interface and system control units, typically including a keyboard, display terminal, control console, console computer, and printer.
Contactor
A heavy duty relay used to control high-power electrical circuits.
Contamination
A foreign substance dispersed where it is undesirable: for example, unwanted electrons in the photon beam or unwanted photons in an electron beam.
Contamination, Radioactive
Deposition of radioactive material in any place where it is not desired, particularly where its presence may be harmful. The harm may be in vitiating an experiment or a procedure, or in endangering personnel.
Control Console
The Clinac system control unit, which includes operator controls for starting and stopping an exposure.
Controlled Area
A defined area in which the occupational exposure of personnel (to radiation) is under the supervision of the Radiation Protection Supervisor.
Control System
A coordinated group of components designed to exert a directing influence on other components. A system of apparatus for automatically controlling an accelerator by a servo system that adjusts the control elements to maintain the flux level near a desired value.
Corpuscle
A blood cell.
Corpuscular Emission, Associated
The full complement of secondary charged particles (usually limited to electrons) associated with an x-ray or gamma ray beam in its passage through air. The full complement of electrons is obtained after the radiation has traversed sufficient air to bring about equilibrium between the primary photons and secondary electrons. Electronic equilibrium with the secondary photons is intentionally excluded.
Coulomb (C)
Unit of quantity in current electricity. A quantity afforded by 1 ampere of current in 1 second flowing against 1 Ohm of resistance with a force of 1 Volt.
Count (Radiation Measurements)
The external indication of a device designed to enumerate ionizing events. It may refer to a single detected event or to the total number registered in a given period of time. The term often is erroneously used to designate a disintegration, ionizing event, or voltage pulse. Spurious Count: In a radiation counting device, a count caused by any agency other than radiation.
Counter, Gas Flow
A device in which an appropriate atmosphere is maintained in the counter tube by allowing a suitable gas to flow slowly through the sensitive volume.
Counter, GeigerMuller
Highly sensitive, gas filled radiation measuring device. It operates at voltages sufficiently high to produce avalanche ionization.
Glossary: Console Group
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Counter, Proportional
Gas-filled radiation detection device; the pulse produced is proportional to the number of ions formed in the gas by the primary ionization particle.
Counter, Scintillation
The combination of scintillator, photomultiplier tube, and associated circuits for counting light emissions produced in the phosphors.
Counting, Coincidence
A technique in which particular types of events are distinguished from background events by coincidence circuits, which register coincidences caused by the type of events under consideration.
Counting Ratemeter
An instrument that gives a continuous indication of the average rate of ionizing events.
Cross-Sectional Area (of an x-ray beam)
An area in the plane of the beam perpendicular to its direction of travel.
Curie
The former special unit of activity. One curie equals 3.7 × 1010nuclear transformations per second. Several fractions of the curie are in common usage. (Abbreviated Ci) Microcurie: One-millionth of a curie (3.7 × 104 disintegrations per second). Abbreviated Ci. Millicurie: One-thousandth of a curie (3.7 × 107 disintegrations per second). Abbreviated mCi. Picocurie: One-millionth of a microcurie (3.7 × 10–2 disintegrations per second or 2.22 disintegrations per minute). Abbreviated pCi; replaces the term µµCi.
Cutie Pie
An ionization chamber device commonly used for detecting radiation exposure rate.
Cyclotron
A particle accelerator in which charged particles receive repeated synchronized accelerations or “kicks” by electrical fields as the particles spiral outward from their source. The particles are kept in the spiral by a powerful magnet.
Daughter
Synonym for Offspring. (See Decay Product.)
Deadman Switch
Synonymous with Motion enable switch.
Decay, Radioactive
Disintegration of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons.
Decay Constant
The fraction of the number of atoms of a radioactive nuclide which decay in unit time. Symbol: λ . (See also Decay Curve and Disintegration Constant.)
Decay Curve
A curve showing the relative amount of radioactive substance remaining after any time interval.
Decay Product
A nuclide resulting from the radioactive disintegration of a radionuclide, formed either directly or as the result of successive transformations in a radioactive series. A decay product may be either radioactive or stable.
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Glossary: Counter, Proportional
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Decrement Lines
Imaginary lines drawn through parts where the absorbed energy (radiation dose) is a certain percent of the energy absorbed at the same depth along the central axis of the radiation beam.
Delta Ray
Any secondary ionizing particle ejected by recoil when a primary ionizing particle passes through matter.
Densitometer
Instrument utilizing a photocell to determine the degree of darkening of developed photographic film.
Density (Physical)
The mass per unit volume of a substance. Usually kg/m–3 or g/cc–l. (Symbol: p)
Depth Dose
A radiation dose at some specified depth in tissue relative to the dose at a fixed reference point on the beam axis. It is usually expressed as a percentage of surface dose.
DeQing
A circuit in the modulator cabinet that regulates the size of dc voltage pulses delivered by the high-voltage power supply to the pulse forming network.
Detector
An instrument capable of registering the presence of radiation. The two common modes of operation for a detector are: Mean-Level or Integrating: The average effect of the radiation is cumulated over time. Pulse-Type: Individual radiation interactions are separated or resolved in time.
Detector, Radiation
Any device for converting radiant energy to a form more suitable for observation. An instrument used to determine the presence, and sometimes the amount, of radiation.
Deuterium (D)
A heavy isotope of hydrogen with one proton and one neutron in the nucleus.
Deuteron
An isotopic form of hydrogen in which the nucleus contains one proton and one neutron. When deuterons are substituted for the common form of hydrogen in the water molecule, the substance is known as “heavy” water.
Directly Ionizing Particles
Charged particles such as alpha or beta particles which cause ionization of an atom without any intermediate interaction taking place.
Disintegration, Nuclear
A spontaneous nuclear transformation (radioactivity) characterized by the emission of energy and/or mass from the nucleus. When numbers of nuclei are involved, the process is characterized by a definite half-life.
Disintegration Constant
The fraction of the number of atoms of a radioactive nuclide which decay in – λt unit time; λ in the equation N = N 0 e , in which N0 is the initial number of atoms present, and N is the number of atoms present after some time, t. (See also Decay Constant.)
Dose
A general form denoting the quantity of radiation or energy absorbed. For special purposes it must be appropriately qualified. If unqualified, it refers to absorbed dose. (See also Maximum Permissible Dose.)
Glossary: Decrement Lines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Absorbed Dose: The energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The former unit of absorbed dose is the rad. One rad equals 100 ergs per gram. The new unit of absorbed dose is the gray. One gray equals 1 joule per kilogram. (See also Rad and Tissue Dose.) Cumulative Dose (Radiation): The total dose resulting from repeated exposures to radiation. Depth Dose: The radiation dose delivered at a particular depth beneath the surface of the body. It is usually expressed as a percentage of surface dose. Dose Distribution: The variation of dose in any region of an irradiated object. Dose Equivalent (H): A quantity used in radiation protection It expresses all radiations on a common scale for calculating the effective absorbed dose. It is defined as the product of the absorbed dose and certain modifying factors. (The former unit of dose equivalent is the rem. The new unit of dose equivalent is the Sievert [Sv].) Exit Dose: Dose of radiation at surface of body opposite to that on which the beam is incident. Integral Absorbed Dose (Volume Dose): A term used mainly in radiation biology to mean the total energy absorbed by an individual or other biological object or phantom during exposure to radiation. It is frequently obtained by integrating the absorbed dose with respect to mass throughout an irradiated region. It may be stated in joule or kilogram gray. Maximum Permissible Dose Equivalent (MPD): The greatest dose equivalent that a person or specified part thereof shall be allowed to receive in a given period of time. Median Lethal Dose (MLD): Dose of radiation that would be required to kill, within a specified period, 50% of the individuals in a large group of animals or organisms; also called LD50. Midline Absorbed Dose: The absorbed dose calculated or measured for a point in tissue and at the “midline” or “center” of the biological specimen, i.e., for the point lying equidistant from the exterior points on the specimen. The designation is for dosimetric purposes and implies no particular biological significance for the midline location. Percentage Depth Dose: The ratio expressed as a percentage, of the absorbed dose rate at a point, at depth along the beam axis, to the absorbed dose rate at a fixed reference point in the beam axis. Permissible Dose: The dose of radiation that an individual may receive within a specified period with expectation of no significantly harmful result. Skin Dose (Radiology): Absorbed dose at center of irradiation field on skin. It is the sum of the dose in air and scatter from body parts. Surface Dose: The absorbed dose delivered by a radiation beam anywhere the radiation passes through the superficial layer of the phantom or patient. Threshold Dose: The minimum absorbed dose that will produce a detectable degree of any given effect. Tissue Dose: Absorbed dose received by tissue in the region of interest, expressed in rads. (See also Absorbed Dose and Rad.)
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Glossary: Dose
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Dose Meter, Integrating Ionization chamber and measuring system designed for determining total radiation administered during an exposure. In medical radiology the chamber is usually designed to be placed on the patient’s skin. A device may be included to terminate the exposure when it has reached a desired value. Dose, Protraction
A method of administering radiation by delivering it continuously over a relatively long period at a low dose rate.
Dose Rate
Absorbed dose delivered per unit time.
Dose Ratemeter
Any instrument that measures radiation dose rate.
Dosimeter
Instrument to detect and measure accumulated radiation exposure. In common usage, a pencil-size ionization chamber with a self-reading electrometer, used for personnel monitoring.
Dosimetrist
An individual with training and knowledge in treatment planning. Under the supervision of a qualified radiological physicist, a dosimetrist makes dose calculations and assists in calibration and verification of dose distribution within the patient.
Dosimetry
The calculations, measurements, and other activities required for determining the radiation dose to be delivered.
Dosimetry Interlock
A machine condition is identified in which the ability of the Clinac to deliver or measure dose may be impaired.
Dosimetry, Photographic
Determination of cumulative radiation dose with photographic film and density measurement.
Drive Stand
The stationary unit on the Clinac that holds the gantry.
Dual Dosimetry
The use of two independent signals from the Clinac dosimeter proportional to the integrated dose. The primary channel is programmed to terminate the beam at the dose set by the operator. If the beam continues, the secondary channel is programmed to terminate the beam at a preset number of monitor units beyond the set dose.
Dyne
The unit of force which, when acting upon a mass of one gram, will produce an acceleration of one centimeter per second per second.
Eddy Current
An induced electric current circulating wholly within a mass of metal. Such currents are converted into heat, and thus cause serious waste.
Efficiency (Counters)
A measure of the probability that a count will be recorded when radiation is incident on a detector. Usage varies considerably, so it is well to ascertain which factors (e g., window transmission, sensitive volume, energy dependence) are included in a given case.
Electricity
One of the forces of nature developed by chemism, magnetism, or friction; also said to be electrons in motion.
Electrode
A conductor used to establish electric contact with a nonmetallic part of a circuit.
Glossary: Dose, Protraction
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Electrolyte
A substance capable of conducting an electric current and being decomposed by it.
Electromagnetic Interference (EMI)
Electromagnetic radiation that can degrade the performance of a radiotherapy accelerator or other nearby electronic equipment.
Electromagnetic Radiation
Transport of energy through space as a combination of an electric and magnetic field: for example, visible light and x-rays.
Electromagnetic wave
A wave produced by the oscillation of an electric charge.
Electrometer
Electrostatic instrument for measuring the difference in potential between two points. Used to measure change in electric potential of charged electrodes resulting from ionization produced by radiation.
Electromotive Force
Potential difference across electrodes tending to produce an electric current.
Electron
A stable elementary particle having an electric charge equal to –1.60 × 10– C and a rest mass equal to 9.1091 × 10–31 kg.
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Secondary Electron: An electron ejected from an atom, molecule, or surface as a result of an interaction with a charged particle or photon. Valence Electron: Electron that is gained lost, or shared in a chemical reaction. Electron Affinity
The tendency of a neutral atom to attract a free electron to itself.
Electron Beam Therapy
Treatment by electrons accelerated to high energies in a linear accelerator. Primarily used for lesions situated at or near the surface.
Electron Equilibrium
A condition established in a standard ionization chamber whereby the number of electrons entering a specified volume equals the number of electrons leaving that volume.
Electron Gun
A structure that injects electrons into the linear accelerator.
Electron Volt
A unit of energy equivalent to the energy gained by an electron in passing through a potential difference of 1 volt. Larger multiple units of the electron volt are frequently used: keV for thousand or kilo electron volts; MeV for million or mega electron volts. (Abbreviated: eV, 1 eV = 1.6 × 10–19 J.)
Electroscope
Instrument for detecting the presence of electric charges by the deflection of charged bodies. It has two metallic leaves hanging at the end of a very slender vane. When like charges are placed on the leaves, they move apart or repel. As the charge is reduced, the leaves move closer together until they are finally side by side when the charge has been reduced to zero.
Electrostatic Field
The region surrounding an electric charge in which another electric charge experiences a force.
Element
A category of atoms all of the same atomic number.
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Glossary: Electrolyte
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Emergency-off Circuit
An electrical circuit in the Clinac that causes all high power to be removed from the system whenever an emergency off button is pressed.
Energy
Capacity for doing work. “Potential energy” is the energy inherent in a mass because of its spatial relation to other masses. “Kinetic energy” is the energy possessed by a mass because of its motion; SI units: kg × m2 × S–2 or joules. Binding Energy: The energy represented by the difference in mass between the sum of the component parts and the actual mass of the nucleus. Excitation Energy: The energy required to change a system from its ground state to an excited state. Each different excited state has a different excitation energy. Ionizing Energy: The average energy lost by ionizing radiation in producing an ion pair in a gas.
Energy Dependence
The characteristic response of a radiation detector to a given range of radiation energies or wavelengths compared with the response of a standard free-air chamber.
Energy Fluence
The sum of the energies, exclusive of rest energies, of all particles passing through a unit cross-sectional area.
Energy Flux Density (Energy Fluence Rate)
The sum of the energies, exclusive of rest energies, of all particles passing through a unit cross-sectional area per unit time (energy fluence per unit of time).
Energy Imparted
See Dose, Integral Absorbed Dose.
Entrance Port
The area on the surface of a patient or a phantom on which a radiation beam is incident.
Enzyme
A biological catalyst of great specificity for a particular substance or a particular group of closely related substances which generally activates or accelerates a biochemical reaction.
Epidermis
The outermost layer of cells of the skin.
Epilation (Depilation)
The temporary or permanent removal or loss of hair.
Epithelium
A term applied to cell that line all canals and surfaces having communication with external air; also, cells specialized for secretion in certain glands as the liver, kidneys, etc.
Erg
Unit of work by a force of one dyne acting through a distance of one cm. Unit of energy which can exert a force of one dyne through a distance of one cm; ergs units: dyne-cm or gm-cm2/sec2.
Erythema
An abnormal redness of the skin due to distension of the capillaries with blood. It can be caused by many different agents: heat, drugs, ultraviolet rays, and ionizing radiation.
Erythrocyte
A red blood corpuscle.
Glossary: Emergency-off Circuit
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Eugenics
The science which deals with the influences that improve the hereditary qualities of a race or breed.
Excitation
The addition of energy to a system, thereby transferring it from its ground state to an excited state. Excitation of a nucleus, an atom, or a molecule can result from absorption of photons or from inelastic collisions with other particles.
Exit Dose
The absorbed dose at the point where the beam axis emerges from the patient.
Exposure
A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electric charges on all ions of one sign produced in air when all electrons liberated by photons in a volume element of air are completely stopped in air, divided by the mass of the air in the volume element. The former special unit of exposure is the roentgen. The new special unit of exposure is C · kg–1. Acute Exposure: Radiation exposure of short duration. Chronic Exposure: Radiation exposure of long duration.
Fallout
Radioactive debris from a nuclear detonation, which is airborne or has been deposited on the earth. Special forms of fallout are “Dry Fallout,” “Rainout,” and “Snowout.”
Field
A plane section of the beam perpendicular to the beam axis.
Field Block
A solid object of attenuating material used to shape a treatment beam.
Field Light
A light system that illuminates an area on the patient’s surface identifying the area of therapy beam entry.
Field Size
The size of an area irradiated by a given beam, usually measured by one of the following conventions: geometric field size, which measures the geometric projection on a plane perpendicular to the central axis, or physical field size, which measures the area included within the 50 percent maximum dose isodose curve at the depth of maximum dose.
Film Badge
A pack of photographic film that measures radiation exposure for personnel monitoring. The badge may contain two or three films of differing sensitivity and filters to shield parts of the film from certain types of radiation.
Film Ring
A film badge in the form of a finger ring.
Filter (Radiology)
Primary: A sheet of material, usually metal, placed in a beam of radiation to absorb preferentially the less penetrating components. Secondary: A sheet of material of low atomic number (relative to the primary filter) placed in the filtered beam of radiation produced by the primary filter.
Filtration, Inherent (Xrays)
The filter permanently in the useful beam; it includes the window of the xray tube and any permanent tube or source enclosure.
Fissile (Fissionable) Material
Any material readily fissioned by slow neutrons, for example, 235U, 239P.
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Glossary: Eugenics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Fission
The splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy and frequently one or more neutrons. Fission can occur spontaneously, but usually it is caused by the absorption of gamma-ray photons, neutrons, or other particles.
Flattening Filter
A cone-shaped attenuator placed in the x-ray beam to achieve uniform intensity over the specific treatment field at a specific depth and a specific energy.
Fluence
The number of particles passing through a unit cross-sectional area.
Fluorescence
The emission of radiation of particular wavelengths by a substance as a result of absorption of radiation of shorter wavelength. This emission occurs essentially only during the irradiation.
Fluorescent Screen
A sheet of material coated with a substance (such as calcium tungstate or zinc sulfide) which will emit visible light when irradiated with ionizing radiation.
Fluorography (photofluorography)
Photography of image produced on fluorescent screen by x or gamma radiation.
Fluoroscope
A fluorescent screen, suitably mounted with respect to an x-ray tube for ease of observation and protection, used for indirect visualization (by xrays) of internal organs in the body or internal structures in apparatus or in masses of material.
Flux Density (fluence rate)
The number of particles passing through a unit cross-sectional area per unit of time. (Fluence per unit of time.)
Focal Spot (Xrays)
The part of the target of the x-ray tube struck by the main electron stream.
Fractionation
A technique of administering radiation therapy in multiple doses over a number of days or weeks to achieve a maximum therapeutic ratio.
Frequency
In harmonic motions, the number of cycles, revolutions, or vibrations completed in a unit of time. (See also Hertz.)
Function Key Assignments
A bar on the bottom line of the screen indicating the command currently assigned to each function key on the keyboard.
Fusion, Nuclear
Act of coalescing two or more atomic nuclei.
Gamete
Either of the two germ cells (sperm or ovum).
Gamma-ray
Short wavelength electromagnetic radiation of nuclear origin (range of energy from 10 keV to 9 MeV) emitted from the nucleus.
Gantry
The entire rotating unit of the Clinac that emits the treatment beam. The upper part of the gantry includes linear accelerator, and the lower part contains a counterweight or a retractable beam stopper.
Glossary: Fission
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Gantry Rotation Readout
A display that indicates the degrees of rotation of the gantry about the isocenter.
Gas Amplification
As applied to gas ionization radiation detecting instruments, the ratio of the charge collected to the charge produced by the initial ionizing event.
Geiger Region
In an ionization radiation detector, the operating voltage interval in which the charge collected per ionizing event is essentially independent of the number of primary ions produced in the initial ionizing event.
Geiger Threshold
The lowest voltage applied to a counter tube for which the number of pulses produced in the counter tube is essentially the same, regardless of a limited voltage increase.
Geiger Tube
An ionization type radiation detector with a very high sensitivity for photons in the energy range 10 to 1000 keV.
Gene
Fundamental unit of inheritance which determines and controls hereditary transmissible characteristics. Genes are arranged linearly at definite loci on chromosomes.
Genetics
The branch of biology dealing with the phenomena of heredity and variation.
Generator (“Cow”)
A device in which a daughter radionuclide is eluted from an ion exchange column containing a parent radionuclide long lived compared to the daughter.
Genetic Effect of Radiation
Inheritable change, chiefly mutations, produced by the absorption of ionizing radiations. On the basis of present knowledge these effects are purely additive; recovery does not occur.
Genetically Significant Dose
That absorbed dose equivalent which, if received by every member of the population, would be expected to produce the same total genetic injury to the population as the actual absorbed dose equivalent received by various individuals.
Genotype
The fundamental hereditary (genetic) constitution of an organism.
Germ Cells
The cells of an organism whose function is reproduction.
Given Dose
The applied dose delivered by one beam in a complete treatment or in a treatment session.
Glove Box
An enclosure used for working with radionuclides particularly those in the form of powders and volatile liquids.
Gonad
A gamete-producing organ in animals; testis or ovary.
Gray (Gy)
The new unit of absorbed dose equal to 1 joule per kilogram in any medium. (See Absorbed Dose.)
Grenz Rays
X-rays produced at voltages of 5 to 20 kVp, intended primarily for surface therapy.
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Glossary: Gantry Rotation Readout
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Ground State
The state of a nucleus, atom, or molecule at its lowest energy. All other states are “excited.”
Half-Life
A general term used to describe the time elapsed until some physical quantity has decreased to half of its original value. Here the concept of half-life will be applied to radionuclides. Half-Life, Biologic: The time required for the body to eliminate one-half of an administered dosage of any substance by regular processes of elimination. Approximately the same for both stable and radioactive isotopes of a particular element. Half-Life, Effective: Time required for a radioactive element in an animal body to be diminished 50% as a result of the combined action of radioactive decay and biologic elimination. Biological half-life × Radioactive half-life Effective half life = -----------------------------------------------------------------------------------------------------------------Biological half-life + Radioactive half-life Half-Life, Radioactive: Time required for a radioactive substance to lose 50% of its activity by decay. Each radionuclide has a unique half-life.
Half-Value Layer (Half Value Thickness) (HVL)
The thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduces the exposure rate by one-half.
Hand Pendant
A hand held control device that allows the operator to adjust the treatment couch, collimator, and gantry for a patient.
Hardness (Xrays)
A relative specification of the quality of penetrating power of x-rays. In general, the shorter the wavelength the harder the radiation.
Health, Radiologic
The art and science of protecting human beings from injury by radiation, and promoting better health through beneficial applications of radiation.
Heat Exchanger
A cooling device that uses city water to carry off the heat generated by certain Clinac systems.
Heel Effect
The cathode end of the x-ray tube has a slightly visible tendency to make a more dense image than has the anode end.
Heredity
Transmission of characters and traits from parent to offspring.
Hertz
Unit of frequency equal to 1 cycle per second. (See also Frequency.)
Heterogeneous Radiation
Beam of x-rays consisting of many x-rays of different wavelengths.
Highlight
A reverse-video bar shown on the monitor screen to identify a choice in a window or an input space.
Hysteresis
A lagging or retardation of the effect. The magnetization of a piece of iron or steel due to a magnetic field that is made to vary through a cycle of values; lags behind the field.
Glossary: Ground State
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Immunity
The power which a living organism possesses to resist and overcome infection.
Implant (Radiology)
Encapsulated radioactive material embedded in a tissue for therapy. It may be permanent (seed) or temporary (needle).
Indirectly Ionizing Particles
Particles that cause ionization to occur only after an intermediate interaction producing a charged particle has taken place.
Infrared Radiation
Invisible thermal radiation whose wavelength is longer than the red segment of the visible spectrum.
Input Space
A reverse-video box on the monitor screen that provides a space for the operator to enter input to the system.
Integral Dose
A measure of the total energy absorbed by a patient or object during exposure to radiation.
Integrating Circuit
An electronic circuit that records the total number of ions or events collected for a given time from which an average value for the number of ions or events per unit time can be found.
Intensification Factor
The quantity of intensification, expressed numerically as light energy, of the applied source of energy when it passes through the screen emulsion.
Intensifying Screen
Sheet of cardboard or other substance coated with fluorescent material, placed in contact with the film in radiography. The x or gamma rays excite the fluorescent substance. The light thus emitted adds to the radiation effect on the film and produces an image of greater density for a given exposure. Sheets of thin lead may be used in industrial radiography and radiation therapy with very high energy radiation. In this case, the increased effect is due largely to secondary electrons and x-rays emitted by the lead.
Intensity
Amount of energy per unit time passing through a unit area perpendicular to the line of propagation at the point in question.
Interlock
A electrical circuit or mechanical device to prevent operation or application of power until the circuit or device is placed in a certain state.
International Commission on Radiological Protection
(ICRP) An international organization, founded in 1928, and supported financially by the World Health Organization (WHO), the International Atomic Energy Agency (IAEA), the United Nations Environment Program the International Society of Radiology, and others, which operates under rules approved by the International Congress of Radiology. Members of ICRP are selected from nominations submitted to it by the National Delegations to the International Congress of Radiology and by the ICRP itself. The International Executive Committee of the Congress approves the selections.
International Commission on Radiation Units and Measurements
(ICRU) An international organization, founded in 1925, with the principle objective of development of internationally acceptable recommendations regarding: (l) quantities and units of radiation and radioactivity, (2) procedures suitable for measurement and application of these quantities in clinical radiology and radiobiology, and (3) physical data needed in the application of these procedures, the use of which, tends to assure uniformity in reporting. The ICRU works closely with the ICRP in its consider-
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Glossary: Immunity
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ation of and recommendations for the radiation protection field. Financially, ICRU is supported by the United States National Institutes of Health and many national and international societies, foundations, and companies. International System of Units (SI)
The standard metric system of measurement adopted in 1975 for worldwide use. SI units commonly used in radiotherapy include the gray (measures absorbed dose), sievert (measures the dose equivalent), coulomb per kilogram (measures exposure), and the becquerel (measures the disintegration rate of a radionuclide).
Inverse Square Law
1. A rule relating two physical entities by a particular proportionality constant. This constant is one divided by the square of some other physical quantity, usually the distance between the two physical entities. 2. A formula for the relationship that the intensity of radiation is inversely proportional to the square of the distance from a point source.
Ion
Atomic particle, atom, or chemical radical bearing an electric charge, either negative or positive.
Ion Chamber
See Monitor ion chamber.
Ion Pair
Two particles of opposite charge, usually referring to the electron and positive atomic or molecular residue resulting after the interaction of ionizing radiation with the orbital electrons of atoms.
Ionization
The process by which a neutral atom or molecule acquires a positive or negative charge. Primary Ionization: (l) In collision theory: the ionization produced by the primary particles as contrasted to the “total ionization,” which includes the “secondary ionization” produced by delta rays. (2) In counter tubes: The total ionization produced by incident radiation without gas amplification. Secondary Ionization: Ionization produced by delta rays. Specific Ionization: Number of ion pairs per unit length of path of ionizing radiation in a medium, e.g., per centimeter of air or per micron of tissue. Total Ionization: The total electric charge of one sign on the ions produced by radiation in the process of losing its kinetic energy. For a given gas, the total ionization is closely proportional to the initial ionization and is nearly independent of the nature of the ionizing radiation. It is frequently used as a measure of radiation energy.
Ionization Density
Number of ion pairs per unit volume.
Ionization Path (Track)
The trail of ion pairs produced by an ionizing radiation in its passage through matter.
Ionization Potential
The potential necessary to separate one electron from an atom, resulting in the formation of an ion pair.
Ionizing Event
Any occurrence of a process in which an ion or group of ions is produced.
Ion Pump
A vacuum pump that maintains the high vacuum in the accelerator by removing gas molecules.
Glossary: International System of Units (SI)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Irradiation
Exposure to radiation.
Isobar
One of two or more chemical elements that have the same mass number but different atomic numbers.
Isocenter
The intersection of the gantry axis of rotation and the collimator bearing axis.
Isodose Curve
A line connecting points of equal radiation doses.
Isodose Chart
A graphic display containing a series of isodose curves that maps out the relative intensities of a radiation field in a phantom or patient.
Isomers
Nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. Commonly, the isomer of higher energy decays to one with lower energy by the process of isometric transition.
Isotones
Nuclides having the same number of neutrons in their nuclei.
Isotopes
Nuclides having the same number of protons in their nuclei, and hence the same atomic number, but differing in the number of neutrons, and therefore in the mass number. Almost identical chemical properties exist between isotopes of a particular element. The term should not be used as a synonym for nuclide. Stable Isotope: A nonradioactive isotope of an element.
Joule
The unit for work and energy, equal to 1 Newton expended along a distance of 1 meter (1J = 1N × 1M)
Kerma (kinetic energy released per unit mass)
The kinetic energy of charged ionizing particles liberated per unit mass of specified material by uncharged ionizing particles such as photons and neutrons. Kerma is measured in the same units as absorbed dose, joule per kilogram (J/kg-1) and its special name is gray (Gy). Kerma can be quoted for any specified material at a point in free space or in an absorbing medium. Since air kerma and tissue kerma differ by less than 10% over a wide range of photon energies, these two may be considered equal in magnitude for radiation protection purposes. In this respect, air kerma means air kerma in air. Kerma is independent of the complexities of geometry of the irradiated mass element, and permits, therefore, specification for photons or neutrons in free space or in an absorbing medium and hence has a wider applicability than exposure.
Keylock Switch or Keyswitch
A rotary switch that requires a special key to operate. Similar to an ignition switch on a car.
Kilo Electron Volt (keV)
One thousand electron volts, 103 eV.
Kilovolt (kV)
A unit of electric potential difference, equal to 1000 volts.
Kilovolt Constant (kVcp)
The value in kilovolts of the potential difference of a constant potential generator.
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Glossary: Irradiation
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Kilovolt Peak (kVp)
The maximum value in kilovolts of the potential difference of a pulsating potential generator. When only half the wave is used, the value refers to the useful half of the cycle.
Laboratory Monitor
(See Survey Meter.)
Laser
A device for transforming incoherent light of various frequencies into a very narrow, intense beam of coherent light.
Latent Image
Development occurring between the time of exposure of a film to radiation and the processing of that film.
Latent Period
The period or state of seeming inactivity between the time of exposure of tissue to an injurious agent and response.
LD50 (Radiation Dose)
Dose of radiation required to kill, within a specified period, 50 percent of the individuals in a large group of animals or organisms. Also called the Median Lethal Dose.
Lead Equivalent
The thickness of lead affording the same attenuation, under specified conditions, as the material in question.
Lesion
A hurt, wound, or local degeneration.
Leukemia
A disease in which there is great over-production of white blood cells, or a relative over-production of immature white cells, and great enlargement of the spleen. The disease is variable, at times running a more chronic course in adults that in children. It can be produced in some animals by long continued exposure to low doses of ionizing radiation.
Linear Accelerator Structure
A linear series of adjacent cylindrical microwave resonant cavities (called the guide) in which charged particles are accelerated by applying a highfrequency voltage during the particle transit inside the structure.
Linear Energy Transfer (LET)
The quotient of dE by dL, in which dL is the distance traversed by a particle and dE is the average energy loss in dL due to collisions with energy transfers less than some specific value. Simply, it is a conventional expression for energy deposition measured along the track of an ionizing particle. Gamma and x-ray photons generate low LET electron tracks. Natural alpha particles and fast neutrons and protons give high-LET tracks.
Localization Films
X-ray films taken with radiopaque markers to define the tumor position relative to external markings.
Major Interlock
A machine condition is identified that could damage the Clinac if not corrected.
Mass
The material equivalent of energy; different from weight in that it neither increases nor decreases with gravitational force.
Mass Numbers
The number of nucleons (protons and neutrons) in the nucleus of an atom. (Symbol: A)
Glossary: Kilovolt Peak (kVp)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Maximum Permissible Dose (MPD)
(See Dose.)
Mean Free Path
The average distance that particles of a specified type travel before a specified type (or types) of interaction in a given medium. The mean free path may thus be specified for all interactions (i.e., total mean free path) or for particular types of interaction such as scattering, capture, or ionization.
Mean Life
The average lifetime for an atomic or nuclear system in a specified state. For an exponentially decaying system the average time for the number of atoms or nuclei in a specified state to decrease by a factor of e (2.718).
Mega Electron Volt (MeV)
One million electron volts, 106 eV.
Meson
One of a class of medium-mass, short lived elementary particles with a mass between that of the electron and that of the proton. Examples: Pi mesons (pions) and K mesons (kaons)
Metabolism
The sum of all physical and chemical processes by which living organized substance is produced and maintained and by which energy is made available foe the uses of the organism.
Metastasis
The transfer in the body of malignant neoplastic cells from the original or parent site to one more distant.
Micron (µ)
Unit of length equal to 10–6 meters.
Microwave
Radio waves in the frequency range of approximately 1000 megahertz and upward.
Milliampere
A unit of current. Generally the current flowing between the filament and anode of an x-ray tube is stated in this unit.
Milliroentgen (mR)
A submultiple of the Roentgen, equal to one one-thousandth of a Roentgen.
Minor Interlock
A machine condition is identified that prevents Clinac beam-on, but the condition is normally user-correctable.
Modulator
A system in the Clinac that generates a succession of short pulses of high current and voltage for operating the klystron.
Molecule
Smallest quantity of a compound which can exist by itself and retain all properties of the original substance.
Molybdenum Breakthrough
This term refers to the amount of parent nuclide, molybdenum, contained in an eluted sample of its offspring 99mTc. 37 kBq (1 µCi) of Mo is allowed per 37 MBq (l mCi) of 99mTc eluate. However, no more than 185 kBq (5 µCi) of Mo are allowed per patient dose.
Momentum
The product of the mass of a body and its velocity; SI units, kg × m · s–l.
Monitor
A cathode-ray tube device used to view data produced by a computer.
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Glossary: Maximum Permissible Dose (MPD)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Monitoring
Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in an occupied region. Area Monitoring: Routine monitoring of the radiation level or contamination of a particular area, building, room, or equipment. Some laboratories or operations distinguish between routine monitoring and survey activities. Personnel Monitoring: Monitoring any part of an individual, his breath, or excretions, or any part of his clothing.
Monitor Ion Chamber
A special radiation measuring device in which the collected electrical charge from ionization in a gas filled cavity is taken to be proportional to some parameter (for example, dose or exposure) or radiation field.
Monitor Unit (MU)
A unit of radiation exposure. A table for the conversion of monitor units into units of absorbed dose (gray or rad) can be generated by a dose calibration of the machine by a qualified physicist.
Monoenergetic
Having only one energy associated with it.
Monte Carlo Method
A method permitting the solution by means of a computer of problems of particle physics, such as those of neutron transport, by determining the history of a large number of elementary events by the application of the mathematical theory of random variables.
Motion Enable Switch
A safety switch that allows motion of certain motorized functions only so long as the operator continues to press the switch.
Multiple-port Treatment
Directing more than one radiation beam toward the tumor from different angles for the purpose of increasing the dose without irrevocably destroying normal tissue.
Mutation
Alteration of the usual hereditary pattern, usually sudden.
National Council on Radiation Protection and Measurements (NCRP)
This committee was granted a United States Congressional charter in 1964. It is operated as in independent organization financed by contributions from government, scientific societies, and manufacturing associations.
Natural (Napierian) Logarithms
A system of logarithms using the base e.
Negative Ion
Negative charged ion; commonly termed “anion.”
Neoplasm
Any new and abnormal growth, such as a tumor; “neoplastic disease” refers to any disease that forms tumors, whether malignant or benign.
Neutrino
A neutral particle of very small rest mass originally postulated to account for the continuous distribution of energy among particles in the beta decay process.
Neutron
An electrically neutral or uncharged particle of matter existing along with protons in the atoms of all elements except the mass 1 isotope of hydrogen. The isolated neutron is unstable and decays with a half-life of about 13 minutes into an electron, proton, and neutrino. Neutrons sustain the fission chain reaction in a nuclear reactor.
Glossary: Monitoring
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Epithermal Neutron: A neutron having an energy between 0.5 and 1O5 eV. (Sometimes the energy range is given as 0.5 to 100 eV and the energy range from 100 eV to 105 eV is called Intermediate.) Fast Neutron: A neutron having an energy above 0.1 MeV (105 eV). Thermal Neutron: A neutron having an energy of about 0.025 eV which corresponds to a velocity of 2200 m/s. Newton
The unit of force that, when applied to a 1 kilogram mass, will give it an acceleration of 1 meter per second per second. (1 N = 1 kg × 1m · 1s–2).
Nomogram
Conversion scale between two sets of units.
Nonionizing Radiation
Radiation that does not cause ionization when it interacts with matter.
Nuclear Reaction
See Reaction, Nuclear.
Nuclear Reactor
A device by means of which a fission chain reaction can be initiated, maintained, and controlled. Its essential component is a core with fissionable fuel. It usually has a moderator, a reflector, shielding, and control mechanisms. Thermal Nuclear Reactor: A nuclear reactor in which the fission chain reaction is sustained primarily by thermal neutrons. Most existing reactors are thermal reactors.
Nucleon
Common name for a constituent particle of the nucleus. Commonly applied to a proton or neutron.
Nucleus (Nuclear)
That part of an atom in which the total positive electric charge and most of the mass is concentrated.
Nuclide
A species of atom characterized by the constitution of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content; or, alteratively, by the atomic number (Z), mass number A = (N + Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable time. Thus nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not so considered.
Occupational Exposure
The exposure of an individual to ionizing radiation because the occupation of the individual includes duties or activities that necessarily involve the likelihood of exposure.
Ohm
The unit of electric resistance.
Oncology
Preferred name for tumor treatment. (See Therapy, Radiation Therapy.)
Offspring
Synonym for Daughter. (See Decay Product.)
Operating Software
The integrated collection of programs used by the Clinac system computer to interface the system with the operator and control the machine.
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Glossary: Newton
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Organ
Group of tissues which together perform one or more definite functions in a living body.
Osmosis
The passage of pure solvent from the lesser to the greater concentration when two solutions are separated by a membrane which selectively prevents the passage of solute molecules, but is permeable to the solvent.
Osmotic
Pertaining to osmosis.
Owner
A person or organization having title to or administrative control over one or more radiotherapy installations.
Ozone
A gas produced by an electrical discharge in ordinary oxygen or air. Pure ozone is an unstable, faintly bluish gas with a characteristically fresh, penetrating odor.
Pair Production
An absorption process for x and gamma radiation in which the incident photon is annihilated in the vicinity of the nucleus of the absorbing atom with subsequent production of an electron and positron pair. This reaction only occurs for incident photon energies exceeding 1.02 MeV. (See also Absorption, Compton Effect, and Photoelectric Effect.)
Parent
A radionuclide that, upon disintegration, yields a specific nuclide — either directly or as a later member of a radioactive series.
Password
A number or word used to gain entry to a certain program or a restricted part of a program.
Path, Mean Free
Average distance a particle travels between collisions.
Patient Support Assembly (PSA)
See Treatment couch.
Penumbra
The region, at the edge of a radiation beam, over which the absorbed dose rate changes rapidly as a function of distance from the axis. It may be defined geometrically and dosimetrically. Geometric Penumbra: That region in space which could be irradiated by primary photons or particles coming from part of the source only. By analogy, the transmission penumbra is the region irradiated by photons or particles which have traversed part of the thickness of the collimator, i.e., at its outer edge. Geometric Penumbra Width: The width of the geometric penumbra in a plane perpendicular to the beam axis at any distance of interest from the source. It is a geometrical concept only and is calculated from the expression W = c(SSD + d – SCD)/SCD in which c is the source diameter (or effective diameter), SSD + d is the distance from the source to point of interest, and SCD is the distance from the source to the edge of the collimator. Physical Penumbra: This is a dosimetric concept; the physical penumbra width is the lateral distance between two specified isodose curves at a specified depth.
Glossary: Organ
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Periodic Table
An arrangement of chemical elements in order of increasing atomic number. Elements of similar properties are placed one under the other, yielding groups and families of elements. Within each group, a gradation of chemical and physical properties exists but, in general, chemical behavior is similar. From group to group, however, a progressive shift of chemical behavior occurs from one end of the table to the other.
Permeable
Affording passage or penetration.
Personnel Monitor
A dosimeter (usually a film badge, thermoluminescent device, or ionization chamber) used for determining the exposure to an individual. Such monitoring is required for all persons who are radiation workers.
Phantom
A volume of material approximating as closely as possible the density and effective atomic number of tissue. Ideally a phantom should behave in respect to absorption of radiation in the same manner as tissue. Radiation dose measurements made within or on a phantom provide a means of determining the radiation dose within or on a body under similar exposure conditions. Some materials commonly used in phantoms are water, perspex polystyrene, Masonite, pressed wood, and beeswax.
Phosphorescence
Emission of radiation by a substance as a result of previous absorption of radiation of shorter wavelength. In contrast to fluorescence, the emission may continue for a considerable time after cessation of the exciting irradiation.
Photoelectric Effect
Process by which a photon ejects an electron from an atom. All energy of the photon is absorbed in ejecting the electron and in imparting kinetic energy to it. (See also Absorption, Compton Effect, and Pair Production.)
Photon
A quantity of electromagnetic energy (E) whose value in joules is the product of its frequency (v) in hertz and Planck’s constant (h). The equation is: E = hv. (See also Radiation.)
Photosynthesis
The production of carbohydrates by green plants in the presence of sunlight through the agency of chlorophyll.
Physics, Health
A science and profession devoted to the protection of man and his environment from unnecessary radiation exposure.
Pig
A lead-lined container used for storing radionuclides.
Planck’s Constant
A natural constant of proportionality (h) relating the frequency of a quantum of energy to the total energy of the quantum. – 34 h = E --- = 6.6256 × 10 J ⋅ s v
Plateau
As applied to radiation detector chambers, the level portion of the counting rate-voltage curve where changes in operating voltage introduce minimum changes in the counting rate.
Polycythemia
A disease characterized by overproduction of red blood cells.
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Glossary: Periodic Table
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Port Film Exposures
A radiograph taken with the patient interposed between the machine portal and an x-ray film. The purpose is to demonstrate that the treatment field on the patient adequately encompasses the treatment volume and at the same time avoids adjacent critical structures.
Positive Ion
Positively charged ion; commonly called cation.
Positron
Particle equal in mass to the electron and having an equal but positive charge.
Potential Difference
Work required to carry a unit positive charge from one point to another.
Potential Ionization
The potential necessary to separate one electron from an atom, resulting in the formation of an ion pair.
Power, Stopping
A measure of the effect of a substance upon the kinetic energy of a charged particle passing through it.
Printed Circuit Board (PCB)
A sandwich-like set of insulated boards onto which circuits are etched and various components are soldered.
Proportional Region
Voltage range in which the gas amplification is greater than one, and in which the charge collected is proportional to the charge produced by the initial ionizing event.
Proton
Elementary nuclear particle with a positive electric charge equal numerically to the charge of the electron and a rest mass of 1.67474 × 10–27 kg.
Pulse Forming Network (PFN)
An electrical circuit in the modulator that supplies accurately shaped pulses of high voltage necessary for klystron operation.
Pulse Height Selector
A circuit designed to select and pass voltage pulses in a certain range of amplitudes.
Pulse Repetition Frequency (PRF)
A signal applied to the thyratron tube that causes the capacitors in the pulse-forming network to discharge. This provides a nearly flat high-power dc pulse to the electron gun and the klystron of the accelerator.
Purpura
Large hemorrhagic spots in or under the skin or mucous tissues.
Quality (Radiology)
The characteristic spectral-energy distribution of x radiation. It is usually expressed in terms of effective wavelengths of half-value layers of a suitable material; e.g., up to 20 kV, cellophane; 20 to 120 kVp, aluminum; 120 to 400 kVp, copper; over 400 kVp, tin.
Quality Factor (Q)
The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that expresses, on a common scale for all ionizing radiations, the effectiveness of the absorbed dose.
Quantum
An observable quantity is said to be “quantized” when its magnitude is, in some or all of its range, restricted to a discrete set of values. If the magnitude of the quantity is always a multiple of a definite unit, that unit is called the quantum (of the quantity). For example, the quantum or unit of orbital angular momentum is h, and the quantum of energy of electromag-
Glossary: Port Film Exposures
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 netic radiation of frequency v is hv. In field theories, a field (or the field equations) is quantized by application of a proper quantum mechanical procedure. This quantization results in the existence of a fundamental field particle, which may be called the field quantum. Thus, the photon is a quantum of the electromagnetic field, and in nuclear field theories the meson is considered the quantum of the nuclear field. Quantum Theory
The concept that energy is radiated intermittently in units of definite magnitude called quanta, and absorbed in a like manner.
Quenching
The process of inhibiting continuous or multiple discharge in a counter tube which uses gas amplification.
Quenching Vapor
Polyatomic gas used in Geiger-Muller counters to quench or extinguish avalanche ionization.
Rad
The former unit of absorbed dose equal to 0.01 joule per kilogram in any medium. (See also Absorbed Dose and Tissue Dose)
Radiant Energy
The energy of electromagnetic radiation, such as radio waves, visible light, x and gamma rays.
Radiation
(l) The emission and propagation of energy through space or through a material medium in the form of waves; for instance, the emission and propagation of electromagnetic waves, or of sound and elastic waves. (2) The energy propagated through space or through a material medium as waves; for example, energy in the form of electromagnetic waves or elastic waves. The term radiation or radiant energy, when unqualified, usually refers to electromagnetic radiation. Such radiation commonly is classified, according to frequency, as hertzian, infrared, visible (light), ultraviolet and x-ray or gamma-ray. (See also Photon.) (3) By extension, corpuscular emissions, such as alpha and beta radiation, or rays of mixed or unknown type, as cosmic radiation. Annihilation Radiation: Photons produced when an electron and a positron unite and cease to exist. The annihilation of a positron-electron pair results in the production of two photons, each of 0.511 MeV energy. Background Radiation: Radiation arising from radioactive material other than the one directly under consideration. Background radiation due to cosmic rays and natural radioactivity is always present. Background radiation may also be due to the presence of radioactive substances in other parts of the building or in the building material itself. Characteristic (Discrete) Radiation: Radiation originating from an atom after removal of an electron or excitation of the nucleus. The wavelength of the emitted radiation is specific, depending only on the nuclide and particular energy levels involved. External Radiation: Radiation from a source outside the body — the radiation must penetrate the skin. Internal Radiation: Radiation from a source within the body (as a result of deposition of radionuclides in body tissues). Ionizing Radiation: Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter.
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Glossary: Quantum Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Leakage (Direct) Radiation: All radiation coming from the source housing except the useful (primary) beam. Monochromatic Radiation: Electromagnetic radiation of a single wavelength, or radiation in which all the photons have the same energy. Monoenergetic Radiation: Radiation of a given type (e.g., alpha, beta, neutron, gamma) in which all particles or photons originate with and have the same energy. Primary Radiation: The useful beam of an x-ray tube. Scattered Radiation: Radiation that, during its passage through a substance, has been deviated in direction. It may also have been modified by a decrease in energy. Secondary Radiation: Radiation resulting from absorption of other radiation in matter. It may be either electromagnetic or particulate. Radiation Beam
The flow of therapeutically useful radiation energy through a defined area.
Radiation Protection Survey
An evaluation of the radiation hazards in and around an installation.
Radiation Oncologist
A physician who has received specific training and experience in therapeutic radiology.
Radiation Sickness
(Radiation Therapy): A self-limiting syndrome characterized by nausea, vomiting, diarrhea, and psychic depression, following exposure to appreciable doses of ionizing radiation, particularly to the abdominal region. It usually appears a few hours after irradiation and may subside within a day. It may be sufficiently severe to necessitate interrupting the treatment series or to incapacitate the patient. (General): The syndrome associated with intense acute exposure to ionizing radiations.
Radiation Therapist
An individual who has received specific training in radiation therapy technology and who is certified by a recognized specialty board as being competent in radiation therapy technology.
Radiation Therapy
Treatment of disease with any type of radiation.
Radioactivity
The property of certain nuclides of (l) spontaneously emitting particles or gamma radiation or (2) emitting x radiation following orbital electron capture or (3) undergoing spontaneous fission. Artificial Radioactivity: Man-made radioactivity produced by particle bombardment or electromagnetic irradiation, as opposed to natural radioactivity. Induced Radioactivity: Radioactivity produced in a substance after bombardment with neutrons or other particles. The resulting activity is “natural radioactivity” if formed by nuclear reactions occurring in nature, and “artificial radioactivity” if the reactions are caused by man. Natural Radioactivity: The property of radioactivity exhibited by more than 50 naturally occurring radionuclides.
Glossary: Radiation Beam
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Radiobiology
That branch of biology which deals with the effects of radiation on biological systems.
Radio Frequency (rf)
Any frequency at which coherent electromagnetic radiation of energy is possible. Usually considered to denote frequency above 150 kilohertz and extending up to the infrared range.
Radiography
The making of shadow images on photographic emulsion by the action of ionizing radiation. The image is the result of the differential attenuation of the radiation in its passage through the object being radiographed.
Radiological Physicist
An individual who devotes the majority of occupational time to the physics of radiology, including therapeutic radiological physics, diagnostic radiological physics, and medical nuclear physics.
Radiology
That branch of medicine which deals with the diagnostic and therapeutic applications of radiant energy, including x-rays and radionuclides.
Radionuclide
A nuclide that displays the property of radioactivity.
Radiopharmaceutical
A pharmaceutical compound that has been tagged with a radionuclide.
Radioresistance
Relative resistance of cells, tissues, organs, or organisms to the injurious action of radiation. The term may also apply to chemical compounds or to any substances.
Radiosensitivity
Relative susceptibility of cells, tissues, organs, organisms, or any living substances to the injurious action of radiation. Radioresistance and radiosensitivity are currently used in a comparative sense, rather than in an absolute one.
Radiotherapy Accelerator Service Technician
An individual with the following minimum qualifications: training equivalent to an associate’s degree in electronics, training in servicing and maintaining the machine, and a demonstrated understanding of the emergency and safety regulations adopted by the owner of the accelerator.
Range
The depth in any material measured from the entrance of an ionizing particle to the stopping position of that particle after it has lost all of its energy.
Rare Earth
Any of the series of very similar metals ranging in atomic numbers from 57 through 71.
Reaction (Nuclear)
An induced nuclear disintegration, i.e., a process occurring when a nucleus comes in contact with a photon, an elementary particle, or another nucleus. In many cases the reaction can be represented by the symbolic equation: X + a Y + b or, in abbreviated form, X(a,b) Y. X is the target nucleus, a is the incident particle or photon, b is an emitted particle or photon, and Y is the product nucleus.
Recombination
The return of an ionized atom or molecule to the neutral state.
Recovery Rate
The rate at which recovery takes place after radiation injury. It may proceed at different rates for different tissues. “Differential recovery rate:” Among tissues recovering at different rates, those having slower rates will ultimately suffer greater damage from a series of successive irradiations. This
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Glossary: Radiobiology
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 differential effect is considered in fractionated radiation therapy if the neoplastic tissues have a slower recovery rate than surrounding normal structures. Regenerative Process
The process by which damage or destroyed cells are replaced by new ones of the same type.
Relative Biologic Effectiveness (RBE)
The factor used to compare the biologic effectiveness of absorbed radiation doses (i.e., rads) due to different types of ionizing radiation; more specifically, it is the experimentally determined ratio of an absorbed dose of radiation in question to the absorbed dose of a reference radiation required to produce an identical biologic effect in a particular experimental organism or tissue. This term should not be used in radiation protection. (See also Quality Factor.)
Rem
A special unit of dose equivalent. The dose equivalent in rem is numerically equal to the absorbed dose in rad multiplied by the quality factor, the distribution factor, and any other necessary modifying factors.
Repair
The partial or complete restoration of functional integrity in cells following damage caused by radiation. Operationally, repair means that after irradiation a cell responds as though it had received a smaller dose than under conditions in which damage is more fully expressed. The ability to observe repair implies, therefore, that a comparison is made with a treatment of reference. Full repair indicates that cells respond as though they had not been previously irradiated. (Repair embraces processes sometimes referred to as bypassing of damage, shedding of damage, compensating for damage, elimination of damage, and/or the specific biochemical reversal of damage.)
Resolving Time, Counter
The minimum time interval between two distinct events which will permit both to be counted. It may refer to an electronic circuit, a mechanical indicating device, or a counter tube.
Rest Mass
The intrinsic mass of any physical entity; the mass possessed by that entity apart from any motion it may have.
Roentgen (R)
The special unit of exposure. One roentgen equals 2.58 × 10–4 coulomb per kilogram of air. (See also Exposure.)
Roentgenography
Radiography by means of x-rays.
Roentgenology
That part of radiology which pertains to x-rays.
Roentgen Rays
X-rays.
Scattering
Change of direction of subatomic particles or photons as a result of a collision or interaction. Coherent Scattering: Scattering of photons or particles in which definite phase relationships exist between the incoming and the scattered waves. Coherence manifests itself in the interference between the waves scattered
Glossary: Regenerative Process
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 by two or more scattering centers. An example is the Bragg scattering of xrays and neutrons by the regularly spaced atoms in a crystal, for which constructive interference occurs only at definite angles, called “Bragg angles.” Compton Scattering: The scattering of a photon by an electron. Part of the energy and momentum of the incident photon is transferred to the electron, and the remaining part is carried away by the scattered photon. Elastic Scattering: Scattering caused by elastic collisions and, therefore, conserving kinetic energy of the system. Rayleigh scattering is a form of elastic scattering. Incoherent Scattering: Scattering of photons or particles in which the scattering elements act independently of one another; no definite phase relationships exist among the different parts of the scattered beam. The intensity of the scattered radiation at any point is obtained by adding the intensities of the scattered radiation reaching this point from the independent scattering elements. Inelastic Scattering: The type of scattering that results in the nucleus being left in an excited state and the total kinetic energy being decreased. Scattering Coefficient, Compton
That fractional decrease in the energy of a beam of x or gamma radiation in an absorber due to the energy carried off by scattered photons in the Compton effect. (See also Compton Absorption Coefficient.)
Scattering Foil
In electron beam therapy, a thin metal plate used to disperse the electron beam before it passes through the collimator jaws. The function of the scattering foil is to flatten intensity over the field, analogous to the function of the flattening filter with x-rays.
Scintillation Counter
An instrument that detects and measures ionizing radiation by counting the light flashes (scintillations) induced by the radiation in certain materials.
Sealed Source
A radioactive source sealed in an impervious container which has sufficient mechanical strength to prevent contact with and dispersion of the radioactive material under the conditions of use and wear for which it was designed.
Series, Radioactive
A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results. The first member is called the “parent,” the intermediate members are called “daughters,” and the final stable member is called the “end product.”
Shield
A body of material used to prevent or reduce the passage of particles or radiation. A shield may be designated according to what it is intended to absorb (as a gamma-ray shield or neutron shield), or according to the kind of protection it is intended to give (as a background, biologic, or thermal shield). The shield of a nuclear reactor is a body of material surrounding the reactor to prevent the escape of neutrons and radiation into a protected area, which frequently is the entire space external to the reactor. It may be required for the safety of personnel or to reduce radiation enough to allow use of counting instruments for research or for locating contamination or airborne radioactivity.
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Glossary: Scattering Coefficient, Compton
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Sievert
The new special unit of dose equivalent. The sievert equals the absorbed dose in gray times the quality factor for the radiation in question. (Symbol: Sv)
Sigmoid Curve
S-shaped curve, often characteristic of a dose-effect curve in radiobiological studies.
Simulation
Use of a simulator to determine the various treatment field outlines and orientations to be used during radiation therapy.
Simulation Films
X-ray films taken on a simulator with the same field size, target-to-skin distance, and orientation as a therapy beam.
Simulator
A radiation generator operating in the diagnostic x-ray range that can direct a radiation beam toward a patient with parameters imitating those proposed for therapy, and providing direct x-ray fluoroscopic visualization and roentgenographic images. The simulator x-rays do not contribute to the therapy dose required.
Skin Dose
Absorbed dose at the center of the irradiation field on skin. It is the sum of the dose in air and scatter from body parts.
Softness
A relative specification of the quality or penetrating power of x-rays. In general, the longer the wave length the softer the radiation.
Somatic Effects
Effects that may become evident in the irradiated individual.
Source
Synonymous with Target.
Source Axis Distance (SAD)
Synonymous with Target axis distance (TAD).
Source Film Distance (SFD)
Synonymous with Target-film distance (TFD).
Source Surface Distance (SSD)
The distance measured along the beam axis, from the front surface of the source to the surface of the irradiated object Synonymous with TSD.
Specific Activity
Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide.
Specific Gamma-ray Constant
For a nuclide emitting gamma radiation, the product of exposure rate at a given distance from a point source of that nuclide and the square of that distance divided by the activity of the source, neglecting attenuation.
Spectrum
A visual display, a photographic record, or a plot of the distribution of the intensity of radiation of a given kind as a function of its wavelength, energy, frequency, momentum, mass, or any related quantity.
Split Course
A course of radiotherapy delivered in two parts separated by a rest period of several weeks.
Standard
Something established as a measure; a model to which other similar things should conform.
Glossary: Sievert
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Standard, Radioactive: A sample of radioactive material, usually with a long half-life, in which the number and type of radioactive atoms at a definite reference time are known. It may be used as a radiation source for calibrating radiation measurement equipment. Sterility (Biological)
Temporary or permanent incapability to reproduce.
Sublethal Damage
Cellular damage, the accumulation of which, may result in lethality.
Survey, Radiologic
Evaluation of the radiation hazards incident to the production, use, or existence of radioactive materials or other sources of radiation under specific conditions. Such evaluation customarily includes a physical survey of the disposition of materials and equipment, measurements or estimates of the levels of radiation that may be involved, and sufficient knowledge of processes using or affecting these materials to predict hazards resulting from expected or possible changes in materials or equipment.
Survey Meter (Laboratory Monitor)
A detection instrument used to monitor an area for unsuspected radiation or to search for a lost radiation source or contamination.
Syndrome
The complex of symptoms associated with any disease.
Target
A metal plate placed in the beam of high-speed electrons to produce x-rays. For electron therapy, the target is retracted from the beam.
Target Axis Distance (TAD)
The distance measured along the central axis from the center of the front surface of the target to the isocenter.
Target Film Distance (TFD)
The distance measured along the central axis from the center of the front surface of the target to an x-ray film.
Target Skin Distance (TSD)
The distance measured along the central axis from the center of the front surface of the target to the surface of the irradiated object.
Target Theory (Hit Theory)
A theory explaining some biological effects of radiation on the basis that ionization, occurring in a discrete Volume (the target) within the cell, directly causes a lesion which subsequently results in a physical response to the damage at that location. One, two, or more “hits” (ionizing events within the target) may be necessary to elicit the response.
Tenth-Value Layer (TenthValue Thickness) (TVL)
The thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduce’s the kerma rate by ten.
Therapy
Medical treatment of a disease. Brachytherapy (therapy at short distances): The treatment of disease with sealed radioactive sources placed near, or inserted directly into, the diseased area. Contact Radiation Therapy: X-ray therapy with specially constructed tubes in which the target-skin distance is very short (less than 2 cm). The voltage is usually 40 to 60 kV.
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Glossary: Sterility (Biological)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Radiation Therapy: Treatment of disease with any type of radiation. Rotation Therapy: Radiation therapy during which either the patient is rotated in front of the source of radiation or the source is revolved around the patient. In this way, a larger dose is built up at the center of rotation within the patient’s body than on any area of the skin. Teletherapy (therapy at long distance): The treatment of disease with gamma radiation at a distance from the patient.
Thermionic Emission
Release of electrons from the cathode filament by heat.
Thermoluminescent Dosimetry
A method of determining dose by exposing certain phosphoric materials to radiation, and then heating the materials and measuring the light emitted. The luminescence is proportional to the dosage delivered.
Threshold, Photoelectric
The quantum of energy hv0 that is just enough to release an electron from a given system in the photoelectric effect. The corresponding frequency, vO, and wavelength, 8O, are the threshold frequency and wavelength respectively. For example, in the surface photoelectric effect, the threshold hv0 for a particular surface is the energy of a photon which, when incident on the surface, causes the electron to emerge with zero kinetic energy.
Thyratron
A gas-filled electron tube in which the grid controls only the start of a continuous current, giving the tube a trigger action. A thyratron is used to discharge the pulse-forming network in the modulator.
Tissue Equivalent Material
Material made up of the same elements in the same proportions as they occur in a particular biologic tissue. In some cases, the equivalence may be approximated with sufficient accuracy on the basis of effective atomic number.
Townsend Avalanche
(See Avalanche.)
Track
Visual manifestation of the path of the ionizing particle in a chamber or photographic emulsion.
Transformer
An electrical device for increasing or decreasing the incoming voltage.
Transmutation
Any process in which a nuclide is transformed into a different nuclide, or more specifically, when transformed into a different element by a nuclear reaction.
Transport Group
This is any one of seven groups into which normal form radionuclides are classified according to their radiotoxicity and potential hazard in transportation.
Transport Index
The number to be placed on a package label to designate the degree of control to be exercised by the carrier during transportation and indicating the following: (l) the highest radiation absorbed dose equivalent rate in microSievert per hour at three feet from any accessible external surface of the package, or (2) for Fissile Class II packages only, the number calculated by
Glossary: Thermionic Emission
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dividing the number “50” by the number of similar packages that may be transported together. Treatment Beam Parameters
The data required for complete specification of an individual treatment beam, including radiation energy, field size, use of wedges and blocks, orientation with respect to patient, and prescribed exposure time, dose, and distance.
Treatment Couch
The Clinac unit that supports the patient during therapy.
Treatment Field
A plane section of a beam, perpendicular to the beam axis, as defined by the collimator.
Treatment Head
The section of the Clinac gantry from which the treatment beam exits. The treatment head includes the carrousel, collimator, an ionization chamber, the range finder and field-defining light, and other supporting components.
Treatment Plan
An ensemble of radiation beams or sources designed to produce a prescribed dose pattern in and for the patient; includes spatial and temporal distributions.
Treatment Planning
A complex process carried out prior to the administration of radiation therapy. The planning process usually includes such items as tumor localization, treatment volume determination, contour preparation, and treatment dose determination to prescribe the dosage pattern required.
Treatment Room
An enclosed space specially designed and dedicated for patient treatment by a radiotherapy accelerator.
Treatment Type
Standard and specialized therapies available on the Clinac 2100C, including fixed x-ray, fixed electrons, arc x-ray, port film exposures, arc electrons, total body x-ray, total body electrons, and high dose rate total skin electrons.
Tritium (T)
The hydrogen isotope with one proton and two neutrons in the nucleus.
Tube, Photomultiplier
An electron multiplier tube in which the electrons initiating the cascade originate by photoelectric emission.
Umbra
The region within the beam receiving the full strength of the primary x-ray or gamma-ray photons.
Uncontrolled Area
An area not under the authority of the Radiation Protection Officer and not subject to restriction due to the presence of radiation.
Valence
Number representing the combining or displacing power of an atom; number of electrons lost, gained, or shared by an atom in a compound; number of hydrogen atoms with which an atom will combine, or which it will displace.
Van De Graaff Accelerator
An electrostatic machine in which electrical charge is carried into the high voltage terminal by a belt made of an insulating material moving at a high speed. The particles are then accelerated along a discharge path through a vacuum tube by the potential difference between the insulated terminal and the grounded end of the accelerator.
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Glossary: Treatment Beam Parameters
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Velocity
The ratio of displacement to the time required for this displacement. Time rate of motion in a given direction and sense. Average velocity equals the total distance passed over, divided by the whole time taken.
Video Display
See Monitor
Volt
The unit of electric pressure or electromotive force; the force necessary to cause 1 ampere of current to flow against 1 Ohm of resistance. –1 1V = 1J ⁄ C
Voltage
The potential difference, in volts, between two different points in an electric circuit or between two different electrodes. Voltage, Operating: As applied to radiation detection instruments, the voltage across the electrodes in the detecting chamber required for proper detection of an ionizing event. Voltage, Starting: For a counter tube, the minimum voltage that must be applied to obtain counts with the particular circuit with which it is associated.
Volume, Sensitive
That portion of a counter tube or ionization chamber which responds to a specific radiation.
Watt
The unit of power equal to 1 joule per second (1W = 1J/s).
Waveguide
Metal pipe of rectangular or circular cross section that transfers radio-frequency energy to the accelerator.
Wavelength
Distance between any two similar points of two consecutive waves ( λ ). For electromagnetic radiation, the wavelength is equal to the velocity of light (c) divided by the frequency of the wave (v), λ = c/v. The “effective wavelength” is the wavelength of monochromatic x-rays that would undergo the same percentage attenuation in a specified filter as the heterogeneous beam under consideration.
Wave Motion
The transmission of a periodic motion or vibration through a medium or empty space. Transverse: Wave motion in which the vibration is perpendicular to the direction of propagation. Longitudinal: Wave motion in which the vibration is parallel to the direction of propagation.
Wedge Filter
A tapered block of attenuating material, designed to produce wedge shaped isodose curves.
Window
(1) A plate, usually made of ceramic or glass, placed across the microwave waveguide to separate the air in the waveguide from the vacuum in the linear accelerator and the klystron. (2) A plate, made of thin aluminum or beryllium, through which electrons are extracted. (3) A rectangular portion of a monitor screen used for input from the operator or to display information to the operator.
Glossary: Velocity
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 X-rays
Penetrating electromagnetic radiations whose wavelengths are shorter than those of visible light. They are usually produced by bombarding a metallic target with fast electrons in a high vacuum. In nuclear reactions, it is customary to refer to photons originating in the nucleus as gamma-rays, and those originating in the extranuclear part of the atom as x-rays. These rays are sometimes called roentgen rays after their discoverer, W.C. Roentgen.
Some of these definitions are reproduced with permission of the U.S. Department of Health Education and Welfare Public Health Service Food and Drug Administration Bureau of Radiological Health. from the Radiological Health Handbook rev. ed. 1970.
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Glossary: X-rays
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Eight Index
This chapter contains an index of terms used in the C-series Clinac Accelerator System Basics manual.
Index
8-i
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Index Numerics 3dB Quadrature Hybrid ................................................................ 4-21
A Advances in Linear Accelerator Design for Radiotherapy ............... 2-19
B Beam Trans port Magnet Systems ................................................. 2-44
C Circulators ................................................................................... 4-22
D Definitions, Intensity vs. Energy ..................................................... 2-3
E Electron Injection and Bunching ................................................... 2-18 Emergency and Safety .................................................................... 1-1 Emergency Off Button .................................................................. 1-10 Enabling emergency pendant ........................................................ 1-11
F Fill Time ....................................................................................... 2-16
G Gas Flow, defined ......................................................................... 6-17 Gas Laws ...................................................................................... 6-13 Gas, defined ................................................................................... 6-7
8-ii
Index
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
I Injection Timing ............................................................................ 2-17 Ion Chamber Theory ....................................................................... 6-1 Ion Pump, defined ......................................................................... 6-21
K Karzmark, Dr. C. J. ....................................................................... 2-19 Kinetic Energy Relationships ........................................................... 2-3 Klystron Theory ............................................................................ 4-24
L Load Line Considerations .............................................................. 2-16
M Machine Physics ............................................................................. 2-1 Microtrons .................................................................................... 2-42 Microwave Accelerator Structures ................................................. 2-20 Modulator Theory ........................................................................... 3-1
N Nature of Vacuum ........................................................................... 6-3
P Pendant, emergency ...................................................................... 1-11 Pressure, defined ............................................................................ 6-7
R Resonant Circuits ......................................................................... 4-14 Rest Energy Relationships ............................................................... 2-4 RF Theory ....................................................................................... 4-1
Index
8-iii
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF Transmission Theory ............................................................... 4-16
S Standing Wave Accelerator ............................................................. 2-9
T TE10 Mode ................................................................................... 4-18 Temperature, defined ...................................................................... 6-6 Thermocouple Gauge .................................................................... 6-26 Thyratron Theory .......................................................................... 3-19 Total Energy Relationships ............................................................. 2-4 Transmission Lines ........................................................................ 4-5
V Vacuum Gauges ........................................................................... 6-26 Vacuum, defined ............................................................................ 6-3 Vapor Pressure, defined ................................................................ 6-10
W Widely Variable Energy Linacs ...................................................... 2-33
8-iv
Index
C-series Clinac® Accelerator System Basics
Revision AF: September 2005 Copyright © 2005 Varian Medical Systems C-series Clinac® Accelerator System Basics
i
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
WARNING! Do not attempt to operate or repair the Clinac using the descriptive material in this Course Manual. Refer only to the specific operation and maintenance manuals supplied with your equipment. Uninformed or careless operation of the machine can expose the operator, patient and other persons to hazards that can cause serious injury or death. Clinacs used for training purposes are specially configured for that purpose. Specifically, protective screens and covers have been removed from some areas where dangerous voltages may be present, and safety interlocks may be disabled. Also, 3-phase primary power is present in the Power Distribution Chassis even when the Clinac is in the Emergency Off state. Do not reach into the Power Distribution Chassis without first turning off the main circuit breaker on the wall, and do not reach into any area of the Clinac until your instructor has told you that it is safe to do so.
Compiled and Edited by: Bill Kirkness Varian Medical Systems Oncology Systems Customer Support Education Department
ii
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Statement of Objectives C-series Clinac® Maintenance Course This course is provided for those personnel directly responsible for the maintenance of the Clinac. It will cover Modulator and RF System theory, beam generation and control, motion control, dosimetry, position readouts, power distribution, accessories, computer control system, diagnostics, interlocks and troubleshooting. This material is for Training Purposes Only. For on-site operation and/or maintenance procedures, refer to the specific operation and maintenance manuals supplied with your equipment.
Statement of Policy This material is the exclusive property of Varian Associates and is loaned subject to return upon demand and with the express condition that information contained herein not generally known in the trade shall be treated confidentially and shall not be reproduced, redistributed or used in any manner, directly or indirectly, detrimental to Varian’s interests. This material must be returned to Varian Medical Systems immediately upon demand at any time.This material is registered to:
Name: ________________________ Date: _________________ C-series Clinac® Accelerator System Basics
iii
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
East
Ed. Dept.
Education Department
Ed. Dept. Northern Region Service
Conf. Room
Clinac 600C/D Lab
bc
Book Room
Vision Lab
Clinac 23EX Lab
Classroom
Classroom
2
3
Clinac 2100C Lab
Classroom 1 Admin. Offices
1b
5
6
Storage Areas
7
1a
Classroom
Copy Room
Classroom
Cafeteria
Classroom
Electrical Breakers
Area 2
Clinac 2300C/D Lab
Shipping & Receiving
Open 8 am5 pm
Main Conf. Lobby Room
North
Info. Svc’s
Classroom
bc Copy Room
Accounting
b c
4 VAR iS Lab
H.R.
Support Engineering Logistics
Conf. Room
ONCOLOGY SYSTEMS CUSTOMER SUPPORT 596 ALDER DRIVE MILPITAS, CA 95035 (408) 321-9400
iv
Storage Area
S.A.P. Command Center, Help Desk, Facilities
Work Shop
Area 1
West
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Revision History Revision
Date
Description
A
May 1996
Initial Version
B
Feb 1997
General reformat and minor corrections. Added Title Page and Table of Contents to each chapter.
C
Aug 1997
Updated Chapter 1 from new Varian Safety Manual, including Lockout/Tagout and Gantry Pin Procedures.
D
Jul 1999
Added Klystron Theory section (formerly in High Energy Beam Delivery System Manual) to Chapter 5.
E
Jan 2000
Updated Title Page, added Table of Contents to this section, Tables of Illustrations to all chapters with captioned illustrations.
F
Jan 2002
General update. Increased font size for better legibility.
AA
May 2002
Added abbreviation list. Converted from WordPerfect to Adobe FrameMaker resulting in some page and paragraph re-numbering. Added index.
AB
Nov 2003
Minor corrections. Redrew illustrations with CorelDraw.
AC
Aug 2004
General update for 2004.
AD
Jan 2005
General update for 2005. Also reduced footer font size.
AE
May 2005
Updated Chapter 1 from new Clinac Safety Manual. Removed Chapter 2, “Controls & Indicators” and renumbered remaining chapters. Re-formatted and added explanatory text to Chapter 2, “Machine Physics.”
AF
September 2005
General update for consistency with Varian standards affecting notes, cautions and warnings.
C-series Clinac® Accelerator System Basics
v
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Visual Cues
This document uses the following visual cues to help you locate and identify information. This symbol identifies comments about a specific task. Some examples include notes, required data, messages, substitutions and shortcuts.
CAUTION: Describes actions or conditions that can result in minor or moderate injury or can result in damage to equipment.
WARNING:Describes actions or conditions that can result in serious injury or death. Italics: Used for emphasis, defining new terms, or book titles. Bold: Identifies menu commands, items you can select on the screen, and buttons to press.
Abbreviations
vi
The following abbreviations are used throughout this manual: Abbreviation
Meaning
AMC
Advanced Motion Controls, Inc.
BNC
A standard coaxial cable connector developed by the Berkeley Nucleonics Corp.
CPU
Central processing unit (today often used to mean “microprocessor”)
CRT
Cathode-Ray Tube Monitor
EEPROM
Electrically Erasable Programmable Read-only Memory
EPROM
Erasable Programmable Read-only Memory
EMI
Electronic Measurements, Inc.
FPGA
Field Programmable Gate Array
LED
Light-emitting diode
MCU
Microcontroller (a microprocessor especially designed for control system applications)
MPU
Microprocessor (unit)
PCB
Printed Circuit Board (sometimes referred to as a “card”)
PSA
Patient support assembly (Treatment couch)
PWM
Pulse Width Modulation
SCR
Silicon Controlled Rectifier or Thyristor
STD
Pro-Log Corp. Control System Bus: “Simple To Design”
C-series Clinac® Accelerator System Basics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents Chapter 1: Emergency and Safety ........................................1-1 1. Abstract........................................................................................... 1-5 2. Introduction..................................................................................... 1-7 2.1. Overview .................................................................................. 1-7 2.2. Operators and Treatment Personnel ......................................... 1-7 2.3. Maintenance and Service Personnel.......................................... 1-8 2.4. Customer Support .................................................................... 1-8 2.5. Related Publications ................................................................. 1-9 3. Emergency Procedures..................................................................... 1-9 3.1. Terminating the Treatment Beam ............................................. 1-9 3.2. Performing an Emergency-Off ................................................. 1-10 3.3. Using the Emergency Hand Pendant....................................... 1-10 3.4. Lowering the Treatment Couch in an Emergency .................... 1-11 3.5. X-Ray and Electron Beam Radiation ....................................... 1-13 3.6. Induced Radiation in Accelerator Components........................ 1-14 3.7. Radio Frequency (RF) Radiation.............................................. 1-15 3.8. Electromagnetic Interference .................................................. 1-16 3.9. Sulfur Hexafluoride and Freon 12 Gases ................................ 1-16 3.10. Lead ..................................................................................... 1-17 3.11. Beryllium ............................................................................. 1-18 3.12. Dielectric Insulating Oil ........................................................ 1-18 3.13. Depleted Uranium (Low-Energy Clinacs) ............................... 1-19 3.14. Ozone and Oxides of Nitrogen (High-Energy Clinacs)............. 1-19 3.15. Implosion ............................................................................. 1-19 3.16. Electric Shock ...................................................................... 1-20 3.17. Service Precautions .............................................................. 1-20 3.18. Electrical Fire ....................................................................... 1-21 3.19. Remote Movements............................................................... 1-21 3.20. LaserGuard .......................................................................... 1-21 3.21. On-Board Imager Precautions............................................... 1-22 3.22. High Dose Precautions ......................................................... 1-22
Table of Contents
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.23. Falling Parts or Accessories .................................................. 1-23 3.24. Deterioration of Plastic Parts from Radiation ........................ 1-23 3.25. Patient Fall from the Treatment Couch ................................. 1-23 3.26. Treatment Couch Pinch Points ............................................. 1-24 3.27. High Temperature Surfaces .................................................. 1-24 3.28. Laser Beams ........................................................................ 1-25 3.29. Software Integrity................................................................. 1-25 3.30. Microwave Tube Operating Hazards ..................................... 1-26 4. Service and Maintenance Guidelines.............................................. 1-27 4.1. Clinac Accelerator Specifications ............................................ 1-27 4.2. Lockout/Tagout Procedures ................................................... 1-28 5. Owner Guidelines .......................................................................... 1-48 5.1. Planning Operations .............................................................. 1-48 5.2. Radiation Protection Survey ................................................... 1-48 5.3. Safety and Emergency Training .............................................. 1-49 5.4. Routine Use ........................................................................... 1-49 5.5. Quality Assurance.................................................................. 1-50 5.6. Accidental Radiation Overdose ............................................... 1-50 5.7. Backup Interlocks .................................................................. 1-51 5.8. Emergency Beam Termination................................................ 1-51 5.9. Emergency Plan ..................................................................... 1-51 6. Appendix A: Venting Waveguide Gases .......................................... 1-52 6.1. Testing for Waveguide Arcing ................................................. 1-52 6.2. Prerequisites: Parts Ordering Procedure ................................. 1-52 6.3. Venting a Clinac With a Venting System................................. 1-53 6.4. Venting a Clinac Without a Venting System............................ 1-53 6.5. Retrofitting the Clinac With a New Venting System................. 1-55 6.6. Contents of 872031-01 and 872031-02 Kits ........................... 1-55
Chapter 2: Machine Physics .................................................2-1 1. Introduction .................................................................................... 2-3 2. Definitions:...................................................................................... 2-3
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Table of Contents
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3. Kinetic Energy Relationships............................................................ 2-3 4. Rest Energy Relationships ............................................................... 2-4 5. Total Energy Relationships............................................................... 2-4 6. Conversion of Energy to Electron Volts............................................. 2-5 7. Measurement of Energy Change....................................................... 2-6 8. Example of Simple Acceleration ....................................................... 2-8 9. The Standing Wave Accelerator ........................................................ 2-9 10. Impedance Matching.................................................................... 2-11 11. Plotting ........................................................................................ 2-12 12. Accelerator Equivalent Circuit...................................................... 2-13 13. Load Line Considerations............................................................. 2-16 14. Fill Time ...................................................................................... 2-16 15. Injection Timing ........................................................................... 2-17 16. Electron Injection and Bunching .................................................. 2-18 17. Advances in Linear Accelerator Design for Radiotherapy............... 2-19 17.1. Electron Therapy .................................................................. 2-59
Chapter 3: Modulator Theory ...............................................3-1 1. Introduction..................................................................................... 3-5 2. Basic Concepts ................................................................................ 3-5 3. DeQing Principles ............................................................................ 3-7 4. Non-resonant Transmission Line Principles...................................... 3-8 5. Pulse Shape Definitions ................................................................. 3-16 6. Line Type Modulator Load Element Principles ................................ 3-17 7. Fault Conditions ............................................................................ 3-18 8. Thyratron Theory ........................................................................... 3-19 8.1. Introduction ........................................................................... 3-19 8.2. Operating Notes on Hydrogen-filled Tubes .............................. 3-23
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Chapter 4: RF Theory...........................................................4-1 1. Introduction .................................................................................... 4-5 2. Traveling Waves on Transmission Lines ........................................... 4-5 3. Waveforms ...................................................................................... 4-8 4. Waveguides ................................................................................... 4-11 5. Resonant Circuits.......................................................................... 4-14 6. RF Transmission Theory ................................................................ 4-16 7. RF Waveguide Design .................................................................... 4-17 7.1. Modes .................................................................................... 4-17 7.2. The TE10 Mode ....................................................................... 4-18 7.3. Coupling ................................................................................ 4-18 7.4. Determining the TE10 Dominant Mode of a Waveguide ............ 4-19 8. Transmission Lines ....................................................................... 4-20 8.1. VSWR .................................................................................... 4-21 9. Vector Analysis – 3dB Quadrature Hybrid...................................... 4-21 10. Circulators .................................................................................. 4-22 11. Klystron Theory ........................................................................... 4-24 11.1. Theory of Klystron Operation................................................ 4-24 11.2. Associated Equipment.......................................................... 4-34 11.3. Power Supplies..................................................................... 4-35 11.4. Cooling ................................................................................ 4-39 11.5. RF Circuits .......................................................................... 4-45 11.6. Tuning ................................................................................. 4-49 11.7. Noise in Klystron Amplifiers ................................................. 4-51 11.8. Summary ............................................................................. 4-52
Chapter 5: Ion Chamber Theory ...........................................5-1 1. Introduction .................................................................................... 5-3 2. Present Configuration...................................................................... 5-3 3. Efficiency ........................................................................................ 5-5 4. Upper Limit of Dose Range (Saturation) ........................................... 5-6
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 5. Applied Electric Field ....................................................................... 5-7 6. Effects of Temperature and Pressure ................................................ 5-7 7. Beam Opacity .................................................................................. 5-8 8. Inverse Square Law.......................................................................... 5-8 9. Insulation Materials ......................................................................... 5-9 9.1. Mica ......................................................................................... 5-9 10. Pulse Shape ................................................................................. 5-10 11. Concluding Remarks.................................................................... 5-11 12. References ................................................................................... 5-12
Chapter 6: Vacuum Theory...................................................6-1 1. Introduction..................................................................................... 6-3 2. The Nature of Vacuum ..................................................................... 6-3 2.1. What Is Vacuum? ..................................................................... 6-3 2.2. What About Pressure? .............................................................. 6-4 2.3. How Is a Vacuum Produced? .................................................... 6-4 2.4. Different Types of Vacuum ....................................................... 6-4 2.5. Where Is Vacuum Used?........................................................... 6-5 2.6. Why Is Vacuum Needed? .......................................................... 6-5 3. Temperature .................................................................................... 6-6 4. Pressure .......................................................................................... 6-7 4.1. What is Gas? ............................................................................ 6-7 4.2. Atmospheric Pressure............................................................... 6-7 4.3. Pressure Measurement ............................................................. 6-8 4.4. Partial Pressure ........................................................................ 6-9 4.5. Vapor Pressure....................................................................... 6-10 4.6. Effects of Pressure.................................................................. 6-12 4.7. Pressure Ranges..................................................................... 6-12 5. Gas Particles.................................................................................. 6-13 6. Gas Laws ....................................................................................... 6-13 6.1. Avogadro’s Law....................................................................... 6-13
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 6.2. Boyle’s Law ............................................................................ 6-14 6.3. Gas Expansion....................................................................... 6-14 6.4. Charles’ Law .......................................................................... 6-15 6.5. Gay-Lussac’s Law .................................................................. 6-16 6.6. General Gas Law.................................................................... 6-16 7. Gas Flow ....................................................................................... 6-17 7.1. Viscous Flow.......................................................................... 6-17 7.2. Molecular Flow....................................................................... 6-17 7.3. Mean Free Path...................................................................... 6-18 8. Conductance ................................................................................. 6-18 8.1. Conductance in Viscous Flow................................................. 6-19 8.2. Conductance in Molecular Flow ............................................. 6-20 9. Review of the Nature of Gases........................................................ 6-20 10. Ion Pump .................................................................................... 6-21 10.1. Components......................................................................... 6-22 10.2. How the Pump Works........................................................... 6-22 10.3. Vacuum System Use ............................................................ 6-26 10.4. Summary ............................................................................. 6-26 11. Vacuum Gauges .......................................................................... 6-26 11.1. Thermocouple Gauge ........................................................... 6-26
Chapter 7: Glossary..............................................................7-1
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Chapter One
Emergency and Safety
The material in this chapter is taken from the Varian Clinac Emergency and Safety Manual, and is included in this chapter for reference to supplement the Safety Lecture material presented during the Maintenance Training Course.
Emergency and Safety
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Abstract: .......................................................................................................................... 1-5 2. Introduction:.................................................................................................................... 1-7 2.1. Overview:.................................................................................................................. 1-7 2.2. Operators and Treatment Personnel:......................................................................... 1-7 2.3. Maintenance and Service Personnel: ......................................................................... 1-8 2.4. Customer Support: ................................................................................................... 1-8 2.5. Related Publications: ................................................................................................ 1-9 3. Emergency Procedures: .................................................................................................... 1-9 3.1. Terminating the Treatment Beam:............................................................................. 1-9 3.2. Performing an Emergency-Off: ................................................................................ 1-10 3.3. Using the Emergency Hand Pendant: ...................................................................... 1-10 3.4. Lowering the Treatment Couch in an Emergency: ................................................... 1-11 3.5. X-Ray and Electron Beam Radiation: ...................................................................... 1-13 3.6. Induced Radiation in Accelerator Components: ....................................................... 1-14 3.7. Radio Frequency (RF) Radiation: ............................................................................. 1-15 3.8. Electromagnetic Interference:.................................................................................. 1-16 3.9. Sulfur Hexafluoride and Freon 12 Gases:................................................................ 1-16 3.10. Lead: .................................................................................................................... 1-17 3.11. Beryllium:............................................................................................................. 1-18 3.12. Dielectric Insulating Oil: ....................................................................................... 1-18 3.13. Depleted Uranium (Low-Energy Clinacs): .............................................................. 1-19 3.14. Ozone and Oxides of Nitrogen (High-Energy Clinacs): ............................................ 1-19 3.15. Implosion: ............................................................................................................ 1-19 3.16. Electric Shock: ..................................................................................................... 1-20 3.17. Service Precautions:.............................................................................................. 1-20 3.18. Electrical Fire: ...................................................................................................... 1-21 3.19. Remote Movements:.............................................................................................. 1-21 3.20. LaserGuard: ......................................................................................................... 1-21 3.21. On-Board Imager Precautions: .............................................................................. 1-22 3.22. High Dose Precautions:......................................................................................... 1-22
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.23. Falling Parts or Accessories: .................................................................................. 1-23 3.24. Deterioration of Plastic Parts from Radiation: ........................................................ 1-23 3.25. Patient Fall from the Treatment Couch: ................................................................. 1-23 3.26. Treatment Couch Pinch Points: ............................................................................. 1-24 3.27. High Temperature Surfaces: .................................................................................. 1-24 3.28. Laser Beams: ........................................................................................................ 1-25 3.29. Software Integrity:................................................................................................. 1-25 3.30. Microwave Tube Operating Hazards: ..................................................................... 1-26 4. Service and Maintenance Guidelines:.............................................................................. 1-27 4.1. Clinac Accelerator Specifications: ............................................................................ 1-27 4.2. Lockout/Tagout Procedures: ................................................................................... 1-27 5. Owner Guidelines: .......................................................................................................... 1-48 5.1. Planning Operations: .............................................................................................. 1-48 5.2. Radiation Protection Survey: ................................................................................... 1-48 5.3. Safety and Emergency Training: .............................................................................. 1-49 5.4. Routine Use: ........................................................................................................... 1-49 5.5. Quality Assurance:.................................................................................................. 1-50 5.6. Accidental Radiation Overdose: ............................................................................... 1-50 5.7. Backup Interlocks: .................................................................................................. 1-51 5.8. Emergency Beam Termination:................................................................................ 1-51 5.9. Emergency Plan: ..................................................................................................... 1-51 6. Appendix A: Venting Waveguide Gases: .......................................................................... 1-52 6.1. Testing for Waveguide Arcing: ................................................................................. 1-52 6.2. Prerequisites: Parts Ordering Procedure: ................................................................. 1-52 6.3. Venting a Clinac With a Venting System:................................................................. 1-53 6.4. Venting a Clinac Without a Venting System:............................................................ 1-53 6.5. Retrofitting the Clinac With a New Venting System:................................................. 1-55 6.6. Contents of 872031-01 and 872031-02 Kits: ........................................................... 1-55
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Table of Illustrations Figure 1.1. Console Dedicated Keyboard:............................................................................ 1-10 Figure 1.2. Emergency Pendant:......................................................................................... 1-11 Figure 1.3. Typical Lockout Devices: .................................................................................. 1-29 Figure 1.4. Main Circuit Breaker Lockout/Tagout: ............................................................. 1-30 Figure 1.5. Gantry Locking Pin with Lock and Tag:............................................................. 1-34 Figure 1.6. ETR or Exact Couch with Safety Braces:........................................................... 1-35 Figure 1.7. PSA Couch with Safety Braces: ......................................................................... 1-36 Figure 1.8. Air Hose Removed and Lockout Tag Attached: .................................................. 1-37 Figure 1.9. Handle Removed and Lockout Tag Attached: .................................................... 1-38 Figure 1.10. Water Valve with Handle Removed and Tag Attached: ..................................... 1-40
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Abstract European Representative
Notice
The Clinac Safety Guide (P/N 1104957-03) provides reference information about safety precautions pertaining to C-Series Clinac accelerators.
Manufacturer:
European Representative:
Varian Medical Systems, Inc. 3100 Hansen Way, Bldg. 4A Palo Alto, CA 94304-1030, US
Varian Medical Systems UK Ltd. Gatwick Road, Crawley West Sussex RH10 9RG United Kingdom
Information in this document is subject to change without notice and does not represent a commitment on the part of Varian. Varian is not liable for errors contained in this document or for incidental or consequential damages in connection with furnishing or use of this material. This document contains proprietary information protected by copyright. No part of this document may be reproduced, translated, or transmitted without the express written permission of Varian Medical Systems, Inc.
FDA 21 CFR 820 Quality System Regulations (CGMPs)
Varian Medical Systems products are designed and manufactured in accordance with the requirements specified within this federal regulation.
ISO 9001 and ISO 13485
Varian Medical Systems products are designed and manufactured in accordance with the requirements specified within ISO 9001 and ISO 13485 quality standards.
CE 0086
Varian Medical Systems products meet the requirements of Council Directive MDD 93/42/EEC.
Trademarks
Clinac, Silhouette, LaserGuard, and VARiS are registered trademarks, and RMS, PortalVision, On-Board Imager, iX, Trilogy, and EDW are trademarks, of Varian Medical Systems, Inc. All other trademarks or registered trademarks are the property of their respective owners. © 1995–2004 Varian Medical Systems, Inc. All rights reserved. Printed in the United States of America.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Important Notice This manual includes information that is critical to ensure safe operation, service, and maintenance of Varian Medical Systems medical linear accelerators. This manual is designed to assist properly trained and equipped personnel in safely completing work on the Varian Medical Systems medical linear accelerators. Comprehensive knowledge of the standards of care required for operation, service, and maintenance of the accelerator is needed to make effective use of this manual; the manual is not a substitute for such knowledge. Only properly trained personnel should operate, service, or maintain any Varian Medical Systems medical linear accelerator. Varian is not responsible for injuries or damages due to activities which do not conform to generally accepted standards of care, or which are inconsistent with specific provisions of this manual.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
2. Introduction
This chapter has been reproduced from the Varian Clinac Safety Manual, P/N 1104957-03, published 12/2004. Within this chapter, the original document words “Manual” and “Chapter” have been changed accordingly.
2.1. Overview
This safety manual provides information about emergency procedures, safety precautions, service and maintenance guidelines, and owner guidelines that apply to the general operation and maintenance of the Clinac radiotherapy accelerator and associated equipment.
2.1.1. Visual Cues
This document uses the following visual cues to help you locate and identify information: Note: This symbol identifies comments about a specific task. Some examples include notes, required data, messages, substitutions, and shortcuts.
CAUTION: Describes actions or conditions that can result in minor or moderate injury or can result in damage to equipment.
WARNING: Describes actions or conditions that can result in serious injury or death. Italics: Used for emphasis, defining new terms, or book titles. Bold: Identifies menu commands, items you can select on the screen, and buttons to press. Preparing Personnel The Clinac is a sophisticated and potentially hazardous piece of equipment. Uninformed or careless operation or service can result in poor performance, equipment damage, serious and possibly fatal injuries. The owner should require the following actions of each person who operates, maintains, or is otherwise associated with the Clinac: Become thoroughly familiar with the material in this manual and the operating instructions detailed in your user documentation. Become thoroughly familiar with and follow the emergency procedures and safety precautions in this manual, as well as warnings and cautions contained in this manual and in the other books in the documentation set. Become thoroughly familiar with and follow the emergency and safety procedures established for local use by the owner.
2.2. Operators and Treatment Personnel
Varian Clinac medical linear accelerators are sophisticated and potentially hazardous pieces of equipment. Unauthorized or careless operation or service can result in poor performance, equipment damage, or serious and possibly fatal injuries. The owner must require the following actions of each person who operates, maintains, or is otherwise associated with the Clinac:
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2.3. Maintenance and Service Personnel
!
Learn the contents of this guide and the operating instructions detailed in your user documentation.
!
Follow the emergency procedures, safety precautions, warnings, and cautions in this guide, and in all other related publications.
!
Follow emergency and safety procedures established for local use by the owner.
Maintenance and service procedures are restricted to service personnel who receive the appropriate maintenance training and are authorized by the owner.
WARNING: Authorized service personnel must become thoroughly familiar with and follow lockout/tagout safety procedures established for local use by the owner during all service and maintenance procedures. They are also required to take all precautions necessary to protect themselves, patients, and other persons from injury, and to protect equipment from damage. To ensure safe operation and maintenance conditions for use of any medical linear accelerator, the owner is responsible for establishing emergency and safety procedures. Use this manual, along with the warnings and cautions in the other books in the documentation set as the starting point for formulating local procedures.
2.4. Customer Support
If you cannot find information in this user guide, you can contact Varian in several ways: !
!
Help Desk Support •
North American toll-free support: 1.888.827.4265
•
Global telephone support:1.702.938.4807
•
Global telephone support, Treatment Planning: 1.702.938.4712
To order additional documents •
From North America:1.800.535.5350
• • !
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Globally: 1.702.938.4700
World Wide Web •
!
and press 1 for “Parts” on your touch-tone phone
If you have access to the Internet, point your browser to Oncology Systems: http://www.varian.com and then select Support.
E-mail •
Information Management Systems, Digital Imaging Management Systems, and Delivery Systems: [email protected]
•
Treatment Planning Systems:
•
Brachytherapy Systems:
[email protected] [email protected]
Emergency and Safety: Introduction
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
United States mail Varian Medical Systems, Inc. 3100 Hansen Way, Bldg. 4A Palo Alto, CA 94304-1030, U.S.A.
!
European representative Varian Medical Systems, UK Ltd. Gatwick Road, Crawley West Sussex, RH102RG, England •
2.5. Related Publications
Phone: +44-1293-531-244
The following Varian publications provide further information about the Clinac and related products: !
C-Series Clinac Instructions for Use (P/N 1102903)
!
C-Series Clinac Technical Reference Guide (P/N 1106795)
!
Exact Couch User Guide/Maintenance Manual for Couch and Couch Top (P/N 1104201)
!
C-Series Clinac Custom Coding Guide (P/N 1106292)
!
MLC User Guide (P/N 1101351)
!
MLC Systems and Maintenance Guide (P/N 1101018)
!
Shaper User Guide (P/N 1101352)
!
DMLC Implementation Guide (P/N 1105417)
!
LaserGuard Clinical Reference Guide (P/N 100011555)
!
Service Technical Bulletins (STBs)
!
Customer Technical Bulletins (CTBs)
3. Emergency Procedures
This section describes procedures recommended for use in the event of an emergency. The owner is responsible for adapting these procedures locally and establishing other emergency procedures.
3.1. Terminating the Treatment Beam
To terminate the beam: From the console dedicated keyboard (Figure 1.1), press the BEAM OFF button and turn the DISABLE/ENABLE keyswitch to the DISABLE position. Remove the key and place it in a secure location.
WARNING: In the event of an emergency during operation of the Clinac, terminate the treatment beam immediately. Examples of an emergency situation requiring termination of the treatment beam include the following: ! Clinac fails to properly terminate a treatment ! Accumulated radiation dose displayed on the console monitor exceeds the dose preset for the treatment ! Fire, smoke, or gas fumes are detected ! Power failure
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Figure 1.1. Console Dedicated Keyboard
WARNING: If the beam remains on, press the nearest Emergency Off button.
3.2. Performing an EmergencyOff
Pressing an Emergency Off button immediately turns off power to all components except the console video monitor and console computer. These remain powered on. Emergency Off buttons supplied by Varian are located on the dedicated keyboard, on both sides of the treatment couch, next to the auxiliary electronics chassis in the drive stand, and on both sides of the drive stand. Other Emergency Off buttons may be installed by the owner of the facility. For additional information about the location of Emergency Off buttons, see Section 5.1.1 on Page 1-48. To verify that the Emergency Off button has interrupted power, listen for mechanical noises (for example, running motors or fans) in the treatment room. If you hear evidence that power is still on, turn off the main facility circuit breaker.
3.3. Using the Emergency Hand Pendant 1-10
!
If a patient is receiving therapy, remove the patient from the treatment couch as soon as possible.
!
Check the dose counters, and record the cumulative number of monitor units received by the patient up to that time.
!
Do not try to operate the accelerator until service personnel have restored proper operation of the machine, including operation of the emergency-off circuits.
If couch motion controls do not function due to power failure or other emergency, use the emergency pendant to lower the couch for safe removal of a patient (Figure 1.2). You cannot use the emergency pendant to raise the
Emergency and Safety: Emergency Procedures
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 couch. The emergency pendant is located in the drive stand of the Clinac. It is battery-powered and remains functional after: !
Power failure.
!
Emergency Off button has been pressed.
Figure 1.2. Emergency Pendant You can also use the emergency pendant to lower the treatment couch if the couch fails to respond to a vertical motion control. The pendant is not operational unless the emergency pendant switch is enabled. Before using the emergency pendant, you must enable it by pressing an EMERGENCY PENDANT ON/OFF switch. The emergency pendant and emergency pendant on/off switch are located as follows:
3.4. Lowering the Treatment Couch in an Emergency
!
On standard Clinacs, the emergency pendant and EMERGENCY PENDANT switch are mounted in the right side of the drive stand.
!
On Silhouette high-energy models, the emergency pendant and EMERGENCY PENDANT ON/OFF switch is mounted behind the door to the immediate right of the accelerator.
You can use the emergency pendant to change the position of the couch so that the patient can be removed from the treatment room, if other controls do not function. First, enable the emergency pendant: 1. On the console dedicated keyboard, turn the DISABLE/ENABLE keyswitch to DISABLE. 2. Enter the treatment room and tell the patient to remain on the couch. 3. Open the right door of the drive stand and toggle the EMERGENCY PENDANT ON/OFF switch to ON. 4. Remove the emergency pendant from its storage location inside the drive stand and return to the treatment couch with the pendant. Do not disconnect the pendant cable from the drive stand.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Next, move the couch: 1. Check the longitudinal position of the couch to see if the patient can leave safely. 2. If the couch top is in the longitudinal position you want, proceed to step 3. If the longitudinal position of the couch top does not allow the patient to safely leave the couch: A. On the emergency pendant (Figure 1.2), rotate the NORM/OUT/DOWN switch to the OUT position. B. Press and hold down the enable button on top of the emergency pendant. C. Continue to hold down the enable button while you manually move the couch top to the desired position along the longitudinal axis. D. Release the enable button to reengage the longitudinal brake. 3. Check the height of the couch top to see if the patient can leave it safely. If the couch top is at the height you want, proceed to step 4. If the couch top is too high for the patient to leave safely, lower the couch top: A. On the emergency pendant, rotate the NORM/OUT/DOWN switch to the DOWN position. B. Press and hold down the enable button on top of the pendant. While you hold down the enable button, the couch top lowers. C. Release the button when the top reaches the height you want. 4. Help the patient from the couch and treatment room. 5. Rotate the NORM/OUT/DOWN switch to the NORM position and toggle the EMERGENCY PENDANT ON/OFF switch to the off position. 6. Return the emergency pendant to its storage location in the drive stand. 7. Safety Precautions This section identifies significant operating and maintenance hazards associated with the Clinac. This information is provided to help you avoid or control these hazards.
Note: The identification of potential hazards and suggested safety precautions described in this manual are provided to assist you in maintaining a safe workplace. Varian recommends that you refer to applicable federal, state, and other local standards for more information and specific safety requirements. Refer also to “Related Publications” on page 1-9 for additional information.
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3.5. X-Ray and Electron Beam Radiation
3.5.1. Personnel Precautions
3.5.2. DISABLE/ENABLE Key Precautions
3.5.3. Treatment Precautions
3.5.4. Cardiac Pacemaker Precautions
The Clinac can produce a lethal radiation dose in a very short time. Never operate the X-ray or electron beam without adequate X-ray shielding. Radiation exposure can cause serious illness or death, though not instantaneously. Radiation exposure may also cause certain types of cardiac pacemakers to malfunction. !
Post signs warning all persons of the radiation hazard in the area.
!
Permit no person other than the patient in the treatment room when the treatment beam is on.
!
When working on or near the machine, wear radiation monitoring devices approved by the cognizant regulatory agency.
!
Allow only trained, qualified personnel to operate or maintain the machine.
!
Before entering the treatment room, operators and service personnel must turn the DISABLE/ENABLE keyswitch to the DISABLE position. Remove the key and place it in a secure location.
!
When entering the treatment room, block the door so that it cannot close and enable the door interlock while you are inside.
!
Insert the DISABLE/ENABLE key into the keyswitch on the dedicated keyboard and turn it to the ENABLE position only when necessary to enable the beam-on condition. Immediately after the beam terminates, turn the DISABLE/ENABLE keyswitch to the DISABLE position.
!
When powering down or placing the Clinac in standby, put the DISABLE/ENABLE and power keys in a secure key-storage enclosure. Lock the enclosure to prevent unauthorized activation of the machine.
!
For each treatment plan, the physicist or dosimetrist needs to give appropriate attention to the dose correction factors resulting from scatter and attenuation through any object placed between or near the radiation source and the patient. Typical objects include devices such as shadow trays, centerspine attachments, centerspine and side rails of the treatment couch, couch panels, tennis racket panels, wedge and compensating filters, and shadow blocks.
!
Operating personnel should monitor the accumulated radiation dose readout continually during beam-on.
!
Operating personnel should terminate the beam according to the local emergency procedure if the beam fails to terminate on the preset dose.
!
Before you treat a patient wearing a pacemaker, contact the manufacturer of the pacemaker about possible hazards that may pertain to treatment by a linear accelerator.
!
Make arrangements to monitor the operation of a pacemaker worn by anyone in the treatment room or in adjacent rooms.
!
If you suspect EMI is affecting the operation of a pacemaker, stop operating the Clinac immediately.
!
Post warnings of the potential danger to individuals wearing a pacemaker.
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3.5.5. Radiation Overdose
In the event a person is exposed or thought to be exposed to excess radiation, the law in most localities of the United States requires the following steps: !
Immediately notify the appropriate local, state, and federal authorities.
!
Request an investigation by a professional qualified in the detection of radiation.
!
Consult with medical experts in radiation treatment.
For more information, see “Accidental Radiation Overdose” on page 1-50.
3.6. Induced Radiation in Accelerator Components
Copper, iron, lead, and tungsten components can become temporarily radioactive when irradiated by X-rays with energies above 10 MeV. Induction of radioactivity is caused by the photoneutron or (c,n) reaction, which produces neutrons. When the product nucleus is left in an unstable state, it becomes radioactive. The primary radioactive components are those which absorb most of the Xray energy: !
Targets
!
Collimator assemblies
!
Compensating filters
!
Other shielding and structural material surrounding the target
Except for copper, the other materials become only mildly radioactive following irradiation. The greatest concentration of radioactivity occurs in the target (tungsten and copper). Immediately after beam shutdown the target can contain up to 15% of the total activity produced in the accelerator. The target is approximately 20 times more radioactive than the next most active component. Most of the measurable radiation streams out of the primary collimator and jaw openings. The copper energy slits in the magnet system can also become highly radioactive because they come in direct contact with the beam and act as secondary targets. However, radiation from the target is wellshielded and the radiation is collimated like the primary beam; it is easy to avoid unnecessary exposure. Some radioactivity is also induced in the tungsten collimator, the tungsten or iron flattening filter, and the iron and copper in the magnet. Most of this energy is induced in the shielding within 30° from the beam axis. The tungsten collimator and jaw system are also subject to high X-ray flux. However, the relatively short half-life of the principal tungsten isotope (115 days) limits the buildup of radioactivity. Other components, such as the tungsten flattening filter and structural parts, are 20 times less active than copper when exposed to the same X-ray field.
3.6.1. Handling Precautions
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You can remove radioactive target assemblies safely, with minimal exposure, if you follow safe well-planned work practices. Use of three basic principles—time, distance, and shielding—results in doses being As Low As Reasonably Achievable (ALARA). To minimize exposure, follow these guidelines:
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3.6.2. Handling the X-Ray Target
!
Always allow the maximum time possible for radioactive decay to occur. Higher energies produce higher amounts of radioactivity. For example, an 18 MV machine requires a longer “cooling” period than a 15 MV machine.
!
Perform a radiation survey to assess the hazard. Using a dose rate meter, open the jaws and survey the nearest accessible area beneath the collimator.
!
Plastic trays, wedges, and other accessories can become radioactive following high-energy X-ray treatments. Inform personnel of the risk of residual exposure and take appropriate precautions.
!
Survey all potentially radioactive disposable components with a pancake GM survey instrument prior to disposal. Radioactive components require proper handling and disposal techniques.
The X-ray target is handled only during major service or repair. The target can become radioactive depending on the Clinac energy levels and how long it has been since the target was irradiated. A radiation survey meter is the only way to determine if the target is safe to handle. Be sure to use a radiation survey meter to detect radioactivity before handling the target.
3.7. Radio Frequency (RF) Radiation
The microwave power tube in the drive stand or gantry produces high levels of microwave energy that is supplied to the linear accelerator through a waveguide system. Since this energy does not appreciably penetrate metal, the power tube body and the entire RF system are made of metal and designed to attenuate or shield RF radiation. Avoid exposure of personnel to RF radiation and prevent anyone from being in the vicinity of open energized waveguides. Exposure to high levels of RF radiation can result in serious bodily injury, including blindness.
3.7.1. General Precautions
To minimize exposure of personnel to RF radiation: !
All input and output RF connections, waveguides, flanges, and gaskets must be tight to prevent RF leakage.
!
Do not operate the magnetron or Klystron tube unless it is properly attached to an appropriate energy-absorbing load.
!
Cardiac pacemakers may be affected.
!
If you suspect leakage of RF energy, do not attempt to operate the Clinac until service personnel have verified proper operation of the machine.
!
Do not expose any part of your body to an energized RF waveguide system that is open or loosely bolted together, or to the window of an energized magnetron or Klystron.
!
Never look into or expose any part of the body to an open waveguide while the tube is energized.
!
Permit only service personnel with the appropriate training and experience to service or repair the magnetron or Klystron tube, the RF waveguide system, or the energy-absorbing load of a Clinac.
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3.8. Electromagnetic Interference
The low levels of electromagnetic radiation present around the Clinac when the beam is on can cause electromagnetic interference (EMI) with cardiac pacemakers and surrounding hospital equipment. EMI can interfere with the operation of cardiac pacemakers or patient monitoring equipment, possibly causing serious bodily injury or death. EMI from other equipment, such as microwave hyperthermia or diathermy equipment, can interfere with the Clinac integrated dose counters, which could result in incorrect doses to patients.
3.8.1. General Precautions
3.9. Sulfur Hexafluoride and Freon 12 Gases
!
Keep covers, doors, and panels on the Clinac closed during operation.
!
If EMI appears to be interfering with the operation of the Clinac or any equipment in the vicinity, cease operation of the Clinac immediately.
!
To reduce EMI, reinstall all fasteners for covers, screens, and panels. Screws in panels and covers must be correctly and completely installed. Check to make sure that they are tight.
!
For EMI pacemaker precautions, see “Cardiac Pacemaker Precautions” on page 1-13.
Sulfur hexafluoride (SF6) and Freon 12 are colorless, odorless, nontoxic gases used in Clinacs as a dielectric to prevent arcing in the RF waveguide. These gases are stored as a liquid under pressure in a disposable metal container in the drive stand. Freon 12 is present in older model Clinacs only. SF6 is now used in Clinac production. The waveguide is a sealed unit. Exposure to breakdown products should not occur unless waveguide integrity is compromised.
Note: Freon 12 is present in Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
3.9.1. General Precautions WARNING: ! Improper handling of SF6 and Freon 12 gas cylinders can
result in the rapid release of pressurized gas, causing serious and possibly fatal injury.
! Contact with gas or liquefied gas may cause serious injuries, including burns or frostbite. ! Handle gas cylinders with care. A ruptured gas cylinder may become a projectile. !
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Although these gases are nontoxic, they can act as an asphyxiant by displacing oxygen.Avoid an inhalation hazard by venting in accordance with standard venting procedures as described in CTB42, Venting Waveguide Gases.
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3.9.2. Gas Cylinder Storage and Handling
3.9.3. First Aid for Gas Exposure
3.10. Lead
3.10.1. Precautions
3.10.2. Lead Handling and Storage
!
High-voltage arcing in the RF waveguide may cause SF6 and Freon 12 to break down into toxic compounds including hydrochloric and hydrofluoric acids; and chlorine, fluorine, and phosgene gases. Treat used waveguide pieces as though they are contaminated and handle with personal protective equipment (PPE). Always wear eye protection and chemical-resistant gloves. For information about PPE, see “Using Personal Protective Equipment” on page 1-45.
!
Attach valve caps to all stored cylinders to prevent damaging the stem and the possible release of stored energy.
!
Store in a clean, dry, and well-ventilated area where the temperature does not exceed 125° Fahrenheit (52° Celsius).
!
Ensure that all cylinders are secured to the drive stand and that all spares are secured with chains.
!
Handle cylinders with care, avoiding any collisions.
!
Do not refill or reuse damaged or dropped cylinders.
!
For inhalation, immediately remove the person to fresh air and seek medical attention.
!
In case of skin contact, flush with copious amounts of water. Treat for frostbite if necessary.
!
In case of eye contact, immediately flush with copious amounts of water. Seek medical attention.
Lead is present in the shielding, balance weights, and some wedge trays in the Clinac. !
Handling of lead shielding, balanced weights, or wedge trays may generate lead dust that can be inhaled or ingested.
!
Exposure to lead dust can cause adverse health effects such as anemia and gastrointestinal abnormalities.
!
Severe overexposure to lead dust may cause neuromuscular dysfunction, paralysis, and birth defects.
!
Observe proper hygiene and wear a disposable mask to avoid inhalation and ingestion of lead-contaminated dust.
!
Always wear gloves to protect against skin contact when handling lead. The gloves worn for lead handling should not be used for other purposes.
!
When handling lead with leather gloves, wear latex or vinyl gloves underneath the leather gloves. Lead will penetrate leather gloves that have been used repeatedly for lead. Always remove gloves and store them in a designated plastic storage bag before leaving the work area.
!
After handling lead or lead-contaminated materials, wash your hands and face thoroughly.
!
Do not use compressed gas to clean lead components.
!
Do not use water or saturated wet cloths to clean lead. Excess moisture may cause lead to oxidize. However, you can use a moist, lintfree cloth to avoid airborne lead oxide particulates.
!
Follow proper lifting practices when lifting and handling lead.
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3.10.3. First Aid for Lead Exposure
!
If lead residue contaminates the skin, wash well with soap and water.
!
If lead is ingested or inhaled, contact a physician immediately for assistance.
3.11. Beryllium
Beryllium is present in the bend-magnet window of all high-energy Clinacs. Normal service and maintenance procedures do not expose you to beryllium compounds. However, removal and manipulation of the window may cause exposure to beryllium particulates.
3.11.1. Precautions
3.11.2. First Aid for Beryllium Exposure
3.12. Dielectric Insulating Oil
!
Beryllium can be absorbed through the skin.
!
It may enter into the body through a cut or puncture, producing hard lesions with central nonhealing areas.
!
Beryllium poisoning may result in serious health problems including neuromuscular dysfunction and paralysis. Beryllium has also been known to cause cancer.
!
Wear disposable gloves to avoid skin contact.
!
Wear safety glasses to prevent eye contact.
!
Observe proper personal hygiene after contact with beryllium.
!
If dust enters the eyes, flush with copious amounts of water.
!
If cut with broken glass, treat with first aid. Wash with copious amounts of water.
!
Immediately seek medical attention.
Dielectric insulating oils are present in the pulse transformer, modulator, rectifiers, and capacitors in all Clinacs. !
Dielectric insulating oil is harmful if swallowed. It is also a skin irritant.
!
Dielectric insulating oil may become hazardous over time by breaking down and becoming infused with heavy metals.
!
Dielectric oils can also produce aromatic hydrocarbons and carbon monoxide upon combustion.
3.12.1. Precautions
!
Wear gauntlet-length rubber gloves when handling oils.
!
Failure of an oil-filled component can result in smoke. If smoke is detected, ventilate the area immediately. Excessive exposure to vapors is moderately irritating to the eyes and mucous membranes, though it does not pose a significant health risk.
3.12.2. First Aid for Dielectric Insulating Oil Exposure
!
In the event of eye or skin contact, flush the affected area with copious amounts of water.
!
If ingested, seek medical attention.
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3.13. Depleted Uranium (LowEnergy Clinacs)
Depleted uranium is present in the X-ray head of older low-energy Clinacs. New production machines no longer use depleted uranium. Depleted uranium is a naturally occurring, processed radioactive material, and must be handled with caution. A major portion of the radiation emanates as alpha and beta particles, with only a small fraction being emitted as gamma rays. External radiation levels are well below those generally considered hazardous. Depleted uranium is radioactive and must be handled with caution. Uranium oxide poisoning can result from oxidation of the depleted uranium components, if the protective coating of plated components is damaged.
3.13.1. Precautions
3.14. Ozone and Oxides of Nitrogen (HighEnergy Clinacs)
!
Under no circumstances should anyone attempt to machine, file, drill, scrape, scratch, or break the protective coating (plating) of the depleted uranium components.
!
When handling depleted uranium, prevent scratching the protective coating by wearing heavy leather gloves without metal snaps or rivets. This prevents scratching the protective coating.
!
Some machines use depleted uranium alloyed with 0.75% titanium and do not have the removable contamination problem associated with earlier machines using unalloyed depleted uranium.
The interaction of the Clinac electron or X-ray beam and air produces ozone and oxides of nitrogen, although normally in negligible quantities unless high dose rates and long exposures are experienced. Because of its high radiolytic yield and chemical reactivity, ozone gas is the most toxic of the gases formed from this interaction. Pure ozone is an unstable, faintly bluish gas with a characteristically fresh, penetrating odor. The average person can detect ozone at a concentration approximately equal to the generally accepted threshold limit value of 0.1 ppm. Ozone affects the respiratory system and irritates the eyes and all mucous membranes. High ozone concentrations enhance the reactivity of combustible materials.
3.14.1. Precautions
3.15. Implosion
!
If ozone is detected, shut down the Clinac immediately and vacate the treatment room.
!
Allow sufficient time for normal room ventilation to exhaust the gas before reentering the treatment room.
!
Do not operate the Clinac again until service personnel have checked room ventilation and verified machine operation.
Because of the internal vacuum in the magnetron or Klystron and the linear accelerator, the ceramic and glass windows that separate the vacuum from the waveguide can shatter inward (implode) if struck with a hard object or subjected to mechanical shock. Other components that can implode are the thyratron tubes in the modulator and the cathode-ray tubes (CRTs) in the console video monitors. Flying debris from an implosion could result in bodily injury, including cuts and puncture wounds.
3.15.1. Precautions
When working with or near any part of the Clinac containing a vacuum, personnel should take every precaution to protect their bodies from flying debris produced by a possible implosion.
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3.16. Electric Shock
During operation, the normal voltages in some areas of the Clinac are over 25,000 Vdc. Pressing an Emergency Off button removes power from the gantry, drive stand, and parts of the console. However, voltages up to 230 Vac (380 Vac at 50 Hz) still remain in the power line from the main facility circuit breaker, the console computer, the console video monitor, the console printer, and the power line supplying these components. With the main circuit breaker turned off, locked, and tagged, many highvoltage capacitors in the Clinac continue to pose a potential shock hazard until they are discharged. Several high-voltage capacitors can recharge spontaneously to dangerous levels even after being discharged. Shorting sticks attached to the drive stand, gantry, and the modulator cabinet are designed to provide a convenient and safe means for discharging the highvoltage capacitors. Varian recommends you use shorting sticks to prevent inadvertent lack of ground. The shorting sticks must be left hanging in place to prevent capacitors from recharging. Contact with high-voltage circuits can cause serious injury or death.
3.16.1. General Precautions
3.17. Service Precautions
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!
Keep equipment covers, doors, and panels closed during operation.
!
Do not bypass interlock switches on the Clinac doors and panels.
!
Do not touch any component inside the Clinac unless you know it does not present a shock hazard. The potential for electrical shock remains until each circuit supplying the Clinac is turned off at the facility circuit breaker and high-voltage capacitors are shorted out and remain shorted out.
!
During service procedures, before you touch any high-voltage circuit component, remove power from the circuit and follow appropriate lockout/tagout procedures in accordance with OSHA 29CFR 1910.147 or equivalent local standards.
!
Hire only properly trained and experienced service personnel with a full understanding of electrical hazards and high-voltage safety practices.
!
Ground the circuit with the nearest shorting stick. Keep the stick attached to the circuit while you work on or near it. Use more than one shorting stick to eliminate the possible loss of ground.
!
Lockout/Tagout the system or subsystem that is being serviced.
!
Interlocks that must be bypassed for maintenance purposes must be restored immediately upon completion of the task. Never leave the machine unattended with any interlock bypassed without first posting appropriate warning signs.
!
Permit only service personnel with the appropriate training and experience to remove the protective housing of laser-beam units, accessories, and power supplies. For additional warnings, refer to the documentation supplied by the laser unit manufacturer.
!
Take special care and always turn off electrical current when working in confined areas of the Clinac, for example, in the Silhouette service bays located on either side of the stand. If any electrical components in these areas are not turned off, inadvertent electrical shock could occur.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
Adequate lighting is required in any area where work is performed. If necessary, provide additional lighting. For example, additional lighting is required for pin-to-pin troubleshooting in the Silhouette cabinet.
!
Turn off any electrical equipment before you move it.
3.18. Electrical Fire
Any electrical equipment carries the risk of an electrical fire. An electrical fire in the treatment room or console area could cause severe burns, asphyxiation, and other injuries or death.
3.18.1. Precautions
!
If you see smoke or smell a burning odor, follow the established local emergency procedure for a fire.
!
Never use water to fight electrical fires.
!
Permit only personnel trained in fire-fighting procedures to attempt to put out an electrical fire.
3.19. Remote Movements
The Clinac can be configured to allow couch position corrections and large couch rotation swings, as well as gantry and imager arm motions, from outside the treatment room. Some treatments (for example, stereotactic treatments) may require that the gantry move very close to the treatment couch, which increases the risk of collision. Although the gantry rotates at a maximum speed of only about one revolution per minute, substantial force is required to stop gantry motion because of the inertia of its large mass. In addition, if you have an imaging system (for example, On-Board Imager or PortalVision), extended imager arms intrude into the space near the treatment couch. 4D Console and On-Board Imager (OBI) software applications do not automatically retract the OBI arms. This presents danger of collision. For instructions on configuring motion limits, refer to the Clinac Technical Reference Guide. Use caution to prevent collisions: !
Retract all imager arms (OBI and/or PortalVision) away from the couch when not in use.
!
Always observe remote motions either directly, or using closed-circuit monitors.
!
Perform a dry-run (a pre-treatment execution of all motions prior to first treatment, or after a Plan correction) to check for potential collisions before delivering treatment.
!
Stop motions immediately if you suspect a collision may occur. The fastest way to stop motion is to release the Motion Enable bar.
If a collision occurs between the treatment couch and the gantry, call maintenance to inspect the system for damage. Do not operate the Clinac again until the machine is fully checked by service personnel. Never use the system for patient simulation until proper and safe operability has been verified by maintenance personnel.
3.20. LaserGuard
LaserGuard is an optional collision detection system for the Clinac. LaserGuard uses an invisible laser beam “shield” to detect potential collisions between the patient, treatment couch, and gantry. LaserGuard is a Class 1 laser product, and exposure for any length of time does not cause injury or damage to the eyes.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 LaserGuard does not replace existing safety systems or the vigilance of the user. You must verify all patient and equipment clearances before performing any remote motion sequence. Configuration of LaserGuard by unauthorized personnel can cause a machine malfunction that may result in damage to equipment, bodily injury, or death. Only authorized personnel should configure LaserGuard. Avoid divulging the configuration password to unauthorized individuals. For detailed information, refer to the LaserGuard Clinical Reference Guide.
3.21. On-Board Imager Precautions
The Varian On-Board Imager (OBI) allows you to take kV setup fields with two “arms” that you can rotate into position on either side of the treatment couch. One On-Board Imager arm delivers kV beam and the other arm transmits the image. Take special care to ensure that a collision does not occur between the OnBoard Imager arms and the patient, operator, or surrounding equipment. Such a collision can cause serious and possibly fatal injuries. Collisions between On-Board Imager and the treatment couch or any other fixed object can cause serious damage to both units, and serious or possible fatal injury to anyone caught between them.
3.21.1. Precautions
!
Exercise extreme caution during any motion of On-Board Imager and associated machine axes.
!
Before taking kV setup fields, rotate the On-Board Imager and associated machine axes to ensure that a collision will not occur during kV beam delivery.
!
Keep the patient under continuous observation whenever On-Board Imager and other machine axes are moved. If a collision appears possible, stop motions immediately by pressing Emergency Off.
!
Should the On-Board Imager arms collide with the treatment couch or any other equipment, shut down the Clinac immediately by pressing Emergency Off. Do not operate the Clinac again until the machine is fully checked by service personnel.
For details about On-Board Imager, refer to the On-Board Imager documentation.
3.22. High Dose Precautions
Depending on your Clinac software configuration, the Clinac allows you to deliver high-dose stereotactic fixed and arc X-rays for single surgical treatments and fractionated radiotherapy. It is important that you take appropriate precautions for delivering stereotactic treatments. When delivery high-dose treatments, Varian recommends that you: !
Use appropriate collimation (such as paraffin blocks, or a Varian MLC) to narrow the high-dose treatment field.
!
Employ reliable patient safety and immobilization precautions for stereotactic treatment: •
Immobilize the patient as comfortably as possible, and briefly explain the treatment procedure. This helps the patient to remain motionless on the treatment couch.
•
Remind the patient to stay on the treatment couch during treatment until it is safe to leave. A patient should not leave the treatment couch unless supervised by authorized personnel.
•
As with all treatments, perform a dry-run to check for potential collisions before delivering treatment.
For detailed information about Stereotactic treatments, refer to the C-Series Clinac: Instructions for Use.
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3.23. Falling Parts or Accessories
After prolonged use and its accompanying vibrations, the nuts, bolts, and other fasteners on the MLC or accessory mounts can become loose and fall from the machine. Similarly, wedge trays, shadow blocks, and other accessories can fall from the treatment head, usually as a consequence of improper installation. A part or accessory falling on a patient, operator, or other person could result in serious injury.
3.23.1. Precautions
3.24. Deterioration of Plastic Parts from Radiation
Check all nuts, bolts, and other fasteners for tightness at least once every six months. !
Take care to install accessories properly.
!
Never install or remove accessories when a patient is on the treatment couch. The only exception to this precaution is when an accessory can be installed only after the field light establishes the treatment area. In this case, use extreme caution.
!
Do not exceed the weight capacity of the accessory mount when loading shadow block trays.
Some Clinac parts are made of plastic and can lose strength as a result of age and prolonged exposure to radiation. Among these parts are the treatment couch top panels, tennis racket strings, compensator trays, and shadow block trays. Deterioration of plastic parts on the treatment couch could cause a patient to fall. Deterioration of plastic parts attached to the treatment head could allow an accessory to fall. A patient fall from the treatment couch or an accessory falling on a patient, operator, or other persons could result in severe injuries or death.
3.24.1. Precautions
3.25. Patient Fall from the Treatment Couch
!
Examine treatment couch panels monthly. Replace any panels that are cracked or show any signs of degradation. Regardless of condition, replace all couch panels after every 1000 hours of beam operation or 5 years of use, whichever comes first.
!
Inspect the strings on the tennis-racket-type panels periodically and replace upon any sign of degradation.
!
Examine all plastic parts periodically. Immediately replace any part that is cracked, discolored, or shows any signs of degradation.
Any of the following conditions could cause a patient to fall from the treatment couch: !
Collision with the gantry
!
Improper positioning or restraint of the patient
!
Top panels weakened from radiation
!
Degraded strings in the tennis-racket-type panels
!
Sudden movements or stops of the treatment couch
!
Overloading of the couch top
A patient fall from the treatment couch could result in severe injuries or death.
3.25.1. Precautions
!
Lock the couch top into position before you place the patient on it.
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3.25.2. Gantry Precautions
3.26. Treatment Couch Pinch Points
!
If you must rotate the couch top, do so before placing the patient on the couch. After rotation, make sure that the top is latched securely. Never apply weight to a couch top that is unlatched.
!
Position the patient carefully on the treatment couch. Distribute the patient's body uniformly over the couch top and secure with the restraining straps provided. For ETR and Exact Couches, the maximum safe patient weight is 400 lb. Other treatment couch weight limits may be less; for maximum patient weight, refer to the documentation for your treatment couch.
!
When the couch top is fully extended, do not exceed the maximum weight of 300 lb. allowed on the end of the couch top. Do not allow anyone to stand or sit on the end of a fully extended couch top.
!
Avoid sudden movements or stops when adjusting the treatment couch. Do not activate couch motions when brakes are released on the couch top.
!
Always keep the patient in sight when rotating the gantry. Stop the rotation immediately if a collision appears possible.
!
During patient setup, rotate the gantry from the hand pendant. Never rotate the gantry from the console.
!
Before beginning arc therapy, always perform a dry run and rotate the gantry through its entire arc to make sure that it cannot collide with the patient or equipment.
!
Always keep the patient in sight when performing setup motions. For remote motions, be sure that the camera angles provide adequate views of the patient.
The lateral and longitudinal carriages and the couch top contain potential pinch points during movement, particularly between the underside of the couch top and the lateral carriage. Additional pinch points are exposed when the protective bellows surrounding the lift mechanism must be lowered during maintenance procedures. The couch carriages, couch top, and lift mechanism contain numerous pinch points that could cause severe injury or death.
3.26.1. General Precautions
When moving the couch's lateral and longitudinal carriages or the couch top, make sure that all parts of your body are clear of the couch and that no person or equipment can be caught in the moving mechanism.
3.26.2. Service Precautions
If the bellows must be lowered for maintenance, install the safety braces and remove all electrical power to the treatment couch before working in or around the exposed lift mechanism. Follow appropriate lockout/tagout procedures. Be sure to keep hands and other body parts clear of potential pinch points.
3.27. High Temperature Surfaces
Many Clinac components operate at high temperatures, including the thyratron tubes in the modulator compartment of the drive stand, the quartz-halogen lamps in the treatment head and their accessories, and the transistor heat sinks in the power supplies. Body contact with the surface of a component operating at a high temperature can cause severe skin burns.
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3.27.1. Precautions
3.28. Laser Beams
!
Take care to prevent body contact with lamps, heat sinks, thyratron tubes, or any other component that provides a sensation of warmth upon approach.
!
Tubes or porcelain components can be damaged by the oils and moisture from your hand. When handling tubes or porcelain components, wear gloves to prevent direct contact with skin.
!
If you must handle a heat-producing component, allow a reasonable cool-down period after turning off the power.
!
Follow appropriate lockout/tagout procedures.
The optional localizer system uses up to five Class II laser beams to visually locate the Clinac isocenter and to aid in setting up the patient. Each of these beams is visible to the eye as a narrow line of bright red or green light.
WARNING: The radiation from one of these localizer laser beams can cause permanent retinal damage. The optional LaserGuard collision detection system uses an invisible laser beam ‘shield’ to detect potential collisions between the gantry head and the patient. Note: LaserGuard is a Class 1 laser product. Exposure for any length of time does not cause injury or damage to the eyes. For more information about LaserGuard, see “LaserGuard” on page 1-21.
3.28.1. Precautions
3.29. Software Integrity
!
Never stare directly into one of the visible green or red laser beams from the localizer system. Advise patients of this precaution. The invisible LaserGuard laser beams are safe and do not cause retinal damage.
!
For additional warnings, refer to the documentation supplied by the laser unit manufacturer.
Software and computer equipment included with the Clinac are installed by Varian, and developed and tested exclusively for operation of the Clinac system. This software remains the property of Varian and is licensed to the user strictly for the purpose intended. Modifying any of the software provided with the Clinac or using any other software on the Clinac computer can seriously compromise the integrity of data stored on the Clinac computer and can result in uncertain, unreliable, and potentially hazardous system operation. Any attempt to modify the Clinac computer, its operating system, or the Clinac software, or to operate non-Clinac software on the Clinac computer is considered by Varian to be a product alteration. This results in termination of the remainder of the warranty on a Clinac system. To protect the hospital, its patients, and Varian from the potential consequences of unauthorized modifications, software that has been altered shall be removed by Varian.
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3.29.1. Precautions
3.30. Microwave Tube Operating Hazards
!
Do not modify the software supplied with the Clinac, including the operating system and files placed on the fixed disk.
!
Operation of the Clinac computer is restricted to Clinac software provided by Varian. No other software is allowed.
!
Do not alter the configuration of the Clinac computer installed by Varian personnel, including interior printed-circuit boards, peripherals, and switch settings.
Serious hazards exist in the operation of microwave tubes. Safe operating practices require careful attention to hazards associated with microwave tubes. Persons who work with microwave tubes or equipment that uses them must protect themselves against serious injury. Careless operation of microwave tubes can result in damage to tubes, the linear accelerator, or other property; and potentially fatal injury. The operation of microwave tubes involves one or more of the following hazards:
3.30.1. Precautions
Microwave tubes in the Clinac operate at voltages high enough to cause fatal injury by electrical shock. !
Always break the primary circuits of the power supply and discharge high-voltage circuits when direct access to the tube is required. Follow appropriate lockout/tagout procedures.
!
Keep a safe distance from the voltages encountered.
!
Use grounded safety screens during tube operation.
3.30.2. Radio Frequency (RF) Radiation
Refer to “Radio Frequency (RF) Radiation” on page 1-15.
3.30.3. X-Ray Radiation
Never operate high-voltage tubes without adequate X-ray shielding in place.
3.30.4. Corrosive and Toxic Compounds
External output waveguides, cathodes, and high-voltage bushings of microwave tubes are sometimes operated in systems that use dielectric gas to impede microwave or high-voltage breakdown. Release of dielectric gas in the presence of moisture and arcing can create toxic and corrosive compounds. To prevent damage from dielectric gas breakdown:
3.30.5. Hot Water
!
When a leak in the dielectric system is detected or the waveguide is disassembled for service, ventilate the area and avoid breathing any fumes or touching any liquids that develop.
!
Take precautions for highly toxic and corrosive substances before permitting personnel to perform any work on or near the tube.
Extreme heat occurs in the electron collector portion of microwave tubes during operation. Water channels used for cooling reach temperatures as high as the boiling point of water (100° Celsius or above), and the hot water is under pressure (typically as high as 100 psi). A rupture of the water channel or contact with hot portions of the tube can scald or burn.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Take precautions to prevent and avoid such rupture or contact.
3.30.6. Hot Surfaces
Portions of microwave and thyratron tube surfaces can reach extremely high temperatures, especially the cathode insulator and cathode/heater surfaces. All heated surfaces can remain hot for an extended time after the tube is shut off. To prevent serious burns, take care to avoid any bodily contact with these surfaces, both during tube operation and for a reasonable cool-down period after operation.
4. Service and Maintenance Guidelines
This section provides service and maintenance safety guidelines for the Clinac and related systems.
Note: Lockout/tagout procedures are required for servicing or maintaining the Clinac. Refer to OSHA regulation 29 CFR 1910.147, The Control of Hazardous Energy (Lockout/Tagout) and 29CFR1910.333, Selection and Use of Work Practices and other applicable state and federal occupational safety and health regulations (domestic US), or other local (or international) standards for additional information and requirements.
4.1. Clinac Accelerator Specifications
The following specifications must be observed for safe Clinac operation.
4.1.1. Electrical Specifications
Electrical operating specifications: !
Type of protection against electric shock: Class I
!
Degree of protection against electric shock: Type B
!
Operation: The Clinac is classified as being suitable for continuous connection to the supply main in the standby state and for specified permissible loadings.
!
The Clinac is not for use in the presence of flammable anesthetic mixtures.
!
Electrical requirements: •
Low Energy Clinac Series input voltage: 200 to 240 Vac 50 or 60 Hz 42 Amps max @208V; or 360 to 440 Vac 50 or 60 Hz 22 Amps max @ 400V.
•
High Energy Clinac Series input voltage: 200 to 240 Vac 50 or 60 Hz 125 Amps max @ 208V; or 360 to 440 Vac 50 or 60 Hz 65 Amps max @ 400V.
4.1.2. Environmental Specifications
Environmental operating requirements:
4.2. Lockout/Tagout Procedures
Lockout/tagout is required when servicing or maintaining the Clinac or a Clinac subsystem if unexpected startup or release of stored energy could cause injury to personnel. All sources of hazardous energy to which personnel could potentially be exposed, including electrical, mechanical, and
!
Humidity range: 15% to 80% relative humidity, non-condensing
!
Temperature range: 60°F to 80°F (16°C to 27°C)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 potential energy due to gravity or pressure, must be shut down and secured by authorized personnel. Depending upon the work being performed, it may be necessary to use multiple procedures to isolate all energy sources. The rules and standards established in lockout/tagout procedures apply to everyone associated with the maintenance and operation of the Clinac. Failure to follow proper lockout/tagout procedures can result in serious and possibly fatal injuries. The lockout/tagout procedures in this section are organized according to the type of energy source identified:
4.2.1. General Lockout/Tagout Procedure
!
Main Electrical Power
!
Electrical Power to Subsystems
!
Electrical Power to Facility Subsystems
!
Gantry Locking Pin
!
Patient Couch
!
Air System
!
Gas System
!
Water System
1.
The general procedure for lockout/tagout is as follows:
2.
Notify affected personnel that a lockout procedure is about to begin and that the Clinac subsystem or facility power will be shut down for service.
3.
Locate all energy sources associated with the system or subsystem.
4.
Operate the energy-isolating devices necessary to isolate the system or subsystem.
5.
Attach a lockout/tagout device to each energy-isolating device involved. Typical lockout devices are shown in Figure 1.3.
6.
Dissipate any stored energy.
7.
Verify that the system or subsystem is isolated and deenergized.
WARNING: Do not remove, bypass, or ignore lockout/ tagout devices (Figure 1.3). Only the authorized service person installing the lockout/tagout devices is permitted to remove them. Unauthorized removal can result in damage to equipment, and injury or death to patients or personnel.
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Figure 1.3. Typical Lockout Devices
4.2.2. Releasing Lockouts
4.2.2.1. Releasing Lockout During Service
This manual includes specific details of lockout/tagout procedures in the section for each lockout/tagout procedure. General steps for releasing a lockout are as follows: 1.
Prepare the system or subsystem for operation. This includes inspecting the system or subsystem to verify that all equipment components are fully assembled and operational, that all safety panels are in place, and that all tools and other nonessential items have been retrieved.
2.
Ensure that personnel are positioned safely.
3.
Remove lockout devices and tags.
4.
Notify affected personnel that the system or subsystem has been released from lockout.
A lockout/tagout procedure is not required if service and maintenance personnel will not be exposed to the unexpected release of hazardous energy. Some service or maintenance functions require equipment, or portions of the equipment, to be turned on. Under these circumstances, the work must be conducted with appropriate safeguards to prevent exposure to hazardous energy sources. Sometimes it may be necessary to remove lockout/tagout devices temporarily and turn on the machine or equipment for a limited time for testing or positioning of equipment or components. Under such circumstances, the work must be completed with appropriate safeguards to prevent exposure to hazardous energy sources. As soon as possible, once the testing is done, the authorized service personnel must again turn off the equipment and resume lockout/tagout procedures.
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4.2.3. Main Electrical Power
When service is performed that requires removing main power to the Clinac and where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout the main electrical power to the Clinac.
WARNING: The activation of main electrical power to the Clinac while someone is servicing the Clinac can result in serious and possibly fatal injury.
4.2.3.1. Lockout/Tagout Main Power
To lockout/tagout the main power: 1.
Notify affected personnel in the area that main electrical power to the Clinac will be under lockout/tagout.
2.
Place the Clinac in standby mode.
3.
Turn off the main circuit breaker, switchbox disconnect, or other energy isolating device that provides power to the Clinac.
4.
Attach an appropriate lockout/tagout device and tag to the energyisolating device (Figure 1.4).
5.
Verify that power has been removed.
Figure 1.4. Main Circuit Breaker Lockout/Tagout
4.2.3.2. Restoring Main Electrical Power
1-30
To restore main electrical power: 1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
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4.2.3.3. Electrical Power to Subsystems
3.
Remove lockout devices and tags.
4.
Turn on the main power.
5.
Notify affected personnel that service has been completed and that the Clinac is ready for startup.
When service is performed on a subsystem within the Clinac, where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout the electrical power to the system at the applicable energy source.
WARNING: The activation of electrical power to a subsystem while someone is servicing the subsystem may result in serious and possibly fatal injury. To lockout/tagout the power to a subsystem: 1.
Notify affected personnel in the area that main electrical power to the Clinac will be under lockout/tagout.
2.
Identify and locate the source of electrical power to the individual subsystem to be worked on.
Note: For detailed information, refer to the electrical schematic diagrams in your product data book or service/systems manual.
4.2.4. Power to Facility Subsystems
3.
Turn off power to the subsystem by turning off the appropriate energy-isolating device.
4.
Attach appropriate lockout/tagout devices to the appropriate energy-isolating devices.
5.
Use shorting sticks to dissipate any remaining energy (see “Using Shorting Sticks” on page 1-41).
When service is performed on facility subsystems, where unexpected startup or release of stored energy could cause injury to personnel, lockout/tagout electrical power to the subsystem at the applicable energy source. Typical facility subsystems include but are not limited to: !
Setup lights
!
Laser positioning lights
!
Closed-circuit television
!
In-room monitors
WARNING: The activation of electrical power to a facility subsystem while someone is servicing the Clinac or subsystem may result in serious and possibly fatal injury.
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4.2.4.1. Cordand-Plug Subsystems
4.2.4.1.1. Cordand-Plug Subsystem Precautions
Lockout/tagout procedures do not apply to cord-and-plug subsystems, as long as the cord to the subsystem is pulled and appropriate safety precautions taken. Typical cord-and-plug subsystems include: 1.
Clinac control console and display.
2.
Multileaf collimator (MLC) console and display.
3.
PortalVision console and display.
4.
VARiS console and display.
5.
Respiratory Gating, LaserGuard, and other additions to the Clinac.
When servicing cord-and-plug subsystems, the following safety precautions apply: !
Ensure that the electrical subsystem is unplugged.
!
Plug must remain in the exclusive control of the person performing service or maintenance work.
!
If the plug is not under exclusive control, lock it out (with a plug cap) and tag it.
To lockout/tagout the cord-and-plug subsystems: 1.
Notify affected personnel that electrical power to the facility subsystem will be under lockout/tagout.
2.
Identify and locate the source of electrical power to the facility subsystem to be worked on.
3.
Turn off power to the subsystem by turning off the appropriate energy-isolating device.
4.
Attach appropriate lockout devices and tags to appropriate energyisolating devices.
5.
Verify that power has been removed by measuring the absence of voltage at the applicable subsystem.
To release the lockout on cord-and-plug subsystems:
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1.
Check the Clinac and facility subsystem to ensure that nonessential items (such as tools or service equipment) have been removed and that the Clinac, Clinac subsystem, or facility is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove lockout devices and tags.
4.
Turn on power to the facility subsystem.
5.
Verify proper operation of the subsystem.
6.
Notify personnel that service is complete and that the subsystem is ready for startup.
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4.2.5. Gantry
You can prevent an unexpected gantry rotation during service by locking out and tagging the gantry locking pin. Lockout/tagout is always required when servicing the gantry drive system (for example, gantry motors, drive chain, harmonic drive, and clutch). With the gantry locking pin disengaged and the drive system components intact, minor imbalances to the gantry due to the removal of fiberglass or other components may result in slow gantry rotation. The speed of rotation is directly related to the amount of the imbalance. If drive system components are not intact (for example, if the drive chain is removed), gantry imbalances may result in a more rapid rotation.
WARNING: A collision between the gantry and service personnel could cause serious and possibly fatal injuries. A collision between the gantry and the treatment couch or any other fixed object could cause serious damage to both units, and serious or possible fatal injury to anyone caught between them.
To lockout/tagout the gantry locking pin: 1.
Notify affected personnel that the gantry will be under lockout/tagout.
2.
Ensure that the gantry rotation axis is unobstructed.
3.
Rotate the gantry to one of the four locking positions (at 0, 90, 180, and 270°). Choose the angle that provides the most convenient access for the maintenance or service to be performed.
4.
Engage the locking pin.
5.
Visually confirm that the gantry locking pin is engaged. While staying outside the arc of the gantry rotation, verify that the gantry is locked in place by moving or rocking the gantry by hand, or by operating the hand pendant.
Note: On low-energy Clinacs, the gantry locking pin can be seen protruding approximately 1/4 in. to 1/2 in. past the inside surface of the gantry sprocket hub, when engaged. 6.
Attach appropriate lockout devices and tags to the gantry locking pin (Figure 1.5).
To release the gantry lockout, see “Releasing Lockouts” on page 1-29.
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Figure 1.5. Gantry Locking Pin with Lock and Tag
4.2.6. Treatment Couch Lift
There are three types of Clinac treatment couches: !
PSA couch
!
ETR couch
!
Exact couch
When service is performed on the treatment couch, where unexpected energization, startup or release of stored energy could cause injury to personnel, block the couch with safety braces and tag it.
WARNING: Unexpected movement of the patient couch can cause serious and possibly fatal injury.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 To lockout/tagout the treatment couch: 1.
Notify personnel that the treatment couch will be blocked and tagged.
2.
For the PSA couch, raise the couch skirting. For the Exact or ETR couch, lower the bellows.
3.
Raise the couch sufficiently to allow insertion of the braces.
4.
Insert the couch safety braces (Figure 1.6 and Figure 1.7).
5.
Lower the couch until it is resting against the safety braces.
6.
Attach lockout tags to the rear of the couch.
Figure 1.6. ETR or Exact Couch with Safety Braces
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Figure 1.7. PSA Couch with Safety Braces To release the lockout on the treatment couch:
4.2.7. Air System
1.
Check the Clinac and the area around the machine to ensure that nonessential items have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the couch safety braces and lockout tags.
4.
Notify affected personnel that service is complete and the patient couch is ready for use.
When service is performed on the compressed air system (high-energy Clinac models only), where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the air system.
CAUTION: The release of air pressure while someone is servicing the air system can result in injury.
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To lockout/tagout the compressed air system:
2.
Notify affected personnel that the air system will be under lockout/tagout.
3.
Turn off the air flowing to the machine.
4.
Remove the valve handle or air hose (if present).
5.
Attach an appropriate tag to the valve (Figure 1.8).
6.
Release air pressure in the line.
7.
Confirm that no air is flowing to the machine by checking that the pressure gauge reads zero.
Figure 1.8. Air Hose Removed and Lockout Tag Attached To remove the lockout: 1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout tag.
4.
Replace the valve handle or air hose.
5.
Open the valve.
6.
Verify that there are no air leaks.
7.
Notify personnel that service is complete and that the system is ready for use.
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4.2.8. Gas System
When service is performed on the SF6 or Freon 12 gas system, where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the gas system.
CAUTION: The release of gas pressure while someone is servicing the gas system can result in injury. Varian recommends the use of an electronic “sniffer” device to detect gas leaks. To lockout/tagout the gas system: 1.
Notify affected personnel that the gas system will be under lockout/tagout.
2.
Turn off the gas flowing to the machine.
3.
Remove the valve handle (Figure 1.9).
4.
Attach an appropriate lockout tag to the valve.
5.
Release any stored gas pressure or other energy in the line.
6.
Confirm that there is no gas flowing to the machine.
Figure 1.9. Handle Removed and Lockout Tag Attached
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 To remove the lockout:
4.2.9. Water System
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout tag.
4.
Replace the valve handle.
5.
Open the valve.
6.
Verify that there are no gas leaks.
7.
Notify personnel that service is complete and that the system is ready for use.
When service is performed on the water system, where the unexpected energization, startup, or release of stored energy could cause injury to personnel, lockout/tagout the water system.
WARNING: Release of water and water pressure on exposed electrical components while someone is servicing the water system may result in serious and possibly fatal injury.
To lockout/tagout the water system: 1.
Notify affected personnel that the water system will be under lockout/tagout.
2.
Turn off the water flowing to the machine.
3.
Turn off the water pump.
4.
Lockout/tagout the appropriate pump circuit breakers.
5.
Confirm that there is no water flowing to the machine.
6.
Remove the valve handle.
7.
Attach an appropriate lockout tag to the valve (Figure 1.10).
8.
When opening water system lines, use caution to prevent water from dripping onto, or spraying into, electronic components.
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Figure 1.10. Water Valve with Handle Removed and Tag Attached To remove the lockout:
4.2.10. Shift Changes
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the machine is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the lockout devices and tags.
4.
Replace the valve handle, if applicable.
5.
Open the valve, replace the locking pin, or otherwise restore equipment to normal operating conditions.
6.
If applicable, restore power to the system. For example, turn on the appropriate pump circuit breaker to restart the water pump.
7.
Verify that there are no leaks or other unexpected hazards.
8.
Notify affected personnel that service is complete and that the system is ready for use.
If maintenance on a Clinac extends beyond one shift, or if there is a personnel change, all additional personnel are required to place their locks on the lockout device before they begin work on the equipment. If maintenance on a Clinac requires multiple service personnel, all are required to place their locks on the lockout device.
4.2.11. Working with Contractors 1-40
When outside personnel (contractors or others) will be working on the Clinac, the on-site employer or owner and the outside contractors must inform each other of the lockout/tagout procedures they will use.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Authorized personnel responsible for performing the lockout must ensure that any affected personnel understand and comply with appropriate lockout procedures in accordance with this manual and applicable regulations. Coordinate lockout activities with all involved parties.
4.2.12. Using Shorting Sticks
When working on the modulator, gun deck, or VacIon subsystems, use shorting sticks to dissipate any residual stored energy, and leave the shorting sticks in place to prevent the reaccumulation of charge. Note: Varian recommends that you use more than one shorting stick to prevent the inadvertent loss of ground. Verify that power has been removed by measuring the absence of voltage at the subsystem. To use shorting sticks:
4.2.13. Chemical Hazards
1.
Check the Clinac and the immediate area around the machine to ensure that nonessential items (such as tools or service equipment) have been removed and that the Clinac, Clinac subsystem, or facility is operationally intact.
2.
Ensure that personnel are positioned safely.
3.
Remove the shorting sticks (if in use), lockout devices, and tags.
4.
Turn on power to the subsystem.
5.
Verify proper operation of the subsystem.
The Clinac contains substances that can be hazardous to your health. Information located on container labels and material safety data sheets (MSDSs) are provided for your protection and safety. Always read the information contained on labels and MSDSs provided by the manufacturer of hazardous substances. Failure to understand and identify the presence of hazardous substances in the Clinac and its accessory products can result in serious injury. This information is provided as general guidance only. Refer to OSHA regulation 29 CFR 1910.1000 and other applicable state and federal occupational safety and health regulations (domestic US), or other local (or international) standards for additional information and requirements. The guidelines presented in the next section, “Communication Guidelines for Hazardous Chemicals,” apply to all Clinac service personnel.
4.2.13.1. Communication Guidelines for Hazardous Chemicals
The owner must develop and maintain a written hazard communication program. This program must accomplish the following: !
Inform employees about hazard-communication standards.
!
Explain implementation of the program as it applies to their specific workplace.
!
Inform and train employees how to recognize, understand, and use the labels and MSDSs provided by vendors or otherwise provided with the Clinac.
!
Inform and train employees about safety procedures for working with hazardous substances.
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4.2.13.2. Employee Responsibilities
Operators and maintenance and service personnel who come into contact with the Clinac are required to read the labels and the MSDSs provided with the Clinac. They are also required to follow pertinent instructions and warnings.
4.2.14. Material Safety Data Sheets (MSDSs)
Material safety data sheets provided with your Clinac contain the following information:
4.2.14.1. Container Labels
4.2.14.2. Hazardous Substances in the Clinac
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!
Identification of the substance, including the manufacturer’s name, address, and emergency phone number
!
Hazardous components, including the chemical ID, common names, and worker exposure limits
!
Physical and chemical characteristics, including boiling point, vapor pressure, vapor density, melting point, evaporation rate, water solubility, appearance, and odor under normal conditions
!
Physical hazards, including safe handling methods
!
Reactivity, indicating whether the substance is stable or not
!
Health hazards, including information on: how the chemical could enter the body, for instance through inhalation, skin contact, or by swallowing; whether or not the chemical is considered a carcinogen; signs and symptoms of exposure; and existing medical conditions that could be aggravated by exposure
!
Precautions for safe handling and use, including information on: what to do if the substance spills or leaks, disposal of the substance, equipment and procedures needed for cleaning up spills and leaks, how to handle the substance properly, storage, and any other precautions
Containers of hazardous chemicals are labeled with the following information: !
Chemical name
!
Manufacturer or importer information, including name, address, and emergency contacts
!
Physical hazards
!
Health hazards
!
Personal protective clothing, equipment, and procedures recommended
!
Storing or other special handling instructions
Depending upon the Clinac model, the following hazardous substances may be present: 1.
Freon 12 is present in the waveguide system in Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
2.
Sulfur hexafluoride (SF6) is present in the waveguide systems of all Clinacs other than Clinacs 4/100 (prior to S/N 81), 6/100 (prior to S/N 457), and 600C (prior to S/N 66).
3.
Lead is present in the shielding and balance weights in all Clinacs.
4.
Beryllium (99% pure) is present in the window of the bend magnet vacuum chamber of all high-energy Clinacs.
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4.2.14.3. Other Substances in the Clinac
4.2.15. Preventing Exposure to Bloodborne Infections
5.
Dielectric insulating oil is present in the pulse transformer in all Clinacs.
6.
Dielektrol III capacitor oil is present in the modulator, rectifiers, and capacitors in all Clinacs.
7.
Depleted uranium is present in the X-ray head of older low energy Clinacs.
Depending upon the Clinac model, other substances, which may pose negligible health risks under normal operating conditions, may also be present. These substances may include: !
Ethylene glycol
!
Lubricants
!
Paint (touch-up)
!
Paint (black, aerosol)
!
Asbestos
Some bloodborne infections are caused by viruses carried in the bloodstream. These viruses are usually transmitted by contact with infected blood, although in some cases they may be transmitted by other bodily fluids as well. Individuals infected with bloodborne pathogens, such as the hepatitis B virus (HBV) or the human immunodeficiency virus (HIV), can develop serious health problems that could ultimately prove fatal. This information is provided for general guidance only. Refer to OSHA regulation 29 CFR 1910.1030, “Occupational Exposure to Bloodborne Pathogens” and other applicable state and federal occupational and health regulations (domestic US), or other local (or international) standards, for additional information and requirements. The guidelines presented in this section apply to all Clinac operators and maintenance and service personnel. Occupational situations that may present a risk of exposure to bloodborne pathogens include: !
Periodic maintenance inspections (PMIs).
!
Routine maintenance.
!
Cleaning of the treatment couch and the collimator.
!
Patient contact.
Since the Clinac and its accessories are located in a hospital environment, the following occupational situations may apply: !
Service-related duties that bring you into contact with human blood or other bodily fluids
!
Service-related duties that bring you into contact with hypodermic needles or other sharp instruments that might be contaminated with infected blood
!
Service-related duties that bring you into contact with materials (towels, sheets, clothing) contaminated with blood or other bodily fluids
!
Potential occupational exposure to bloodborne pathogens
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4.2.15.1. Safety Precautions
!
Providing emergency first-aid assistance to coworkers
!
Dealing with blood, unfixed tissue, or organs from humans (living or dead); tissue cultures; or solutions that contain blood or other human tissue
!
Handling experimental animals infected with HIV or HBV, or the blood, organs, or tissues from these animals
!
Assume all bodily fluids are infectious and act accordingly. Avoid contact with blood or other bodily fluids. Keep all cuts and scrapes covered.
!
Follow proper procedures when in contact with infected blood or other bodily fluids or contaminated materials. Avoid procedural shortcuts, which may save you time, but place you at risk of becoming exposed to potential bloodborne pathogens.
!
Follow proper housekeeping procedures. Recommended housekeeping guidelines include:
!
!
•
Clean and decontaminate all equipment and work surfaces that have been contaminated with blood or other potentially infectious materials. Use the disinfectant required by the employer (or, if none is specified, with a solution of 5.25% sodium hypochlorite [household bleach] diluted between 1:10 and 1:100 with water).
•
Remove and replace protective coverings, such as plastic and foil, that may have become contaminated.
•
Always use mechanical means, such as tongs, forceps, or a brush and dust pan, to pick up contaminated broken glassware. Never use your hands to pick up broken glassware even when wearing gloves.
•
Place all potentially contaminated wastes in designated disposal containers.
•
Store sharp objects in a manner that ensures safe handling.
Dispose of waste properly. Recommended waste disposal procedures include: •
Place all infectious waste in closable, leakproof containers or bags that are color-coded and appropriately marked.
•
Place all sharp items in puncture-resistant containers for disposal.
•
Double-bag infectious wastes if the outside of a bag is contaminated with blood or other potentially infectious materials.
•
Inspect and decontaminate bins, pails, and cans used for storage or disposal of potentially infectious materials on a regular basis.
Observe proper hygiene. Infected matter can enter the bloodstream through cuts and other openings in the skin, through the eyes, or through the mucous membranes of the nose and mouth. •
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It is especially important to wash hands frequently, since they are the most likely source of contact with infected blood or other bodily fluids. Wash carefully after any contact with body fluids or other potentially infectious materials and after removing gloves that may have been contaminated.
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4.2.16. Using Personal Protective Equipment
•
If blood splatters in the eyes, flush thoroughly with clean water as soon as possible after the contact.
•
Wear appropriate personal protective equipment when the job involves contact with blood or potentially infected material, or when giving first aid.
Personal protective equipment (PPE) prevents blood or other potentially infectious material from coming into contact with the skin, eyes, mouth, mucous membranes, or clothing. This section details procedures for using PPE to protect yourself from possible hazards. Wear the appropriate PPE for the task you are performing. This information is provided for general guidance only. Refer to OSHA regulation 29 CFR 1910.132-1910.139 and other applicable state and federal occupational and health regulations (domestic US), or other local (or international) standards, for additional information and requirements.
4.2.16.1. Safety and Service Guidelines
4.2.16.2. Eye Protection
These guidelines for using personal protective equipment apply to all Clinac operators and maintenance and service personnel: !
Obtain PPE that fits well.
!
Inspect and clean PPE before and after each use.
!
Use the correct PPE for the situation. Depending on the hazards of the job, you may need more than one type of PPE to protect yourself.
Safety glasses or goggles must be worn while servicing the Clinac where eye hazards exist. !
Wear safety glasses fitted with side shields to provide maximum protection from flying particles or dust.
!
Keep protective eyewear clean.
!
Contact lenses are not a substitute for protective eyewear.
4.2.16.3. Foot Protection
Shoes that provide protection for the whole foot, such as work shoes, are recommended during service, maintenance, and installation of the Clinac. For electrical service on equipment involving potentials above 20 kV, wear safety-toe boots. Do not wear steel-toe boots.
4.2.16.4. Respiratory Protection
Masks should be worn whenever there is a risk of exposure to airborne substances, such as dust or chemical vapor. The mask should cover the nose and mouth.
4.2.16.5. Hand Safety
Wear protective gloves while servicing the Clinac where hazards to your hands exist, and follow these guidelines: !
Treat used waveguide pieces as though they are contaminated and handle with personal protective equipment (PPE). Always wear eye protection and chemical-resistant gloves. For information about PPE, see “Using Personal Protective Equipment” on page 1-45.
!
Leather gloves used for handling lead must be set aside and not used for other purposes. With prolonged use, leather gloves can absorb lead. Always wear disposable latex, vinyl, or nitrile gloves under leather gloves that have been used to handle lead.
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Before servicing the Clinac, remove jewelry such as rings, bracelets, and watches.
!
Stay clear of pinch points and crushing hazards including, for example, motors and gears in the collimator, gantry, and couch. Lockout/tagout the couch and gantry locking pin, when appropriate, to prevent hand injuries.
!
If you anticipate coming into contact with blood or other potentially infectious materials, wear latex, vinyl, or nitrile gloves. Check items for slivers, jagged edges, or burrs before lifting.
!
Always wash your hands immediately after removing gloves.
!
Dispose of single-use gloves in the proper containers. If gloves are contaminated, do not dispose of them in regular trash.
Table 4-1 lists recommended types of gloves.
Note: For those with rubber or latex allergies, use vinyl or nitrile gloves.
Table 1.1. Hand Protection
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Operation
Type of Glove
Protect Against
Notes
Handling lead
Leather gloves with latex, vinyl, or nitrile gloves underneath
Lead particulates or dust
Latex, vinyl, or nitrile gloves prevent contamination from leaching of lead through dedicated leather gloves
Handling depleted uranium
Leather
Low-level radiation and uranium poisoning
Handling ion chamber
Latex, vinyl, or nitrile
Contaminated dust, low-level radiation, and moderate heat
Handling hot thyratron tubes
Leather
Moderate heat
Handling sharp or rough objects
Leather
Cuts and abrasions
Contact with blood or other bodily fluids
Latex, vinyl, or nitrile
Bloodborne pathogens
Cleaning couch or collimator
Latex, vinyl, or nitrile
Bloodborne pathogens
Handling waveguide parts
chemical-resistant gloves, such as rubber or neoprene
Chemical hazards
Handling chemicals
Rubber, neoprene
Chemical hazards
Also wear eye protection
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4.2.17. Heavy Lifting and Handling
There are a number of situations that may require lifting or handling heavy objects during the service and maintenance of the Clinac.
4.2.17.1. Guide lines for Heavy Lifting
General guidelines for lifting and moving heavy equipment or objects safely include:
Lifting and handling heavy equipment or objects improperly can cause personal injuries, including sprains, strains, fractures, wounds, and hernias.
!
Dress for safety. Shoes with reinforced toes and nonslip soles may help to reduce the risk of injuries.
!
Think ahead. Plan a clear and unobstructed route to your destination.
!
Examine the object. Judge its weight and stability. Look for sharp edges. Decide how best to hold the object.
!
Get a good grip. Use your palms and fingers. Wear only properly fitting gloves.
!
Get help if you need it. For objects over 20 kg (44 lb.), use suitable handling devices and techniques. If you have any doubt about your ability to move the object, ask for help or use a mechanical aid.
Practice proper lifting techniques to safely protect your back and spine: !
Stand close to the load with feet wide apart.
!
Squat, bending at the hips and knees.
!
As you grip the load, curve your lower back inward by pulling your shoulders back and pushing your chest out.
!
Be sure to keep the load close to your body.
!
When you set the load down, squat, bending at the hips and knees. Keep your lower back curved inward.
If you are unable to bend your knees easily or get very close to an object, use alternative lifting techniques:
4.2.17.2. Precautions
!
Stand as close as you can.
!
Brace your knees against a solid object.
!
Bend at the hips, keeping your head and back straight.
!
Lift slowly, using your legs, buttocks, and stomach muscles.
When lifting or handling heavy objects (those weighing over 20 kg, or 44 lb.), obtain or use equipment appropriate for the task at hand. Also refer to information provided during specific product safety training. Situations that require lifting or handling heavy objects include, but are not limited to, the following: !
Lifting or handling the turntable or collimator: Use appropriate Aframe handling equipment.
!
Lifting or handling the sled assembly: Use appropriate bracket-fixture handling equipment.
!
Lifting or handling the Klystron or Klystron solenoid: Use appropriate handling equipment.
!
Lifting or handling gas cylinders (high-energy Clinacs), e-rack power supplies, RF drives, or fiberglass covers (canoes): Request assistance when necessary.
Emergency and Safety: Service and Maintenance Guidelines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.2.18. Tripping and Other Hazards (Silhouette Edition Clinac)
Tripping hazards exist when working in either service bay of the cabinet of a Silhouette Edition Clinac. The RF side has a step-up plate so that the hazard is decreased, but the modulator side has a stand-alone kick plate where special care must be taken. Ignoring the size and location of this plate can cause someone to trip and cause bodily injury.
5. Owner Guidelines
This section provides guidelines for establishing emergency and safety procedures for Clinac operation and maintenance.
5.1. Planning Operations
To prepare for the safety of patients and staff in the treatment room and surrounding areas, the owner is responsible for installation planning and installation of certain emergency and safety equipment.
5.1.1. Emergency-Off Buttons
Provide sufficient Emergency Off buttons in the treatment room and console area. These buttons should complement the Emergency Off buttons supplied by Varian on the dedicated keyboard, on both sides of the treatment couch, and on both sides of the drive stand. For additional information about the location of Emergency Off buttons, refer to National Council on Radiation Protection and Measurements (NCRP) Report Number 51.
5.1.2. Main Circuit Breaker
Install the main facility circuit breaker within 10 feet (3 meters) of the control console. The main circuit breaker for the Clinac should have an undervoltage trip that causes the main breaker to trip if an Emergency Off button is pressed.
5.1.3. Ventilation and Temperature Regulation
Maintain adequate room ventilation. Heat and air conditioning should be available to maintain the Clinac at room temperature (65–70° F).
5.1.4. Fire Extinguishers
Provide suitable fire extinguishers in the treatment room and near the control console. In the United States, the type of extinguisher must be approved for electrical fires by federal, state, and local codes and regulations.
5.1.5. Radio FrequencyEmitting Equipment
Ensure that nearby rooms with RF-emitting equipment have proper room shielding to prevent leakage and door interlock switches to prevent operation with doors open. Adopt operating procedures that minimize leakage potential.
5.1.6. Emergency Lighting
Provide automatic emergency lighting (with flashlights as backup) in the treatment room and console area.
5.2. Radiation Protection Survey
Before beam calibration and routine use of the Clinac, the owner must have a radiation protection survey completed by a qualified radiation expert.
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Service personnel could bump their heads on the portion of the waveguide that crosses the RF cabinet service bay. Before working in this area, Varian recommends that you first remove this portion of the waveguide, and then reinstall it when the work is completed.
Emergency and Safety: Owner Guidelines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the United States, the radiation survey report indicates if the installation meets the recommended standards of the NCRP and the applicable local, state, and federal regulations. Outside the United States, the owner is responsible for compliance with the applicable statutory and regulatory requirements. Before routine use of the Clinac, the owner must:
5.3. Safety and Emergency Training
!
Have a qualified radiological physicist calibrate the dose rate and integrated dose measured by the transmission ionization chamber.
!
Conduct checks at least daily during the first month of operation to establish that the ionization chamber response is constant within specified limits.
!
Make constancy checks during the course of each day to compare monitor response from the start to the end of the working day.
!
Conduct daily, or at least weekly, calibration checks after the constant output of the machine is established. Record all calibration measurements in a log.
Personnel who work with or near the Clinac must receive formal training on the emergency and safety procedures adopted by the owner. Each person should receive further training on a periodic basis. Training should include at least the following items: !
Location and use of Emergency Off buttons
!
Location and use of the main facility circuit breaker
!
When to use lockout/tagout procedures as required by OSHA 29 CFR 1910.147 and/or other applicable state and federal occupational safety and health regulations (domestic US) or other local (or international) standards
!
Local evacuation procedures for fire, smoke, or chemical fumes
!
Location and use of the emergency lighting system (including backup lighting such as flashlights)
!
Procedure to remove a patient from the treatment couch in an emergency
5.4. Routine Use
The owner must establish operational and maintenance safety procedures for routine use of the Clinac. Combine the following items with the other safety precautions described in this section to reduce the likelihood of injury to personnel and damage to the equipment.
5.4.1. Addressing Equipment Malfunctions
Cease machine operation immediately if any equipment malfunction is detected or suspected, and call service personnel to correct the problem.
5.4.2. Recording Observations
Record any unusual machine behavior or other observations in the log book when you perform the daily checkout procedure and throughout the treatment day.
5.4.3. Securing Keys
When the machine is placed in standby mode and left unattended, remove the DISABLE/ENABLE and POWER keys from the equipment and deposit
Emergency and Safety: Owner Guidelines
1-49
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 them in a key storage enclosure. Lock the enclosure to prevent unauthorized activation of the machine.
5.4.4. Testing Emergency-Off Circuits
Test the emergency-off circuits at least once every three months to ensure proper functioning.
5.4.5. Checking Fasteners
Check all fasteners for tightness at least semiannually.
5.4.6. Posting Signs
Post signs on doors to the treatment room and in the console area to inform:
5.5. Quality Assurance
!
All persons that a radiation hazard exists in the area.
!
Personnel to wear radiation monitoring instrumentation when they enter the treatment room.
!
Operators to have the DISABLE/ENABLE key in their possession when they enter the treatment room and to make sure that the door cannot close while in the room.
!
Any person wearing a pacemaker to remain out of the area until the effect of radiation and radio-frequency interference on pacemakers is known.
Because of the importance of precisely administered treatments in radiation therapy, the owner should establish a comprehensive quality assurance (QA) program for each radiation therapy facility. Sources of errors in radiation therapy include: !
Tumor localization
!
Patient immobilization
!
Field placement
!
Human errors in calibration
!
Patient setup
!
Equipment use
A program of periodic checks can minimize many of these errors. References: “AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Committee Task Group,” Medical Physics, Vol. 21, No. 7, July 1994 “Comprehensive Quality Assurance for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group,” Medical Physics, Vol. 21, No. 4, April 1994 “Physical Aspects of Quality Assurance in Radiation Therapy,” AAPM Report No. 13, American Association of Physicists in Medicine, May 1984
5.6. Accidental Radiation Overdose
1-50
The owner must establish the procedures to follow in case of an accidental overexposure of a patient or personnel to radiation. Post these procedures conspicuously in the console area.
Emergency and Safety: Owner Guidelines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the event that a person is thought to be exposed to an excess of radiation, the law in most localities of the United States requires the following steps:
5.7. Backup Interlocks
5.8. Emergency Beam Termination
!
Immediately notify the appropriate local, state, and federal authorities.
!
Request an investigation by a professional qualified in the detection of radiation.
!
Consult with medical experts in radiation treatment.
!
Check the following references: •
National Council on Radiation Protection and Measurement (NCRP) Reports Number 38 and 102
•
Appropriate state regulations
The Clinac is designed to terminate the exposure when the total dose displayed on the console monitor equals the set dose. Should this normal (DOS1) termination fail to occur, the following backup interlocks terminate the beam: !
When the total dose exceeds the dose set by the operator (DSFA interlock).
!
At a fixed percentage of monitor units beyond the value set by the operator or a fixed number of monitor units, whichever is less (DOS2 interlock).
!
When the numbers of monitor units (MU) from the primary and secondary dosimetry channels differ from each other by more than 5% or 2 MU, whichever is greater (DS12 interlock).
!
Upon coincidence between the displayed time and the time value set by the operator (TIME interlock).
!
In dynamic therapy, when there is a disagreement between the intended dose and position and the actual dose and position (DPSN interlock).
The operator must be aware of the progress of treatment at all times. If the beam does not terminate correctly, the operator should take immediate action (press the nearest Emergency Off button) and not wait for a backup system to activate. Beam termination by an interlock or means other than normal termination may be a sign of a significant equipment malfunction. Varian recommends that you suspend patient treatments after any abnormal termination until qualified personnel determine that it is safe to continue using the Clinac.
5.9. Emergency Plan
An emergency situation can arise at any time. The owner must establish procedures for handling emergencies. Personnel operating or working around the Clinac must be trained in these procedures. !
The emergency plan should include:
!
Emergency procedures (modified for local conditions) described in this manual
!
Periodic testing of emergency equipment, including the emergency pendant system, emergency lighting system, fire extinguishers, and emergency-off circuits
Emergency and Safety: Owner Guidelines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6. Appendix A: Venting Waveguide Gases
!
Evacuation routes in case of an emergency situation, with the routes posted near the control console
!
Scheduling and content of periodic drills and training
!
Identification of qualified personnel to be contacted in the event of a fire, medical emergency, or other situation requiring outside help
!
Procedures to restore operation of the Clinac following an emergency
!
Procedures for using a hazardous spill kit
!
Procedures for maintaining up-to-date copies of Clinac MSDSs in the event of a hazardous substance leak or spill
All Clinacs use either Freon 12 or SF6 as a dielectric gas in the RF waveguide system. Waveguide arcing causes Freon 12 to break down into hydrogen chloride gas as well as hydrochloric and hydrofluoric acids. With SF6, waveguide arcing produces hydrofluoric acids and other hazardous fluoride products. These gases could be toxic and should not be inhaled. The procedures for venting waveguide gases will reduce exposure to hazardous gases that can be present when the RF waveguide system is purged. Abnormal noises during BEAM ON often indicate RF arcing in the waveguide. Specifically, listen for: !
Distinct or repetitive metallic “dinging” noise.
!
Continuous racket or banging noises other than the normal high-volt pulsing sound.
If venting the system is needed, follow the procedures. Varian recommends reading the entire procedure (for the correct machine configuration) before beginning work. Varian can provide any of the service contained in this procedure. Call the Regional Varian Customer Support Office (see “Customer Support” on page 1-8) for details.
6.1. Testing for Waveguide Arcing
6.2. Prerequisites: Parts Ordering Procedure
Use an oscilloscope to confirm if the RF is arcing in the waveguide. Determine if the following waveforms are tearing-off or have abnormally high spikes. !
For Low Energy machines, observe Magnetron I, Forward Power, and Reflected Power.
!
For High Energy machines, observe Klystron I, Forward Power, Load Power 1, Load Power 2, and Reflected power.
If venting the Clinac is needed, some parts are required. Determine if there is a venting device on the Clinac. !
All high energy Clinacs have a built-in venting device at the stand.
!
All low energy Clinacs that use SF6 also have the venting device at the stand.
!
All low energy Clinacs that use Freon 12 do not have the venting device at the stand.
If the Clinac has a venting device: !
1-52
Order the 872031-01 kit from Varian and perform the section “Venting a Clinac With a Venting System” on page 1-53.
Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 !
A plastic bag may be used instead of the Cubitainer (listed in the material list at the end of this procedure).
If the Clinac does not have the venting device and retrofit is not being performed: !
Neither kit needs to be ordered. Complete the section “Venting a Clinac Without a Venting System” on page 1-53.
!
For this procedure, provide a large plastic bag to contain the gas, and a rubber band to close the bag. A 30 – 50-gallon trash bag is ideal.
To retrofit the Clinac with the venting device, order the 872031-02 kit and complete the section “Retrofitting the Clinac With a New Venting System” on page 1-55.
6.3. Venting a Clinac With a Venting System
Clinacs with a venting device can be vented in two ways: If the room has an exhaust vent, use flexible tubing to route the discharged gas directly into the room exhaust vent. 1.
If there is no exhaust vent in the room:
2.
Order the 872031-01 kit to properly vent the gas.
3.
Capture the gas into the plastic Cubitainer or a 30–50 gallon trash bag.
4.
Be sure to close the bag securely with adhesive tape or a rubber band.
5.
Carry the bag to an exhaust vent or an unpopulated area outdoors, and open it.
WARNING: For your safety and the safety of others, be sure to discharge the gas away from people or animals. 6.
Discharge the container through at least 15 feet of flexible tubing to allow a safe upwind breathing zone. Allow 5–10 minutes to discharge all the gas.
6.4. Venting a Clinac Without a Venting System WARNING: Read this entire section before performing the steps. Before you begin: 1.
Decide where the gas will be vented: Into the ventilation system, or outdoors away from people or animals.
2.
Shut off the gas bottle on the Clinac.
Emergency and Safety: Appendix A: Venting Waveguide Gases
1-53
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6.4.1. Option 1 for Venting Gas
This option may be the easiest since the relief can be used to control the gas flow. 1.
Rotate the gantry to either 90° or 270°.
2.
Remove the top gantry cover.
3.
Locate the gas pressure relief valve at the RF system.
4.
Wrap the opening of the plastic bag over the relief valve so that the venting gases will fill the bag without escaping into the treatment room; cover the valve handle as well.
5.
Use one hand to secure the bag over the relief valve and the other hand to carefully pull the valve handle.
6.
Holding the bag shut, carefully remove it from the relief valve.
7.
Twist the bag shut and seal it with the rubber band or adhesive tape.
8.
Carry the bag to an exhaust vent or an unpopulated area outdoors.
WARNING: For your safety and the safety of others, be sure to discharge the gas away from other people or animals.
6.4.2. Option 2 for Venting Gas
1-54
9.
Discharge the container through at least 15 feel of flexible tubing to allow safe, upwind breathing zone.
10.
Allow 5–10 minutes to discharge all the gas.
11.
If the gas venting system is being added to the Clinac, see “Retrofitting the Clinac With a New Venting System” on page 1-55.
1.
Use one of the hose connections on the stand instead of the relief valve.
2.
Determine which hose connection will be used to vent the gas. The connection chosen should allow enough room to comfortably hold the bag on the hose as well as allow for inflation of the bag.
3.
Test the hose connection selected:
4.
Try to loosen the hose connection slightly and immediately retighten it. This test helps ensure that full control over the hose connection can be turned on and off when venting the gas into the bag.
5.
Wrap the opening of the plastic bag over the hose so that the venting gases fill the bag without escaping into the treatment room.
6.
Discharge the container through at least 15 feel of flexible tubing to allow safe, upwind breathing zone.
7.
Allow 5–10 minutes to discharge all the gas.
Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
6.5. Retrofitting the Clinac With a New Venting System
6.5.1. Inspecting the New Venting Device for Leaks
1.
Order the 872031-02 kit.
2.
Complete the venting procedure in “Venting a Clinac Without a Venting System” on page 1-53.
3.
Assemble the mounting bracket, clamps, and venting manifold as shown in drawing 1101708.
4.
Find the filter trap at the pressure regulator in the stand.
5.
On the Clinac, disconnect the yellow hose at the outlet of the filter trap that routes to the gantry.
6.
Connect the yellow hose to one end of the venting manifold.
7.
Use the new yellow hose (supplied with the kit) to connect the filter trap outlet to the open end of the venting manifold.
8.
Mount the manifold assembly next to the pressure regulator.
1.
Slowly refill the gas system to operating pressure, 39 psi, for Clinacs with 3-port circulators, and 32 psi for Clinacs with 4-port circulators.
2.
Perform a bubble test on the gas lines by applying the provided Snoop on all the line joints and fittings.
To seal a leak, tighten the joints and connections or apply new Teflon tape.
6.5.2. Verify the Gas Leak Rate
1.
Set the gas pressure to operating pressure, 39 psi, for Clinacs with 3-port circulator, or 21 psi for Clinacs with a 4-port circulator.
2.
Allow approximately ten minutes for the gas pressure to stabilize and warm up.
3.
Close the gas bottle main valve.
4.
Turn the gas pressure regulator all the way down.
5.
Write down the regulated pressure indicated on the regulator gauge.
6.
Observe the pressure and compare it with the previous day’s pressure.
If the leak rate exceeds 2 psi per day, call the regional Varian office for service.
6.6. Contents of 872031-01 and 872031-02 Kits
Order the 872031-01 kit if the Clinac already has the venting system. Order the 872031-02 kit if the Clinac is being upgraded with the venting device.
Emergency and Safety: Appendix A: Venting Waveguide Gases
1-55
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 1.2. Contents of 872031-01 and 872031-02 Kits -02
-01
Part Number
Description
1
1
88-301200-00
Cubitainer, 5 gallon*
1
1
88-301201-00
Faucet for the 5 gallon Cubitainer*
3ft
3ft
28-158988-00
Tygon tubing, 3/8 ID x 1/2 OD
1
28-628983-00
Cap, 1/4 flare
1
28-611673-00
Union, 1/4 flare x 1/4 MPT
1
27-109614-00
Valve, Shutoff
1
28-201311-00
Nipple, pipe, 1/4 inch
1
28-207010-00
Tee, 1/4 x 1/4 x 1/4 FPT
2
28-610922-00
Elbow, 1/4 Flare x 1/4 MPT
1
00-886892-02
Clamp, Tee Mounting
4
13-311160-00
Nut, KEPS, 10-32
1
00-834931-05
Hose Assembly, 20-inch
1
88-902021-00
Snoop
88-189795-00
Teflon Tape
00-886891-01
Bracket, Tee Mounting
Drawing
Documents included with kit
1
*The Cubitainer and faucet are optional: A 30—50 gallon trash bag can be used instead.
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Emergency and Safety: Appendix A: Venting Waveguide Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Two
Machine Physics
In this chapter, Accelerator Physics will be discussed to give the reader a greater understanding of how the Clinac treatment beam is produced.
Machine Physics
2-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Introduction:.................................................................................................................... 2-3 2. Definitions:: ..................................................................................................................... 2-3 3. Kinetic Energy Relationships:........................................................................................... 2-3 4. Rest Energy Relationships:............................................................................................... 2-4 5. Total Energy Relationships:.............................................................................................. 2-4 6. Conversion of Energy to Electron Volts: ............................................................................ 2-5 7. Measurement of Energy Change: ...................................................................................... 2-6 8. Example of Simple Acceleration:....................................................................................... 2-8 9. The Standing Wave Accelerator: ....................................................................................... 2-9 10. Impedance Matching: ................................................................................................... 2-11 11. Plotting: ....................................................................................................................... 2-12 12. Accelerator Equivalent Circuit: ..................................................................................... 2-13 13. Load Line Considerations: ............................................................................................ 2-16 14. Fill Time:...................................................................................................................... 2-16 15. Injection Timing: .......................................................................................................... 2-17 16. Electron Injection and Bunching: ................................................................................. 2-18 17. Advances in Linear Accelerator Design for Radiotherapy:.............................................. 2-19
Table of Illustrations Figure 2.1. The Basic Accelerator: ........................................................................................ 2-3 Figure 2.2. Electrostatic (DC): .............................................................................................. 2-8 Figure 2.3. Alternating Current (AC):.................................................................................... 2-8 Figure 2.4. Phase Velocity: ................................................................................................... 2-9 Figure 2.5. Forward Power Polarity:.................................................................................... 2-10 Figure 2.6. Reflected Power Polarity:................................................................................... 2-10 Figure 2.7. Summation of Forward and Reflected Power: .................................................... 2-10 Figure 2.8. Source vs. Load Impedance: ............................................................................. 2-11 Figure 2.9. Load Power vs. Load Resistance:....................................................................... 2-12 Figure 2.10. Power Calculation Parameters: ....................................................................... 2-12 Figure 2.11. Accelerator Equivalent Circuit: ....................................................................... 2-13 Figure 2.12. Equivalent Circuit Showing Resonant Components:........................................ 2-15 Figure 2.13. Load Line Showing Effect of Energy Slit: ......................................................... 2-16 Figure 2.14. Accelerator Equivalent Circuit: ....................................................................... 2-16 Figure 2.15. Fill Time: ........................................................................................................ 2-17 Figure 2.16. Fill Time Equivalent Circuit: ........................................................................... 2-17 Figure 2.17. Electron Injection Timing:............................................................................... 2-17 Figure 2.18. Effect of Energy Slit on Injection Timing: ........................................................ 2-18 Figure 2.19. Effect of Injection Timing on Target Current Waveform: .................................. 2-18 Figure 2.20. Velocity Vectors: ............................................................................................. 2-19
2-2
Machine Physics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
A basic electron accelerator consists of a vacuum chamber with two electrodes connected across a voltage source. Since electrons carry a negative charge, an electron entering the cavity between the plates will be attracted toward the plate with the positive charge and repelled by the negatively charged plate, as shown in Figure 2.1 below. In this example, with a 1-volt battery, the electron will be accelerated by 1 electron volt (eV).
e
–
e
–
1 eV
Vacuum
1 Volt
Figure 2.1. The Basic Accelerator
2. Definitions:
Before proceeding, it is important for the reader to understand the difference between the terms energy and intensity, used to describe treatment beam parameters. These can be defined in two ways: 1.
2.
Basic electron beam: Intensity:
The number of electrons passing a point within a given unit of time (beam current).
Energy:
The total energy possessed by each individual electron in the beam.
Treatment beam (electrons or photons): Intensity:
The amount of radiation delivered per unit of time to a point.
Energy:
The ability of the beam to penetrate.
The total energy possessed by an electron (or any other object) is the sum of its kinetic and rest energies. These will now be defined and discussed in some detail.
3. Kinetic Energy Relationships
Definition: Statement: Example 1:
Kinetic energy is that energy an object possesses by virtue of its motion. 1 2 The KE (kinetic energy) of an object in joules = --- mV at ev2 eryday velocities. Take a laboratory size object. (e.g., a baseball which weighs approximately 0.2 kilograms . 0.5 pounds moving at 30 meters/second . 60 miles/hour): 1 2 ∴ KE = --- ( 0.2kG ) × ( 30 m/sec ) 2 KE = 90 joules
Machine Physics: Introduction
2-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Example 2:
Take an electron (e–) which weighs 9.1 × 10–31 kG moving at 30 meters/second: 1 – 31 2 ∴ KE = --- ( 9.1 × 10 kG ) × ( 30m/sec ) 2 – 28 KE = 4.09 × 10 joules
Observation: Compare the kinetic energy of the baseball to that of the electron:
4.09 × 10-28 joules
90 joules vs. (Baseball)
(Electron)
Conversion:1 joule = 1 watt-second 1 kilowatt hour = 3.6 × 106 watt-seconds 2.5 × 10-5 kilowatt hours
1.14 × 10-34 kilowatt hours vs.
(Baseball)
4. Rest Energy Relationships
(Electron)
Definition:
Rest energy is that energy an object possesses by virtue of its mass.
Statement:
A. Einstein’s equation states the total energy: Et = mc2
Example 3:
where c is the speed of light (.3 × 108 meters/sec).
Take the baseball again and calculate the rest energy (RE): RE = (0.2 kG)(3 H 108 m/sec)2 = 1.8 H 1016 joules = 5 H 109 kilowatt hours
Example 4:
or
Now calculate the rest energy of the electron: RE = (9.1H 10–31 kG)(3 H 108 m/sec)2 = 81.9 H 10–15 joules = 2.275 H 10–20 kilowatt hours
or
Observation: Compare the rest energy of the baseball to that of the electron: 1.8 × 1016 joules 5 × 109 kilowatt hours
81.9 × 10–15 joules vs.
(Baseball)
5. Total Energy Relationships 2-4
Definition:
2.27 × 10–20 kilowatt hours (Electron)
Total energy is that energy an object possesses by virtue of its motion and its mass. Et (total energy) = KE + RE
Machine Physics: Rest Energy Relationships
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Example 5:
Take the baseball again and calculate its total energy (Et): Et = 90 joules + 1.8 H 1016 joules = 18,000,000,000,000,090 joules
Example 6:
Now calculate the total energy of the electron: Et = 4.09 H 10–28 joules + 81.9 H 10–15 joules Et . 81.9 H 10–15 joules
Observation: Almost all of the total energy of an object is due to its mass at these velocities.
6. Conversion of Energy to Electron Volts
Definition:
An electron volt is the kinetic energy an electron acquires by being accelerated in a vacuum through a potential difference of 1 volt (See Figure 2.1 on Page 2-3).
Statement:
The conversion factor to go from joules to electron volts (eV) is: 1.6 × 10–19 joules = 1 eV.
An electron has a rest energy of 81.9 × 10–15 joules; therefore, we must divide the rest energy by the conversion factor to obtain the same thing in eV.
Example 7:
Converting the rest energy of the electron: – 15
81.9 × 10 joules RE = -------------------------------------------------– 19 1.6 × 10 5
= 5.11 × 10 eV = 0.511MeV
Example 8:
Converting the rest energy of the baseball: 16
1.8 × 10 joules RE = -------------------------------------------– 19 1.6 × 10 35
= 1.12 × 10 eV 29
= 1.12 × 10 MeV
Observation: The difference of equivalent energies in eV between the electron and the baseball: 1.12 × 1029 MeV
0.511 MeV vs.
(Baseball)
Machine Physics: Conversion of Energy to Electron Volts
(Electron)
2-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Observation: In Example 2 the equivalent kinetic energy of the electron moving at 30 meters/second or 60 miles/hour in electron volts (eV) would have required a battery potential of 2 nanovolts (See Figure 2.1 on Page 2-3.) Example 9:
Now let us calculate the total energy an electron possesses by passing through a potential of 22 million volts: Et = KE + RE Et = 22 MeV + 0.511 MeV Et = 22.511 MeV
7. Measurement of Energy Change
Definition:
Gamma ( γ ) is the ratio of total energy (Et) to the rest energy (Er) of an electron. E γ = -----t Er
however,
Er = mo c
2
as stated before, where mo = original mass
2
and
E t = mc
therefore
mc γ = -------------2 mo c
and
m γ = ------mo
2
substituting mc2 for E canceling identical terms
Example 10: Calculating the effective increase in mass as a result of accelerating an electron to typical linear accelerator values: 4MeV + 0.511MeV γ = ----------------------------------------------------- = 8.828 0.511MeV + 0.511MeV- = 20.569 γ = 10MeV -------------------------------------------------------0.511MeV + 0.511MeV- = 36.225 γ = 18MeV -------------------------------------------------------0.511MeV 22MeV + 0.511MeV γ = --------------------------------------------------------- = 44.053 0.511MeV Observation: An acceleration of 22 MeV gives an equivalent increase in mass of approximately 44 times. The following equation is derived from Newton’s second law of motion (as applied to momentum): Statement:
γ =
1 --------------------v 2 1 – ⎛ ---⎞ ⎝ c⎠ 1
⎛ ⎞ --2⎜ 1 ⎟ γ = ⎜ --------------------2-⎟ ⎜1 – ⎛v ---⎞ ⎟ ⎝ ⎝ c⎠ ⎠ These equations are normally referred to as proof that an object with mass can never attain the speed of light.
2-6
Machine Physics: Measurement of Energy Change
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Needed:
A way to express velocity as a ratio of the speed of light.
Definition:
$ (beta) is the ratio of the velocity of an electron to the speed of light.
$ has other meanings different from this one and is not to be confused as it is defined here.
1
⎛ ⎞ --21⁄2 ⎜ 1 ⎟ 1 ⎜ --------------------2-⎟ = ⎛⎝ -------------------2-⎞⎠ ⎜1 – ⎛v 1 – (β) ---⎞ ⎟ ⎝ ⎝ c⎠ ⎠
v substituting $ for --c
1⁄2 1 Conclusion: γ = ⎛ -------------------2-⎞ ⎝ 1 – (β) ⎠ 2 1 γ = ------------------2 1 – (β)
1 1⁄2 β = ⎛ 1 – -----⎞ 2⎠ ⎝ γ
Exercise:
squaring both sides
solving for $
Calculate the percentage of the speed of light (c) as a function of kinetic energy.
Example 11: Ek + Er γ = -----------------Er 22MeV + 0.511MeV γ = -------------------------------------------------------0.511MeV γ = 44.053 1 1⁄2 β = ⎛ 1 – ----2-⎞ ⎝ ⎠ γ 1⁄2 1 β = 1 – ⎛ ----------------------2-⎞ ⎝ 44.053 ⎠
β = 0.9997
Conclusion: An electron accelerated to 22.511 MeV is traveling at approximately 0.9997 times, or 99.97% of, the speed of light.
Machine Physics: Measurement of Energy Change
2-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Observation: Accelerated Energy 17 KeV 100 KeV 0.511 MeV 4 MeV 6 MeV 10 MeV 18 MeV
8. Example of Simple Acceleration
Ratio of c .252 .548 .866 .993 .997 .998 .999
(X-ray tube) (Klystron) (Doubling rest energy)
In a simple accelerator, more energy can be achieved in two ways: increasing the accelerating voltage ( V a ), or increasing the number of accelerating cavities, as shown in Figure 2.2 below.
e–
e–
KE = (V1 + V2) eV
Vacuum
V2
V1
etc.
Figure 2.2. Electrostatic (DC) When high electron beam energies are required, batteries are not practical; however, since high-voltage AC power is easy to generate, it can be used to generate high-strength electrical fields in the cavities, as shown in Figure 2.3 below. Vacuum
e
–
e
V1
V2
V3
–
etc.
Figure 2.3. Alternating Current (AC)
2-8
Machine Physics: Example of Simple Acceleration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The best efficiency is achieved when the electron enters each cavity at the time when V a is at maximum strength. Therefore, the electrical field must propagate through the cavities at the same velocity as the electron. This is called the phase velocity as shown in Figure 2.4 below. Accelerators using this principle are called traveling wave accelerators.
Vacuum
d
–
e
e
φ1 = 0°
φ2
φ3
–
φ4
Figure 2.4. Phase Velocity
Definition:
v ϕ = Phase velocity which equals Distance -----------------------Time d v ϕ = --t
at 60 Hz
λ = 3000 miles
at 3 GHz*
λ = 10 centimeters
Conclusion: Thus a particle accelerator requires microwave frequency generators and distances. *Actually 2.856 GHz in high-energy and 2.998 GHz in low-energy Clinacs
9. The Standing Wave Accelerator
Statement:
E n = cos ( ωt – ϕ n )
If the cavities are one-quarter of the of the AC field wavelength, at the time when the first and fifth cavities are at maximum forward-acceleration potential, the third cavity is at maximum reverse acceleration potential. By the time the electron reaches the third cavity, the polarities will be opposite. Thus, the electron acquires additional energy in every other cavity. In a traveling wave accelerator, the forward power is absorbed in the final cavity. In a standing wave accelerator, the power in the final cavity is allowed to reflect back, creating a standing wave and doubling the field strength in each cavity. However, in order for the reflected power to match the phase of the forward power, one of the end cavities must be either half as long, or one and one-half times as long, as the others. If all cavities are the same size, the reflected power will cancel the forward power. Also, in order to allow the forward and reflected power to travel between the cavities, the connecting openings must be at least one-quarter of the wavelength.
Machine Physics: The Standing Wave Accelerator
2-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 λ 4
Electric Fields
+
0
-
0
+
0°
-90°
-180°
-270°
0°
Forward Power Figure 2.5. Forward Power Polarity
Reflected Power
λ 4
0°
-270°
-180°
-90°
0°
+
0
-
0
+
Electric Fields Figure 2.6. Reflected Power Polarity
λ 4
0°
-270°
-180°
-90°
0°
+2E
0
-2E
0
+2E
0°
-90°
-180°
-270°
0°
Electric Fields Figure 2.7. Summation of Forward and Reflected Power
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Machine Physics: The Standing Wave Accelerator
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Impedance Matching
When power is being transferred from its source to its load, the most efficient transfer occurs when the impedance of the load (Rl) is equal to the impedance of the source (Ro). This explained in example 12, below:
Ro (Source)
Vo (Source)
Rl
(Load)
Il Figure 2.8. Source vs. Load Impedance Vo I l = ----------------Ro + Rl 2
Pl = Il Rl 2
Vo Rl P l = ------------------------2 ( Ro + Rl )
Vo - for I l substituting ----------------Ro + Rl
Example 12: Reference Figure 2.8 above. Case 1: Rl = Ro
Given:Ro = 1 ohm Rl = 1 ohm Vo is constant
1 - = 1 P l = ---------------------- = 0.25 watts 2 4 (1 + 1)
Case 2: Rl < Ro
Given:Ro = 1 ohm Rl = 0.5 ohm Vo is constant
0.5 0.5 P l = --------------------------2- = ------------- = 0.222 watts 2.25 ( 1 + 0.5 )
Case 3: Rl > Ro
Given:Ro = 1 ohm Rl = 2 ohms Vo is constant
2 2 P l = --------------------- = --- = 0.222 watts 2 9 (1 + 2)
Machine Physics: Impedance Matching
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11. Plotting
Figure 2.9, below, illustrates this effect.
.25 .222
P1
.5
1
2
R1
Figure 2.9. Load Power vs. Load Resistance Conclusion: For maximum transfer or power the load impedance must equal the source impedance. Needed:
Expression to find actual maximum power.
Ro
Vo
Rl Io Figure 2.10. Power Calculation Parameters
Statement:
Vo - if R o = R l I o = --------2R o 2
P max = I o R o VO ⎞ 2 2 - R I o R o = ⎛ --------⎝ 2R o⎠ o 2 Vo ⎞ 2 V o ⎛ --------- R o = --------⎝ 2R o⎠ 4R o
Vo - for I o substituting --------2R o and multiplying by R O
2
V ∴P max = ---------o4R o
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Machine Physics: Plotting
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
12. Accelerator Equivalent Circuit
Figure 2.11, below, illustrates the relationships between the parameters.
Ro
1:n
Va n
Vo
I2
Ra
Ia
Va
Ira
Io
Figure 2.11. Accelerator Equivalent Circuit Statements: 1 V o = I o R o + ---- V a N I 2 = I ra + I a V I ra = ------aRa V substituting ------a- for I ra Ra
V I 2 = ------a- + I a Ra I o = nI2 V I o = n ⎛ ------a- + I a⎞ ⎝ Ra ⎠ V V ∴V o = n ⎛ ------a- + I a⎞ R o + ------a ⎝ Ra ⎠ n V a R o⎞ V V o = n ⎛ ------------+ nI a R o + ------a ⎝ Ra ⎠ n Ro 2 2 nV o = n V a ⎛ -------⎞ + n I a R o + V a ⎝ R a⎠ 2R 2 nV o = V a ⎛ n ------o- + 1⎞ + n I a R o ⎝ Ra ⎠
2
nV o n Ia Ro - – ----------------------V a = ----------------------R 2 Ro 2 o n ------- + 1 n ------- + 1 Ra Ra
Machine Physics: Accelerator Equivalent Circuit
Think about it!
rearranging terms
multiplying by n
combining and rearranging terms 2R dividing by n ------o- + 1 and solvRa ing for V a
2-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Definition: β (beta) is equal to the shunt impedance divided by the source resistance times the turns ratio squared. Ra β = -----------2 n Ro 2
n R 1 --- = ------------oβ Ra
inverting
R 2 n R o = ------aβ
rearranging 2
nV o n Ia Ro V a = ----------------------- – ----------------------2 Ro 2 Ro -----n -----+ 1 n +1 Ra Ra 2
nV o n Ia Ro V a = ------------------ – -----------------⎛1 --- + 1⎞ ⎛ 1 --- + 1⎞ ⎝β ⎠ ⎝β ⎠
Ro I a -----nV o β V a = ------------------- – ------------------1 1 ⎛ --- + 1⎞ ⎛ --- + 1⎞ ⎠ ⎝β ⎠ ⎝β βnV Ia Ra V a = -------------o- – -----------1+β 1+β
restating the equation for V a (see above) 2
n Ro substituting β for ------------Ra
R 2 substituting ------a- for n R o β
multiplying by β
2
Vo P max = --------4R o
restating the equation for P max (see above)
V o = ( 4P max R o )
1⁄2
solving for V o
P max will be denoted as P o . V o = ( 4P o R o )
1⁄2
nV o = n ( 4P o R o )
1⁄2
2
nV o = ( 4P o n R o ) Ra nV o = ⎛ 4P o -------⎞ ⎝ β⎠
1⁄2
βnV o = ( 4P o R a β )
2-14
1⁄2
1⁄2
multiplying by n combining terms Ra 2 substituting ------- for n R o β multiplying by β
Machine Physics: Accelerator Equivalent Circuit
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 βnV Ia Ra V a = -------------o- – -----------1+β 1+β
restating the accelerator voltage equation
2
Ia Ra ( 4P o R a β ) – -----------V a = ---------------------------1+β 1+β
Statement:
1⁄2
substituting ( 4P o R a β ) in accelerator equation
for βnV o
1⁄2
( 4R a β ) K 1 = --------------------------1+β Ra K 2 = ⎛ -------------⎞ ⎝ 1 + β⎠ Va = K1 ( Po ) Statement:
1⁄2
– K2 ( Ia )
substituting K1 and K2 in accelerator equation
Where Po equals the input microwave power to the accelerator and Ia equals the electron accelerated beam current.
This is not the same as target current!
Statements:
2
Va P cavity = -------Ra
P beam = V a I a The ratio of the accelerator impedance to the source impedance is given by $. At $ = 1, the accelerator is matched. All of the above calculations relate to an accelerator operating at resonance.
Ro
1:n
Va n
Vo
Io
I2
Ra
Ia
Va
Ira
Figure 2.12. Equivalent Circuit Showing Resonant Components
Machine Physics: Accelerator Equivalent Circuit
2-15
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ω–ω where δ = ---------------oωo
Ra Z = -------------------------------------------2 1⁄2 [ 1 + ( 2Q o δ ) ]
Figure 2.12 on Page 2-15 defines the off-resonant conditions.
13. Load Line Considerations
+5%
Operating Point
Va (Energy) -5%
±3% Energy Slit
Ia
(Beam Current)
Figure 2.13. Load Line Showing Effect of Energy Slit
14. Fill Time Ro
1:n
Va n
Vo
I2
Ra
Ia
Va
Ira
Io
Figure 2.14. Accelerator Equivalent Circuit
See reference Figure 2.15.
t
Statement:
2-16
--------------Ra ⎞ ⎛ R C⎞ - ⎜ 1 – e EQ ⎟ V a = ⎛ V o ------------------⎝ R o + R a⎠ ⎝ ⎠
Ro Ra where R EQ = ------------------Ro + Ra
Machine Physics: Load Line Considerations
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
RF Power Level
Va
t Figure 2.15. Fill Time
15. Injection Timing
Ro
S1
Vo
Ra
C
Ia
Ic
Figure 2.16. Fill Time Equivalent Circuit Vo β Ia Ra V a = ------------ – ------------1+β 1+β
for the steady state condition.
When capacitor current (Ic) equals the desired value of Ia, S1 is closed.
Desired RF Power Level
Va
Magic time to close S2 for gun pulse timing
t Figure 2.17. Electron Injection Timing
Machine Physics: Injection Timing
2-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figures 3.18 and 3.19 illustrate how of Gun timing, RF loading and the Energy Slit affect the target current.
±3% Slit
Injection too late
No Injection
Desired RF Power Level
Va
Injection too early
t Figure 2.18. Effect of Energy Slit on Injection Timing
4.5 – 5 µSec
Injection too early Optimal target current
Injection too late (or bad light pipe)
Figure 2.19. Effect of Injection Timing on Target Current Waveform
16. Electron Injection and Bunching
Statement:
The wave field is initially moving faster than the electron. so the electron appears to be moving backward with respect to the wave field.
Injection energy (velocity) must be correct to deposit the electron at the proper point on the wave front. The percentage of current captured is a function of the field. Speed of Light Injection Equation: 2
2πM o c ⎛ 1 – β⎞ 1 ⁄ 2 - ------------cos θ α = cos θ o – -------------------Eλ ⎝ 1 + β⎠
The cos2 approaches 1 if bunching parameters are correctly selected.
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Machine Physics: Electron Injection and Bunching
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Wave field velocity vector
Resultant with reference to wave field velocity –
e
Electron velocity vector
RF power amplitude Region of bunched electrons
Figure 2.20. Velocity Vectors In the highest energy modes only about 1/3 of the electrons that enter the guide are captured. For lower energy modes, this number is even less.
17. Advances in Linear Accelerator Design for Radiotherapy
This section has been compiled from an article by Dr. C. J. Karzmark, published in 1984 by the Department of Radiology, Stanford University School of Medicine, Stanford, California 94305, and is included in this manual for reference only. In this section the paragraph numbering scheme of the original article is used and the bibliographic references are omitted.
I. Introduction
The microwave-powered electron linear accelerator, or linac, is becoming the dominant radiotherapy treatment unit. In the U. S., linacs now comprise over one-half of all megavoltage treatment units in service and about 90% of newly installed units. Several technical advances, combined with attention to how patients are most effectively set up and treated, have led to continuing improvements in linac radiotherapy during the three decades since their introduction in England and in the United States. A simplified exposition of linac theory and operation can be found in a contemporary reference by Karzmark and Morton. In it, the major modules of a medical linac are identified, their principles of operation are described individually, and then their collective functioning in providing a radiation treatment beam is discussed. Additional aspects are presented in an earlier technical review of the subject by Karzmark and Pering. The microwave accelerator structure (sometimes referred to as the “guide” or as the “waveguide”), the central component of linacs, consists of a linear array of microwave cavities. Such structures have undergone extensive de-
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 velopment and the result has been physically shorter accelerators having higher energy gradients along the accelerating axis. Most treatment units are isocentrically mounted, and many of these machines employ a horizontally mounted accelerator structure and a beam bending system. Here, improved beam transport magnet systems and treatment head designs help ensure flatness uniformity of treatment beams and their stability with time independent of gantry motion and position as well as of beam-limiting device orientation. Various methods have been developed to permit varying the electron beam energy over a wide range. For example, in single-pass (single traversal) linacs, the ratio of radio-frequency (rf) power to two portions of the accelerator structure is varied so as to keep a constant energy gradient in the first injection portion while providing a variable energy gradient in the second portion. In two-pass linacs, the phase of the microwave electric field encountered by electron beam is varied as it returns for the second pass. In microtrons, the beam extraction path is shifted from one orbit to another. By providing both high and low x-ray energies together with a broad range of electron beam energies, a wide variety of treatment plans can be implemented. The incorporation of digital electronics and computer techniques has led to improved reliability together with desirable monitoring, control and safety features. Human engineering improvements have involved aesthetics, function, patient safety, and comfort, as well as convenience for the radiotherapy treatment technologist. Several new research directions suggest opportunities for continued improvement in linear accelerators for radiotherapy.
II. Microwave Accelerator Structures
We can understand how linacs accelerate electrons by first examining how an electric E field wave pattern travels down a hollow cylindrical pipe, or waveguide as it is called. Such waveguides, and their hollow rectangular pipe counterparts, are used to transmit microwave power from an rf source to an accelerator structure.
A. Introduction
Waveguides replace conventional wires and cables which are inefficient for transmitting power at microwave frequencies. Figure 2.21(a) shows the E field pattern and charge distribution at one instant of time in a plane containing the axis of a cylindrical waveguide. The accompanying magnetic H field pattern, in this case circling around and orthogonal to the axis, is not shown here or in later illustrations since it is not directly involved in the accelerator process. Electrons injected along the axis would be accelerated by the moving E field. Unfortunately, however, the velocity of this E field pattern, the phase velocity of the wave exceeds that of light and is therefore unsuitable for continuing acceleration of charged particles. The wave is slowed by inserting washerlike discs into the waveguide, as shown in Figure 2.21(b), so that the wave stays in step with the accelerating electrons. These discs divide the waveguide into a series of cylindrical cavities, the basic structure of a linear accelerator. Virtually, all medical linacs operate at frequencies of approximately 3000 MHz in the so-called S band where typical accelerating cavities are about 10 cm in diameter and 2.5 to 5 cm in length. These cavities are arranged to serve two purposes: (1) to couple and distribute microwave power between adjacent cavities and hence, along the length of the structure; (2) to provide an E field with suitable axial distribution for accelerating electrons. The particular spatial configuration of E and H fields in a cavity is called the mode and denoted by abbreviations such as TEM (transverse electric and magnetic) field pattern as in a coaxial line or cavity or TM010 (fundamental transverse magnetic only field pattern), as in Figure 2.21(b).
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Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 By varying the aperture and length of the cavities initially traversed, the continuum of injected electrons is concentrated into discrete bunches as well as accelerated. This initial portion of the structure is called the buncher and its cavities are non-uniform. Their inner diameter, aperture diameter, and axial spacing vary to provide the increasing phase velocity E field to accommodate the accelerating electron bunches. Since the electron velocity is almost constant and near the speed of light beyond the buncher, subsequent cavities are made uniform for further acceleration of the bunches of electrons. Early buncher designs contained many cavities (see Figure 2.22); later only several cavities were used and more recently, a single half cavity. Improved understanding of bunchers has resulted in a reduction of electron injection voltage from the 100–200 kV region to the 1– 30 kV region. Typically, one-third of the injected beam is captured, bunched, and accelerated. (See Ref 5 for an elementary description of bunching action.) 8 8/2
(a)
+ +
–
–
–
–
+ +
+ +
–
–
+ +
–
–
–
–
+ +
+ +
–
–
8 8/2
(b)
+
–
+
–
+
–
+
–
Figure 2.21. (a) Spatial traveling wave electric E field pattern and charge distribution at one instant of time along the axis of a smooth cylindrical waveguide. (b) Spatial traveling wave electric E field pattern and charge distribution at one instant of time along the axis of a disk-loaded cylindrical waveguide. The direction of the electric E field is reversed every half wavelength 8/2. The pattern repeats every wavelength and there are four cavities per wavelength in the diskloaded structure. The direction of the E field also reverses every half cycle in time.
B. Traveling Wave Structure Linac
An early prototype traveling wave (TW) structure, cut in half along its cylindrical axis, is shown in Figure 2.22. The buncher section is on the left and the uniform section is on the right. Figure 2.23 shows an elementary TW accelerator with electrons and microwave power injected on the left. Accelerated electrons emerge on the right and the residual microwave power not transferred to the electron beam or structure walls is absorbed in a load at that end. The discs serve to separate the cylinder into a linear array of coupled cavities which both transport the microwave power and accelerate electrons down the structure.
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 2.22. Cutaway traveling wave accelerator structure; the buncher section is on the left and the uniform section is on the right.
Figure 2.23. Short traveling wave accelerator structure. The rectangular waveguide on the left is the input coupler for conveying microwave power from a source to the structure. The acceleration of an electron bunch in a TW linac is similar to the way a boy on a surfboard, positioned just forward of the crest on a water wave of velocity vw, moves forward as shown in Figure 2.24(a). Similarly, Figure 2.24(b) shows how two electron bunches of velocity ve, are accelerated by the negative portion of the E wave moving at phase velocity vp. This E wave is created by the charge distribution shown in Figure 2.24(c). For clarity in Figure 2.24(c), the instantaneous axial E field patterns, such as depicted in Figure 2.21(b), have been omitted. The boy and the electron bunches move forward at the velocity of their respective wave motions, the phase velocity vw and vp, respectively. Traveling wave structures of 2B/4 design, in which one cavity in four contains an electron bunch at any one time, dominated early linac structure designs. Some later structure designs incorporated three cavities per wavelength 8, a phase shift of 2B/3 rad (120°) per cavity with one cavity in three containing an electron bunch at any one time. More recently, standing wave (SW) designs have appeared in which one axial cavity in two contains an electron bunch at any one time.
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Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In general, TW structures have a larger frequency bandpass and hence, are less sensitive to frequency change than comparable SW structures for the same mode of operation. Since the buncher in TW linacs is adjacent to the power feed at the gun end, the magnitude of the electric field in the early cavities relatively independent of beam current so the buncher action is not greatly affected by beam loading. As a result, bunching in TW structures is efficient over a comparatively broad energy range, typically ±35%.
Figure 2.24. Traveling wave principle: (a) for a boy surfing on a water wave advancing to the right, (b) for electron bunches occupying a similar position on an advancing electric E field (for simplicity, E– is drawn upward), (c) the associated charge distribution which pushes (– charge) and pulls (+ charge) the electron bunches along the cylinder. There are four cavities per wavelength 8 and one electron bunch every four cavities.
C. Standing Wave Structure Linac
We can convert the traveling wave linac just described to a standing wave linac. We do so by arranging for both a forward moving (to the right) f and backward moving (to the left) b E wave, each of which is reflected at both ends of the structure. The two moving E field maxima and resultant wave are shown in Figure 2.25 at three sequential times one-quarter cycle apart. The resultant E field pattern is the sum of the forward and backward components and ideally, in the absence of the I2R copper losses and beam loading, is double the amplitude found in a compatible TW structure. Note that every other cavity (e.g., #2 and #4) in Figure 2.25 has zero field at all times, at times (e.g., t1, and t3), because both forward and backward waves are each zero, at other times (e.g., t2), because they are equal to magnitude but opposite in direction and hence completely cancel.
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Such zero-field cavities couple microwave power between cavities but play no role in particle acceleration. They may be moved off axis to form a sidecoupled or bimodal SW structure, thus shortening the overall length for a given energy gain. Figure 2.26 illustrates a cutaway section of a side-coupled SW structure. The apertures between cavities of TW structures involve a design compromise. They should be large to transport microwave power efficiently between cavities but should be small enough to “squeeze down” the E field, thus maximizing its value along the beam axis. This design conflict is removed for the SW structure; axial acceleration cavities may be optimized for electron acceleration and coupling cavities for microwave power transport. The latter are made small for convenience, since to first order they contain zero electric field and hence, can be of much lower Q than on-axis cavities without entailing much I2R loss in their copper walls since almost no wall current flows.
TIME
PHASE
t1
ωt1
1
2
3
4
5
E
t2
ωt1 + π/2
E b
t3
f
b
f
ωt1 + π
E NEG.
POS. E FIELD MAXIMA
Figure 2.25. Standing wave electric E field patterns in an accelerator structure for combined forward and backward waves at three sequential instants of time. Two traveling waves moving in opposite directions (f forward, b backward) generate such a standing wave. At time t2, the E field is zero in all cavities. The pattern shown at time t1 will recur one-half cycle after time t3. Individual cavities are numbered across the top.
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Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The axial apertures of SW accelerating cavities can be made small and equipped with extended “nose cones” which increase their axial length. They now function as zero-field drift tubes, and the electron bunch crosses the cavity gap as E is near its maximum value (see Figure 2.26). Figure 2.27 illustrates the evolution of such an SW structure from a TW structure and the associated E field pattern at one instant of time. The coupling cavities in Figure 2.27(d) are staggered, in contrast to Figure 2.27(c), to reduce asymmetries introduced by the coupling slots. Figure 2.28 shows the time variation of the E field in an SW structure for one complete microwave cycle. The pattern varies sinusoidally in magnitude but remains stationary in space just as a vibrating violin string. The actual field axial distribution resembles more that of Figure 2.27 than Figure 2.25. These somewhat rectangular shaped patterns can be represented by a sum of spatial harmonics comprising a sinusoidal fundamental as in Figure 2.25 and higher order sinusoids which do not produce net acceleration of synchronous electron bunches. The E field pattern recurs along the beam axis twice as frequently as in TW structures; that is, an accelerating electron bunch is contained in every second cavity instead of every fourth cavity. In SW structures, all the cavities tend to have the same electric field, since the rf power bounces back and forth from each end of the structure many times (typically 100 times) to fill the cavities with energy as expressed in E and H fields. Hence, beam loading of any cavity contributes to the reduction of the E field in all cavities, including the buncher cavities. As a result, bunching in SW structures is efficient over only a narrow energy range, typically ±5%. Thus, to maintain effective bunching action in SW structures, the input rf power must be increased with an increase in beam current.
Figure 2.26. Cutaway bimodal or side-coupled standing wave accelerator structure. The axial accelerating cavities are shaped for optimum efficiency and the off-axis coupling cavities are staggered to reduce asymmetries introduced by the coupling slots (courtesy of Los Alamos Scientific Laboratory, Los Alamos, New Mexico).
Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
(a)
(b)
(c)
(d)
NEG.
POS. E FIELD MAXIMA
Figure 2.27. Evolution of a standing wave accelerator structure from a traveling wave structure and related E wave patterns along the axis. The E field patterns below each structure show the spatial field along the axis at the same time in the microwave cycle.
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Machine Physics: Advances in Linear Accelerator Design for Radiotherapy
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t1
0°
t2
45°
t3
90°
t4
135°
t5
180°
t6
225°
t7
270°
t8
315°
t9
360°
0
π/2
π
3π/2
2π
Figure 2.28. Axial E field pattern for one microwave cycle of time for the bimodal structure depicted in Figure 2.27(d). This cycle includes the sequential E field patterns shown. The corresponding phase angles in degrees and radians are shown on the right. The pattern at time t9 is a repetition of that at time t1.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
D. Structure Designs
Several new structure designs, in addition to those described, have been developed recently. Two side-coupled structures of the Los Alamos (LASL) design, such as illustrated in Figure 2.29(a) (and also Figure 2.26), may be interlaced in the Varian design as shown in Figure 2.29(b). A
(a)
A'
A
(b)
A'
Figure 2.29. Cross section and end views of two SW structures: (a) sidecoupled Los Alamos (LASL) design; (b) side-coupled interlaced Varian design. The sections A–A’, which include the cylindrical axes of symmetry, are shown on the left. By arranging the side cavity coupling to connect every other axial cavity as shown in Figure 2.29(b) instead of adjacent cavities and feeding each group of cavities with microwave power 90° out of phase, a remarkably short, high-gradient structure results. It withstands high average E field gradients without breakdown along its length, in part because the ratio of the peak E field at the cavity surface to average accelerating E field value on axis of almost four in the LASL design is reduced to about 1.3 for the Varian design. However, the latter is more difficult to manufacture and has higher I2R dissipative copper losses associated with its larger ratio of cavity wall area to cavity volume. Benguang et al. have recently studied the interlaced structure with the aid of a model and a computer program. They conclude that the structure is best suited for short straight-through design linacs. Schriber has provided a useful comparison of SW and TW structures.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 2.30 shows some structure variations which have been investigated or adopted for the relativistic (constant velocity), section of the linac structure. The on-axis coupling designs, (a), (b), (c) and (d), are smaller in diameter and entail simple machining and brazing operations than off-axis coupling designs (e), (f), (g), and (h). The constant impedance uniform TW structure [Figure 2.30(a)] characterized early structure designs. It provides a decreasing energy gain per meter along its length as the microwave power is attenuated by the structure and by transfer to the ever-more-energetic electron beam. The constant gradient non-uniform TW structure Figure 2.30(b)] provides a constant energy gain per meter for a specific loading, i.e., beam current.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 2.30. Linac structure variations showing E field maxima at one instant of time: (a) TW constant impedance; (b) TW constant gradient; (c) SW biperiodic with on-axis coupling cavities; (d) SW tri-periodic with on-axis coupling cavities; (g) disk and washer cross section; (h) disk and washer structure. The arrows represent the maximum E field values at one instant of time. Structures (e) and (f) are shown in more detail in Figures 3.29(a) and 3.29(b) respectively.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Here, the size of aperture varies regularly along the length, but the phase velocity is constant. The 10,000-ft. Stanford Linear Accelerator Center (SLAC) physics research accelerator is divided into 10-ft. sections of this design. The bi- and tri-periodic SW structures, Figures 3.30(c) and 3.30(d), respectively, are simple to manufacture and have smaller outside diameters than the side-coupled variations [Figures 3.30(e) and 3.30(f)] but their shunt impedance, a measure of efficiency, is usually lower and they have lower energy gains for a given power and beam loading. The on-axis coupling cavities of radiotherapy linac structures [Figures 3.30(c) and 3.30(d)] can be “thin” since their resonant frequency depends only on their diameter in the dominant TM010 mode. They contain only rf coupling fields and not accelerating fields and hence, can be of low-Q design. Power loss in these cavities is small since the E field and associated wall currents within them are small. Coupling between axial cavities is usually magnetic via peripheral slots [see Figures 3.26 and 3.27(b)] and axial apertures can be optimized for acceleration. The transverse magnetic modes (sometimes called E modes), having only circular H field lines, are suited for such magnetic coupling. Although most bi-periodic structures employ magnetic coupling using peripheral slots, an on-axis coupled structure has been built and tested at 3000 MHz. Schriber et al. have compared the performance of S band, standing wave linacs having either on-axis or off-axis coupling. They conclude that optimized on-axis coupled accelerating structures have about a 20% higher effective shunt impedance (hence, higher energy gradient) and are an attractive choice. A disc and washer test cavity [Figures 3.30(g) and 3.30(h)] has been built and tested at 1350 MHz for use on a proton linac where it compares favorably with the side-coupled design. However, for 3000-MHz electron linacs, its relatively large diameter, difficulties in supporting and cooling the inner washers, as well as the presence of higher order spatial modes due to the washer supports, lower the shunt impedance and mitigate against its use. Nevertheless, it is being studied for use in a high-power (300 W) 2856-MHz linac or 36-orbit racetrack microtron intended to produce negative pi-mesons (pions) for radiotherapy.
E. Structure Design Parameters and Operating Principles
Microwave cavities are efficient devices for accelerating charged particles. An accelerating potential of about 1 MV can be established across the gap of a 5-cm-long cavity with about 0.2 MW of pulse power loss in that cavity. We characterize this efficiency by a figure of merit or quality factor, Q, defined by Q = f 0 ⁄ ∆f ( Energy stored in cavity ) = 2πf ⋅ ----------------------------------------------------------------------( Energy loss per cycle )
(1)
where f 0 is the resonant frequency and ∆f the frequency difference between the half power points. The energy storage is in the E and H fields in the cavity volume. The energy loss is to the cavity interior surface from the currents which flow there, giving rise to the E and H fields. Microwave cavities for S-band accelerators may have unloaded values as large as 2 × 104 or larger, but this value decreases with beam loading and external coupling to adjacent cavities and back to the power source. Almost all medical linacs operate in the microwave S-band at a frequency of approximately 3000 MHz. If a beam-loaded cavity has a Q of 104, its half power ∆f is 300 kHz. For stable output with less than 1% reduction in energy, an automatic frequency control (AFC) system would be expected to limit frequency excursions to ±20 kHz. Using the same criterion, the dimensional tolerance for achieving compatible resonant frequencies for a group of S-band cavities is about 10–3 cm and precision machining is required. For copper, the temper-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ature coefficient of expansion is such that a change in resonant frequency f 0 of about 60 kHz/°C results and temperature control of the cavity inner surface of better than ½°C is desired for the structure. Alternatively, the AFC system can follow the resonant frequency f 0 of the structure as the temperature of the structure changes. Electron energy is denoted by V in this section to avoid confusion with the electric field E, and all powers are peak values during the pulse unless stated otherwise. The energy V, gained by an electron traversing a structure, is given by the integral of the first spatial harmonic of the axial electric field Ez, over the length of the structure, L in meters, multiplied by the electron charge e. Ez is equal to E z0 – sin θ , where E z0 is its input maximum value and θ is the phase factor, often near 90°, and described in connection with Figure 2.32. L
V = e ⋅ ∫ E z ⋅ dz
(Mev).
(2)
0
A useful figure of merit for structures and one that establishes the power needed for the requisite E field for a given energy gain is the shunt impedance r. 2
r = E z ⁄ ( dP ⁄ dz )
(MS/m).
(3)
Here, r is expressed as the square of the axial field divided by the power dissipated per unit length. Expressed in megohms per meter, shunt impedance values at S-band typically vary from 50 to 120 MS/m. Both Q and the power dissipation, dP/dz, depend on the precise shape of the conducting walls of the cavities and on the way current is distributed on them. For the general case, it is useful to consider the maximum energy V o gain for zero beam current and then subtract the effect of a beam current i in reducing Ez. V = Vo – iK 1
(MeV).
(4)
The constant K1 is a voltage attenuation coefficient and is characteristic of the particular structure. Strictly speaking, Eq. (4) defines the load line for a TW structure. When impedance matched, TW linacs look like a non-resonant, pure resistive load to the power source, and are characterized by a straight voltage-current load line. The SW load line is curved and falls slightly below that for the TW structure, except where it is tangent at the beam loading for the correct impedance match. SW linacs look like a resonant circuit with a reactive component absent only at one beam condition. The zero current energy V o may be expressed by V o = K 2 L ⋅ r ⋅ P Cu
(MeV).
(5)
Here, K 2 is a constant typically slightly less than unity, r is in megohms per meter, and P Cu is in megawatts copper loss to the total structure length L in meters. Microwave input power is expended as P o = P Cu + P l + P e
(6)
P e = V ⋅ i (MW),
(7)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 where P Cu represents copper losses associated with the intense cavity wall currents which give rise to the cavity E and H fields. The term P l represents residual or reflected power dissipated in a load, and P e , the power ultimately transferred to the electron beam. Linac structures are compared in terms of V o values, the zero beam condition. Thus, for L = 1 m, P o = 2 MW, and r = 50 MS/m, a V o value of 10 MeV is obtained. Electron energy is a square root function of each parameter in Eq. (5), and there are linear trade-offs between them. For example, we might halve the structure length and double the peak power P Cu , leaving the zero beam current electron energy unchanged. The effect of beam loading in a constant group velocity TW structure [Figure 2.30(a)] is described by Eq. (4), as a linear reduction in the rate of energy gain as the bunch proceeds down the length of the structure. We may compensate for this reduction by progressively reducing the aperture size which operates to reduce the group velocity and increase the stored energy in later cavities. The result is a constant gradient structure as depicted in Figure 2.30(b). The effect of beam loading in SW structures operates differently. Here, the effect is to reduce the average value of the E field in all cavities. A high shunt impedance denotes high efficiency in producing high-energy gain at zero beam current and is a significant parameter when comparing the performance of two structures under similar conditions of length, beam loading, and power. At times, we may choose to optimize one parameter at the expense of another. For example, the Varian structure depicted in Figure 2.29(b) has a peak to average axial E field ratio of 1.3 compared to 3.8 for the LASL structure of Figure 2.26(a), although the shunt impedance values do not differ greatly, for 83 MS/m for Varian versus 78 MS/m for LASL structures. Hence, the Varian structure allows much higher average field gradients without electrical breakdown, and a shorter linac structure can be constructed. A 10-cm-long 4-MeV linac using the Varian structure would require 1.8 MW of microwave power at zero beam current. Operationally, we require 0.8 MW of electron beam power; 4 MeV at 200 mA, during the pulse to provide an adequate x-ray dose rate. Hence, total power input to the structure would be 2.6 MW requiring about 3.2-MW magnetron power considering transmission losses, a value considerably in excess of that available from a typical 2-MW magnetron. At the present time, the Varian structure is not commercially employed. We can construct a more conservative 28-cm-long 4 MeV linac which requires only 0.75 MW of microwave power at zero beam current at 2-MW magnetron power at full beam current using either it or the LASL structure. The tradeoff here is between structure length and microwave power. Large amounts of power are expended in the normal metallic resistance of copper. Therefore, electron linacs are operated on a pulse basis with a typical duty factor of 0.001. Hence, the average power consumption in kilowatts is typically numerically equal to the peak value in megawatts. Individual microwave cavities are designed for operation in a particular resonant mode which connotes a specific geometrical pattern of E and H fields within the cavity. Most common for electron linacs is the TM010 mode which has only transverse H fields, as well as the requisite axial E2 fields. We design the structure for operation in this, the dominant mode, and find that other cavity resonant modes, such as the TM010 or TM011, are relatively far removed in frequency. However, the structure operating as a series of coupled cavities may also support additional modes at frequencies near the resonant mode desired, e.g., TM010. These coupled cavity, operational modes consume energy, and usually do not contribute to beam ac-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 celeration. This problem becomes more severe as the number of coupled cavities is increased; the number of coupled cavity modes increases and their frequency separation from the dominant resonant frequency TM010 decreases. This problem is known as “moding,” and design efforts are directed toward its reduction or suppression. For example, the tapered constant gradient TW structure depicted in Figure 2.30(b) is less subject to moding problems than the constant impedance structure of Figure 2.30(a). The periodicity found in constant impedance structure is interrupted since sequential constant gradient cavities differ slightly and coupled cavity mote resonances are not reinforced along the structure. A mid-structure feed point is one method of reducing moding in SW linacs. Before an injected beam of electrodes can gain maximum energy, the structure must be filled with electromagnetic energy. Each structure design has a characteristic fill time, typically about 1 Fs, which is determined by the structure design and the velocity of propagation of rf energy down the structure, the group velocity vg. TW linacs involve one-way propagation of waves, and their fields build up in space during the fill time. A typical group velocity in a TW linac is 0.01c where c is the velocity of light. SW linacs involved two-way propagation of waves. Their fields build up in time to a constant equilibrium value throughout the structure by the two waves bouncing back and forth many times between the two ends of the structure during the fill time. A typical group velocity for these waves is 0.05c in an SW structure and there are typically ten bunches in a 1 m structure. Electrons are injected into the structure with a delay approximating the fill time. The precise time is chosen such that the unloaded E field has built up to about the stable beam loaded value, thereby minimizing electron beam energy spread, and optimizing capture and bunching of injected electrons.
III. Widely Variable Energy Linacs
Clinical considerations suggest the need for a wide variety of beam energies, particularly for electron therapy where tumors of different sizes extend to different depths adjacent to or near a body surface. A range from 2 MeV to more than 30 MeV electron beams has been employed.
A. Clinical Need
Clinically, most tumors may be adequately irradiated with 4–6 MV x-ray beams and treatment units having these energies comprise the majority of linacs in use. However, thick body sections such as the lateral pelvis are advantageously treated with 10–25 MV x-rays. Marks has noted the advantage of treating certain tumors with combined low- and high-energy x-rays. Gale and Innes earlier cited the advantages of employing mixed high-energy x-ray and electron beams. Tapley and others have demonstrated an advantage of adding an electron boost irradiation to some specific lesions treated with megavoltage x-rays. It would appear from this perspective, that a single versatile linac design could advantageously provide a low and a high x-ray energy together with a wide variety of electron beam energies without the necessity of moving the patient. Such requirements pose difficult design problems but several units have been developed recently to solve them.
B. Technical Difficulties
The major problem arises because of the wide energy limits inherent in these specifications for both electrons and x-rays together with significant differences in beam current requirements for the two modalities. For x-ray dose rates comparable to those for electron therapy, the beam current re-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 quirement may be 100 or more times greater than for electron therapy. The overall energy gained by an electron is the integral of the axial E field encountered over the path length through the structure. The E field itself is proportional to P Cu where P Cu is the microwave power delivered to the copper walls of the structure; that is, after beam loading and other losses are considered. The major parameters in determining the range and magnitudes of electron energy are the type and length of the accelerator structure, the available microwave power, and beam loading as it depends on the beam current. Either a klystron or magnetron may be employed as the microwave power source. A klystron usually functions as an amplifier in conjunction with a low-power oscillator. This combination is more stable than a magnetron oscillator and its higher output can be varied more readily over a wider power range than can a magnetron. Although the klystron and associated electronics are more costly than a magnetron, the longer klystron life, sometimes ten times longer, may represent good value. Efficient capture of electrons injected into a linac structure, i.e., the bunching action performed by the first few cavity, is effective over a relatively narrow range of E field values and hence limited microwave power range. Outside this range, it may be difficult to capture and bunch sufficient numbers of electrons to satisfy the high-current requirement for low-energy x-ray therapy, the most demanding modality in this regard since x-ray production varies approximately as V 3 where V is the electron energy. The bunching action of the conventional TW structure is less susceptible to beam loading than the SW structure as explained in Sec. II.-B..
C. Early Designs
In view of these considerations, one early approach in covering a wide electron and x-ray energy range was to employ a relatively long TW structure whose bunching action encompasses the requisite beam currents and beam energy range. Beam energy is varied by changing the microwave power input but an increased energy spread of the beam also results because of poor electron bunching action as the energy range is increased. Beam energy is a square root function of input power and therefore changes slowly with changes in power. Klystrons function well down to half power and magnetrons over a much narrower range. A 50% reduction in klystron power achieves only a 30% reduction in beam energy. A typical energy requirement for electron therapy from 6 to 18 MeV would require a ninefold decrease in power and is beyond the capability of either klystrons or magnetrons themselves. Such an energy range can be accommodated by varying the linac input power through the use of an additional microwave attenuator and ancillary hardware. A very early method of reducing the electron beam energy was to detune the power source frequency from the structure resonant frequency f 0 . Since the resonance curve, Ez versus frequency, for these high-Q structures is very steep, the axial electric field Ez and beam energy change rapidly with tuning. However, this method of beam energy reduction proved unsatisfactory since operation on either of the resonance curve slopes is unstable. The attendant complications include reflections from the impedance mismatch, an increasingly non-uniform axial field Ez, and reduced capture and bunching of the injected beam from the gun. A detuned linac may give rise to excessive x-ray leakage radiation originating from unexpected “targets” where a tuned beam would not normally impinge. Another approach was to join two TW structures in cascade as shown in Figure 2.31. The microwave power is divided between them and the power
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 level and phase made variable for the section. The electron bunches accelerated in the first section are stably located just forward of the advancing wave crest as shown in Figure 2.24(b). By suitably adjusting the phase and power to the second section, they may be further accelerated or de-accelerated. Since the electron velocity is virtually independent of electron energy in this section, a stable position of the bunch on the traveling wave is easier to maintain. When de-accelerated in the second section, these bunches are less stable in position on the wave as explained below so that both an attenuator and phase shifter are used to optimize performance.
KLYSTRON MODULATOR
LOAD
CIRCULATOR
POWER DIVIDER
ELECTRON GUN
POWER SUPPLY
PHASE SHIFTER
ACCELERATOR TW SECTION I
ATTENUATOR EXIT ELECTRON BEAM
ACCELERATOR TW SECTION II LOAD
LOAD
Figure 2.31. Variable energy linac composed of two TW structures in cascade.
D. Phase Considerations
The operation of the two-section linac can be understood in terms of the position of an electron bunch with respect to the phase of the microwave E field being experienced. Figure 2.32 illustrates one cycle of the 3000-MHz E field and several positions where an electron, or bunch of electrons, traveling at a velocity ve, might be located and borne along at the phase velocity of the wave vp. Typically, electrons are accelerated at positions 1, 2, and 3; they are de-accelerated at positions 4, 5, and 6. In general, the phase, or phase angle, refers to a position along the wave here expressed in degrees. By convention, an electron at position 1 at 90° is said to be in phase and at position 5 at 270° as out of phase. The rate of energy gain or loss by an electron is determined by the electric field E it is experiencing and therefore by its position along the wave, i.e., the phase, and the amplitude of the wave. By shifting the phase of the wave, we move the electron bunch to a new position along the wave with a higher or lower value of E. By keeping the phase constant and varying the amplitude of the wave, the magnitude of the E field being experienced by the electron can be altered. The E field pattern recurs along the beam axis twice as frequently as in TW structures; that is, an accelerating electron bunch is contained in every second cavity instead of every fourth cavity. In SW structures, all the cavi-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ties tend to have the same electric field, since the rf power bounces back and forth from each end of the structure many times (typically 100 times) to fill the cavities with energy as expressed in E and H fields. Hence, beam loading of any cavity contributes to the reduction of the E field in all cavities, including the buncher cavities. As a result, bunching in SW structures is efficient over only a narrow energy range, typically ±5%. Thus, to maintain effective bunching action in SW structures, the input rf power must be increased with an increase in beam current.
MICROWAVE E FIELD CYCLE
E–
1 2
3
ve vp
0°
E+
180°
360° PHASE ANGLE
4
6 5
Figure 2.32. Electron bunches shown in representative positions on the microwave E field phase diagram. For simplicity, E– is drawn upward. The overall energy gained is determined by the integral of the electric field E over the path length of the electron through the structure. Electrons gain maximum energy if they are concentrated in a spatially narrow bunch along the axis at the wave crest position 1. They gain less energy but their position on the wave is more stable and axial spread minimized if they were located just forward of the crest at position 2, a position comparable to the idealized electron bunches of Figure 2.24(b). This axial phase stability of an electron bunch at position 2 is achieved as follows: An electron which leads the bunch at position 3 on the wave encounters a smaller E field slowing it down; if it lags the bunch at position 1 on the wave, it encounters a larger E field speeding it up. Both effects operate in sequential cavities to ensure a compact axial bunch and phase stable position as well as a constant terminal energy of the exit beam. This phase stable bunch is often called the synchronous bunch since it moves in synchrony with the wave. The two-section linac illustrated in Figure 2.31 depends on first bunching and accelerating a group of electrons located at or near position 2 in section I to a preset energy. Then, by adjusting the amplitude and phase of the E field in section II, we add to or subtract from this energy. In section II, the energy is added to electron bunches at positions 1, 2, and 3 or subtracted at positions 4, 5, and 6. The energy subtractive positions 4 and 5 are phase stable, but position 6 is unstable in phase and tends to spread a low-energy bunch spatially and, hence, energetically. However, for relativistic electrons over a few MeV in energy, the effect is not large. Beam current losses are more severe when operating linac section II in the energy subtractive mode. These lost electrons, which impinge on the structure disks and walls, are high energy and can give rise to significant anomalous leakage radiation levels. Typically, a relativistic electron bunch would occupy a 20°
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 phase angle interval; that is, somewhat longer than the idealized bunches illustrated in Figure 2.32. The feasible energy range of the two-section linac depicted in Figure 2.31 extends between the sum and difference values of the two individual structure energy gains. Representative electron differential energy spectra for operation with several differences between sections I and II have been reported by Lanzl. Since the first section is operated at constant power level, the electron bunching action performs well. However, the necessity of two microwave structures, two speed points, and additional high-power microwave hardware are complicating factors. Moreover, TW structures are inherently less efficient than SW structure in that residual power is dissipated in an external, or internal collinear load. The Varian Clinac 35 and the AECL Therac Sagittaire treatment units are of the TW two-section design as is an early 5–50 MeV linac described by Carpender et al. A hybrid variation consisting of a short backward wave TW structure followed by a long SW structure is shown in Figure 2.33. Here, the backward wave is magnetically coupled by slots placed peripherally in the disks between cavities. As in the side-coupled structure, the conflicting requirements of power transport versus electron acceleration are separated and optimized independently. The short TW structure is designed for low-power consumption and good buncher performance. Such backward wave structures have the unusual property that the phase velocity vp at which electrons are bunched and accelerated, is opposite in direction to the direction of power flow and group velocity vg, the velocity at which the structure fills with energy. Therefore, this TW structure has the electron gun and power feed on opposite ends of the structure. The klystron power is fed at the beam output end of the TW structure. The residual power appearing at the electron injection end is redirected to the center of the SW section. Electron bunches entering the SW section are traveling at an almost constant velocity near that of light and little energy spread ensues. Standing wave structures may be fed at any point along their length, thereby offering more flexibility in mechanical design. However, there is an advantage in feeding at the mid-region of SW structures, in that absorption of energy in coupled cavity modes is less troublesome.
KLYSTRON MODULATOR
ELECTRON GUN
TW SECTION (backward wave)
LOAD
POWER SUPPLY
SW SECTION (side cavity coupled)
EXIT ELECTRON BEAM
CIRCULATOR
PHASE SHIFTER
ATTENUATOR
Figure 2.33. Hybrid variable energy linac composed of a backward wave TW section followed by an SW section.
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E. Clinac 2500
A recent commercial entry, the Clinac 2500 illustrated schematically in Figure 2.34, features a 1.9-m side-coupled SW structure, wherein one of the side coupling cavities functions as an energy switch to provide two xray beam energies of 6 and 24 MV. This side cavity, shown inset in Figure 2.34, is changed mechanically to alter the x-ray energy by inserting a plunger with an attendant change of the E field amplitude or phase. The accelerating E fields in the buncher are unchanged when switching energy and hence, there is little change in energy spread of the beam. The Clinac 2500 can be viewed as a sophisticated two-section SW linac with an extremely compact power divider/phase shifter. A 5.5-MW klystron is the microwave power source. Six electron energies, 6–22 MeV, are provided. ENERGY SWITCHING SIDE CAVITY
DEMOUNTABLE GRIDDED ELECTRON GUN
DUAL PHOTON STANDING WAVE ACCELERATOR
270° ACHROMATIC BENDING MAGNET
COLLIMATOR ASSEMBLY
ISOCENTER AXIS
RF INPUT COUPLER
ENERGY SWITCHING SIDE CAVITY PLUNGER 2 1
ACCELERATING CAVITIES
Figure 2.34. A two x-ray energy linac using an SW structure and an energy switching side cavity (Clinac 2500, courtesy of Varian Associates).
Figure 2.35(a) illustrates two alternative methods of providing the high and low x-ray energy modalities in the Clinac 2500 side-coupled linac structure by employing two different locations of the energy switching side cavity. The high x-ray energy modality depicted in Figure 2.35(b) is provided by operating all of the axial accelerating cavities of sections I, II, and III of the structure approximately in phase (see electron bunch position 2 in Figure 2.32) with the energy switch plunger completely retreated. The average accelerating field E1 experienced is constant over the entire length of the structure and the terminal electron beam energy is proportional to the area under the average E1 rectangle of Figure 2.35(b).
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ENERGY SWITCHING SIDE CAVITY LOCATION 1
LOCATION 2
ELECTRON GUN I
(a)
II
III
EXIT ELECTRON BEAM
(b)
AVERAGE FIELD
SW SIDE-COUPLED STRUCTURE
(c)
AVERAGE FIELD
E1
E2
E1
(d) AVERAGE FIELD
E2
E1
Figure 2.35. Energy switching side-coupled SW structure and average E field distributions for operating conditions described in the text. In one method of providing the low-energy modality, the energy switching side cavity is placed at location 1, as in Figure 2.35(a), and its normal TM010 mode (E field configuration) is unchanged. The phase is held constant but the amplitude of the E field is reduced by inserting the plunger half way into the cavity into dashed position #1 of Figure 2.34. Here, the energy switch operates at location 1 in order to achieve the desired reduction in amplitude of the E field. Sections I, II, and III all operate in phase but the amplitude of the E field is reduced from E1, in section I to E2 in sections II and III as shown in Figure 2.35(c). The terminal energy is represented by the sum of areas under E1 and E2. Figure 2.35(d) illustrates an alternate method of providing a low-energy modality wherein the energy switch is placed at location 2 in Figure 2.35(a). Here, the normal TM010 E field configuration of the energy switching side cavity is changed to the TEM mode by inserting the plunger fully into the cavity in the dashed position #2 of Figure 2.34. The effect of this is to preserve the amplitude, but to reverse the phase in the axial accelerating cavities which follow it by 180°. The average electric E field now varies as shown in Figure 2.35(d). As a result, the terminal energy is now the energy gained in sections I and II, represented by the rectangle under E1 minus that lost in section III represented by the rectangle under E2.
F. Multiplepass Linacs; Therac 25
Recently, two linac designs have been introduced in which the high-energy requirement is achieved by passing the beam two or three times through the same accelerating structure. Such multiple-pass linacs could be con-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 strued as microtrons, but their geometry, construction, and isocentric mounting are similar to that of conventional linacs. Figure 2.36 illustrates one such unit described by Froelich. It employs either one or three passes through a 0.7-m side-coupled SW accelerating structure (2) and is magnetron powered. An energy gain of 2–8 MeV/pass provides readily adjustable electron energies from 2–24 MeV and x-rays of 6, 12, and 20 MV. One distinctive feature of this unit is the hollow cathode annular electron gun (1) which permits passage of the returning electron beam in the three-pass configuration. Such electron guns tend to have higher emittance than their axial counterparts; that is, beam angular divergence is greater and beam cross section is larger for a given beam current. Another feature is the unique beam return “butterfly” magnets (3) so-called because of the butterfly-shaped electron trajectories through them. These magnets are constructed in three sections to provide the 180° beam turnaround and have shaped pole edges to provide transverse focusing and isochrony so that the electron bunch is reproduced after traversing the magnet.
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1
2
3
3
0
(a)
20 cm 40
SCALE
0
0.5 m
1.0
SCALE
(b) Figure 2.36. (a) A multiple-pass linac or shuttle microtron which involves either one or three traversals of the electron beam through the SW structure; (b) multiple-pass linac mounted in an isocentric gantry (after Froelich) A somewhat similar commercial unit, the AECL Therac 25 shown in Figure 2.37(a), employs two passes of the beam, a hollow cathode gun and one return magnet. The 1.2-m SW structure is of the bi-periodic type shown in Figure 2.37(c) [and Figure 2.30(c)] and incorporates thin pancake-like, onaxis coupling cavities. It provides a 25-MV x-ray beam and eight electron energies between 5 and 25 MeV. The Therac 25 is shortened because of the double-pass design and fits into a shorter room than some high-energy units. The unit is powered by a 2.6-MW magnetron. The beam energy is varied by adjusting the distance of the 180° beam return magnet from the accelerating structure over a short interval of about 2.5 cm. This alters the phase of the accelerating E field encountered by the returning beam and hence, the energy gain of the beam during the second pass as described for Figure 2.32. The 2.5-cm adjustment, )x, of the four-sector, doubly achromatic, isochronous return magnet provides approximately a 0 –180° phase
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 angle range of the E field for the return passage of an electron bunch (see Sec. V.). REFLECTING MAGNET DOUBLE PASS ACCELERATING WAVE GUIDE ∆x
270° BENDING MAGNET
(a)
ELECTRON GUN/INJECTOR SYSTEM
PRIMARY COLLIMATOR
BEAM APERTURE ADJUSTABLE COLLIMATOR X-RAY
(b)
COUPLING SLOT
MACHINED SEGMENT PANCAKE COUPLING CAVITY ACCELERATING CAVITY
(c)
Figure 2.37. (a) A double-pass linac incorporating a single reflecting magnet; (b) half-cavity machined segment showing beam aperture and coupling slots; (c) cross section of biperiodic cavity structure (Therac 25, courtesy of Atomic Energy of Canada, Limited). The treatment head consists of a conventional four-jaw movable collimator suspended below a turntable that has three positions corresponding to the three modes of operation: photon therapy, electron therapy, and field illumination. The nominal 270° magnet system adjacent to the treatment head incorporates two dipole magnet sectors. After traversing this 270° magnet, the electron beam is scanned spirally at two fields per second on a scatterer using a quadrupole magnet. Although not providing a low energy x-ray mode, the Therac 25 provides high electron energies from a comparatively short structure using a magnetron, a relatively inexpensive microwave power source. The use of aluminum coated polyamide films (for example, Kapton, DuPont Company, Industrial Film Division, Wilmington, Delaware 19898) in the monitor ionization chamber is expected to significantly prolong its useful life since these films appear to have good electrical properties and outstanding resistance to radiation damage.
IV. Microtrons
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The microtron is an electron accelerator which combines the principles of the electron linac and the cyclotron.
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A. Introduction, Circular and Racetrack Designs
In the circular microtron, the electron gains energy from a microwave cavity (sometimes called a resonator) and describes circular orbits of increasing radius in a uniform magnet field. The cavity voltage, frequency, and magnet field are so adjusted that after each transit through the cavity, the electron gains sufficient energy so that its transit time in the magnet field increases by an integral number of microwave cycles. An increase in energy gain per orbit can be achieved by placing the cathode inside the cavity and allowing the electron beam to be pre-accelerated before reaching the entrance hole of the resonant cavity for the first time. The microtron exhibits axial phase stability characteristics similar to those of accelerating electrons in linacs as described in Sec. III.-D., but with a narrower energy spread. Splitting the magnet into two D-shaped pole pieces and separating them provide greater flexibility in achieving efficient electron injection and higher energy gain per orbit through the use of multi-cavity accelerating structures. This configuration, called a racetrack microtron, consists of two semicircular and two straight section orbits. One early racetrack microtron designed for radiotherapy provided 1.5 to 15 MeV electron beams through a common exit portal using from 1 to 6 accelerating orbits. The small beam emittance of microtrons (product of beam radius and divergence) and minimal energy spread simplifies the beam transport system and encourages the use of a single microtron for supplying several treatment rooms including, for example, an intraoperative electron beam for an operating suite. An additional advantage may be (depending on the energy and design parameters) attainment of high energies in a smaller system volume when compared with linear accelerators, due to the cubical geometry characteristic of microtron units. Depending upon their energy and design, microtrons may require large heavy iron magnet poles and a high degree of field uniformity. For the circular microtron, the required magnet volume, and hence cost, grows as the energy E3. In recent years, modifications of the conventional microwave accelerating structure and improvements in electron gun injection methods and magnet designs have opened up new possibilities for microtrons in radiation therapy.
B. Circular Microtron — 22 MeV
A novel 22-MeV circular microtron for radiation therapy, the Scanditronix MM 22, shown in Figure 2.38, has been described by Svensson et al. Extraction of the electron beam is made through a movable steel deflection tube which can be moved to select any of the orbits between number 10 and 42, with all the electron energies emerging along a single axis. The deflection tube displaces any selected orbit downward by a constant distance. As a result, the electron beam exits through the fixed horizontal extraction tube as shown in Figure 2.38, instead of continuing through the resonator cavity.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 MOVABLE DEFLECTION TUBE
ELECTRON BUNCHES
ELECTRON GUN
RESONATOR
WAVEGUIDE
EXTRACTED BEAM
Figure 2.38. Cross section of a 22-MeV microtron illustrating the path of electrons in the accelerator (left) and through an isocentric treatment unit (right). The range of motion of the deflection tube from orbit 10–42 is shown by arrows on the dashed line. The electron energy gain per turn is approximately 535 keV and the energy spread of the exit beam is about 35 keV [full width at half maximum (FWHM)]. The magnet poles for this circular microtron are 1.8 m in diameter, and a magnetic field uniformity of 1 part in 10,000 is required. Either a klystron or magnetron can be used as the microwave power source. Two x-ray energy options among 6 or 10 and 20 MV, are available and ten electron energies ranging from 2–22 MeV. A composite x-ray flattening filter is employed to improve depth dose characteristics; high atomic number Z in the center and low Z on the periphery. Two separated scattering foils are employed for electron therapy; a high-Z primary foil of constant thickness is used to spread the beam followed by a low-Z secondary foil of variable radial thickness to flatten the electron fields for sizes up to 40 cm in diameter at the isocenter. Capability of uncoupling one x-ray and electron collimator jaw along the beam axis permits easy abutting of x-ray, electrons, or x-ray to electron treatment fields without divergence. In one configuration, a Scanditronix MM 22 microtron feeds two isocentric treatment units and also provides a research beam. Being stationary, this microtron is more easily maintained, magnetron lifetime may be increased but electron gun replacement may be more frequent as compared to a linac.
C. Racetrack Microtron — 50 MeV
In one newly constructed unit (Figure 2.39), a six-cavity accelerating structure rather than a single cavity is used between the separated pole pieces to provide energy gains of 5 MeV per orbit. An exit beam energy ranging from 5–50 MeV for electrons and photons is provided. The beam energy is changed by moving an extraction magnet in or out to select the appropriate orbit for the desired energy as shown in Figure 2.39. The extracted beam exits along a common beam line independent of energy. A thorough description of the development of the 50-MeV microtron prototype is provided by Rosander et al. Attaining desirable radiation beam characteristics for electrons and x-rays becomes more difficult at higher energies. The microtron, by combining its inherent small beam emittance and energy spread with a scanned pencil beam such as described earlier, may provide one approach to improving beam characteristics. Here, the scanned pencil beam of electrons could fa-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 cilitate the provisions of large flat fields of electrons and x-rays. The 50 MeV as well as the MM 22 microtron designs involve moving either an extraction magnet or a steel deflection tube, respectively, within the evacuated system and this feature could present maintenance problems. movable extraction magnet MAGNET
MAGNET
linear accelerator electron bunches extracted beam
electron gun
Figure 2.39. Racetrack microtron in 5–50 MeV range for electron and x-ray therapy.
V. Beam Transport Magnet Systems
The beam transport magnet system includes solenoids and steering coils over the accelerator structure, together with focusing quadrupoles and bending magnets after the accelerator structure, as well as associated power supplies.
A. Introduction
The system confines, steers, and guides the electron beam from injection to the x-ray target or electron scatterer. For simplicity, magnet energizing coils and magnetic return paths, where used, have been omitted in many of the illustrations which follow. A fundamental design consideration for optimal performance is matching the beam transport system capabilities to the characteristics of the linac beam. Most treatment units are isocentric, a circumstance necessitating bending magnets in higher energy machines so that the correspondingly longer structure can lie horizontally in energy machines. However, the introduction of short SW structures has led to straight-through isocentric 4and 6MV units having a 100-cm source-axis distance (SAD) with moderate isocenter and room heights. No bending magnets are required, and the structure is so short that a solenoid need not be used to confine the beam. However, such straight-through designs let low-energy electrons strike the xray target, so the peak electron energy must be slightly higher than in a bent-beam machine to achieve the same x-ray penetration. The beam transport systems of bent-beam medical linacs must adequately respond to operating conditions which affect beam stability and symmetry. Asymmetries arise when the primary x-ray lobe does not strike the flatten-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ing filter symmetrically. This can be due to a change in the angle of incidence of the electron beam on the target or due to the lateral displacement of the position of the electron beam at the target (see Figure 2.40). These changes can result from changes in beam energy because of a change in microwave power or microwave frequency as well as from other sources such as mechanical strains associated with motions of the gantry or temperature variations as well as from the effects of stray magnetic fields. Such changes occur more often in the bending plane than in the transverse plane. Automatic frequency control and other feedback control devices, based on sampling ionization chambers, have become more sophisticated in limiting adverse response, (Sec. VI.-B.).
ELECTRON BEAM
∆φ
∆r
TARGET
FILTER
RESULTANT DOSE AT Dmax
(a)
(b)
(c)
Figure 2.40. Effect on resultant x-ray dose distribution at Dmax of misalignment between electron beam and flattening filter axis: (a) symmetrical flat field for a correctly aligned x-ray flattening filter, (b) asymmetrical field for an angular divergence )N, and (c) asymmetrical field for a radial displacement )r. At high energies, these effects can be pronounced because of the very peaked filter and high attenuation gradient as a function of its radius.
B. Definitions and Conventions
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Magnets bend (deflect) and separate (disperse) particles of differing momenta. These two actions are necessarily intermixed and we often wish to emphasize one action, for example, deflection in bending a small beam of electrons onto an x-ray target. Medical linacs may employ only one dipole magnet but at times two or more magnets are used to better separate the two actions. The electron beam which enters a beam transport system contains a spectrum of particles which must be restricted in momentum (or energy) range. Spatially, their trajectories may diverge from the central axis and be laterally displaced from it. The task of the transport system is to
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 bring this diversity of particles to a small and coaxially directed beam of particles on the central axis of the x-ray target or the scattering foil. The literature readily applicable to medical linac magnets is modest in extent. Some analyses make use of matrix methods and often differing conventions, notations, and terminology are used. Many phenomena involved with the transport of charged particles in magnetic fields can be described with first-order theory in which quantities involving the products of two or more small differentials of momentum or length are ignored. The theory is limited to particle beams with small energy spread and small angular and spatial spread. A general solution to the electron transport problem involves a second-order differential equation. Its two orthogonal solutions are sine- and cosine-like functions and all possible particle trajectories are some linear combination of them. A number of definitions follow which are useful in describing and understanding the performance of beam transport systems by these as well as less sophisticated methods. An electron of charge e and relativistic mass m moving with a velocity v at right angles to and through a uniform field of strength B experiences a magnetic force Bev perpendicular to its instantaneous direction of motion. It gains no energy from such a static field but is constrained to travel in the arc of a circle of radius D. The requisite centripetal force for such motion, mv2/D, is provided by the magnetic field such that: mv2/D = Bev or BD = mv/ e. The quantity BD measures the “stiffness” of the beam and is called the magnetic rigidity. For example, a 15,000-G dipole field will bend a 25-MeV electron beam with a radius of 5.66 cm. A 5-MeV electron beam can be bent with the same radius by a field of 3240 G. In addition to supplying the centripetal force to bend the beam through an angle, the bending magnet system provides forces to focus the beam. These focusing forces can be provided by tilting the pole faces with respect to each other to curve the field lines throughout the magnet or by tilting the pole edges to employ the curved fringe field lines at the magnet entrance and exit. Tilting the pole faces produces a radial gradient n of the magnetic field defined by ρ ∆B n = ---- ⋅ -------- . B ∆x
(8)
Some magnets provide such a non-uniform transverse field in which the magnetic field increases (n < 0) or decreases (n > 0) with bending radius of the particle. However, most dipole magnets provide a uniform field transverse to the bending plane and are characterized by a field gradient value of n = 0. In either case, the radial focusing (often analyzed in terms of the horizontal or axial motion) can be separated to first order from the transverse (vertical) focusing. The strong focusing properties of quadrupoles may also be employed. Electrons whose momenta differ from a central value P0 are deviated from the central orbit by a magnet. This phenomenon is termed energy dispersion. The term achromatic describes systems which bring particles of differing energies, which were originally paraxial, to the same focus. In a zero dispersion system (sometimes called singly achromatic), the particles diverge after reaching this focal point. In an achromatic system (sometimes called doubly achromatic), the particles do not diverge but remain parallel after reaching this focal point. Double focusing refers to an ability to focus both the transverse and radial image components at the same point along the central trajectory. A double-focused image is called stigmatic. The term energy focus is sometimes used for the momentum focus associated with
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dispersion and the term spatial focus is used for the radial and transverse focusing associated with the divergences and lateral displacement of particles with respect to the entering central ray. Divergence refers to the angular departure )n of the beam from a central axis or trajectory, displacement refers to the lateral positional departure )r from this axis. The latter two quantities are illustrated in Figure 2.40. The return magnets of multiplepass linacs or microtrons must be isochronous; that is, the length of time required to traverse the magnet is independent of electron energy. This ensures that the electrons comprising the bunch are not further separated in time and hence, in phase and energy on subsequent passes. A central orbit reference trajectory is defined herein by particles of momentum P0 which enter the transport system axially and are deflected along a central orbit of radius D0 passing through the median plane between poles [see Figure 2.43(b)]. It is convenient in examining beam transport to combine two main characteristics of a beam, its radius and its divergence, into a single parameter, the phase space area, which is termed emittance. For a constant velocity beam and with the reasonable proviso of no coupling between coordinate motions, the phase space is invariant in time as the beam moves through the transport system. Defining z along the central orbit reference trajectory, we can separate the transverse perturbation motion into small )x and )y displacements with respect to this coordinate. We define momentum coordinates Px and Py which are proportional to their divergences from the z axis. thus: x’ = dx/dz and y’ = dy/dz. The ensemble of particles moving through the system has a constant phase space for both x and y and can be defined at any position z in terms of the maximum excursions of their displacement and divergence coordinates at that position. This phase space can be expressed as an ellipse whose shape varies as the beam progresses through the system but whose area remains constant. The individual particles oscillate about the equilibrium orbit as representative points in phase space. We call the four-dimensional space the emittance g, with ε = xx′yy′ ⁄ π
2
(9)
An important corollary is that if we focus the beam to a smaller diameter, its divergence will increase and vice versa, but its emittance will remain constant. We can also reduce the diameter of a beam, and consequently the current and emittance, by intercepting a portion of it. Acceptance is a complementary term to emittance and refers to a characteristic of an equipment, a measure of its ability to accept and transport a beam rather than a property of the beam. For maximum efficiency, there must be a good match between the emittance of the entrant beam and the acceptance of the transport system, its counterpart. Unless the acceptance of the beam transport system is at least as large as the emittance of the entrant beam, some of the particles will not be transmitted. Beams characterized by small values of emittance are more readily transported over long distances. Most isocentric treatment units employ a nominal 90° or 270° beam-bending magnet with the accelerator guide structure mounted approximately horizontally in the gantry. The 270° magnet systems are usually achromatic and the resulting isocenter height is acceptably low. However, when a 90° magnet system is made achromatic, two or more magnets are involved. The beam path through it becomes long and the accelerator structure axis may lie well above the x-ray target. This can raise the isocenter height excessively in a high-energy machine. The sections which follow describe specific magnet systems.
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C. 90° Magnets
Figure 2.41 illustrates the effect of a simple 90° dipole magnet on the exit beam for entrant beams having an energy spread ± E about E0 or a radial displacement )x or a divergence )n. These aberrations, which are easiest to understand individually in the radial plane of simple 90° magnets can appear in combination and are also present in the transverse plane of magnets. They may, in part, be corrected in more complex magnet systems. Typically, the beam energy spread is restricted to ±10% or less about its central value. The trajectories of the low, central, and high-energy components in Figures 3.40(a) and 3.41(a) are denoted by 1, c, and h, respectively. MAGNET POLE ∆x
∆φ
ELECTRON BEAM c
c
c
∆x
l E0–∆E
c E0
(a)
∆φ
h E0+∆E
c
(b)
c
(c)
Figure 2.41. Effect of simple 90° dipole magnet deflection system on entrant beams which are (a) axial and nondivergent with E = E0 ±)E, (b) nondivergent with E constant but parallel and displaced )X above and below the entrant center line, and (c) axial with E constant but diverging )n above and below the equilibrium orbit. The electron beam entrant center line is denoted by c. The electron trajectories shown were constructed using straight-line entry and exit paths connected tangentially to arcs of constant radius when the electron is in the dipole magnet field. A single 90° bend magnet such as shown in Figure 2.42(a) is not achromatic. It can bend a mono-energetic beam on axis to a point at the x-ray target but the spread of energies of the actual beam results in a spread of such focal points at the x-ray target as shown. This spreading effect can be minimized by reducing the bending radius of the magnet, restricting the emittance and the energy spread of the beam, stabilizing the operation of components which affect beam energy or by incorporating a second magnet to provide focusing. However, even with these precautions, changes of energy as well as variation of the angle and position of the entrant electron beam will produce detrimental asymmetries in the exit beam and treatment field in the non-achromatic 90° systems more readily than in the achromatic 270° systems. The principal effect of an energy change of the entrant beam in a non-achromatic 90° magnet system is a lateral displacement at the target although a secondary effect could be the appearance of new leakage foci. To be effective, energy controlling slits must be located near the output of a 90° magnet, but here they tend to become enlarging the effective focal spot size. A uniform field 90° achromatic bending magnet system suggested by K. L. Brown is described by Penner. It consists of a single quadruple magnet positioned in the plane of symmetry between two 45° deflection magnets. The magnet system is achromatic and double focusing but its design requires a high isocenter. The performance of the 90° bent-beam Clinac 6 treatment unit has been reported by Horsley and its magnet described by Avery.
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D. 270° Magnets
Figure 2.42(b) illustrates a 270° triple-focus, zero dispersion (singly achromatic) uniform field magnet wherein a ±10% beam energy spread is brought to a single focal point on the target, in part, by choice of the angle of the entrant and exit pole faces. The higher energy component is deflected through a circle of larger radius, the lower energy component l through a circle of smaller radius, but both converge on the target at the same point on the central energy trajectory c. However, a change in mean energy of the beam from the accelerator structure will result in a change in mean angle of the beam at the target and hence to asymmetry in the x-ray field. ELECTRON BEAM
h c l
MAGNET POLE
MAGNET POLE
ELECTRON BEAM
TARGET
l c h
TARGET
(a)
(b)
d
2
MAGNET SECTORS
d
1
MAGNET POLE a2 SECTION d1-d2 ELECTRON BEAM MAGNET POLES
a1
(c)
(d) d
2
TARGET MAGNET POLE Y Z X
S2
SLITS Z
MAGNET POLES
S1 l c h
S1 a2
X S2
d
1
ELECTRON BEAM a1
BX = BY = 0 BZ = GXn
TARGET
(e)
SECTION d1-d2
(f)
Figure 2.42. Nominal (a) 90° and (b) 270° bending dipole magnets with representative electron orbits. Trajectory c corresponds to the central energy E0, trajectories l and h correspond to E0 –10% and E0 +10%, respectively. One type of 270° achromatic magnet is shown in (c) and (d) and another type in (e) and (f).
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Magnet edges contribute important focusing properties. In one approximation, the transverse field can be assumed to begin and end abruptly at an edge near where the beam enters and where it leaves the magnet. A beam of particles entering normal to such an edge will be radially converged. The edge may be angled other than at 90° with respect to the particle beam and particles will be more rapidly converged (focused) or less rapidly converged (defocused) in the radial plane. The fringing magnetic field which exists beyond the pole edges, provides additional focusing for an angled pole edge. Here, a radial field component exists which interacts with the azimuthal particle velocity to produce a transverse (vertical) force. This force will be focusing (toward the central orbit plane) or defocusing (away from it) depending on whether the pole edge angle, with respect to the beam, is greater or less than 90°. If the edge tilt produces focusing in the transverse plane, then it produces defocusing in the radial direction, and vice versa. As in the case of magnetic quadrupoles, transverse focusing and radial defocusing go together and vice versa. The pole edge angles chosen in Figures 3.42(b) and 3.42(c) provide this edge focusing effect. Cross has shown that some judicious choices of angles can result in two-directional focusing in both the radial and transverse directions. In order to minimize distortions in radiation field flatness due to changes in beam energy, it is desirable to employ an achromatic magnet system. Figure 2.42(c) illustrates one way to build a 270° achromatic magnet system by combining uniform and non-uniform field regions in the same magnet. It focuses a range of entrant momenta, and an input configuration of lateral displacements and angular divergences of the electron beam, to an optically similar configuration at the output focal plane. Its entrant pole face angles, a1, and a2, can be adjusted for optimal radial and transverse focusing of each mono-energetic bundle of rays at the d1 – d2 plane. In addition, two adjustable pole sections of the magnet, shown in section view d1 – d2 [Figure 2.42(d)] provide an adjustable radial field gradient with n > 0. This radial gradient is controlled by adjusting angle a3 [Figure 2.42(d)] to re-converge the different energy bundles of rays into a single spot at the x-ray target, the distribution of rays in this spot being the same as in the beam cross section which enters the magnet. The Siemens Mevatron treatment units employ this type of bending magnet. Establishing and adequately preserving the electron beam position and direction at the x-ray target or scattering foil to maintain a symmetrical radiation field become more difficult for higher energy linacs which operate over a wide range of energies. High-energy treatment units incorporate as many as six discrete magnets in their beam transport system. Because of iron hysteresis and saturation effects, a programmed sequence of magnet current changes may be employed to reliably and accurately establish or change a given beam energy or modality. A three element, double focusing, 270° achromatic magnet system has been described by Hutcheon and Heighway. It employs an input quadrupole doublet and a 180° uniform field magnet separated by a short drift space from a 90° uniform field output magnet to provide a nominal 270° system. The system minimizes the magnet dimensions in the direction opposite to the exit trajectory, facilitating a lower isocenter height. It is suitable for electron beam energies from 5 to 50 MeV, an overall energy spread of 20% and a total bending angle between 230° and 290°. The AECL Therac 25 treatment unit employs this type of bending magnet.
E. Mirror 270° Magnet
The 270° magnet illustrated in Figures 3.42(e) and 3.42(f) is a design of Enge. It is sometimes called a “loop” or “pretzel” magnet but more often an achromatic magnetic mirror since particles which traverse it appear to be
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 reflected from the y–z surface at the entry point. It is employed in the Brown-Boveri Dynaray treatment units. Typically, its non-uniform field increases with x having an n = 1 exponent value near the edge and n = 0.8 elsewhere and angle a1 = a2 = 45°. The x–y proportions of the trajectory loops are constant independent of momentum, i.e., the loops have the same shape, and all particles exit at the origin independent of momentum, i.e., there is no dispersion and the deflection is achromatic. This magnet is capable of faithfully focusing a wide range of momenta as limited by slits S1 and S2 placed on the plane of symmetry [see section d1 – d2 in Figure 2.42(f)]. The Varian Clinac 2500 employs a modified loop magnet wherein a simple two-step gap replaces the smoothly varying gap along the x axis [Figures 3.42(e) and 3.42(f)]. The beam trajectory in the modified loop magnet has been described by Tronc. The Dynaray-4, employing a 270° “mirror” magnet is described by Sutherland.
F. Three-sector 270° Magnet
The illustrations comprising Figure 2.43 describe the nominal 270° magnet system used in the Varian Clinac 18 treatment unit and are shown mounted in the treatment head in Figure 2.45. Figure 2.43(a) is a crosssectional view of this magnet in the median bending plane of the central orbit. It incorporates three uniform field magnet sectors, or pole sets, M1, M2, and M3, with short drift tubes connecting them. A magnetic shunt between poles (not shown) provides a relatively magnetic field free region for passage of the particles between sectors. The emittance of the beam entering the magnet system is typical of the output beam of an electron linac in terms of cross-sectional are, divergence, and energy spread. The performance of the system is analyzed with respect to a particle which enters along the central axis with reference momentum P0 and whose central orbit reference trajectory is shown as a heavy dashed line in Figures 3.43(a) and 3.43(b). It has been shown by Brown that the properties of such a system are completely determined to second order by specifying five representative trajectories or paths relative to the reference trajectory. Spatial departures from the reference trajectory by particles of reference momentum P0 are separated into orthogonal radial and transverse components for initially axial but divergent trajectories Sx,y and for initially parallel but displaced trajectories Cx,y. The momentum trajectories Dx dispersed from the reference trajectory in the median plane are for particles initially axial and with momenta within ±)P of reference momentum P0. Two of these trajectories, shown in Figure 2.43(a), depict divergence from the central orbit (Sx) and lateral displacement from the central orbit (Cx) in the median plane. Figure 2.43(b) is a simplified view similar to Figure 2.43(a) depicting trajectory Dx, of momentum dispersed particles initially on the central axis. The trajectories of all particles through the system are symmetrical about a plane of asymmetry located at 135 midway along and normal to the reference trajectory. As shown, the energy selection slits S1 and S2 which are placed at ± )P about radius D0 of the reference trajectory intercept all particles except a narrow momentum band ±)P centered about P0. An end view of this magnet system in the transverse plane is shown in Figure 2.43(c). The entering angular divergence Sx and lateral displacement Cx in the beam cross-section from the accelerator are reproduced at the target plane as shown in Figure 2.43(a) with no significant increase in their magnitude. Figure 2.43(d) illustrates focusing properties in the transverse plane due to the fringing fields of the shaped pole faces along and near the reference trajectory and depict angular divergence Sy and lateral displacement Cy. Again, these transverse divergences and displacements are repro-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 duced at the target with no significant increase in their magnitude. The entering radial and transverse slop and displacement perturbations are equal in magnitude and remain uncoupled in the transport system to first order. Since their dimension in the transverse plane is equal to that in the bending plane, the focal spot is circularly symmetric. This system is achromatic since both the spatial dispersion and its derivative are zero at the output plane. The energy determining slits shown in Figures 3.43(a) and 3.43(b) are sources of leakage radiation from stopped electrons. However, this bremsstrahlung is directed away from the isocenter and not contributory to patient exposure. A magnetic analysis of the electron treatment beams from a Clinac 18 gave energy dispersion values of ±0.4 to ±0.7 MeV FWHM over the range 6–18 MeV, respectively. These dispersion values include the effect of scatter from the linac thin window and 1 m of air. A description of magnetic and threshold techniques for energy calibration of high-energy radiations has been given by Lanzl.
MAGNET MAGNET COIL SECTOR
PLANE OF SYMMETRY PLANE OF SYMMETRY S2
M
ENTRANT TRAJECTOR Y
M1
TARGET
CX
(a)
S
M
SLITS
S1 M2
M3
SLITS
S
SX
CENTRAL ORBIT REFERENCE TRAJECTORY
D
M
CENTRAL ORBIT REFERENCE TRAJECTORY
n
MAGNET COIL VACUUM CHAMBER
–∆ρ
+∆ρ ρ0
ENERGY SELECTION SLITS
n M3
BEAM APERTURES
–∆ρ
(b)
ρ0
PLANE OF SYMMETRY
M1 CY SY
(c)
+∆ρ
CENTRAL ORBIT REFERENCE TRAJECTORY
(d)
N
N
SY
N
S
S
CY
S
M1
M2
M3
Figure 2.43. Nominal 270° achromatic bending magnet system: (a: cross-section view in the radial (bending) plane of the orbit; (b) simplified view in same plane showing momentum dispersion and the use of energy selection slits to limit the momentum to a narrow band ± P about the central reference momentum P0; (c) transverse section end view of the magnet system; (d) central orbit trajectories (simplified) along transverse section.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Typically in this magnet system, the energy defining slits are set for ±3% transmission and are sometimes placed between dipoles M1 and M2 rather than at the plane of symmetry as shown in Figure 2.43(a). The energy servo system, fed by signals from the ionization chamber monitor system described in Sec. VI.-B., controls the input microwave power level so as to maintain constancy of beam energy. The inherent open-loop stability of the system confines the energy spread to within ±1% during a typical treatment, and to within ±5% during a treatment day. The closed-loop gain confines the energy spread to within ±0.1% during a treatment, and to within ±1% during a treatment day.
G. Other Magnets
Panofsky and McIntyre describe an achromatic beam translation system involving two sector magnets. Their objective is to translate the beam without energy dispersion, in order to dispose of unwanted low-energy electrons. More recently, a system involving two magnetic quadrupoles acting as magnetic mirrors have been combined with two microwave cavities to translate, chop, and bunch an electron beam. Here, use is made of an earlier discovery that a magnetic quadrupole with a rectangular aperture can be provided by current sheets bounded by iron rather than the shaped iron pole faces of more conventional designs with their angle aperture limitations. The multiple-pass linacs described earlier incorporate unusual 180° turnaround magnet designs which were described there. Two early radiotherapy beam transport systems not cited in the preceding sections merit attention. Both are isocentric in design and involve rotation of the magnet system about a fixed horizontal linac axis. In one system, the electron beam from a single-section 45-MeV TW linac is first momentum analyzed by a 45° magnet followed by energy defining slits. The beam is then redirected by a 135° magnet so as to be perpendicular to the axis of rotation. The magnet system can rotate ±45°, with respect to the zenith, about the horizontal linac and patient axis. The second magnet system is incorporated in the AECL Therac Sagittaire, a two-section TW linac. It involves a similar but more sophisticated magnet system incorporating additional magnets and permitting 360 rotation around the recumbent patient.
H. Scanned Pencil Beams
Most beam transport systems provide as output a small pencil beam of electrons which are incident on an x-ray target or scattering foil in a fixed position and direction. In an alternative design, the nominal 90° bending magnet is preceded and followed by a scanning magnet as shown in Figure 2.44. The two scanning magnets are placed orthogonal to each other. The electrons still impinge on the x-ray target at a fixed point but their angle of arrival is varied by the two scanning magnets in order to sweep the direction of the x-ray lobe. By suitably varying their magnetic fields, a raster scan of the x-ray lobe direction can be generated similar to that used in a television picture tube. Alternatively, a spiral scan can be generated by appropriate variation of scan magnet currents. An x-ray scanning system incorporating a quadrupole magnet has been described. By retracting the x-ray target, a scanned electron therapy beam is obtained. Such scanned electron beams provide electron treatment fields of good uniformity, low x-ray contamination and could be “programmed” for the arbitrarily shaped fields characteristic of electron therapy. The x-ray lobe scanning feature can largely compensate for the unflatness arising from the angular distribution of bremsstrahlung production and the variation of energy with angle across the field. A thin flattening filter can then be used thereby avoiding significant changes in the photon energy spectrum in the middle of the field rela-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 tive to the edges of the field. This makes it easier to provide large uniform xray fields at high energies. The scanning feature, however, adds complexity and cost and dosimetry is more difficult. For example, monitor ionization chamber collection efficiency may be significantly impaired with attendant non-linearities of the dose monitor. Isodose measurements in a phantom are more complex because of the scanned nature of the beam. BENDING MAGNET ELECTRON BEAM
SCANNING MAGNETS
(a)
(b)
Figure 2.44. Scanned pencil beam provided by a three-magnet system (adapted from Brahme)
Scanned beam systems are interlocked so that if scanning becomes inoperative, no significant hazard arises from the stationary air-scattered, highenergy beams. This “pencil beam” or electrons is minimally scattered by the vacuum window of the structure and the intervening air. For low electron energies, it may be as broad as 10 cm at l00-cm distance, but narrows with increasing energy. A study of the scanned electron beam from a Therac 20 accelerator has been carried out by Pfalzner and Clarke. This unit employs a scanning quadrupole with sawtooth modulated currents of 0.615 Hz and 4 Hz, respectively, in the x and y directions. They conclude that the electron beams of the Therac 20 do not differ appreciably from those of the nearly mono-energetic 22-MeV MM-22 microtron beams, as reported by Svensson, Brahme et al. A high-energy scanned pencil beam of electrons was used earlier by Carpender et al. on a 5–50 MeV linac. This two-section linac was equipped with a novel magnet system for scanning the pencil beam of electron.
VI. Treatment Head
The characteristics of electron and x-ray treatment beams are strongly influenced by the design of the x-ray treatment head.
A. Introduction
A representative design is illustrated in Figure 2.45. In addition to a bending magnet, fixed shielding, and large movable collimators, the head contains the x-ray target, flattening filter, and in some cases, dual electron scattering foils, often mounted on a large carousel. Included also is a field
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 light with a sizable 45° mirror for illumination of large field sizes and optical distance indicator optics together with an increasingly large diameter and complex ionization chamber assemblies for monitoring and control. Accessibility for service often becomes difficult, but a recent “swing away” collimator design may significantly improve access. An earlier Therac 20 head design allows one of the magnet half yokes and associated shielding to be hinged for easy access. X-RAY TARGET (RETRACTABLE) BENDING MAGNET ASSEMBLY ELECTRON ORBIT
FLATTENING FILTER SCATTERING FOILS
DUAL IONIZATION CHAMBER FIELD DEFINING LIGHT RANGE FINDER COLLIMATORS
Figure 2.45. A representative linac treatment head (Clinac 18) with major components identified. The desire for larger field sizes, improved beam characteristics and convenient, functional accessories has led to a number of studies and improvements in treatment head design. The trend from 4–6 MeV to higher energy x-ray treatment beams, larger field sizes and the increasing use of accessories have resulted in an increase in treatment head size in terms of its height above the accelerator structure axis (radially away from isocenter) and its diameter below it. The former stems from the incorporation of the larger 270° bending magnets, thereby increasing the height of both the treatment head and the isocenter. The increase in diameter stems primarily from the requirement for large fields. The large treatment head diameter may interfere with optimal placement of the treatment beam in some anatomical regions such as lateral breast. Increased length of the treatment head toward the isocenter may limit the use of outboard accessories for short SAD units, such as the Clinac 4, as well as increase the amount of
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 scatter radiation reaching the patient from the head. Large, heavy, movable collimator jaws are needed for megavoltage x-ray beams with field sizes variable from zero area to as much as 40 × 40 cm at 1 m from the source. High-energy isocentric machines incorporate large bending magnets. These must be shielded extensively to attenuate radiation arising from their energy selection slits and elsewhere. A high-density, high atomic number material such as uranium (usable up to about 6 MeV), tungsten, or lead are frequently used to conserve space and to rapidly attenuate x-rays. When tungsten or lead are employed for collimators, head shielding, x-ray targets or flattening filters above 10 MV, photoneutron production increases rapidly. In some machines, low-density hydrogenous shielding materials have been incorporated in the head for neutron shielding, further increasing its size. Some 4–6 MeV units employ short accelerating structures in straightthrough beam designs in both isocentric and non-isocentric treatment units. However, at 8 MeV and above, longer accelerating structures are required and are usually incorporated into bent-beam, isocentric units. At such higher energies, x-ray beam flatness becomes increasingly sensitive to angular and spatial misalignment of the x-ray lobe with respect to the axis of the flattening filter. The effects on flatness can be described in terms of displacement )r and divergence )n of the electron beam with respect to the axis of the filter as illustrated in Figure 2.40. Such misalignments can result from small changes in beam energy but are less likely to do so for achromatic magnet systems which may incorporate narrow energy defining slits. They are more likely to result from mechanical strains which occur from the stress of gantry rotation, from beam stopper extension or retraction, from temperature changes of mechanical and electronic components, as well as from the presence of nearby ferromagnetic materials. The effect of such small-energy, displacement and divergence changes on an electron beam traversing a simple 90° magnet system is illustrated in Figures 3.41(a), 3.41(b), and 3.41(c), respectively. Achromatic magnet systems tend to limit the effect of such changes in entrant )E, )r and )n changes on the output beam image and hence, field symmetry and stability. Their output phase space image is a faithful reproduction of their input phase space image with unity magnification. Beam energy stability of a few percent is required to ensure satisfactory constancy of symmetry in bent-beam linacs. Under normal operation, the energy interval selected by the bending magnet slits is centered about the current maximum as illustrated inFigure 2.43(b). If the energy of the beam entering the bending magnet from the accelerator structure shifts, the primary effect is a reduction in output. A secondary effect is a change in beam symmetry in the radial plane, the sense of the change depending on which side of the current maximum the new distribution is centered. Similar, but far less severe, stability problems apply to electron therapy. Naylor and Chiveralls have examined the variations in x-ray beam flatness and calibration with gantry angulation and with time for an 8-MV unit equipped with a 90° bending magnet. They conclude that such variations are confined to a few percent when averaged over four-week periods, an interval pertinent in treatment. Padikal et al. describe a method for assessing the stability of symmetry with gantry angle rotation. In an early study, Naylor and Williams call attention to the need for frequent symmetry, dose monitor, and beam energy checks of linac treatment units. Characteristics of the Mevatron 15-MV photon beam have been described by Paul et al. The x-ray and electron beam dosimetry characteristics, as well as neutron measurements for a Mevatron 80, are described in a series of recent reports. The treatment
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 head design for this 20-MeV unit has been described earlier. Similarly, the x-ray beam characteristic of the Clinac 18 has been reported by Connor et al., and those of the Therac 20 by Patterson and Shragge Design considerations involving the treatment head and pertinent to electron and x-ray therapy, neutron production considerations, together with ionization chamber, dosimetry, and beam steering aspects, follow.
B. Ionization Chamber, Dosimetry, and Beam Steering System
The monitor ionization chamber of a contemporary, high-energy linac is constructed of several plates or electrodes, whose areas may be divided into sectors so as to serve two different monitoring purposes: (1) dosimetry of the x-ray and electron treatment beams, and (2) monitoring of the intensity distribution of the radiation field. The resulting signals can be used in automatic feedback circuits to steer the beam through the accelerator, bending magnet, and onto the target (or scatterer) in order to ensure beam flatness and symmetry. How these needs are satisfied in one representative treatment unit, the Varian Clinac 18, is described below and illustrated in Figure 2.46, a simplified diagram of the ion chamber, dosimetry, and beam steering system. The transmission ionization chamber, shown in the treatment head diagram of Figure 2.45, subtends the entire useful beam and provides two independent outputs for the dual dosimetry monitor. Located just below the flattening filter or scattering foil, it samples the flattened xray or scattered electron beam. This ionization chamber, shown in more detail in Figure 2.46, consists of three polarizing plates and two collecting plates, the latter divided into four collecting sectors, with the sectors defining four distinct laminar collecting volumes. A 500-V polarizing voltage is used with a plate spacing of 1 mm. The two inner D-like sectors provide signals for both dosimetry and steering and the two outer arclike sectors for steering only. The dosimetry system monitors and displays readings related to the quantity and uniformity of the useful beam of radiation. Sutherland early recommended that integrated dose be based on monitoring only the central portion of the field. Most treatment fields are small, and he found that monitoring the entire beam profile can result in axial calibration errors as large as ±3%. In one construction technique, the collecting plate sectors are formed by vacuum deposition of a thin metallic coating on defined areas of an insulating lamina of mica Additional grounded metallic coatings, not shown in Figure 2.46, surround the collecting areas and serve as guard rings to minimize leakage currents over the insulation. Ionization chambers may be sealed to the outside air making them free of the need for temperature and pressure corrections. The electron beam center line through the linac structure and 270° bending magnet is shown in Figure 2.46. This center line establishes the angular orientation of the semicircular collecting plates with respect to the radial and transverse coordinate planes of the beam through the bending magnet as shown. In general, the upper collecting electrode of Figure 2.46 is concerned with signals pertinent to the radial plane, the lower collecting electrode to the transverse plane signals. Semicircular plates A and B are oriented to provide signals related to the radial plane. Their signals are first amplified via A1, and A2, then summed via A3 to provide a console indication of dose rate and of integrated dose via integrator #1 for MU1 channel as shown. Similarly, semicircular plates C and D are oriented to provide signals related to the transverse plane. They feed MU2 channel via amplifiers A5 and A6 summing amplifier A7 and integrator #2. The two dose channels are completely independent, either can terminate the preset exposure with the second channel lagging the first by a constant 40 monitor units.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Accelerator Guide Angle R Steering Coils
Position R Steering Coils
Buncher R Steering Coils
Buncher T Steering Coils
Position T Steering Coils
n Beam Electro
Angle T Steering Coils Target Dose Rate Meter A A 1 B A 2
A3 A4
F A 9 E A 10
A13
G A 11 H A 12 D A 5 C A 6 -500V P.S.
A14 A8
A+B
MU1 Integrator
A-B
(A - B) + (E - F)
SYM1
E-F
G-H
(C - D) + (G - H)
SYM2
C-D
A7
C+D
MU2 Integrator
Figure 2.46. Five electrode ionization chamber with simplified block diagram of dosimetry and beam steering system. The radial and transverse coordinate planes of the bending magnet orbit are identified in the upper left. As shown in Figure 2.46, two groups of four steering coils are used in a servo-feedback system using signals from the ion chamber sectors to control and limit the lateral displacement (position) and angular divergence (angle) of the electron beam in the radial and transverse directions. Both groups of steering coils provide small angular deflections of the electron beam. The angle steering coils shown on the left in Figure 2.46 are located immediately adjacent to the 270° bending magnet entrant aperture. The position steering coils, shown on the right in Figure 2.46, are located at a distance from this aperture and at the end of the accelerating structure. The principal effect of their small angular deflection is to provide a lateral displacement or position correction at the bending magnet entrant aperture. These coil groups are energized by error signals generated if the electron beam strikes the target, or scattering foil at an angle or at a position that produces an asymmetrical x-ray or electron beam, as shown in Figures 3.40(b) and 3.40(c), respectively.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Two of the beam position (lateral displacement) steering coils are connected to limit the radial displacement, and two the transverse displacement of the electron beam from the field flattening filter axis as shown in Figure 2.40(c). The beam angle steering coils are located near entrant dipole M1 of the 270° magnet system shown in Figure 2.43(a). Two of them are connected to limit the radial angular divergence, and two the transverse angular divergence of the electron beam from the field flattening filter axis as shown in Figure 2.40(b). Not shown is a third group of four coils positioned around the beam at the buncher end of the linac structure. These buncher coils also control motion in the radial and transverse planes, steering the electron beam leaving the gun onto the center line of the structure. Signals from peripheral plates E and F are amplified by amplifiers A9 and A10. Their difference signal (E–F) from A13 feeds the radial position steering coils. The radial position steering coils control the radial component of lateral displacement of the electron beam with respect to the flattening filter axis as shown in Figure 2.40(c). Amplified signals from semicircular plates A and B are subtracted in difference amplifier A4 which feeds the radial angle steering coils. The radial angle steering coils control the radial component of angular divergence of the electron beam with respect to the field flattening filter axis shown in Figure 2.40(b). The angle and position asymmetry signals from amplifiers A4 and A13, respectively, are combined to provide a visual display of radial plane beam asymmetry at the console. They are set to provide an operational radial asymmetry limit beyond which the beam is turned off. In a similar manner as shown in Figure 2.46, but not described here, the transverse position and transverse steering coils are connected to amplifiers and ion chamber sectors so as to set servo-control, limit, and display the electron beam asymmetry in the transverse plane. The symmetry meter can be switched to indicate either radial or transverse symmetry. It and the associated steering coils are connected to use all signals from the collecting electrodes. The buncher, beam position, and beam angle steering amplifiers are each provided with six programmable gain and balance controls corresponding to the one x-ray and five electron modes of operation.
17.1. Electron Therapy
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Since the requisite beam current for electron therapy is typically several hundred times less than for x-ray therapy, most shielding problems, including neutron attenuation, are comparatively insignificant even at the higher energies. One is concerned clinically with providing wide, flat electron fields with modest penumbra, a relatively low surface dose, deep penetration to 80% depth dose for each selected energy, rapid falloff of dose with depth on the distal side of the depth dose curve as well as a low contaminating x-ray background. Achieving and assessing these electron beam characteristics have been the subject of a number of studies. Usually, electron applicator cones are attached to the treatment head with the x-ray collimator jaws set to provide fields a few centimeters wider. There is accompanying improvement in electron field flatness from this procedure, preferentially at shallower depths. A too wide setting may create exposure problems outside the treatment volume as in the case of pregnant patients for one particular linac. Schneider has evaluated such leakage radiation for another model of linac, but measurements by Jones suggests that the leakage may be a machine-specific characteristic which should be evaluated by individual users. Often, a lead or Lipowitz’s alloy (for example, Cerrobend, an alloy of Cerro Copper and Brass Company, 16,000 St. Clair Avenue, Cleveland, Ohio 44110) cutout defines the final field size and shape and is placed in the end of the applicator at or near the patient’s skin surface. The
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dosimetry of such shaped small and irregular fields has been described for electrons from 4 to 10 MeV. Electrons scattered off the applicator walls improve flatness at the periphery of the field at a shallow depth with less penetrating electrons and hence, poorer flatness at greater depth. A continuously variable electron beam collimator has been described by Robinson and McDougall, and at least one manufacturer makes such an option available. Most studies find that a single, high atomic number scatterer will adequately flatten small fields up to about 10 cm in diameter for low electron energies up to about 10 MeV. Additional steps are taken at higher energies and for larger fields. Providing several different scatterer thicknesses, often in a carousel, facilitates optimization of beam characteristics for different energies. A dual-foil scattering system, with a few centimeters or more between the two foils, significantly improves electron beam flatness characteristics, particularly above 15 MeV and for fields 15 cm in diameter and larger. The first high atomic number (Z) foil is selected to minimize energy loss for a given scattering distribution and the second foil made of a low-Z composite, thicker on axis, functions more as a field flattening absorber preferentially scattering electrons peripherally. Hence, the electron applicator in such systems primarily serves to define the field size and only affects flatness secondarily. A thorough analysis of the dual-foil scattering system has been provided by Mandour and Harder. Scanning the treatment field with a pencil beam of electrons is an alternate approach to electron therapy. It offers advantages at higher energies of 25– 50 MeV where bremsstrahlung contamination from scattering foils becomes significant. Earlier, Rozenfeld et al. developed a scanned pencil beam system for arbitrarily shaped fields defined by a full size template, in lieu of using scattering foils. The isocentrically directed beam, up to 50 MeV in energy, employs a complete, non-achromatic magnet system. It combines a linear translation of the magnet up to 21 cm along the direction of the gantry axis with an indexed rotation of the gantry to provide 5-mm spacing between scan lines at the skin surface. A treatment is completed in one scan series covering the field, and different electron energies can be used in different portions of the field. This scanned beam system employs a large, non-achromatic beam transport system with attendant stability problems of the treatment beam. Earlier, Briot et al. investigated the scanned electron beam of a Sagittaire 35-MeV treatment unit. They conclude that an adjustable outboard metallic collimator, which could be attached to the x-ray jaws, improved the depth dose and dose gradient at the edges of the field. Others compared the Siemens betatron and Sagittaire linear accelerator electron beams. Bell and Waggener have described a method for rapid determination of the energy of electron treatment beams from medical linacs. Microtrons, having a wide energy range, have come to be employed for electron therapy. A controversial aspect of linac versus microtron treatment beams concerns dependence of the characteristics of the electron beam depth dose on the energy spread of the electron beam. Brahme and Svensson contend that the microtron beam has a depth dose significantly improved (sharper buildup and steeper falloff regions) over that of the linear accelerator and attributed, in part, to the narrower energy spread of the microtron beam. Fregene suggests that a small difference in energy spread, which is of the same order of magnitude in the microtron and linear accelerator beams, should not lead to appreciable differences in depth dose at 10 MeV. Some of the disagreement between observers might be explained on the basis that measurements were performed on different accelerators having different treatment heads (different collimators, scatterers, geome-
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 try, etc.) using different phantom materials and dosimetry techniques. Recent measurements by Johnsen et al. of electron beams for a single linac operated at nominal peak energies of 6 and 12 MeV appear to have resolved this controversy. Based on magnetic analysis of the beams, they compare narrow energy spread beams (0.1 and 0.2 MeV FWHM) with broad (0.8 and 1.2 MeV FWHM) for 6 and 12 MeV, respectively. They conclude that, when consistent measurement techniques are used for the beams tested on the same beam-collimator system, the electron depth dose characteristics are not significantly affected by these relatively large changes in the width of the accelerator’s energy spectrum. Earlier, Bjarngard et al. found no significant difference between the Mevatron XII electron depth dose curves and those from a microtron.
C. X-ray Therapy
A number of studies have focused on x-ray target selection, beam penetration (e.g., percent depth dose at 10 cm or depth of 50% depth dose), and flatness aspects. Podgorsak et al. examined the effects of different atomic number targets and flattening filters on small 10-cm-diam fields 100 cm from the target for energies of 25 MeV. They found that x-ray output on the central axis does not depend significantly on the Z of the targets. However, an Al target gives a more penetrating beam, although high-Z targets emit more radiation at large angles. They also found that an Al filter hardens the beam and a high-Z filter softens it. They recommend a thick Al target and flattening filter above 15 MeV for the most penetrating beam. Below 15 MeV, a high-Z target and low-Z filter are recommended. At 25 MeV, an Al flattening filter is 25 cm in length, an impractical size to incorporate in most linac treatment heads. Ideally, one wishes flattened fields for all field sizes, at all depths, an impossible requirement because of energy changes and scattering in the phantom. One early 4-MV filter design provided satisfactory flatness at 10-cm depth but resulted in excessive dose at shallow depths near the edges of large fields. Invariably, flattening filter choice involves a compromise in order to achieve uniform small and large fields over a range of depths. Two flattening filters are provided in some treatment units to optimize beam flattening with the changeover at about 10 × 10 cm fields, or an additional filter may be attached to the accessory ring for assuring large field flatness. McCall et al. examined linac depth doses using a semi-empirical analytic depth dose model correlated with experimental measurements. They find that an Al filter at 25 MeV produces a great deal more beam hardening on axis than do Ni and W and consequently, there is a larger variation of E with production angle for Al, a significant effect for large field sizes. Al is a good flattener, but the penalty is significant energy spread across large flattened fields. For the Clinac 35, operating at 25-MV x-ray energy, they recommend a Cu target with an Fe filter containing a W conical insert for an optimum combination so as to minimize energy variation with angle and restrict beam hardening on the centerline. Other important aspects of a good target-flattener system are that the flattener must not become too radioactive in operation, and neutron production must be minimized. Thus, other good properties notwithstanding, copper filters are not commonly used above 10 MeV since the gamma-ray dose rate from 9.76 min Cu-62 activation becomes very high. Iron, on the other hand, has essentially the same absorption properties as copper, and the induced activity is much weaker.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Flattening filters may absorb 50%–90% of the central axis photon intensity. Brahme et al. recommended scanned photon beams at high energies pointing out that the flatness and intensity problems are greatly alleviated. Hence, some radiation shielding problems (including those for neutrons) may be as much as 2–5 times less for high-energy scanned photon beams than for a heavily filtered unscanned beam. Methods of measurement and characteristics of leakage radiation for a Clinac 18 treatment unit have been described by Lane et al. The spectral change problem associated with flattening filters was cited earlier by Hansen et al. who identified its importance for a 4-MV linac beam. Later, Larsen et al. developed a calculative program for filter design and applied it to a 4-MV beam. Their program sums the primary and scattered components in an iterative manner to fit the dose profile. Jones points out that the dominant effect is selective hardening of the beam by the flattening filter, and that for thin targets the energy increases with distance off axis. More recently, Hanson and Berkley measured the off-axis quality change for 4 to 10 MV beams and suggested a technique to correct for the effect in treatment planning calculations. Megavoltage x-ray beams exhibit an increased depth dose in the buildup region and a shift of dmax toward the surface as the field size is increased. Recent investigations indicate that these effects are largely due to low-energy electron contaminants originating in components of the treatment head, primarily the flattening filter. Treatment heads can be equipped with electron filters to attenuate this component. Reductions of 10%–20% in surface dose and an increased depth of dmax from 2.5 to 4.5 cm at 25 MV have been observed. Moyer has identified systematic patient x-ray dose errors for 4 and 10-MeV linacs associated with elongated rectangular collimator settings. This collimator-exchange effect, which depends on which movable collimator pair forms the larger or smaller field dimension, may be as large as several percent for highly elongated rectangular fields.
D. Neutron Leakage and Contamination
A significant number of neutrons are produced by high-energy x-ray beams. For most relevant materials, the neutron production threshold occurs at 8–10 MeV, rises rapidly and then plateaus above 20-MeV photon energy. Neutrons which originate in the primary collimator, target, and flattening filter contaminate the useful beam. Others are filtered through the treatment head, some are generated in the patient and most are multiply scattered by barriers comprising the room. These neutrons, together with x-ray leakage, affect the patient as well as those outside the treatment room. McCall and Swanson provide a thorough description of neutron sources and their characteristics originating in linac treatment heads. They find that high-Z materials do not significantly alter the neutron fluence but do substantially reduce the average energy of the transmitted spectrum. The principal source at 25 MeV is the primary collimator which contributes one-half or more of the fluence, often followed by the target and flattening filter, in that order. Depending on the material, energetic neutrons can induce radioactivity in the treatment head and patient support components of the linac as well as the treatment room barriers. Such radioactivity could constitute an appreciable source of exposure, particularly for the technologist (see Sec. VI.-C.). With respect to neutron production, a recent measurement showed that a tungsten flattening filter five radiation lengths thick produced two and one-half times as many neutrons as an equivalent
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 steel filter at 25 MV, although ratios as high as seven times have been predicted. Recent examination of neutron biological effectiveness has led to a scrutiny of neutron protection requirements by regulatory agencies. This scrutiny and the associated need to verify neutron production data, as well as to establish appropriate measurement, calculation, and dose reduction techniques were the subjects of a recent conference. A consensus appears to be developing wherein proposed neutron leakage requirements might reasonably be met in contemporary high-energy linacs. Often, the shielding provided for x-ray leakage is sufficient for neutron leakage as well, and satisfies the total leakage limitation of 0.1% measured in rads. Rawlinson and Johns note that the energy imparted outside the useful beam due to scatter can be more than 20 times greater than the energy due to leakage. Some simplifications in assessment have resulted from defining two pertinent measurement surfaces for protection purposes. One is a plane circular “patient” surface of radius 2 m centered on and perpendicular to the axis of the beam at the normal treatment distance and relates to patient protection. A second cylinder-like complex surface is defined by all points at 1 m from the path of the electrons between the electron gun and the target or electron window and relates to room shielding. A recent report of AAPM Task Group No. 21 on neutrons from high-energy x-ray medical accelerators provides a careful reasoned, quantitative analysis of the problem together with recommendations regarding risk to the radiotherapy patient. The report concludes that the principal risk of carcinogenesis is extremely small, and the implementation of more restrictive regulation is unnecessary and would be counterproductive.
Acknowledgments
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Several commercial treatment units are cited herein to illustrate design principles and variations of them. Their selection in no way constitutes an endorsement of a particular equipment. The choice, at times, depended on the availability of detailed information and numerical data relating to the design. I am grateful to those who have responded to my requests. Many individuals have contributed to my understanding of medical linacs and to the preparation of this review. Their generous efforts have often facilitated my exposition of specific topics. Several have critiqued portions or all of this manuscript. I am grateful to all of these individuals: L. Atherton, A. Brahme, K. Brown, E. Dally, P. Fessenden, D. Goer, R. Jean, S. Johnsen, P. La Riviere, R. Levy, R. McCall, A. McEuen, G. Meddaugh, N. Pering, T. Smith, E. Tanabe, L. Tauman, J. Ting, and W. Turnbull. I am particularly grateful to Craig Nunan whose broad background and critical appraisal have added immensely to my perspective. Pieter Huisman’s talents have provided many of the illustrations. Juanita Clack’s outstanding secretarial skills, patience, and untiring efforts are much appreciated and have greatly simplified my task.
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Chapter Three
Modulator Theory
This chapter will discuss the theory of non-resonant series line type modulators used in Varian C-series Clinacs.
Modulator Theory
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Table of Contents 1. Introduction:.................................................................................................................... 3-5 2. Basic Concepts: ............................................................................................................... 3-5 3. DeQing Principles:............................................................................................................ 3-7 4. Non-resonant Transmission Line Principles: ..................................................................... 3-8 5. Pulse Shape Definitions: ................................................................................................ 3-16 6. Line Type Modulator Load Element Principles: ............................................................... 3-17 7. Fault Conditions: ........................................................................................................... 3-18 8. Thyratron Theory: .......................................................................................................... 3-19 8.1. Introduction: .......................................................................................................... 3-19 8.2. Operating Notes on Hydrogen-filled Tubes: ............................................................. 3-23
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Modulator Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Illustrations Figure 3.1. Resonant Charging Circuit:................................................................................. 3-5 Figure 3.2. Voltage and Current Waveforms of Capacitor C1: ................................................ 3-6 Figure 3.3. Resonant Charging Circuit with Series Charging Diode: ...................................... 3-6 Figure 3.4. Voltage and Current Waveforms on C1 with Series Charging Diode: .................... 3-7 Figure 3.5. Addition of DeQing Switch Circuit:...................................................................... 3-7 Figure 3.6. DeQing Waveforms: ............................................................................................ 3-8 Figure 3.7. Distributive Transmission Line: .......................................................................... 3-8 Figure 3.8. Finite (Lumped) Network: .................................................................................... 3-9 Figure 3.9. Resonant Charging with PFN: ............................................................................. 3-9 Figure 3.10. The Pulse Discharge Circuit: ........................................................................... 3-10 Figure 3.11. PFN Discharge Waveform Formation: .............................................................. 3-11 Figure 3.12. PFN Discharge Waveforms vs. Load Impedance: .............................................. 3-13 Figure 3.14. Typical Line Type Modulator for Linear Accelerator Applications:..................... 3-14 Figure 3.13. Basic Line Type Modulator Circuit: ................................................................. 3-14 Figure 3.15. Pulse Shape Definitions, Theoretical: .............................................................. 3-16 Figure 3.16. Pulse Shape Definitions, Practical: .................................................................. 3-16 Figure 3.17. Equivalent Circuit of Magnetron as Load:........................................................ 3-17 Figure 3.18. Magnetron Voltage-Current Characteristics:.................................................... 3-17
Modulator Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 3.19. Conventional Industrial Thyratron. (a) Anode. (g) Control Grid, (k) source of electron emission: .............................................................................................................. 3-19 Figure 3.20. Critical Grid Potential in Relation to Anode Voltage:........................................ 3-20 Figure 3.21. Reference Points Associated with the Interpretation of Pulse Shapes: .............. 3-22 Figure 3.22. Schematic Diagram of Line Discharge Circuit: ................................................ 3-23 Figure 3.23. Charging Voltage Waveforms: ......................................................................... 3-24 Figure 3.24. Circuit for Recovery Time Measurement:......................................................... 3-27 Figure 3.25. Methods of Providing Negative Grid Voltage Swing: ......................................... 3-28 Figure 3.26. Waveforms with Pulse Transformer Trigger Drive: ........................................... 3-30 Figure 3.27. Inductive Overswing with Bias: ....................................................................... 3-30 Figure 3.28. Inductive Overswing without Bias:.................................................................. 3-30 Figure 3.29. Waveforms Normally Seen During Modulator Adjustments:............................. 3-31 Figure 3.30. Schematic Diagram of Tetrode: ....................................................................... 3-33 Figure 3.31. Single Pulse Drive for Tetrode: ........................................................................ 3-33 Figure 3.32. Simplified Single Pulse Drive: ......................................................................... 3-34 Figure 3.33. Double Pulse Drive for Parallel Operation: ...................................................... 3-35 Figure 3.34. Single Pulse Drive for Parallel Operation: ........................................................ 3-35 Figure 3.35. Anode Circuit Using Separate PFN’s: .............................................................. 3-36 Figure 3.36. Arrangement for Adjustment of Current Sharing:............................................ 3-36 Figure 3.38. Thyratrons in Series: ...................................................................................... 3-37
3-4
Modulator Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
The material in this chapter should be read and thoroughly understood by the student before proceeding to the specific modulator descriptions in the High and Low Energy Clinac Beam Delivery System course manuals.
2. Basic Concepts
Consider the circuit of Figure 3.1 below.
S1
L1
B1 C1
Edc
Figure 3.1. Resonant Charging Circuit When switch S1 is closed, current will begin to flow through inductor L1 to charge capacitor C1. Initially the reactance of L1 will limit current flow resulting in a voltage drop equal to the battery voltage appearing across L1. As time passes, current does begin to flow. This results in the buildup of a magnetic flux or field in the core of L1 (storage of energy). Eventually, the charge across C1 approaches a value equal to the battery voltage Edc. The current through L1 at this point is maximum and both L1 and C1 have stored energy. Current in the circuit begins to decrease because there no longer exists a voltage difference between C1 and the battery. This causes the magnetic field in L1 to collapse. The collapsing field produces a continuation of current flow in L1 that creates a voltage source that adds to the battery voltage. C1 now begins to charge higher than the battery voltage Edc, eventually approaching 2Edc when all the stored energy in L1 has been transferred to C1. There now exists a voltage difference between the battery voltage Edc and the capacitor voltage 2Edc. This causes current flow to reverse in the circuit resulting in the extra stored energy in C1 now causing a reverse current to flow in L1. A magnetic field is again developed in L1 in the reverse direction and L1 now has an excess of stored energy. This cycle of events will continue until all the original stored energy in L1 is dissipated in the I²R losses of the circuit. As a result of this cyclic reaction, a damped oscillation of current will flow between L1 and C1. The oscillation has a resonant time period determined by the value of L1 and C1 defined by the relationship: 1 TC = -----------------2π LC
Modulator Theory: Introduction
3-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π
2π
2Edc Edc 0
t
E (Voltage) I (Current) Resonant Time Period
2π LC
Figure 3.2. Voltage and Current Waveforms of Capacitor C1 It can be seen by the waveform of Figure 3.2 that the capacitor C1 will eventually have a charge voltage equal to the battery voltage Edc after several cycles have occurred. This resonant charging condition can be put to use to allow C1 to maintain the stored energy in L1 by adding a diode in series between L1 and C1 as indicated in Figure 3.3.
S1
L1
B1 Edc
C1
Figure 3.3. Resonant Charging Circuit with Series Charging Diode The sequence of events in the circuit of Figure 3.3 is exactly the same as the circuit of Figure 3.1 except that when the capacitor C1 reaches a voltage equal to 2Edc the diode CR1 will block the reverse flow of current back through L1. The waveforms of Figure 3.4 indicate these events. There are three major advantages to the use of the circuit in Figure 3.3:
3-6
a.
The power source voltage required is only half that of the capacitor stored value.
b.
The efficiency of the transfer of energy is raised from approximately 50% to almost 100%.
c.
It is possible to regulate the voltage charge on the capacitor.
Modulator Theory: Basic Concepts
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π 2Edc E (Voltage) Edc
I (Current)
0
t π LC Time Period Figure 3.4. Voltage and Current Waveforms on C1 with Series Charging Diode
3. DeQing Principles
Regulation of the charge voltage on Capacitor C1 can be accomplished by the circuit of Figure 3.5. Consider switch S2 and resistor R1 shown in parallel with L1. It was stated that the current through L1 was maximum when C1 was charged to Edc and zero when charged to 2Edc (refer to Figure 3.4). If the switch S2 is closed at some time after the current in L1 has reached maximum value and the charge voltage on C1 has reached Edc, the stored energy in L1 will be routed through S2 and be dissipated in resistor R1.
S1
L1
B1 Edc
S2
R1
C1
Figure 3.5. Addition of DeQing Switch Circuit By controlling the exact time when S2 is closed, any charge voltage level from Edc to approximately 2Edc can be placed on C1 resulting in regulation of the stored energy in C1. The shorting circuit consisting of S2 and R1 has been identified by the term “DeQing” circuit. The term “DeQing” has been created to identify, in a simple manner, the action of the circuit composed of S2 and R1 across L1. The word “Q” or “Q Factor” is defined as the ratio of the reactance to resistance of an inductor, capacitor, or resonant circuit. With reference to Figure 3.5, it can be seen that when switch S2 is closed, R1 is placed in parallel with L1. This lowers the “Q” of the inductor. Thus, the circuit is called a “DeQing” circuit, etc.
Modulator Theory: DeQing Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
π 2Edc E (Charged Voltage Level of C1) Edc I (Current through R1) 0
I (Charging Current to C1)
t
π LC Time Period Figure 3.6. DeQing Waveforms Observe the current and voltage waveforms of Figure 3.6. The current waveform indicates an abrupt drop to zero when switch S2 is closed. The energy still remaining in the inductor L1 is the area under the curve indicated by the shaded area under the dashed line. This energy is dissipated by resistor R1.
4. Non-resonant Transmission Line Principles
Non-resonant transmission lines are lines that are either infinitely long or terminated in some impedance. A uniform transmission line has what is called a “characteristic impedance”. This is the impedance that would be measured at the end of such a line if it were infinitely long. The importance of this characteristic impedance lies in the fact that if any length of line is terminated in an impedance of this value, then all the energy flowing along the line will be absorbed at the termination and none is reflected back along the line. A result of this is that the input impedance of any length of transmission line terminated in its characteristic impedance is equal to that characteristic impedance. Transmission lines such as coaxial cables can be represented as a network of distributive series inductive and parallel capacitive elements as diagramed in Figure 3.7. This distributive network can be further simplified or simulated by constructing a finite number of series and parallel elements in the form of the network diagramed in Figure 3.8. This network is commonly called a PFN (Pulse-Forming Network) when used for energy storage purposes in a line type modulator.
OPEN CIRCUIT
IN
Figure 3.7. Distributive Transmission Line
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Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The development of a pulse-forming network that simulates a transmission line is a problem of network mathematics. No network having a finite number of elements can exactly simulate a transmission line which in reality has distributed rather than lumped parameters. The pulse-forming network serves a dual purpose of storing exactly the amount of energy required for a single pulse and of discharging this energy into a load in the form of a pulse of specified shape. The required energy may be stored either in the capacitances or in the inductances, or in combinations of these circuit elements. Networks in which the energy is stored in the electrostatic field of the capacitors are referred to as voltage-fed networks. Networks in which the energy is stored in the magnetic field of the inductors are referred to as current-fed networks. Networks of the voltage-fed type are generally used in practice because only with this type of network can gaseous switches such as thyratrons be used to switch the energy.
OPEN CIRCUIT
IN
Figure 3.8. Finite (Lumped) Network The network of Figure 3.8 can be defined as a voltage-fed network where the stored energy is in the capacitors. If the capacitor C1 of Figure 3.5 is replaced by the network of Figure 3.8 to form Figure 3.9 and the circuit switch S1 is closed, the same sequence of events occur as originally happened in Figure 3.5. Now the current begins to charge each capacitor of the network through the series inductance of each section of the network. The result is that each capacitor will charge to 2Edc at a charge time that is increasingly longer for each capacitor down the network. However, the difference in charging time of each network section is relatively insignificant to the total charge time of the entire circuit. This is due to the large charge time defined by the charging choke L1 and the total value of all the capacitance within the network. The PFN inductors have no significant effect during the charge cycle because of their small inductance value compared to the large inductance of L1.
S1
CR1
L1
B1
L2
C1 Edc
S2
L3
C2
L4
C3
C4
R1
PFN
Figure 3.9. Resonant Charging with PFN
Modulator Theory: Non-resonant Transmission Line Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Since the application for which these modulators are usually designed requires the application of an essentially rectangular pulse of energy, the open-ended transmission line (PFN) considered in the foregoing example may be taken as the starting point in a discussion of the discharge circuit. Figure 3.10 represents a simple discharging circuit consisting of a PFN, which is in a charged state of 2Edc, a switch S1, and a load resistance R1 which has the same impedance as the PFN characteristic impedance.
L1
S1
L2
C1 Edc
L3
C2
L4
C3
C4
PFN
R1
Figure 3.10. The Pulse Discharge Circuit When S1 is closed the following events occur: 1.
C1 begins to discharge through L1 into the load resistor R1. Initially, all the charge voltage across C1 will be developed across L1. As time passes, current flows through L1 and begins to dissipate in load resistor R1. The time required is dependent on the values of L1–C1 and R1. Eventually the voltage across R1 will reach Edc because the impedance of the load resistance R1 is equal to the impedance of L1 and C1.
2.
At this point C1 will stop discharging and C2 will begin to discharge through L2 and L1 into R1. Similarly, C2 will stop discharging at Edc and C3 will begin to discharge. This sequence of events continues until the last capacitor C4 begins to discharge. When C4 reaches Edc, it will continue to discharge to zero. Then C3 will discharge to zero. Eventually all capacitors will discharge to zero and all the energy will have been dissipated in R1.
The above sequence of events results in a rectangular current pulse of energy being supplied to R1 with a duration which is twice the transmission time (aggregate discharge times) of the network. Reference Figure 3.11. From the foregoing discussion, as illustrated by Figure 3.11, it can be seen that if the load impedance is equal to the line impedance (Rl=Zo), that is if the line is matched to the load, the current out of the line consists of a single rectangular pulse of 2 time periods (The time to discharge each capacitor down the line and then back). These conditions satisfy Ohm's law where half the voltage appears across the load but for twice as long thus dissipating all the stored energy in the network.
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Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 EC1
EC3
EC2
EC4 EC3
EC1
EC2
Voltage t
IC2 IC1
IC4 IC3
IC2 IC3
IC1
Current t
IL2 IL1
IL4 IL3
IL2 IL3
IL1
Current t
T1
T2
Current
2T
t
Discharge Figure 3.11. PFN Discharge Waveform Formation
Modulator Theory: Non-resonant Transmission Line Principles
3-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The effect of mismatching the load is to introduce a series of steps into the transient (rectangular) discharge. As can be seen in Figure 3.12, these steps will all be of the same sign when the mismatch is Positive (the load resistance is greater than the line impedance) Rl>Zo, and of alternate sign when the mismatch is negative (the load resistance is less than the line impedance) Rl
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Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
(+)EL RL = ZO Matched
0
2T
(+)EL RL = 2ZO Positive Mismatch
0
2T
4T
6T
8T
(+)EL RL = ½ZO Negative Mismatch
0 (–)
2T
4T
6T
8T
Figure 3.12. PFN Discharge Waveforms vs. Load Impedance
Modulator Theory: Non-resonant Transmission Line Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
S1
L1
CR1
B1
L2
C1 Edc
S2
L3
C2
L4
C3
C4
R1
PFN S3
R2 (Load)
Figure 3.13. Basic Line Type Modulator Circuit In the following discussion refer to Figure 3.14.
3-Phase High Voltage Power Supply
Clipper Current Fault Monitor System
Compensated Voltage Divider Circuit 3000:1 Ratio
˜ 10 KV L1
CR1
V1 R1
˜ 20 KV
PFN
T2
CR2 C5 R4
DeQing Switch Trigger Gen. System
˜ 6.7V R5
HVPS Current Monitor Circuit
Current Toroid
C1
V2
C6
C3
C2
CR3
C4
R3
End Clipper Circuit
Main Switch Trigger Gen. System Pulse Transformer Turns Ratio = 1:11
R2
Klystron Equivalent Circuit De-Spiking Network
HVPS O/C Fault Monitor System
C5 R4
T1 RL CRL
CL
Figure 3.14. Typical Line Type Modulator for Linear Accelerator Applications
3-14
1.
A 10 KV DC high voltage power supply has been substituted for the battery.
2.
Switches S2 and S3 have been replaced with high voltage high current type thyratron tubes.
Modulator Theory: Non-resonant Transmission Line Principles
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 3.
The load resistor R2 has been replaced by a pulse transformer T1 and the equivalent circuit of klystron (magnetron in low energy Clinacs), Rl, CRl and Cl.
From the foregoing discussion on impedance matching, it is clear that the PFN must be terminated with a load impedance equal to its characteristic impedance. The impedance associated with high power line type modulator used for klystron-driven accelerators will be typically around 12.5 ohms and for magnetron-driven accelerators, 25 ohms. (The klystron will have an impedance of 1400 ohms and the magnetron 400 ohms.). The pulse transformer will require a step up turns ratio of approximately 1:11 for klystrons and 1:4 for magnetrons because the impedance transformation of a pulse transformer changes as the square of the turns ratio of its windings. Note the new components: 1.
A high voltage power supply current monitor resistor R2 and a fault detector system has been placed in the return path of the power supply. The voltage across the resistor will be proportional to current flow from the power supply. If the current exceeds a specific limit the monitor system will turn off the high voltage power supply. Thus, any failure of components in the charging circuit will be detected.
2.
The DeQing thyratron is controlled by a DeQing trigger system which receives input information on the DC charge level of the PFN through a compensated voltage divider circuit R4,C5–R5,C6 that has a voltage division ratio of 3000 to 1.
3.
The main switch thyratron is controlled by a trigger generator system. This will enable repetitive operation of the modulator circuit at any desired PRF (pulse repetition frequency) within the design limitations of the components of the charge circuit.
4.
A diode CR3, resistor R3 and a current toroid T2 have been added across the end of the PFN. The diode is polarized so current will only flow through the resistor when the charge on the PFN goes negative. The toroid will detect any current flow and the clipper current fault monitor system will turn off the high voltage power supply thus protecting the components in the discharge circuit.
5.
A resistor R4 and capacitor C5 have been placed in series across (in parallel with) the primary of the pulse transformer to form a despiking network. In a practical modulator the pulse output of the PFN is of the order of 1 to 5 microseconds duration. The rise and fall time of the pulse will usually be in tenths of microseconds. The pulse transformer must be designed to transmit these very short pulses. However, during the rise and fall time of the pulse, the transformer will tend to appear as a very high impedance which will cause overshooting of the pulse voltage due to mismatched conditions. This can cause serious problems with the klystron or magnetron as well as the pulse transformer. The de-spiking network will appear as a matched impedance during the fast rise and fall time of the pulse due to the low reactance of C5 effectively placing resistor R4 across the PFN during these times. R4 will be equal in value to the characteristic impedance of the PFN.
Modulator Theory: Non-resonant Transmission Line Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Pulse Shape Definitions
A pulse can be considered as a trapezoidal shape with a finite rise and fall time and a defined width and peak amplitude as indicated by Figure 3.15.
100%
50%
Pulse Width
0% Flat Top Width
Rise Time
Fall Time
Figure 3.15. Pulse Shape Definitions, Theoretical Generally the rise time is defined as the time between 10% and 90% of the rising portion of the waveform and the fall time is likewise defined as 90% to 10% of the fall portion of the waveform. The pulse width is considered to be defined as the width at the 50% amplitude point. Additionally, pulses of a practical nature have definitive values of overshoot and droop as well as reverse tails as indicated in Figure 3.16. % Overshoot 100% 90% % Droop
50%
Pulse Width
10% 0% Reverse Tail Rise Time
Fall Time
Figure 3.16. Pulse Shape Definitions, Practical
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Modulator Theory: Pulse Shape Definitions
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6. Line Type Modulator Load Element Principles
The output pulse of a line type modulator can be applied to either a passive element such as a resistance that will dissipate the pulse energy in the form of heat, or an active device such as a klystron or magnetron that converts the pulse energy into a form of high frequency electromagnetic energy. A magnetron will be utilized as the active load element for our discussions. The circuit elements of Figure 3.17 define the equivalent circuit of a magnetron as a load element of the modulator.
Magnetron Oscillator
RL
Line Type Modulator
Pulse Output
CRL +
CL
VS
–
Figure 3.17. Equivalent Circuit of Magnetron as Load The behavior of a magnetron as a load element of the modulator is equivalent to that of an ideal diode CRL, that is a diode that has a linear E–I (voltage–current) characteristic, in series with a battery of voltage Vs whose polarity is in opposition to the applied voltage pulse of the modulator. For circuit analysis it is possible to represent such a load as a resistance RL in series with the diode CRL and battery of voltage (Vs). The stray capacity of the magnetron can be represented as a capacitor CL in parallel across the network. The effect of the equivalent circuit of Figure 3.17 can be defined by the voltage–current characteristic of Figure 3.18.
Current Actual Theoretical 0 Voltage (VS) Figure 3.18. Magnetron Voltage-Current Characteristics
Modulator Theory: Line Type Modulator Load Element Principles
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The magnetron does not begin to conduct current until the applied pulse voltage approaches the effective value of the bias voltage Vs. At this point the magnetron begins to conduct heavily with only the small additional rise in the applied voltage. During the rise time of the pulse the stray capacitance of the magnetron CL tends to limit the rate of rise of the applied voltage to a realistic value. When the modulator pulse begins to fall the stray capacitance CL tends to prevent the rapid fall of the pulse.
7. Fault Conditions
Whenever a magnetron arcs its internal impedance drops toward zero, causing a reflected low impedance to appear across the PFN. This results in the circuit action as defined under negative mismatch conditions. The PFN discharges negative and at this point the thyratron will stop conducting. This results in a large amount of energy being left in the PFN capacitors stored as a reverse charge. Referring to Figure 3.14, the end clipper diode CR3 conducts this energy through resistor R3 where it is all dissipated as heat. This condition must not be allowed to continue. Some form of monitor system will be used to detect this current flow and turn off the high voltage power supply, thus preventing serious damage to any components. Whenever the magnetron misfires, that is, does not conduct during a pulse, the result is a reflected high impedance mismatch at the PFN, resulting in a slow rate of discharge of the PFN. The charge circuit normally has a charge time several orders of magnitude longer than the discharge time. Typical values would be charging time 1 millisecond and discharge time of 4 to 6 microseconds. The charging choke L1 appears as an open circuit during the short discharge time while the thyratron is conducting thus isolating the power supply from the thyratron which would look like a short to the power supply. Whenever the PFN is mismatched positive it tends to discharge very slowly, resulting in possible continuous firing of the main thyratron. This results in excessive current being monitored by the power supply current monitor resistor R2. The HVOC (High Voltage Over-Current) fault monitor circuit will turn off the high voltage power supply, thus protecting the components from a major failure. The preceding discussion has dealt with only the key factors in the operation of a line type modulator driving a magnetron. Most of the principles discussed also apply to klystron systems, except that the klystron is basically an amplifier tube and behaves accordingly, e.g., there are no problems associated with misfiring and internal arcing within the tube is abnormal and cannot be tolerated. There are many other aspects of its operation that must be dealt with to fully understand any given system or accelerator machine performance. This is only a preliminary exercise.
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Modulator Theory: Fault Conditions
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8. Thyratron Theory
The following information is compiled from The Operation and Use of Hydrogen-Filled Tubes by R. E. Lake, B.Sc. and H. Menown, M.Sc., and edited for this manual.
8.1. Introduction
The conventional thyratron with mercury vapor or rare gas filling was first introduced in the early 1920’s, and the basic geometry of the industrial type of tube has changed very little since then. The tube consists essentially of an anode (a), control grid (g), and source of electron emission (k), as shown in Figure 3.19. With a positive voltage on the anode, the tube will remain in a nonconducting state if a suitable voltage (usually negative) is applied to the grid. This voltage depends on the anode voltage, and for every value of anode voltage there is a critical grid potential (Figure 3.20). The electrons leaving the cathode are prevented from reaching the grid/anode space by the potential barrier at the grid. As the grid voltage is made less negative, or anode voltage more positive, the field due to the anode attracts an increasing number of electrons from the cathode. These collide with gas atoms and when the resultant electrons receive sufficient energy and sufficient collisions occur, cumulative ionization takes place and the tube fires through. The voltage across the tube then drops to a value (10-15V for mercury and rare gases), which is dependent on the application and the nature and pressure of the gas filling. The current passed is then largely determined by the external circuitry.
Figure 3.19. Conventional Industrial Thyratron. (a) Anode. (g) Control Grid, (k) source of electron emission
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 3.20. Critical Grid Potential in Relation to Anode Voltage During breakdown the negative potential on the grid attracts positive ions and these form a sheath through which the grid potential cannot penetrate. Thus, any increase in the negative value of the grid supply voltage has no effect on the current passing. The tube will return to its non-conducting state after removal of anode voltage for a sufficient time (known as the recovery time) to allow the charged particles to disappear. The voltage on the grid then returns to its original value, and a positive voltage can be reapplied to the anode without conduction taking place. The tube, therefore, acts as an electronic switch which may be closed by the application of a positive signal to the grid but which can only be opened by the removal of anode voltage for a minimum time. The grid is almost always a far more massive affair than is found in vacuum tubes since it has to withstand recombination heating and bombardment without allowing a rise of temperature sufficient to cause primary grid emission. The apertures in the grid may consist of perforations of various shapes and sizes or may even be a single large hole or an annulus. Sometimes, a baffle in the form of a disc is attached to the cathode side of the grid with a small spacing between the baffle and the grid proper. Such baffling modifies the characteristic of the tube so that the potential of the grid in the non-conducting state may be either zero or positive. At the same time, this baffle helps to prevent deposition of cathode material on the grid proper. In this case ionization must occur between grid and cathode near the grid apertures before breakdown to the anode can take place. The first reported use of hydrogen as a filling for gas tubes was in 1936. Nothing further was done until the 1940’s when the rapid growth of radar required the development of a switch tube Able to operate at higher frequencies and with more reliable triggering than the mercury tubes then in use. (Pulse generators — Radiation Laboratory Series p. 335 et seq. K. J. Germeshausen. Published by McGraw Hill.)
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 With mercury, the maximum voltage that can be developed across the tube is about 30V. At a higher voltage, positive ion bombardment of the cathode becomes very intense and cathode destruction rapidly occurs. In pulse operation this is invariably about 100V so that destruction of the cathode is inevitable. With hydrogen this critical voltage is at least twenty times greater due to the much lower mass of the hydrogen ion. The recovery time of the hydrogen tube is an order of magnitude less than a similar mercury or rare gas tube because of the higher mobility of the proton. The chemical activity of hydrogen immediately introduces the problem of gas clean-up. To overcome this, all the materials used for the tube structure have to be of a very high degree of purity which is closely controlled. The processing of all the components used and of the tube itself during the exhaust and filling schedule is also very critical. In addition, tube geometry has a considerable influence on the rate of gas clean-up for given operating conditions. The common method of improving the life of hydrogen-filled tubes is by means of a hydrogen replenisher or reservoir. This consists essentially of a controlled weight of (usually) titanium hydride which is maintained at a relatively constant temperature within the tube envelope. Under these conditions titanium, hydrogen and the hydride are in thermal equilibrium and any variation of the amount of hydrogen within the tube envelope causes an adjustment of the equilibrium conditions. By this means the pressure within the envelope is maintained relatively constant. The hydride is usually contained in a metal cylinder which may be closed at one or both ends depending on the cylinder material. The capsule is heated by placing it in series or parallel with the cathode heater or by means of a separate supply. The latter arrangement is sometimes convenient if the thyratron is required to operate over a wide power range, and allows for a less critical design of the reservoir. It is obviously less convenient from the user's view point. Hydrogen thyratrons have been manufactured in the United Kingdom since about 1951. Development in recent years has concentrated on the multielectrode structure, which offers advantages over the conventional triode form. This is discussed in more detail in a later section. The particular nature and application of hydrogen tubes has resulted in the use of terms and definitions peculiar to these devices. The more important and less familiar of these are given below.
8.1.1. Peak Forward Grid Drive
The peak positive value of the unloaded grid pulse with respect to the cathode. This must be added to the bias voltage (if any), to determine the amplitude of the output pulse from the trigger generator.
8.1.2. Recovery Impedance
The ratio of the potential difference between the instantaneous potentials appearing at the thyratron grid and the on-load grid bias voltage to the grid current, at the same instant during the recovery period. This can be expressed as: Inst. ( grid potential – bias voltage )--------------------------------------------------------------------------------current
8.1.3. Recovery Time
The time interval between the cessation of anode current and the instant when the grid regains control under specified anode and grid circuit conditions.
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.1.4. Grid 2 Pulse Delay
The time interval between the voltage pulses on the terminals of grid 1 and grid 2 with the thyratron removed, measured at the 26% level on the leading edge of each pulse. It is essential that the grid pulses overlap.
8.1.5. Anode Delay Time
The time interval between the 26% point on the leading edge of the unloaded grid pulse and the instant when anode conduction occurs. (In tetrodes the grid 2 pulse is used as reference.)
8.1.6. Anode Delay Time Drift
The change in anode delay time over a specified period of time as a result of continued operation of the thyratron under specified conditions.
8.1.7. Time Jitter
The pulse-to-pulse variation of the instant when anode conduction occurs referred to the 26% point on the leading edge of the unloaded grid pulse. (In tetrodes the grid 2 pulse is used as reference.)
8.1.8. R.M.S. Current
Normally computed as: ( peak current ) × ( mean current )
The reference points associated with the interpretation of pulse shapes are shown in Figure 3.21.
Spike Amplitude
Spike Duration Amplitude
70.7%
26%
Pulse Duration Time of Rise
Time of Fall
Figure 3.21. Reference Points Associated with the Interpretation of Pulse Shapes
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.2. Operating Notes on Hydrogenfilled Tubes
The most important application for hydrogen thyratrons is in line-type pulse generators for radar transmitters and particle accelerators where high powers have to be switched rapidly and with precise timing and where small size, high efficiency and tolerance of ambient temperature variations are obvious advantages. A growing use is to be found for thyratrons as protection devices for “Single Shot” use, e.g., to discharge condenser banks in thermonuclear research.
8.2.1. Thyratrons in Line Type Modulators
The basic circuit for the DC charging line type modulator is shown in Figure 3.22, together with the simpler relationships most often used.
LC
LN CN
E
ZL
Figure 3.22. Schematic Diagram of Line Discharge Circuit E
= DC power supply voltage
LC
= charging choke inductance
LN
= total network inductance
CN
= total network capacitance
ZN
= network impedance
f
= pulse repetition frequency
epy i b = -----------------ZN + ZL
tp
= pulse width
epy = 2E + epx
epy
= thyratron peak anode voltage
epx = thyratron peak inverse voltage
ZN =
LN -----CN
t p = 2 L N C N = 2C N Z N
2 epy 2 epy P L = ⎛ ----------------⎞ × Z L = ----------⎝ Z N + L⎠ 4Z L
I
= thyratron mean anode current
ib
= thyratron peak anode current for matched load
ZL
= load impedance
PL
= peak load power
f
= π ( L C )1 ⁄ 2 C N
1 ------------------------------
I = i b ft p ZN × ZL epx = ------------------- × epy ZN + ZL
Although simple in diagrammatic form, this circuit has many components which are distributive in nature and interact with each other in different ways which depend upon their relative values. The modulator circuit must be able to cope with the situations that arise from switching on high voltage at a low value through normal full load operation to mismatch or short circuits produced by the RF load tube. (For a fuller treatment see Pulse Generators — Radiation Laboratory Series.) Those features of the circuit which most affect the performance and reliability of hydrogen thyratrons will now be discussed.
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
8.2.2. The Charging Circuit
The pulse forming network (PFN) is charged to approximately twice the DC supply voltage in a time which equals half the period of the resonant circuit formed by the charging choke inductance and network capacitance. After the thyratron has fired and the voltage has collapsed, sufficient time must be made available for the thyratron to recover. This means that the anode voltage must be kept below cathode potential until the thyratron has recovered.
Supply Voltage
epy
T (a)
TIME
Supply epy Voltage
epy
T
TIME
(b)
Supply Voltage
T
TIME
(c) T = charging period epy = thyratron peak voltage
Figure 3.23. Charging Voltage Waveforms If the thyratron is triggered at the instant the network voltage reaches a maximum then the condition of resonant charging is achieved, see Figure 3.23(a) If this triggering is delayed then the peak voltage will commence to fall unless a charging diode is present, see Figure 3.23(b). (The same effect is produced by a low value of charging choke inductance for a given triggering frequency.) If triggering frequency is increased or inductance increased then a condition of linear charging is approached, Figure 3.23(c). Therefore, in order to make most time available for recovery of the switch tube, the charging choke inductance should be calculated for the highest repetition frequency that will be used. Where the repetition frequency is high (greater than 5000 p.p.s.) or the duty ratio is high (of the order of 0.01), the time available for recovery becomes a very important factor. (See section on Recovery.) In these cases, to enable the switch tube to recover, it may be necessary to delay the charging of the PFN by using a triggered charging diode or a saturable reactor. The power supply should have good regulation to avoid any increase in anode voltage under conditions of interrupted triggering, and should also be free from appreciable overshoot when ‘'snap start” conditions are required.
8.2.3. Charging Diodes
The charging diode must be rated to withstand a peak inverse voltage (p.i.v.) of not less than the full network voltage since at the end of its charging period the anode may swing down to near earth potential. The mean current requirements of high-power modulators may preclude the use of vacuum tubes. In such cases, a gas tube may be used which often takes the form of a “triggered diode” such as the FX297. When it is necessary for the hold-off period to be long, the grid of the charging “diode” may be triggered through a suitably insulated pulse transformer but for conditions not far from resonant a high resistance potential divider will be satisfactory. The inclusion of an anode inductor helps to protect the charging diode from the initial spike of charging current produced by the self capacity of the charging reactor, and also from the full effect of the inverse voltage swing which occurs after the pulse forming network is
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 charged. When delayed triggering of the charging diode is required in order to effect an increase of time available for recovery of the modulator switch tube, care must be taken in the design of the trigger circuit. It is possible for triggering to be caused by RF generated in the modulator when the switch tube fires, due to the ease with which the charging “diode” is triggered. This will give prefiring before the delayed trigger voltage appears. A low impedance drive circuit will help to overcome this.
8.2.4. Thyratron Anode Circuit
Associated with the thyratron anode in conventional arrangements are the pulse forming network and inverse diode, and the most important parameters to be considered are rate of rise of current through the thyratron (di/dt) and inverse voltage at the anode (epx) after the main pulse. A high rate of rise of current will increase leading-edge heating and also the rate of gas clean up in the tube. The latter is particularly important in nonreservoir tubes. The mean rate of rise of current as shown on an oscilloscope trace is not necessarily a good indication, since the peak value may be considerably higher than this due to stray capacitance from the network to ground, and from charging and inverse diode heater transformers. For this reason the three-terminal network shown in Figure 3.22 is preferred since the bulk of these stray capacitances are removed from the thyratron anode. The end inductive section of the network should be mounted close to the epy - . If di/dt is in thyratron anode, and will have a minimum value L A = ------------di ⁄ dt amperes per microsecond, then LA is calculated directly in microhenries. It must be realized that the mean current through the thyratron is the sum of the DC power supply current and the inverse diode current (if any).
8.2.5. Inverse Diode Circuit
The inverse diode or clipper tube is normally found in high-power equipment to protect both the load and thyratron. In the event of a short circuit in the load the circuit impedance is halved and the peak current through the thyratron is doubled. With no inverse diode in the circuit the thyratron anode voltage swings negative to a value approaching the peak positive value and the succeeding charging cycle starts from there. This cycle will provide a positive voltage tending to double the previous value and, if the short in the load persists, this amplification will continue until limited by circuit losses or component failure. The inverse diode circuit must dissipate the energy involved. Depending on the modulator conditions, the resistance of this circuit may lie between 4Zn and about 40Zn. The lower value is to be preferred for fast removal of inverse voltage but higher values may be necessary to permit recovery of the switch tube. A value of around 40Zn will limit the increase in peak positive voltage to approximately 1% following a short circuit where the modulator duty ratio is 0.001. An inductive overswing circuit usually results in the inverse voltage rating of the thyratron being exceeded, due to the time of application being longer than in the resistive case. The primary inductance of the pulse transformer may be sufficient to produce this effect, particularly If the anode of the inverse diode is connected directly to earth. Any excess of inverse voltage in amplitude or time may result in arc-back and overheating of the grid structure, with the possibility of distortion in extreme cases. The rise of negative voltage appearing on the thyratron may be very rapid and the amplitude of the spike is approximately the same whether a diode is used or not. However, as soon as the diode conducts, the voltage rapidly
Modulator Theory: Thyratron Theory
3-25
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 collapses to a value determined by the inverse diode circuit impedance. When pulse transformer loads are used the effect is to delay the rise of the inverse voltage and thus tend to reduce the possibility of arc-back in the switch tube. Vacuum diodes are able to withstand higher voltages than gas tubes but present a higher impedance and are more likely to suffer failure in the event of a prolonged period of reduced load impedance. For this reason the FX297 is widely used, preferably triggered by a simple pulse transformer fed from the main current pulse, or alternatively by a capacity divider.
8.2.6. The Trigger Circuit
The values of jitter and anode delay time drift quoted in tube data sheets are extreme values to cover variation during life and are measured under conditions of minimum trigger amplitude, drive current and rate of rise of voltage. In practice, the trigger signal applied to the thyratron grid should be from a low impedance source and should have a high rate of rise of voltage and a pulse amplitude sufficient to cause rapid ionization of the gas in the grid-cathode space. This will minimize jitter and anode delay time drift. A trigger pulse duration very much in excess of the minimum value specified on the tube data sheets is wasteful and may, in certain circumstances, inhibit recovery of the thyratron after the main current pulse has been switched. In tetrodes, excessive current to grid 1 or excessive delay in the application of the grid 2 pulse may lead to a deterioration in tube performance. Any modulation superimposed on these parameters or on the bias supply will show up as jitter on the main current pulse. The minimum trigger pulse amplitudes quoted in data sheets refer to cathode potential, and the value of any negative bias used should be added to this figure to give the required minimum unloaded pulse amplitude from the trigger generator. This amplitude must always be checked at the thyratron socket with the tube removed. At the instant of firing, the grid potential rises rapidly. A voltage spike of the order of 20ns duration and an amplitude which is an appreciable fraction of the thyratron anode voltage, may be observed on the oscilloscope trace of the grid waveform. This spike can cause breakdown in the trigger unit. The amplitude of this spike increases as the rate of rise of current in the tube is increased. A series resistor, placed adjacent to the grid terminal and aided by the stray capacity, may provide a filter against the spike. A filter network is sometimes necessary, but should be kept as small as possible since it will degrade the grid firing pulse front and thereby increase anode delay time drift. Alternatively a nonlinear resistor may be used in parallel with the trigger unit output.
8.2.7. Recovery
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At the end of the current pulse, a plasma exists throughout the tube which presents a short circuit to positive anode voltages. Therefore, the circuit is arranged so that the thyratron anode is held at a slight negative potential until recovery is complete. The tube has recovered when re-application of a positive anode voltage does not cause further conduction. During the recovery period the plasma decays with time as recombination of ions and electrons mainly on electrode surfaces. Since there are relatively close spacings in the tube between anode and grid, and also in the grid, the plasma density drops rapidly in this region with a time constant of the order of a microsecond. The grid-cathode plasma decays much more slowly because of the wider gaps involved and retains the inherent nature of a plasma by having approximately equal numbers of electrons and ions. Recovery is complete when the grid cathode plasma has shrunk away from the grid apertures so as not to come under the influence of any applied anode voltage,
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 but deionization is not complete until all the ions have disappeared from the tube. It should he noted that a tube has recovered long before it has deionized. Negative bias applied to the grid decreases recovery time. The bias voltage does not pull ions out of the discharge but when the grid potential goes negative with respect to the cathode, the anode field penetration is reduced. Grid bias may be applied via a resistor or an inductor. Both of these must inherently retard the application of negative bias but they are necessary to enable a forward drive pulse to be applied economically. The trigger unit output stage may be a cathode follower, a blocking oscillator or a pulse transformer. Care is needed in the design of the associated circuitry since not only must it apply a positive pulse to the grid but it must also be capable of passing several amperes of deionization current which follow the pulse. A paper by Malter and Johnson (RCA. Rev., June 1950) shows the change in plateau length and exponential decay of positive ion current to the grid of a small thyratron. The wave shape is shown in Figure 3.24.
g
rRg
rs
Grid Voltage
–150V
Es
Time
Cathode Potential
Bias Potential
Ecc
2µF
Exponential Decay
Grid Waveform after Current Pulse Figure 3.24. Circuit for Recovery Time Measurement
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Grid Voltage
(a)
Inductive Swing Time
Cathode Potential Grid Voltage
(b)
Inductive Swing Time
2µF
Grid Voltage
Cathode Potential
(c)
Bias persisting because of capacitor Inductive Swing Time
2µF
Grid Voltage
Cathode Potential
(d)
Bias persisting because of capacitor Inductive Swing Time
Cathode Potential Grid Voltage
(e)
Time
Cathode Potential
Exponential decay
Figure 3.25. Methods of Providing Negative Grid Voltage Swing
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The published data shows recovery curves for various values of bias and recovery resistance. All these measurements have been made in a specially developed circuit which presents a purely resistive impedance between grid and bias (if any) during the recovery period (Figure 3.24). It will be noted that the zero bias recovery time is independent of recovery resistance. These recovery times are much longer than those achieved from a pulse transformer drive since the energy stored in the pulse transformer causes an inductive swing of the grid which produces an effective bias in the grid and facilitates recovery Figure 3.25(a). This improvement may also be achieved by the insertion of an inductance of around 6mH directly between grid and cathode Figure 3.25(b). Figure 3.25(c) and Figure 3.25(d) show similar effects when a capacitor is included as a virtual bias source. Some advantage may be gained from the longer persistence of the bias and control of positive overshoot. Cathode follower outputs can also produce appreciable bias, especially at high repetition frequencies (e.g., 50kHz), from the energy stored in the coupling capacitor (Figure 3.25e). When a bias is applied via an inductor, a voltage (e = L di/dt) builds up due to the deionization current more negative than the bias and providing the subsequent swing back to the bias level is slow enough, a reduction in recovery time will be obtained. In the case of bias applied through the secondary of a pulse transformer excessive damping may inhibit recovery (Figure 3.26). A saturable inductance provides a relatively high impedance during the forward grid pulse but by suitable design this will saturate at the end of the main anode current pulse and present a low impedance to the grid and facilitate recovery (Figure 3.27). Again in the absence of any bias supply the forward pulse drive may be used to produce the bias source (Figure 3.28). e –E
g cc - where eg is the value of the grid Recovery impedance is defined as -----------------i c
voltage at any instant during the recovery period, ic is the value of the grid current at the same instant and Ecc is the grid bias supply voltage. The published recovery characteristics of thyratrons show that decreasing the resistance in series with the bias supply or increasing the bias voltage reduces the recovery time. From inspection of these a value of voltage and resistance may be found which will guarantee recovery in a particular circuit for which the time spent negative by the anode is known. The bias supply must of course be capable of passing a large current without appreciable voltage drop for the duration of the recovery period and should, therefore, be shunted by a suitable capacitor (between 0.1 and 10 µF depending on requirements). It must also be capable of recharging the capacitor rapidly and should be free from ripple which would cause jitter of the output pulse.
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Time
R
Bias Voltage
Grid Voltage
Plateau as deionization current is limited by circuit impedance If R is too great, exponential decay
Steady bias level
2µF Exponential decay + inductive bias
Normal pulse transformer driven circuit
Figure 3.26. Waveforms with Pulse Transformer Trigger Drive
Bias Voltage
Grid Voltage
Time
Inductive swing + switching action of saturation
2µF
Figure 3.27. Inductive Overswing with Bias
Grid Voltage
Time
Bias persisting because of capacitance
2 µF
Figure 3.28. Inductive Overswing without Bias The modulator engineer will, however, always have a simple check on his design by using an oscilloscope to display the grid wave shape See Figures 4.29(a) and 4.29(b). A long “plateau” indicates that either the thyratron is still conducting because its anode is being held slightly positive or the grid bias is being applied through a very high impedance. The appearance of instabilities on the grid wave shape as the high voltage is increased is also indicative of insufficient time allowed for recovery of the switch tube. Further increase of high voltage invariably results in trip outs under these circumstances.
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 AMPS
CATHODE CURRENT PULSE
VOLTAGE
Arc level
Start of current pulse
Plateau
TIME
CATHODE CURRENT PULSE
GRID VOLTAGE WAVEFORM Arc level Thyratron anode ceases to conduct Plateau
Bias level
Figure 3.29. Waveforms Normally Seen During Modulator Adjustments
8.2.8. Thyratron Heater Voltage
Although not strictly a circuit element, the effects of heater voltage variations are worthy of mention. A tolerance of ±7.5V is normally allowed on heater voltage but every endeavor should be made to keep within closer limits. This applies particularly to tubes which include a hydrogen reser-
Modulator Theory: Thyratron Theory
3-31
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 voir in series or parallel with the cathode heater. Any variation of cathode heater voltage thus affects gas pressure as well as cathode temperature. Anode delay-time drift will be affected by changes in heater voltage. A large proportion of the observed jitter is caused by the field from the cathode heater and in cases where jitter requirements are to be reliably less than 1.0ns. DC heater supplies should be used.
8.2.9. Cooling
The current passing through the tube and in particular the rate of rise of current are dependent on a sufficient availability of ionized particles. The discharge path between cathode and anode represents the region of high temperature and thus there will normally be a distribution of gas density within the envelope, the density being highest at the coolest spot. If the envelope is cooled artificially, then a higher than normal gas density will occur at the envelope giving a low density in the discharge region This may limit the rate of rise of current demanded by the circuit. thereby increasing the dissipation within the tube, and accelerating the rate of gas clean-up.
8.2.10. Mounting
Most thyratrons may be mounted in any position although the size of the larger tubes generally dictates a base-down position. Care should be taken that strong RF or magnetic fields are kept away from the thyratrons. These could cause ionization within the tube envelope seriously affecting the hold-off capabilities of the tube and increasing the recovery time. Some gas clean-up may also occur. For medium- and high-power tubes the anode connector should preferably be of large surface area.
8.2.11. Warmup Time
The time quoted on tube data sheets is the minimum necessary for the cathode to reach operating temperature and for the gas pressure to reach a minimum value where reservoirs are used. If trigger pulses are applied before the expiry of the warm-up time then grid/cathode breakdown may be observed, but this does not mean that the cathode temperature or gas pressure are high enough for full power operation. If the ambient temperature before warm up is very low (say below 20° C) then some increase in warm-up time may be necessary.
8.2.12. The Tetrode Thyratron
The advantages offered by the tetrode over the triode in pulse modulator use are sufficiently important to warrant special mention. The reduction of firing time variations in thyratron circuits is always foremost in the minds of both tube designer and users and, while much can be done by careful circuit design, the variations obtained with triode thyratrons may still be excessive for certain applications. Firing time variations fall into two main classes: (a) long-term variations, both repetitive and cumulative. The first of these is largely due to the warming up of tubes and components each time an equipment is used. The second, a much slower variation is due to gradual reduction of gas pressure during the life of the thyratron; (b) short-term or transient variations due to various causes including interference from the A.C. heater field.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Anode
Figure 3.30. Schematic Diagram of Tetrode Variations due to all these causes are reduced when tetrode thyratrons are employed. Tetrode thyratrons have two grids which are pulsed successively with a delay of about 1.0 µs. Grid 1 has a positive characteristic with respect to the cathode as in the triode, and grid 2 is given a negative characteristic with respect to grid 1 (Figure 3.30). Grid 1 performs its “priming” function very much as in the triode while grid 2 performs what may be termed a “gating” function when ionization by the grid 1 pulse is well under way. This results in very rapid and precise takeover by the anode. The negative bias applied to grid 2 assists recovery in the inter-pulse period and thus makes the tetrode thyratron eminently suitable for use in high p.r.r. precision radar equipment. In order to switch pulses which are successively accurate in time over a long period the two grids should be pulsed from separate sources with the grid 2 drive delayed by about 1.0 µs. When only one grid drive pulse is available, this may be made use of as in Figure 3.31. 1kΩ
Pulse input –100V bias
0.1µF
G2
0.001µF 1kΩ
G1 5kΩ
Figure 3.31. Single Pulse Drive for Tetrode
Modulator Theory: Thyratron Theory
3-33
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A more simple drive circuit is shown in Figure 3.32. It must be stressed, however, that some of the inherent advantages of the tetrode may be lost by the use of these circuits but they will be satisfactory for many applications. An increase in time jitter up to about 5.0ns may result and anode delay time drift may be as much as 0.1 µs as against 0.02 µs. 1kΩ
Pulse input –100V bias
G2
0.1µF 1kΩ
G1 5kΩ
Figure 3.32. Simplified Single Pulse Drive These three circuits all depend on transferring the discharge from the grid 1 electrode to grid 2 and thence to the anode. The grids should not be simply strapped together, since this may result in a discharge to the grid 1 electrode which would not necessarily transfer to grid 2 and would result in erratic firing.
8.2.13. The Parallel Operation of Hydrogen Thyratrons
In common with all gas-filled devices, thyratrons cannot be connected directly in parallel without some form of impedance in series with each tube. In repetitive pulse applications, the triode thyratron does not provide a sufficiently precise pulse-to-pulse triggering facility for parallel operation to be successful. The tetrode tube does not have this defect. In theory, any number of tetrodes may be parallel connected, although in practice six is a convenient maximum. Separate pulse-forming networks, each with its own charging diode, are recommended, since fault conditions will result in each thyratron discharging its own network and this will not involve as much energy as in the case of one large network. Further, the triggering requirements are less complicated as will be shown below. The following circuits show suitable arrangements for two tubes, but they may be extended for additional tubes as required. Figure 3.33 shows a suitable circuit where two driving pulses are available each pulse being split between the two thyratrons. Figure 3.34 shows an arrangement using a single driving pulse which automatically provides a delay on grid 2 to each tube. Figure 3.35 shows the general arrangement of the anode circuit.
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Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 R2
Pulse 2 I/P –100V bias
G2
1kΩ
V2 R1
G1
1kΩ
R2
G2
1kΩ
V1 R1
Pulse 1 I/P
G1
1kΩ
Figure 3.33. Double Pulse Drive for Parallel Operation Where the use of a single network cannot be avoided, it is essential that each tube has an anode inductor of the same value and arrangements should be made for adjustment of the current sharing between tubes. This is most conveniently achieved using the circuit of Figure 3.34, and making R2 variable. This provides adjustment of the delay to grid 2 on each tube. Figure 3.36 shows suitable component values. This arrangement has been successful in the parallel operation of six CX1140 tubes to provide a pulse output power of 75MW. R2
Pulse I/P 0.1µF
G2 0.001µF
R1
V2 G1
R3 R2 0.1µF
G2 0.001µF
R1
V1 G1
R3
Figure 3.34. Single Pulse Drive for Parallel Operation When several thyratrons are being used, a check should be made on the triggering supplies by removing one of the thyratrons from its socket to ensure that the amplitude of the available voltage pulse at that socket is greater than the minimum specification requirements when all the other tubes are conducting between grids and cathode.
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Charging diode
Anode Inductor
PFN
I/P Charging choke
G2 G1
V2
Anode Inductor
PFN
G2 G1
V1
RL
Figure 3.35. Anode Circuit Using Separate PFN’s The rate of rise of current rating of the assembly is the sum of the individual tube ratings, and this often provides a capability in excess of a single larger tube.
5kΩ
Pulse input –100V bias
G2
0.001µF
0.1µF 1kΩ
G1 5kΩ
Figure 3.36. Arrangement for Adjustment of Current Sharing The driving pulse should have a fast rising front and be of near maximum rated amplitude to minimize anode delay time and jitter. This applies particularly to the circuit of Figure 3.33, due to the effect of capacitive loading on the leading edge. This effect may be considerably reduced as shown in Figure 3.37 by the use of a variable delay line in series with R2 instead of the 0.001 F capacitor for applications requiring minimum delay time and jitter. The values of series resistors given will, in general, provide satisfactory performance, but if other values are used, the grid 1 current must be kept within the specified limits. With separate networks and separate identical anode inductances, current sharing between tubes is automatic, and small differences in delay time between tubes do not affect the performance.
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1 µS Delay Line
R2
Hydrogen thyratron (tetrode)
R4 0.1µF
R1
R3
2µF Bias –60 to 100V
Figure 3.37. Using a Delay Line to Minimize Time Jitter
8.2.14. The Series Operation of Hydrogen Thyratrons
Where extremely high voltages have to be switched, thyratrons may be operated in series without the necessity of insulated trigger supplies. The general arrangement is shown in Figure 3.38 with suggested component values. Tetrodes are preferred since they generally have a lower anode delay time than triodes. Furthermore, in the arrangement shown, it is essential that the switch tube should fire with low current triggering. a property not normally found in triodes. The operation is as follows. Tube (1) at the lower voltage is triggered normally, using separate pulses to each grid, or a single pulse and the conventional RC arrangement. When this tube fires, the voltage at its anode falls and thus the voltage at the cathode of tube (2) falls. Capacitor (C1) between grid 1 and grid 2 of tube (2) helps to maintain the resulting high potential between grid 1 and cathode and thus the tube fires to grid 1, and thence to grid 2 resulting in complete breakdown.
HV
C1 100µµF
C3
10 M Tube 2
10 M
C2 Tube 1
Figure 3.38. Thyratrons in Series
Modulator Theory: Thyratron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The insulated heater transformer for tube (2) will have capacitance to ground (C2) which, if not minimized, will require an equal compensating capacitor (C3) to be placed in parallel with tube (2). This divider, assisted by the tube interelectrode capacitance, will automatically provide equal voltage distribution between tubes. Added capacitance should be minimized since this will discharge through the tube, and must be connected to the network side of the anode inductors. To avoid the presence of isolated elements, a resistive divider is preferable. The value of this should not be so low as to cause a serious drain from the power supply, and should not be so high as to introduce (with the various capacitances) time constants at variance with the PRF required.
8.2.15. Hydrogen and Deuterium
The advantages of hydrogen over mercury and the rare gases have already been mentioned in Part 1 of this article. However, an increasing use is made of deuterium as the gas filling in place of hydrogen. Chemically the gas is similar to hydrogen, so that conventional hydrogen reservoirs operate equally well with deuterium. The Paschen breakdown curve, however, shows that deuterium is capable of higher hold-off voltages than hydrogen for the same pressure. The greater mass of the ion, however, means less mobility and so recovery time is increased (by a factor of 2 for the same geometry). At the same time, surface recombination effects are reduced and the arc loss is lower. For this reason, deuterium is mostly used in high-power tubes, where recovery time is usually of less importance than hold off voltage and dissipation.
8.2.16. Single Shot and Crowbar Applications
The increasing use of high-power klystrons, traveling wave tubes and other such devices has demanded that some protection should be given in the event of an internal flash-over or similar fault. This protection may be provided by a gas-filled spark-gap or thyratron across the main high voltage supply, which, when triggered by the fault current, effectively short circuits the high voltage until the main circuit breakers operate. Since the “crowbar” device may have to remain quiescent for many thousands of hours until suddenly required, it is desirable that it should consume no power, and essential that it should function with complete reliability when needed. The spark gap has an advantage over the thyratron in its zero power consumption on standby. However, for both spark gap and triode thyratron, there is no simple method of ensuring that the crowbar circuit is in a state of readiness, e.g., the device may have become “leaky” during a long standby period. With tetrode thyratrons the switching action may be controlled completely by the second grid and it is thus possible to run a discharge continuously to grid 1 without affecting the peak hold-off voltage under DC conditions provided bias is applied to the grid. It has been found that a current of about 100mA continuously to grid 1 in no way affects the life or performance of the tube, and that this grid 1 current may then be used as a simple monitor, e.g., by lighting an indicator bulb, to show that the crowbar circuit is in a state of readiness. It is possible for a reversal of high voltage to take place following crowbar action and before circuit isolating elements have responded. If the trigger voltage to the crowbar tube has been removed, then high voltage will be reapplied to the load as this voltage becomes positive again. It is thus desirable that the trigger voltage waveform should be a train of pulses rather than a single pulse. The peak forward voltage given in the data sheets for these thyratrons is the rating for repetitive pulse work, and is normally reduced for DC holdoff applications. At the same time the peak current may be considerably increased provided the repetition frequency is not greater than 1 per 10 seconds.
3-38
Modulator Theory: Thyratron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Four RF Theory
This chapter will discuss microwave RF system theory, including microwave technology, in order to familiarize the student with the application of this technology to the functional operation of the Clinac RF System.
RF Theory
4-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents
1. Introduction:.................................................................................................................... 4-5 2. Traveling Waves on Transmission Lines:........................................................................... 4-5 3. Waveforms: ...................................................................................................................... 4-8 4. Waveguides: ................................................................................................................... 4-11 5. Resonant Circuits: ......................................................................................................... 4-14 6. RF Transmission Theory: ............................................................................................... 4-16 7. RF Waveguide Design: .................................................................................................... 4-17 7.1. Modes:.................................................................................................................... 4-17 7.2. The TE10 Mode:...................................................................................................... 4-18 7.3. Coupling:................................................................................................................ 4-18 7.4. Determining the TE10 Dominant Mode of a Waveguide: .......................................... 4-19 8. Transmission Lines: ....................................................................................................... 4-20 8.1. VSWR: .................................................................................................................... 4-21 9. Vector Analysis – 3dB Quadrature Hybrid: ..................................................................... 4-21 10. Circulators: .................................................................................................................. 4-22 11. Klystron Theory:........................................................................................................... 4-24 11.1. Theory of Klystron Operation: ............................................................................... 4-24 11.2. Associated Equipment: ......................................................................................... 4-34 11.3. Power Supplies: .................................................................................................... 4-35 11.4. Cooling: ................................................................................................................ 4-39 11.5. RF Circuits: .......................................................................................................... 4-45 11.6. Tuning:................................................................................................................. 4-49 11.7. Noise in Klystron Amplifiers: ................................................................................. 4-51 11.8. Summary:............................................................................................................. 4-52
4-2
RF Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Illustrations Figure 4.1. Propagation of a Step Wavefront on a Transmission Line:.................................... 4-5 Figure 4.2. Infinitely Long Line: ............................................................................................ 4-6 Figure 4.3. Finite Line Terminated by its Characteristic Impedance: ..................................... 4-6 Figure 4.4. Traveling Waves on an Open-end Line:................................................................ 4-7 Figure 4.5. Traveling Waves on a Shorted-end Line: .............................................................. 4-9 Figure 4.6. Voltage and Current Waves on a Shorted-end Line:........................................... 4-10 Figure 4.7. Parallel Strip Lines:........................................................................................... 4-11 Figure 4.8. Waves on a Parallel Strip Transmission Line: .................................................... 4-12 Figure 4.9. Electric Field Distribution: ................................................................................ 4-13 Figure 4.10. Electric and Magnetic Fields in a Closed Rectangular Waveguide: ................... 4-13 Figure 4.11. Electric and Magnetic Fields of the Dominant TE10 Mode in a Rectangular Waveguide:......................................................................................................................... 4-14 Figure 4.12. Tuned Resonant Circuit: ................................................................................. 4-15 Figure 4.13. Resonant Circuit Having Two Natural Frequencies: ......................................... 4-15 Figure 4.14. WR 284 RF Waveguide: ................................................................................... 4-16 Figure 4.15. RF Waveform Divided into ¼-Wavelength Intervals:......................................... 4-16 Figure 4.16. RF Rectangular Waveguide Dimensions: ......................................................... 4-17 Figure 4.17. The Transverse Electric (TE) Mode: ................................................................. 4-18 Figure 4.18. The Transverse Magnetic (TM) Mode:............................................................... 4-18 Figure 4.19. Loop Coupling:................................................................................................ 4-19 Figure 4.20. Probe Coupling: .............................................................................................. 4-19
RF Theory
4-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 4.21. Frequency Determining Parameters: ............................................................... 4-19 Figure 4.22. Characteristic (Surge) Impedance: .................................................................. 4-20 Figure 4.23. PFN Parameters:............................................................................................. 4-20 Figure 4.24. 3dB Quadrature Hybrid Vector Analysis: ........................................................ 4-21 Figure 4.25. 3-Port Circulator: ........................................................................................... 4-22 Figure 4.26. Low Energy Clinac 4-Port Circulator and Phase Diagram: ............................... 4-22 Figure 4.27. High Energy Clinac 4-Port Circulator and Phase Diagram (Used in Shunt Tee Clinacs):............................................................................................................................. 4-23 Figure 4.28. High Energy Clinac 4-Port Circulator and Phase Diagram: .............................. 4-23 Figure 4.29. Triode Vacuum Tube Amplifier: .......................................................................... 4-25 Figure 4.30. Sectional View of a Klystron: .............................................................................. 4-26 Figure 4.31. Generation of Alternating Current in a Cavity:.................................................... 4-28 Figure 4.32. High-Power Four-Cavity Klystron: ...................................................................... 4-30 Figure 4.33. High-Power Four-Cavity Klystron, Simplified: ..................................................... 4-31 Figure 4.34. Effect of Tuning on Klystron Performance:.......................................................... 4-32 Figure 4.34. Effect of Tuning on Klystron Performance:.......................................................... 4-32 Figure 4.35. Klystron Power Supply Connections: .................................................................. 4-35 Figure 4.36. Typical Liquid Cooling System for a Klystron Amplifier: ...................................... 4-42 Figure 4.37. Typical RF Circuitry for a Klystron Amplifier: ..................................................... 4-46
4-4
RF Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
The material in this chapter should be read and thoroughly understood by the student before proceeding to the specific RF system descriptions in the High and Low Energy Clinac Beam Delivery System course manuals.
2. Traveling Waves on Transmission Lines
Figure 4.1 has been drawn to show the mechanism by which a step wavefront (pulse) travels along a real transmission line. q
i=I
S1 Edc
i=0
e=0
e=E
i=I
e
q
E
0
S
i I 0
S
Figure 4.1. Propagation of a Step Wavefront on a Transmission Line When switch S1 is closed, the battery with a terminal voltage of Edc is applied to the input of the transmission line. The voltage Edc does not appear instantly at all points along the line. Instead, a wave of voltage progresses along the line. The farther from the battery a given point on the line is, the later the time at which the line voltage at that point jumps from 0 to Edc. A current wave also travels along the line exactly in step (in phase) with the voltage wave. Current flows away from the battery in the top conductor and returns to the battery in the bottom conductor. Plus and minus signs are placed on the conductors at points where voltage exists between the conductors, and magnetic flux lines are shown encircling the conductors wherever current is flowing. The reason that the line cannot charge all at once is due to the existence of series inductance along each conductor as well as shunt capacitance across the conductors. The voltage wave can progress only as fast as the line current can carry a charge to the wavefront to produce the change in voltage. The current wave can progress only as fast as the voltage can develop across each short section of conductor at the wavefront, thus starting current in that corresponding section of line inductance. The voltage and current waves must move in phase with each other along the line. The rate of travel (propagation) along the transmission line is proportional to the square root of the inductive and capacitive reactance per unit length of line. T =
RF Theory: Introduction
LC
4-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The characteristic impedance of the line is proportional to the square root of the quotient of the inductive and capacitive reactance per unit length of line. Rz =
L--C
Thus far, only the start of a wave from the input terminals of a line and the propagation along a line of infinite length have been discussed. The effects that occur when a wave reaches the end of a finite length of line must also be considered. In Figure 4.2 a 100-volt battery has been connected through a 1K series resistance to an infinitely long transmission line with the same characteristic impedance as the series resistance.
1K
id
i
ed
e
S1
100V
RC = 1K
Figure 4.2. Infinitely Long Line The circuit of Figure 4.3 is the same except that the line has been shortened to a length equal to 1 microsecond of propagation time and terminated in its characteristic impedance of 1K ohms. In both circuits, a 50-volt wave, and 50-mA wave travel along the line from the input terminals after the switch is closed.
1K
100V
S1
id
i
ed
e
RC = 1K
1 µS Travel Time
Figure 4.3. Finite Line Terminated by its Characteristic Impedance For the first microsecond of time both lines have the same propagation characteristics, because the remainder of the infinitely long time is equivalent to a 1K resistance also. However, the circuit of Figure 4.3 will develop a steady-state condition as soon as the wave reaches the 1K termination resistance. Once in the steady state, 50 volts appears across the conductors of the line, and a 50-mA current will continue to flow through the conductors and terminating resistance, as expected when two 1K ohm resistors are connected in series across the 100 volt battery. The circuit of Figure 4.4 (A) uses the same 1 microsecond line segment with the end left unterminated. When the switch is closed, the line first appears as a 1K impedance connected in series with the 1K resistors. This is because the end terminal conditions do not affect the wavefront before it reaches the end of the line. A 50-volt, 50-mA wave starts along the line from the input terminals. The current must be zero at the end of the line at all times due to the open circuit. To maintain this zero current condition when the wavefront reaches the end of the line, a second 50-mA current wave must start back along the line toward the battery. There must also be a re-
4-6
RF Theory: Traveling Waves on Transmission Lines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 flected voltage wave associated with the reflected current wave. This is because the characteristic impedance of the line also relates to the quotient of the voltage and current wave. ( across line -) R c also = E --------------------------------I ( across line )
The voltage-wave amplitude is 50 volts thus causing the total line voltage to rise to 100 volts. These traveling waves are indicated in Figure 4.4 (B) and corresponding time variations of e and i are shown by Figure 4.4 (C). 1K
id
i
ed
e
S1
100V
RC = 1K
A. Open End Transmission Line
e 50V
t = ½ µS
s
0
i 50mA
s
0
ed
100V 50V
t = 1½ µS
s
0
id 50mA
0
s
B. Traveling Wave
ed
100V 50V
0
2 µS
t
2 µS
t
id 50mA
0
C. Time Waveforms Figure 4.4. Traveling Waves on an Open-end Line
RF Theory: Traveling Waves on Transmission Lines
4-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
3. Waveforms
The incident (forward) wave of Figure 4.4 (A), in traveling from the battery to the open end of the line, charges the line capacitance from 0 to 50 volts because of the 50-mA current supplied by the battery. When the wave reaches the open end, no capacitance remains to be charged. The 50-mA current is, however, maintained by the line inductance and thus charges the capacitance near the open end of the line to twice the battery voltage. This voltage is produced across the line near the open-end in a direction to reduce the line current toward zero. This increase of voltage and decrease of current near the open end of the line corresponds to a reflected wave traveling back toward the battery. As the reflected wave progresses back to the battery, the entire line is charged to 100 volts and the total current on the line is reduced to zero. Thus, a static condition is reached with the line at 100 volts, and no further changes occur in the circuit. In the circuit of Figure 4.5 on Page 4-9 a short circuit is placed at the end of the section. As a result, the voltage at the short circuit must always be zero, and at the instant the 50-volt, 50-mA incident wave reaches the short circuit, a reflected voltage wave of 50 volts amplitude and of a reversed polarity starts toward the battery. A current wave of 50 mA amplitude is also associated with the reflected voltage wave, and the current in this wave flows in the same direction as the incident current wave. After the reflection is over, a current of 50 mA flows from the battery into the moving wavefront, and a current of 100 mA flows from the wavefront to the short circuit. Thus, the line capacitance at the wavefront is discharged by the net current of 50 MA, and the voltage across the conductors drops to zero. When the reflected wave reaches the battery, the line voltage is zero at all points and the current is 100 mA. No further changes occur because 100 mA is exactly the current drawn by the 1K resistor connected in series with the shorted line section across the 100 volt battery. A fact of fundamental importance is that a wave of any shape can be propagated along a transmission line without any change of shape or magnitude. The voltage wave is always accompanied by a current wave of similar shape. Thus for RF frequencies a sine wave generator rather than a battery can be connected to the transmission line, and accordingly the same functions occur with the sine wave as occurred with the battery. When a line is terminated in an impedance that is different from the characteristic impedance (mismatched) the voltage and current on the line are no longer the result of a single wave traveling from generator to load. Instead, the total voltage and current are the algebraic sums of two waves traveling in opposite directions. As an example, consider the diagram of Figure 4.6 on Page 4-10 in which a sine wave generator with an internal resistance of RL is connected through a switch S to a transmission line of impedance RZ and terminated in a short circuit. The behavior of the line after the switch is closed is similar to that of a shorted line with a battery as the source (reference Figure 4.5), except that sinusoidal waves of voltage and current travel along the line. In order that the voltage at the short circuit may be zero at all times, the voltage reflected back at the short circuit must be equal in magnitude to the incident voltage, but reversed in polarity. The generator has an internal resistance which is equal to the line impedance RZ. This causes the line to behave with respect to the reflected waves exactly as if the line were terminated in its characteristic impedance resulting in steady state conditions when the reflected wave reaches the input terminals.
4-8
RF Theory: Waveforms
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 1K
id
i
ed
e
S1
100V
RC = 1K
A. Shorted Transmission Line 1 µS Travel Time
e 50V
t = ½ µS
s
0
i 50mA
s
0
ed t = 1½ µS
50V
s
0
id 100mA 50mA
0
ed
s
B. Traveling Wave
50V
0
2 µS
t
id 100mA 50mA
0
2 µS
t
C. Time Waveforms Figure 4.5. Traveling Waves on a Shorted-end Line
The incident and reflected voltage and current waves are shown by Figure 4.6 on Page 4-10 (B) and (C) respectively. The resultant sum of these waves is indicated by Figure 4.6 (D). The total voltage at any point on the line varies as the individual waves move in opposite directions along the line. At time (t1), an instant slightly later than that for which Figure 4.6 (B) and (C) are drawn, the peak of the incident voltage wave is at the short circuit end, and the component voltages have equal magnitude and opposite polarity at each point on the line. Thus, the voltage would be zero everywhere on the line as indicated by the line marked t1 in Figure 4.6 (D).
RF Theory: Waveforms
4-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 At other instants, cancellation of the individual waves does not occur. For example, at time (t2) a quarter period later than time (t1) a zero value of the component waves in Figure 4.6 (B) and (C) is at the short circuit end, and the total voltage marked by the curve (t2) in Figure 4.6 (D) is a sine wave of twice the amplitude of either component wave. Looking at additional instants of time (t3) and (t4) a quarter period later, etc., is indicated by the curves (t3) and (t4) respectively in Figure 4.6 (D).
S RS
i e
A. Circuit Diagram
+
–
e wave
e wave
e
B. Traveling Waves of Voltage
+
–
i wave
i wave
i
C. Traveling Waves of Current
t2
+
2em
e
t1,t3 t4 +
2i m
t1 t2,t4 i t3
D. Standing Waves
Figure 4.6. Voltage and Current Waves on a Shorted-end Line The total voltage and current patterns of Figure 4.6 (D) are defined as standing waves. By performing additions at other intermediate times to the quarter period instants, it may be shown that the total voltage and current have a sine wave distribution along the line with zeroes at the short circuit and half-wave intervals from the short circuit. The amplitude and polarity of this sine wave varies as the voltage at each point changes sinusoidally with time. The points of zero voltage are called voltage nodes, and the points of maximum voltage are called antinodes.
4-10
RF Theory: Waveforms
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Referring back to Figure 4.1 on Page 4-5, it should be noted that a magnetic flux or field is associated with the inductance of a transmission line and that an electric field is associated with the capacitance of the line. Thus, the energy in terms of volts and amperes can be translated into units of electric field flux and magnetic field flux. The latter two conditions are to be found within an RF field that is propagated through free space.
4. Waveguides
A waveguide performs the same function at microwave frequencies as a two-wire transmission line does at lower RF frequencies. The waveguide can be considered to possess all the same principles of operation discussed in the preceding sections. However, the transmission of energy is dealt with in terms of the electric and magnetic fields rather than in terms of voltage and current. A special form of transmission line, called a parallel strip line and pictured in Figure 4.7, can be used to define the electric and magnetic field intensities related to the voltage and current waves on a transmission line. The line of Figure 4.7 (A) is composed of two parallel flat-plate conductors of width (w) and spacing (h). The line is considered perfect with no losses with zero resistance in the conductor plates and zero conductance in the dielectric (air) between the plates. Incident and reflected waves of voltage and current can be propagated along the line in a like manner to the two wire transmission line of Figure 4.1 on Page 4-5. Consider that sinusoidal waves are traveling along the line and let the voltage be defined between the plates and the current along the plates be denoted by e and i respectively. i (out) w
i e
h
e i
Direction of Propagation
(A) Pictorial Representation
h
i (in)
w
(B) Cross Section
Figure 4.7. Parallel Strip Lines The two conductors can be considered as plates of a capacitor. The charge on the capacitor appears on the inner surfaces of the plates, and an electric field is developed between the plates because of the voltage (e). The charge and field flux at any cross section point increases and decreases in phase with the voltage wave at that section. The pattern of the electric field is indicated by the solid arrows of Figure 4.7 (B). If (w) is much larger than (h), the electric field will be uniform except at the edges of the strip plates. The current in the conductor plates develops a magnetic field that encircles each conductor. The direction of the field is related to the direction of the instantaneous current on the conductor plates by the right-hand rule and is indicated by the dashed line arrows of Figure 4.7 (B). Because of the plane geometry of the conductors, the magnetic field is uniform in most of the region between the conductors. Because the conductors have no resistance, the current will flow entirely on the surface of the conductors. Thus, the magnetic field as well as the electric field is prevented from entering the conductors. Figure 4.8 shows the resultant waves in the strip line.
RF Theory: Waveguides
4-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
i e i
Direction of Propagation
(A) Pictorial Representation
e i
(B) Voltage and Current Waves
E H
(C) Electric and Magnetic Field Waves Figure 4.8. Waves on a Parallel Strip Transmission Line The waves in the parallel-strip line are a very simple example of many forms of electromagnetic waves that can take place. When the conducting boundaries have a more complex shape, a great variety of more complex field patterns can be generated. The waves of the parallel strip line may be described as fundamental or simple transverse linearly polarized waves. A wave of this type for which both the electric and magnetic field vectors lie in the transverse plane of the conductors (the transverse plane is the plane that is tangent to the conductor surface and perpendicular to the direction of propagation of the wave on the conductor) is defined as the TEM wave or mode. If only one field of vectors lies in the transverse plane the wave is either a transverse electric (TE) or transverse magnetic (TM) wave. It is possible to take the parallel strip line and develop a waveguide structure from it. If the frequency of the wave to be propagated along the strip line is high enough so that it has a half wavelength λ ⁄ 2 value that is equal to or less than the width of the plates, there will exist across the plates an electric field distribution as shown in Figure 4.9 on Page 4-13.
4-12
RF Theory: Waveguides
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
λ 2
λ 2
(A)
(B) Figure 4.9. Electric Field Distribution
The magnetic field will be as indicated before at right angles to the electric field as referenced in Figure 4.7 (B). If there is no voltage or electric field present at the sides of the plates, we could connect them as shown in Figure 4.10.
Figure 4.10. Electric and Magnetic Fields in a Closed Rectangular Waveguide Now the magnetic field will be contained within the walls of the structure. We now have a waveguide transmission line that has all the same characteristics and follows the same laws of propagation as a plane transmission line. It is possible to propagate several different types of electromagnetic waves within a waveguide. Each wave is characterized by a different electric and magnetic field configuration. Associated with each wave type is a cutoff frequency below which, for a particular size guide, the propagation becomes impossible. In general the larger the guide, the lower the cutoff frequency of each type of wave. The wave with the lowest cutoff frequency is defined as the dominant wave or mode. To transmit the dominant mode, a rectangular guide must have a width of at least half the free space wave length of that wave. The height is not critical and is usually made about one half the width. In waveguides, the electric and magnetic fields are confined to the space within the guide walls. Thus, all power is transmitted along the guide with no radiation nor dielectric losses of any practical importance. The electric field (voltage) is defined across the height or short dimension of the guide as in the parallel strip line and the magnetic field current is defined parallel to the opposite walls of the waveguide as identified by Figure 4.11 on Page 4-14.
RF Theory: Waveguides
4-13
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 8 2
A
Direction of propagation
A
(A) TOP VIEW
B
B (B) END VIEW SECTION B-B
(C) SIDE VIEW SECTION A-A
Figure 4.11. Electric and Magnetic Fields of the Dominant TE10 Mode in a Rectangular Waveguide Note that the letters “TE” define the electric field as transverse and the subscripts “1” and “0” indicate the number of half cycle variations of the electric field along the width and height dimensions of the guide in accordance with industry convention of identification.
5. Resonant Circuits
Circuits composed of lumped inductive and capacitive elements can be made to resonate at any desired frequency by selection of the values of inductance and capacitance used. To increase the frequency, the size of the inductance and capacitance must be made smaller. At extremely high frequencies the electrical size as well as the physical size of the components becomes so small that the stray inductance and capacitance of the circuit begins to affect the resonant frequency of the circuit. This results in different construction techniques being employed to raise the frequency higher. In the UHF region parallel wire or coaxial cable transmission lines are used in place of the lumped components. At microwave frequencies resonant cavities are used. All forms of resonant circuits, whether lumped components, transmission lines or cavities, have certain natural frequencies of oscillation. A natural frequency of oscillation is that which can be sustained by the circuit (assuming it has no losses) once started that will continue indefinitely even though there is no internal connection to a power source. Each natural frequency of oscillation is called a natural mode or resonant mode. The term “mode” indicates the manner of oscillation. Various modes may occur depending on the nature of the design and excitation parameters. Figure 4.12 on Page 4-15 shows a simple parallel resonant circuit consisting of a single inductor L1 and capacitor C1. Assume the capacitor C1 is charged before switch S1 is closed. When S1 is closed, the circuit will oscillate at some discrete frequency dependent on the fact that both L1 and C1 must have equal reactance. The frequency can be determined by: 1 f 0 = -----------------2π LC
4-14
RF Theory: Resonant Circuits
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
S1
L1
C1
Figure 4.12. Tuned Resonant Circuit Both voltage and current vary sinusoidally with time, and the mode pattern may, therefore, be described by specifying the phase and magnitude of the voltage and current during the oscillation. Because the reactance of L and C are equal at only one frequency, the circuit of Figure 4.12 has only one mode of oscillation. The circuit of Figure 4.13 has two natural modes of oscillation. (Note the circuit is drawn to resemble a two-section transmission line shorted at one end and open-circuited at the other).
L1
C1
L2
C2
Figure 4.13. Resonant Circuit Having Two Natural Frequencies Below the frequency (f0) at which L1 and C1 and also L2 and C2 resonate, the reactance of the parallel combination of L2 and C2 is inductive. The L2 and C2 combination in series with L1 may, therefore, be considered an inductance that resonates with C1 at some frequency less than (f0). Similarly, the L2 and C2 combination and C1 in series is less than C1 and resonates with L1 at a frequency greater than (f0). From the foregoing discussion a shorted transmission line can be considered to have many frequencies at which it could oscillate dependent on how it is excited. Resonant cavities can be considered as an extension of the transmission line into a single physical metal walled chamber fitted with suitable devices for admitting and extracting electromagnetic energy. One example of a resonator cavity is a rectangular box that may be thought of as a section of rectangular waveguide closed at both ends by conducting plates. Because the end plates appear as short circuits for a wave traveling along the waveguide, the cavity is analogous to a transmission line section with a short at both ends. Resonant modes will occur at frequencies for which the distance between the end plates is a multiple of half the guide wavelength. Higher order waves as well as the dominant wave may give rise to several resonant frequencies, and thus a great variety of resonant modes of operation are possible.
RF Theory: Resonant Circuits
4-15
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The cavity formed by closing the ends of a section of waveguide is only one of many cavity configurations that can be used to generate microwave energy. By appropriate choice of cavity shape, advantages such as ease of tuning, compactness, simple mode spectrum, and high Q may be obtained.
6. RF Transmission Theory
The RF waveguide used on our machines is called WR 284 or JAN RG 48/U. It has a usable frequency range (S Band) of 2.6 to 3.95 GHz in the TE10 mode and its dominant TE10 mode is 2.5 GHz.
2.875" I.D.
1.312" I.D.
Figure 4.14. WR 284 RF Waveguide RF waves travel 186,000 miles/sec or 300 × 106 meters/sec in air. Therefore, the wavelength (8) of one cycle of a particular frequency, measured crest to crest, is equal to: 6
Where:
300 × 10 λ = ----------------------f (in Hz) 300 = ------------------------f (in MHz)
8 = the wavelength of one cycle in meters. 300 × 106 = the distance in meters covered at a speed 186,000 miles/sec.
λ
λ 4
λ 2
λ 4
Figure 4.15. RF Waveform Divided into ¼-Wavelength Intervals To convert from length in meters to inches divide by 0.0254. To convert from length in inches to meters multiply by 0.0254.
4-16
RF Theory: RF Transmission Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF waves travel somewhat slower in waveguides or parallel lines, depending on material and conditions, as much as 15% slower. Therefore, the distance measured crest-to-crest will be shorter compared with the same frequency in air. Example: A quarter-wavelength ( λ ⁄ 4 ) of a particular frequency on any transmission line is calculated using the formula: × KL = 246 -----------------f
Where: L
= ( λ ⁄ 4 ) in feet.
K = Transmission line constant (1.000 for air, usually 0.975 for parallel line to 0.85 for air dielectric coax). f
7. RF Waveguide Design
= Frequency in MHz.
Figure 4.16 illustrates the design of a typical rectangular RF waveguide.
b
a Figure 4.16. RF Rectangular Waveguide Dimensions Dimension (a) is usually 1.3 × λ ⁄ 2 of the frequency to be transmitted, in air. The low frequency cutoff of the guide is roughly twice its width converted to wavelength (TE10 dominant). Dimension (b) is determined by the power to be generated and is usually 0.2 to 0.5 times the wavelength in air. Bends required in waveguides should be made so that the radius is no shorter than 2 wavelengths.
7.1. Modes
There are basically two methods of transmitting energy through a waveguide, the TE and TM mode. The TE mode is most commonly used because: !
It is easy to excite.
!
It is plane polarized.
!
It is each to match to a radiator.
!
Its cutoff frequency is dependent on only one guide dimension (a) therefore easy to design.
RF Theory: RF Waveguide Design
4-17
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In the TE mode, the electric field lies in transverse planes that contain the X and Y axes, and the E lines are parallel to the Y axis and perpendicular to the Z axis of the guide, as shown in Figure 4.17.
Magnetic lines E lines Figure 4.17. The Transverse Electric (TE) Mode The magnetic field is composed of closed loops that lie in the transverse plane that contains the X and Y axes and are wholly transverse to the guide's Z axis, as shown in Figure 4.18.
E lines
M lines Figure 4.18. The Transverse Magnetic (TM) Mode
7.2. The TE10 Mode
7.3. Coupling
The digits after the mode specify the following: 1.
The first digit states the number of λ ⁄ 2 variations in the wide or transverse part of the waveguide.
2.
The second digit states the number of λ ⁄ 2 variations in the narrow section of the waveguide.
There are two methods of extracting energy from a waveguide, loop and probe coupling. In loop coupling, a small loop is inserted into the guide that cuts the magnetic lines (H lines) and therefore acts as a transformer, as shown in Figure 4.19 on Page 4-19. In probe coupling, a small antenna is inserted into the guide parallel with the electric field (E lines), as shown in Figure 4.20 on Page 4-19.
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RF Theory: RF Waveguide Design
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.19. Loop Coupling
Figure 4.20. Probe Coupling
7.4. Determining the TE10 Dominant Mode of a Waveguide
a
a = 2.875"
Figure 4.21. Frequency Determining Parameters
RF Theory: RF Waveguide Design
4-19
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 λ --- = 2.875 inches 2 λ = 2 × 2.875 inches λ = 5.750 inches Λ (meters) = 0.0254 × 10
–2
× 5.750
–2
λ = 14.6 × 10 300 f (MHz) = -----------------------λ (meters) 2
3 × 10 f = --------------------------–1 1.46 × 10 3
f = 2.05 × 10 f = 2.05 GHz
The TE10 dominant frequency is 2.05 GHz.
8. Transmission Lines
Transmission lines can be classified as resonant or non-resonant. Resonant lines are used primarily for impedance matching, phase shifters, inverters, wave filters and chokes. Non-resonant lines are lines that are either infinitely long or terminated in its characteristic impedance. The voltage and current waves move in the same phase with each other and all of the energy is absorbed by the load.
Figure 4.22. Characteristic (Surge) Impedance Z 0 = characteristic impedance Z 1 = total inductive reactance of line = 2πfL 1 Z 2 = total capacitive reactance of line = ------------2πfC Z1 2 Z 0 = Z 1 Z 2 + ⎛ -----⎞ ⎝ 2⎠ 2
Z Where the term ⎛⎝ -----1⎞⎠ represents the number of sections; as this number apZ2
proaches infinity then this term will approach zero. The PFN in the Clinac is a transmission line, so let us calculate its impedance.
Ltotal = 36.5 µH
Ctotal = 18 µF Figure 4.23. PFN Parameters
4-20
RF Theory: Transmission Lines
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 As explained in Chapter 3 of this manual, the load resistor(s) on the despiking network can be used as a temporary load while troubleshooting the modulator. The load resistance, 12.5 ohms, would match the impedance of the generator (PFN line).
8.1. VSWR
VSWR is the ratio of the effective voltage at a loop to the effective voltage at a node. It is also equal to the ratio of the characteristic impedance of the line to the impedance of the load, or vice versa. To measure VSWR, a probe can be inserted in a slot cut in the waveguide that acts as an antenna and is excited by the E lines that flow parallel to it. Since the line current flows parallel to the slot, the effective resistance of the waveguide is not appreciably reduced by the presence of the slot.
9. Vector Analysis – 3dB Quadrature Hybrid
4.24 shows a vector analysis of the input and output signals of the 3dB quadrature hybrid. Port 1 Output Vi 2
Port 2 Output Vr 2
V1
Phase Shift
V1 – V2
0°
0V
90°
–2V
270°
+2V
180°
0V
V2
Vr (–90°)
Vi (–90°)
2
2
Vi 2
Vr (–90°) 2 V2
Vr (–180°)
Vi (–90°)
2
2
V1
Vi 2 V1
Vr 2
Vr (–270°)
Vi (–90°)
2
2
Vi 2 Vi (–90°)
Vr (–270°) 2
2 Vr (–180°)
V2
2
Vi = incident signal Vr = reflected signal
Figure 4.24. 3dB Quadrature Hybrid Vector Analysis
RF Theory: Vector Analysis – 3dB Quadrature Hybrid
4-21
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 When the RF Power first reaches the accelerator structure all of it is initially reflected back toward the source. Once its amplitude has stabilized, if its frequency is equal to the resonant frequency of the accelerator it begins to flow into and resonate within the accelerator. At the end of the pulse, as the amplitude begins to decrease, the power is again totally reflected. In order to separate the forward and reflected power, a circulator is employed.
10. Circulators
There are two types of circulators used in Varian Clinacs, 3-port and 4port. All Clinacs being manufactured as of this writing use 4-port circulators. Many older low-energy Clinacs use 3-port circulators, which are simpler in design. Figures 5.25 and 5.26 show the Low Energy Clinac 3-port circulator and 4-port circulator respectively.
To Water Load
From Water Load
RF Power Path
Magnet
Ferrite Beads
To Magnetron
From Accelerator
From Magnetron
To Accelerator
Top View
Cutaway View
Figure 4.25. 3-Port Circulator Magnets
Blocked off
1
4
3 2
From Magnetron
1 3
3
2 4
1
4 2
To Accelerator To Water Load
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
90° Phase Rotation
1
2
3
4 90° Phase Rotation
4
1
2
180° Phase Rotation NOTE: Circles indicate where phase rotation occurs
Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
Figure 4.26. Low Energy Clinac 4-Port Circulator and Phase Diagram (Configured as 3-Port)
4-22
RF Theory: Circulators
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Figure 4.27 shows the High Energy Clinac 4-port circulator used with the shunt tee power attenuator. In 1996, Varian eliminated the shunt tee and began operating the klystron in its linear mode, thus removing the need for the fourth port. On these machines, port 3 of the Magic Tee is blocked, and the RF power from the Klystron is applied to port 4 of the 3dB Quadrature Hybrid, as shown in Figure 4.28. Magnets
From Klystron
1
4
3
To Shunt Tee
1 3
2
3
2 4
1
4 2
To Accelerator To Water Load #2
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
4
90° Phase Rotation
1
2
3
90° Phase Rotation
4
1
2
180° Phase Rotation Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
NOTE: Circles indicate where phase rotation occurs
Figure 4.27. High Energy Clinac 4-Port Circulator and Phase Diagram (Used in Shunt Tee Clinacs) Magnets
Blocked off
1
4
3 2
From Klystron
1 3
3
2 4
1
4 2
To Accelerator To Water Load
3dB Magic Tee
3
Ferrite Phase Shifter
4
1
3dB Quadrature Hybrid
2
3
90° Phase Rotation
1
2
3
4 90° Phase Rotation
4
1
2
180° Phase Rotation NOTE: Circles indicate where phase rotation occurs
Drawn: Bill Kirkness, 05/96 Redrawn: Bill Kirkness, 10/03
Figure 4.28. High Energy Clinac 4-Port Circulator and Phase Diagram (Configured as 3-Port for KLM Clinacs)
RF Theory: Circulators
4-23
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11. Klystron Theory
This discussion is primarily for those engineers and technicians with little or no knowledge of microwaves who may find themselves suddenly confronted with operating and maintaining a high power microwave transmitter using klystron amplifier tubes. It is assumed that the reader is familiar with the theory and operation of triode RF amplifiers and general electronic circuit theory. We will discuss a simplified theory of operation for klystron amplifier tubes, the associated equipment required to make a klystron amplifier tube operate properly, some of the things that must be protected from malfunctions, and some generalized operating procedures associated with this type of equipment. After reading this information, the reader probably could not design a klystron amplifier; however, he should be able to understand better what is going on in the equipment and the reasons behind some of the operating instructions presented in a typical klystron amplifier Instruction Manual. Knowing and understanding this simple theory should help the operator to realize “why” certain functions are built into a klystron amplifier, and the “why” of certain operating instructions that he may receive. We feel that this understanding is important to a man who must operate or service a fairly complicated and expensive piece of electronic equipment. A little history: The klystron amplifier was in its infancy even at the end of World War II, although reflex klystrons had been developed (for use as local oscillators in radar receivers) to a fairly high degree of sophistication by the end of the war. Since World War II the klystron amplifier has undergone a spectacular evolution. It has become one of the most widely used devices for the amplification of microwave signals, particularly for high power applications. Klystron amplifiers currently in production cover microwave frequency ranges from UHF to 100 GHz, or higher; outputs range from a few milliwatts to many megawatts peak and more than 100 kilowatts average; power gains vary from 3 to 90 dB; and sizes vary from extremely small tubes that can be held easily in the palm of the hand to tubes that are more than 12 feet long. The uses of klystron amplifiers cover almost every microwave application, from low level signal generators, to giant radar equipment and huge transmitters for deep space communication and command. Some of the equipment is complicated and quite expensive. A serious shortage of engineers and technicians trained to operate and maintain this equipment has resulted from the rapidity with which this equipment has been developed and produced. Many engineers and technicians, experienced on lower frequency equipment using conventional vacuum tubes, have required retraining to operate this microwave equipment. We hope this information will help slightly in the training process.
11.1. Theory of Klystron Operation
The basic theory of klystron amplification is quite simple. In fact, the klystron amplification principle can be readily explained by an analogy with a simple triode RF amplifier. Obviously there are some differences (which will be explained), and these differences are what make a klystron amplify at microwave frequencies whereas a triode will not. First, let us consider the basic theory of operation of a simple triode amplifier. Figure 4.29 shows a simplified diagram of a triode amplifier with resonant circuits at both the input and the output. Such resonant circuits restrict the bandwidth of the amplifier and increase the gain. Such an amplifier might be part of an intermediate frequency amplifier circuit typically used at frequencies from 10 to 100 megacycles.
4-24
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF Output
Resonant grid circuit
RF Input Resonant Plate Circuit
Figure 4.29. Triode Vacuum Tube Amplifier A triode radio tube consists of three elements: a cathode that emits a stream of electrons, a grid that stands in the path of the stream, and a plate that attracts the electrons and catches them after they pass through the grid. The grid acts as a valve, opening or closing the passage of electrons according to the voltage on it. The RF input signal comes to the grid as a weak alternating current, oscillating at the RF frequency. The oscillating voltage thus applied to the grid modulates the flow of electrons across the tube at the RF frequency. The electron stream then delivers, at the plate, an alternating current that reproduces the weak signal on the grid with amplification. This alternating current at the plate flows through the resonant plate circuit and excites alternating voltages across it; these voltages constitute the RF output from the amplifier. Now the time it takes an electron to cross the tube is in the order of a billionth of a second. This transit time is short compared with the cycle of a long radio wave (around a millionth of a second); therefore the electron is slowed or speeded by the voltage on the grid at a given moment of the RF cycle. The flow of electrons, therefore, can “follow” the voltage fluctuations on the grid. In microwaves, however, the oscillations are so rapid (i.e., the cycle is so short) that the voltage on the grid may go through several complete oscillations while an electron travels across the tube. In other words, the grid voltage changes too fast and produces only chaos among the electrons. The grid voltage can no longer impose its signal pattern on the electron flow. There are other reasons why the conventional triode tube fails in the microwave range, but this is the most fundamental one the simple fact that the transit time of an electron from cathode to plate is long compared with the time of one cycle of the microwave signal. The klystron tube makes a virtue of the very thing that defeats the triode the transit time of the electrons. What it does is to “modulate” the velocity of electrons so that, as they travel through the tube, they sort themselves into groups and arrive at their destination in bunches. These bunches deliver an oscillating current to the output resonant circuit of the klystron. Figure 4.30 on Page 4-26 shows a cutaway representation of a typical klystron amplifier. Schematically it is very similar to a triode amplifier in that it includes an electron gun, resonant circuits, and a collector (which is roughly equivalent to the plate of a triode). In fact, the klystron amplifier consists of three separate sections the electron gun, the RF section and the collector section.
RF Theory: Klystron Theory
4-25
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Control Grid
Buncher Gap
RF Input
Bunch of Electrons
RF Output
Catcher Gap
Electrons
Collector
Heater
Cathode
Electron Gun
Anode
First Cavity (Buncher)
Drift Tube
RF Section
Intermediate Cavity
Last Cavity (Catcher)
Collector
Figure 4.30. Sectional View of a Klystron Let us consider, first, the electron gun structure: As in the triode, the electron gun consists of heater and cathode, a control grid (sometimes), and an anode. Electrons are emitted by the hot cathode surface and are drawn toward the anode that is operated at a positive potential with respect to the cathode. The electrons are formed into a small, dense beam by either electrostatic or magnetic focusing techniques, similar to the techniques used for beam formation in a cathode ray tube. In some klystron amplifiers a control grid is used to permit adjustment of the number of electrons that reach the anode region; this control grid may be used to turn the tube completely on or completely off in certain pulsed-amplifier applications. The electron beam is well formed by the time it reaches the anode. It passes through a hole in the anode, passes on to the RF section of the tube, and eventually the electrons are intercepted by the collector. The electrons are returned to the cathode through an external power supply (not shown on Figure 4.30). It is evident that the collector in the klystron acts much like the plate of a triode as far as collecting of the electrons is concerned. However, there is one important difference; the plate of a triode is normally connected in some fashion to the output RF circuit, whereas, in a klystron amplifier, the collector has no connection to the RF circuitry at all. From the above discussion it is apparent that the klystron amplifier, as far as the electron flow is concerned, is quite analogous to a “stretched-out” triode tube in that electrons are emitted by the cathode, controlled in number by the control grid, and collected eventually by the collector. Now let us consider the RF section of a klystron amplifier. This part of the tube is physically quite different from a triode amplifier. One of the major differences is in the physical configuration of the resonant circuit used in a klystron amplifier. The resonant circuit used with a triode oscillator, at lower frequencies, is generally composed of an inductance and a capacitor, while the resonant circuit used in a microwave tube is almost invariably a metal-enclosed chamber, known as a cavity resonator.
4-26
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A very crude analogy can be made between the resonant cavity and a conventional L-C resonant circuit. The gap in the cavity (see Figure 4.30) is roughly analogous to the capacitor in a conventional low frequency resonant circuit in that alternating voltages, at the RF frequencies, can be made to appear across the cavity gap. Circulating currents will flow between the two sides of the gap through the metal walls of the cavity, roughly analogous to the flow of RF current in the inductance of an L-C resonant circuit. Since RF voltages appear across the sides of the cavity gap it is apparent that an electric field will be present, oscillating at the RF frequency, between the two surfaces of the cavity gap. When a cavity is the correct size, it will resonate to microwaves of a certain frequency. The cavity can be tuned to various microwave frequencies by adjusting its size by some mechanical means. A crude analogy to the cavity resonator would be a glass goblet that resonates at a certain pitch depending on the level of the water in it, i.e., the size of the air cavity in the goblet. As shown in Figure 4.30, electrons pass through the cavity gaps in each resonator, and pass through cylindrical metal tubes between the various gaps. These metal tubes are called “drift tubes.” In a klystron amplifier the low-level RF input signal is coupled to the first resonator, which is called the “buncher” cavity. The signal may be coupled in through either a waveguide or a coaxial connection. The RF input signal will excite oscillating currents in the cavity walls, if the cavity is the correct size (that is, tuned to the right frequency). These oscillating currents will cause the alternate sides of the buncher gap to become first positive, and then negative, in potential at a frequency equal to the frequency of the RF input signal. Therefore, an electric field will appear across the buncher gap, alternating at the RF frequency. This electric field will, for half a cycle, be in a direction that will tend to speed up the electrons flowing through the gap; on the other half of the cycle the electric field will be in a direction that will tend to slow the electrons as they cross the buncher gap. This effect is called “velocity modulation,” and it is the mechanism that permits the klystron amplifier to operate at frequencies higher than the triode. After leaving the buncher gap, the electrons continue toward the collector in the drift tube region. Ignore for the moment the intermediate resonator shown in Figure 4.30, and let us consider the simple case of a two-cavity klystron amplifier. In the drift tube region the electrons that have been speeded up by the electric field in the buncher gap will tend to overtake those electrons that have previously been slowed (by the preceding half of the RF wave across the buncher gap). It is apparent that, since some electrons are tending to overtake other electrons, clumps or “bunches” of electrons will be formed in the drift tube region. If the average velocity of the electron stream is correct, as determined by the original voltage between anode and cathode, and if the length of the drift tube is proper, these “bunches” of electrons will be quite completely formed by the time they reach the catcher gap of the last cavity (which is called the “catcher”). This results in bunches of electrons flowing through the catcher gap periodically, and during the time between these bunches relatively fewer electrons flow through the catcher gap. The time between arrival of bunches of electrons is equal to the time of one cycle of the RF input signal. These bunches of electrons will induce alternating current flow in the metal walls of the catcher cavity as they pass through the catcher gap. If the catcher cavity is of correct size (tuned to the proper frequency) large oscillating currents will be generated in its walls. These currents cause electric
RF Theory: Klystron Theory
4-27
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 fields to exist, at the RF frequency, within the catcher cavity. These electric fields can be coupled from the cavity (to the output waveguide or coaxial transmission lines) resulting in the RF output from the tube. It is not particularly obvious why a bunch of electrons, passing through the catcher gap, should generate an oscillating RF current in the walls of the catcher cavity. Fortunately, a qualitative explanation is easy to understand. Refer to Figure 4.31 which shows the catcher cavity at three instants of time as a bunch of electrons flow across the catcher gap. The electrons are shown passing the catcher gap, traveling from left to right. To simplify the explanation, we have shown grid wires across the gaps; the grid on the left side of the gap is labeled No. 1, while the grid on the right is labeled No. 2. Since the grid wires, and the cavity walls, are made of high-conductivity metal, such as copper, a large number of free electrons will be present in the metal. In Figure 4.31, as the bunch of electrons approaches Grid No. 1 the free electrons in Grid No. 1 will be repelled since negative charges repel each other. This will tend to cause these electrons to flow from the grid wires into the cavity walls and around the cavity walls toward Grid No. 2. This is shown by the current flow path in Figure 4.31A. The result is that Grid No. 2 will tend to accumulate a surplus of negative charges, whereas Grid No. 1 will have a scarcity of negative charges present. Figure 4.31B shows the instant when the bunch of electrons is between Grids 1 and 2. At this instant the electrons in the bunch are repelling free electrons in both grids equally, and the net current flow around the cavity walls is essentially zero. Direction of Electron Flow
Direction of Electron Flow Electron Bunch
Grid 1
Grid 2
A
Grid 1
Grid 2
B
Grid 1
Grid 2
C
Figure 4.31. Generation of Alternating Current in a Cavity Figure 4.31C shows the instant just after the electron bunch has passed to the right of Grid No. 2. Remember that Grid No. 2 has accumulated an excess of free electrons already and these free electrons would tend to redistribute themselves back toward Grid No. 1 even if the electron bunch was not present. However, the electron bunch further repels the excess free electrons in Grid No. 2 and tends to “push” these free electrons back toward Grid No. 1. As the electron bunch moves further to the right, the electrons will redistribute themselves to essentially an equilibrium condition during the time “between bunches.” The process of course repeats every cycle of the RF wave because a bunch of electrons comes past the catcher gap in a time equal to the interval of one cycle of the RF wave. Since the resonant cavity is a high-Q circuit the oscillating currents tend to be essentially sinusoidal although the bunches of electrons arrive in short bursts. The situation is quite analogous to striking a pendulum one blow for each cycle of its oscillation; this will cause the pendulum oscillation to build up although the driving force is not continuously applied. Another analogy is a Class C triode amplifier where bursts of
4-28
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 current generate essentially sinusoidal voltages in the plate resonant circuit. RF power can be taken from the output (catcher) cavity by coupling to the oscillating current flowing in the cavity walls (or to the electric fields inside the cavity which are generated by these oscillating currents). If the amplifier is functioning properly, the oscillating current in the catcher cavity will be considerably larger than the oscillating currents in the buncher cavity; consequently, amplification has taken place. When the bunches of electrons pass through the output gap in the catcher cavity, they deliver energy to this cavity which causes currents to flow in the cavity walls. Since the electron beam is delivering energy to the cavity, it is slowed in velocity; therefore the beam arrives at the collector with less total energy than it had when it passed through the input cavity. This difference in electron beam energy is approximately equal to the RF energy delivered from the output of the cavity. It is appropriate to mention here that the velocity modulation effect does not form “perfect” bunches of electrons. There are some electrons which come through “out-of-phase.” These electrons show up at the last gap between the bunches. The electric field, at the time these out-of-phase electrons come through, is in a direction to accelerate them; so some few electrons will actually have their velocity increased as they come through the output gap. The electrons reaching the collector therefore have a wide spread of energy. Some of them (the out-of-phase electrons) may have velocities almost twice as high as the average electron velocity; other electrons (the “in-phase,” useful electrons) will be materially slowed and will arrive at the collector with a velocity much less than they started with. In the previous discussion we have considered only a two-cavity klystron amplifier, having neglected the intermediate cavity shown on Figure 4.30. Klystron amplifiers have been built (to our knowledge) with as many as seven cavities, i.e., with five intermediate cavities. The effect of the Intermediate cavities is to improve the bunching process; the result is to increase amplifier gain, and to a lesser extent, the amplifier efficiency. Adding more intermediate cavities is roughly analogous to adding more stages to an I-f amplifier, i.e., the gain of the overall amplifier is increased, and the overall bandwidth is reduced, if all stages are tuned to the same frequency. The same effect occurs with the klystron amplifier. However, it is well known that the bandwidth can be increased, and the gain reduced, by staggertuning an I-f amplifier. This analogy carries over to the klystron amplifier. A given klystron amplifier tube will deliver high gain and narrow bandwidth if all the cavities are tuned to the same frequency; this is called “synchronous-tuning.” If the cavities are tuned to different frequencies the gain of the klystron amplifier will be reduced and the bandwidth may be appreciably increased; this is called “stagger-tuning.” Most klystrons which feature relatively wide bandwidth are stagger-tuned. The appropriate method of accomplishing stagger tuning is discussed in more detail later.
RF Theory: Klystron Theory
4-29
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.32. High-Power Four-Cavity Klystron
4-30
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Collector Output Window Water Circuit
Collector Pole Piece Output Iris Output Cavity (Catcher)
Tuning Diaphragm
Electron Bunch
Water Circuit
Third Cavity Magnetic Circuit
Second Cavity
Drift Tube
Input Cavity (Buncher)
Focus Coils
Input Loop
Anode Pole Piece Anode Heater
Figure 4.33. High-Power Four-Cavity Klystron, Simplified
RF Theory: Klystron Theory
4-31
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The klystron is not a “perfect” linear amplifier; that is, the RF power output is not linearly related to the RF power input at all operating levels. Another way of stating this is that the klystron amplifier will “saturate,” just as a triode amplifier will “limit” if the input signal becomes too large. In fact, if the RF input is increased to levels above saturation, the RF power output will actually decrease. Figure 4.34 shows the plot of typical klystron amplifier performance for various tuning conditions. The RF output is plotted as a function of the RF input. Curve A of Figure 4.34 shows typical performance for synchronous tuning. Under these conditions the tube has maximum gain. The power output is almost perfectly linear, with respect to the power input, up to about 70 per cent of saturation. However, as the RF input is increased beyond that point, the gain decreases and the tube saturates. As the RF input is increased beyond saturation, the RF output decreases. The reason for this decrease in output is quite interesting. Remember, in our previous discussion, that the electron bunches were formed by the action of the RF voltage across the buncher cavity gap. This RF voltage speeded up some electrons and slowed other electrons, resulting in formation of bunches in the drift tube region. Obviously this speeding up and slowing effect will be increased as the RF drive power is increased. The saturation point on Figure 4.34 is reached when the bunches are most perfectly formed at the instant they reach the output (catcher) gap. This results in the maximum power output condition. When the RF input is increased beyond this point, the bunches are most perfectly formed before they reach the output gap, i.e., they form too soon in the drift tube region. By the time the bunches have reached the output gap they tend to “debunch” because of the mutual repulsion of the electrons, and because the faster electrons have overtaken and passed the slower electrons. This causes the power output to decrease.
Envelope of Saturation Peaks Saturation Point
RF Output
A
B C D
RF Input
Figure 4.34. Effect of Tuning on Klystron Performance
4-32
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Curves B, C, and D illustrate a phenomenon of klystron amplifiers which is difficult to explain theoretically, but which should be recognized by personnel operating these amplifiers. It turns out that, if we start with a multicavity klystron which is synchronously tuned, and then tune the next-tothe-last cavity to a higher frequency, we find that the gain of the amplifier is reduced but that the saturation power output level may be increased. This effect is shown by curves B and C. Curve B represents a small amount of detuning of the next-to-the-last cavity, and curve C represents even more detuning of that cavity. Note that the gain of the tube has been reduced (it takes more RF input to obtain a given RF output), and that the saturation output power is higher than obtained with synchronous-tuning (curve A). As stated previously, this stagger-tuning also results in a wider bandwidth for the amplifier. Many klystron amplifiers are operated in this fashion because it enables one to obtain more power output, with the same beam power input, and therefore increases the efficiency of the tube; of course this can only be done if enough RF drive power is available to operate under the stagger-tuned condition. As one might intuitively expect, we can go too far with this stagger-tuning, and the saturation output will eventually drop. This is illustrated by curve D of Figure 4.34. Figure 4.30 does not show one very important item which is usually required for high power klystron amplifier operation. This is an axial magnetic field, i.e., one which is parallel to the center line of the klystron. In klystron amplifiers which are physically “long” it is quite difficult to keep the electron beam formed properly during its travel through the RF section. Since electrons are negatively charged particles, they tend to repel each other; this causes the beam to “spread” in a direction perpendicular to the axis of the tube. If this occurs, the electrons will strike the drift tubes and be collected there, rather than passing through the drift tubes to the collector. To overcome this beam spreading an axial magnetic field is used. The action in the magnetic field is to exert a force on the electrons to keep them going in the correct direction during their transit through the RF section. The magnetic field may be developed by a permanent magnet or by one, or more, electromagnet coils. A permanent magnet is generally used on tubes which are physically small or of medium power rating. Unfortunately, the size and weight of a permanent magnet become excessive for long or high power tubes, making it necessary to use electromagnets. In some large tubes several, separate, magnet coils are used; the current in each coil is individually adjustable to optimize the magnetic field shape. The magnetic field is normally terminated as quickly as possible after the catcher cavity so that the beam can spread before it hits the collector. This tends to spread the electron beam interception over a large surface on the collector; this minimizes collector-cooling problems which would result if the beam remained concentrated at the time of interception. Even with an axial magnetic field some electrons will go “astray” and not remain in the main electron beam. These electrons will be intercepted by the anode or the klystron drift tubes. In high power tubes is it particularly important to minimize the number of these stray electrons because they generate heat when they strike the drift tubes. In high-power klystrons this heating can be a very severe problem because drift tubes are difficult to cool. Temperatures can become high enough to melt the metal in the drift tubes and destroy the tube. The collector is normally insulated from the RF section of large klystron amplifiers to permit separate metering of the electrons intercepted by the drift tubes, and those intercepted by the collector. The e1ectrons intercepted by the RF section are normally called “body current,” while those elec-
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 trons intercepted by the collector are normally called “collector current.” Obviously, the sum of the body current and the collector current is equal to the total current in the electron beam which is normally called “beam current.” Klystron amplifier specifications will quite often place a maximum limit on allowable body current. The previous discussion (describing the general theory of klystron operation) implied that klystron amplifiers normally have actual metal grid structures across the gaps in the resonant cavities. Many low power klystrons do indeed have wire-mesh grids. However, most high-power klystrons do not have actual grids across the gaps, because such grids would intercept sizable amounts of the electron beam. It is very difficult to cool grid structures, and large beam interception would cause the grids to melt, destroying the tube. Fortunately, by proper design, the klystron can be made to work efficiently without actual grid wires across the gaps. The absence of these grids does not change the operating principles discussed previously, but it does have a secondary effect on the klystron performance. It turns out that, if the electron beam has a very small diameter compared with the size of the drift tubes, the beam does not “couple” strongly to the gaps and therefore it does not react as strongly with the klystron cavity. Therefore, the performance of a klystron amplifier, which does not have gridded gaps, can sometimes be improved by permitting the electron beam to be as large as possible (while keeping the body current down to the maximum specified for the tube). The size of the beam can be somewhat controlled by the magnetic field strength. We therefore find that the klystron performance can sometimes be improved by adjusting the magnetic field in a way which does not result in the minimum possible body current condition, i.e., by adjusting the field so that the beam shape is somewhat larger than the minimum obtainable. In gridless-gap klystrons therefore, best operation may be obtained with a body current which is not the minimum obtainable; however, body current must be kept within the maximum specified for the tube. Body current usually increases with RF input level which might be expected since RF causes electron bunches to form. The dense electron concentration in the bunch causes the electrons to repel each other, and the diameter of the bunch may become larger than the diameter of the beam without the bunches. Consequently, some of the electrons in the bunch may be lost to the drift tubes, and the body current may increase.
11.2. Associated Equipment
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In the preceding sections, we have discussed the basic theory of operation of the klystron amplifier tube. Considerable additional equipment is required for a complete amplifier system. First, we will need power supplies to deliver the voltages and currents required for the klystron and for the electromagnets. In high-power systems we will need various types of cooling to get rid of the power supply energy which is not converted into RF output power. We will need various RF circuit components to control and measure the RF input to the klystron tube, and to measure the RF output from the tube. For testing we may need a dummy load to dissipate RF output when it may be inconvenient, or impossible, to radiate. We will need a large collection of meters and protective devices to monitor performance and to protect operating personnel (and the equipment itself) in case of equipment malfunction or operator error. This associated equipment will be discussed in this section.
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
11.3. Power Supplies
Figure 4.35 is a simplified diagram showing the power supplies used in a typical klystron amplifier. In most klystron tubes the anode and RF section of the tube are connected inside the vacuum envelope. These parts are normally called the tube “body,” and they are generally operated at ground potential as shown in Figure 4.35. Operating the tube body at ground is convenient because the input and output connections (either waveguide or coaxial) are then at ground potential; this makes it easy to connect into the rest of the system. Also, this keeps the cavity tuners at ground potential, eliminating any danger to personnel who are tuning the tube. The beam power supply, shown in Figure 4.35, generates the voltage required to accelerate the electrons and form the electron beam. It must also deliver the beam current required for the klystron tube itself. As shown, the positive end of the beam supply operates at (nearly) ground potential, whereas the negative output from the supply is the high potential point in the system. RF Input
RF Output
Cathode
Collector
HEATER SUPPLY
Current Limiting Resistors
Crowbar
GRID SUPPLY OR PULSER
ELECTROMAGNET SUPPLIES
Body Current Meter
BEAM SUPPLY Beam Current Meter
Beam Current Overload
Body Current Overload
Collector Current Meter
Direction of Electron Flow
Figure 4.35. Klystron Power Supply Connections The design details for beam power supplies vary widely depending upon the application of the power amplifier. However, in general, they employ fairly conventional circuits. They usually include means of adjusting the ac voltage to the primary of the power transformer, either an auto-transformer (such as a Variac), an Inductrol, or perhaps an ac generator whose output is varied by adjusting the dc field control. Beam supplies incorporate a step-up transformer, a rectifier circuit, and an LC filter. Either solid-state, hardtube, or gaseous diodes are used in the rectifier circuit. Tube rectifiers normally have a lower initial cost; however, they require periodic replacement. Solid-state rectifiers, particularly for high voltage and high current, are usually more expensive initially; however, their reliability is excellent after the initial design and debugging. Filter design is quite conventional; the amount of filtering depends upon the allowable ripple for the system. In some special cases, extremely low ripple and extremely good beam voltage regulation is required. For low- and medium-power systems this can often be achieved by electronic regulation of the dc output voltage. The circuits are, technically, fairly conventional, but they may become quite complicated and expensive for the mediumpower systems.
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 For extremely high-power systems electronic regulation has not proven practical to date. For these systems it is normal to obtain the primary power from a motor generator set. The inertia of the large MG set effectively “smooths out” variations in the incoming line voltage; and the ac output from the generator can be quite easily regulated by a feedback system to the generator field. Brute-force filtering is used to achieve the allowable ripple. Beam voltage and beam current metering are always provided, as well as beam current overload protection. Some systems have beam over-voltage protection to protect the tube against the possibility of power-supply runaway (rare), and against operator error (more common). Since the beam supply is the source of most of the energy in the system, it is usually turned off when any malfunction occurs in the system; this will be discussed in more detail later. A variable voltage beam supply is usually provided so the tube can be operated at whatever power level is desired (within maximum ratings). It is also desirable to have low beam voltage capability; this is useful when a new tube is installed and initial adjustments are being made. Some systems have a feature that automatically starts the beam voltage at a low value when the supply is first turned on; the voltage then slowly increases until it reaches a preset level; and it may regulate to that level for changes in ac line voltage. The voltage automatically runs down to a low level when the supply is turned off. For high power systems it is normal to have some series resistance between the beam supply and the klystron cathode; this limits the tube current to some finite value in case the tube should arc from cathode to ground. Without some limiting resistance the peak current during an arc could be very high and might destroy the cathode surface; with current limiting resistance a tube can often be “cleaned up” even if it is somewhat gassy or if it arcs on initial turn-on. Most klystron amplifiers include a “getter,” and some include a VacIon vacuum pump. A getter will absorb a limited amount of gas that may accumulate after long storage periods, and therefore may permit a slightly-gassy tube to clean up and be perfectly serviceable (if the tube is not damaged during initial start-up). The VacIon pump operates continuously and will absorb a tremendous amount of gas and may allow a tube to continue in service for its normal life even if it has a small vacuum “leak.” Some high power amplifiers use a “crowbar” system to discharge the beam supply very quickly in case of an internal klystron arc, or other high-voltage fault condition. Most crowbar systems consist of a triggered spark gap connected across the power supply. Circuits are provided which will trigger the gap in case of excessive body current, arcs in the output waveguide (to be discussed later), and (sometimes) loss of magnetic field. When the trigger gap is fired, the main spark gap breaks down. This discharges the energy stored in the beam power supply very quickly and prevents damage to the klystron or associated equipment. Crowbars are normally used only on very high voltage, or very high-power systems where the amount of stored energy could cause serious damage. They are normally not used in equipment operating at less than 20-kilowatts average-power; those systems can be adequately protected by simple current-limiting resistors (which are much less complicated and much less expensive than a crowbar system). A crowbar system will normally operate in a few microseconds and therefore will limit the amount of destructive energy delivered to the tube to a very low value. Conversely, it may take several seconds to turn off and discharge a
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 large beam power supply because of the long time required for overload relays and ac contactors and filter capacitors to discharge. The heater supply furnishes power for the klystron heater, which heats the cathode that emits the electrons for the electron beam. Most klystrons have an “indirectly-heated” cathode, i.e., the cathode is heated simply by being close to the heater windings. A few klystrons have “bombarded” cathodes. In tubes of this type a voltage is applied between the heater and the cathode (with the cathode positive). The heater and cathode then function as a conventional diode. The heater becomes hot enough to emit electrons. These electrons are drawn to the cathode by the “bombarder” voltage. When they strike the cathode they liberate their energy (to the cathode) as heat, just as the plate of a diode is heated by the electrons striking it. Bombarded cathodes have been largely displaced by recently developed “impregnated” cathodes, although a few bombarded-cathode tubes are still in production. The heater supply is normally a rather simple unit. It may deliver either alternating or direct current. For many applications, an ac supply is adequate; it consists simply of a variable auto-transformer, a step-down transformer, and appropriate voltage and current metering. In a few systems that require extremely low-noise performance, dc supplies are necessary. The heater supply must be insulated to withstand the full (negative) beam voltage potential. Meters are normally used to show the heater voltage or current or both. Since these meters must operate at a high negative potential, they may sometimes give incorrect readings due to the large electrostatic fields present; special care must be taken in the design of metering circuits to prevent these false indications. Heater voltages are normally adjustable to take care of individual tube-to-tube variations and compensate for variation of incoming ac line voltage. A normal klystron heater presents very nearly a short-circuit to the power supply when the heater is cold (first turned on). Therefore, it is normal to use some type of current limiting in the heater power supply. Many klystron specifications require that this initial “surge” current be limited to 150 per cent of normal operating current. Protective circuits are often used to turn off the beam power supply if the heater supply fails, since some tubes will be damaged if the beam voltage is “on” while the heater and cathode are cooling after a heater supply failure. Some high power klystrons require that the cathode assembly be cooled by an air blower. An airflow protective interlock is normally included to turn off the heater and beam voltage supplies if the klystron blower ceases to operate. The entire “electron gun” section of some very-high-voltage tubes is immersed in oil, for insulation and cooling. Some klystron amplifiers have a grid (or modulating-anode, which performs the same function) to control the number of electrons in the electron beam. Such grids are often used in pulsed systems to turn the tube either full-on or full-off; a few systems employ grid modulation for transmission of intelligence. In most gridded klystron tubes the grid is never allowed to go positive with respect to the cathode, as this might cause undue grid interception and result in burnout of the grid element. A grid power supply is required in those tubes that have grids. These power supplies and pulsers may take many forms depending upon the system application and will not be discussed in detail. It is important to note, however, that the grid power supply must be insulated for the full beam voltage. Fortunately, most klystron amplifiers designed for communication service do not use grids. The collector of most high power klystrons is insulated from the body of the tube. This allows separate metering and overload protection for the body
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 current and for the collector current which would be impossible if the collector and the body were connected internally. In most systems the collector and body operate at very nearly the same potential; any potential difference is normally only the difference in voltage drop across the various metering circuits. Figure 4.35 shows three electromagnet coils, apparently wrapped around the body of the klystron. Some klystrons are indeed made with the electromagnet coils physically a part of the tube itself. However, in most systems the electromagnet coils are separate from the tube, and the klystron is inserted into the electromagnet structure. In Varian klystrons the electromagnet is designed physically to support and center the klystron tube in the correct position; no physical adjustment of the electromagnet coils is provided or required for correct operation. Many modern klystron amplifiers have only one electromagnet coil and therefore require only one power supply; others may have as many as six separate coils, requiring one power supply for each coil. Electromagnet power supplies are usually simple. They are dc supplies using conventional rectifying techniques. Voltage variation is normally accomplished with an auto-transformer on the input. The supplies are usually well filtered so that the output current contains relatively small ripple components. Ripple on the electromagnet current may cause the electron beam in the klystron to “wander” slightly, at the ripple frequency; this can cause undesirable amplitude and phase modulation of the RF output signal. Voltage and current metering is normally supplied for each of the electromagnet power supplies. If an electromagnet power supply should fail, the electron beam would almost certainly spread, and the total beam current would be intercepted on a small section of the drift tube. In most cases, this would cause the drift tube to melt and permanently destroy the tube. Therefore, klystron amplifier equipment normally has under-current protection in each of the electromagnet coil circuits. When the magnet current falls below a predetermined level, the beam supply is turned off to prevent damage to the klystron. Redundant protection is provided by the bodycurrent overload circuits, which also turn off the beam supply in case of magnet current failure or misadjustment. Figure 4.35 shows the method normally used to monitor body current, collector current, and beam current separately; the diagram shows the most frequent arrangement where the klystron body operates at ground potential. In many systems separate monitoring of collector current is not done since the collector current and total beam current are normally almost equal. It is quite unusual, in a relatively high-power klystron amplifier system, to allow the body current to exceed 10 per cent of the beam current, because high body current usually means low efficiency and increases the danger of burning out drift tubes in the klystron. In very-high-power klystrons the body current is often limited to 1 or 2 per cent of the total beam current. Over-current protection is almost always supplied both for body current and beam current. If a tube arcs internally, the arc will always occur between cathode and anode. When this occurs the body current immediately becomes excessive, tripping out the body current overload relay. If an arc occurs, the beam current is also much higher than normal, and the beam current overload will also trip out. In fact, almost any high-voltage system fault (such as an insulation breakdown from high voltage to ground) will cause excessive current through the body current meter and overload relay.
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Because of the possibility of extremely high currents flowing under fault conditions, the protection of the body current and beam current meters (from burnout) presents a somewhat difficult problem. This problem is normally solved by using very high-current solid-state rectifiers, connected back-to-back, across the meters. Sometimes adding a small resistance or inductance in series with the meter is necessary. Surge capacitors are normally placed across the combination. Connecting the rectifiers back-toback is necessary because fault conditions often cause oscillating currents to flow through the meters.
11.4. Cooling
Most low power klystron amplifiers are air cooled, while all high power k1ystron amplifiers are liquid cooled. At the present state-of-the-art, air cooling can be used up to RF output levels of about one kilowatt, CW. However, we find a few special cases where liquid cooling is employed with tubes having a power output as low as 10 watts; these tubes are used in special applications that are beyond the scope of this bulletin. Remember that the main source of power (and therefore heat) in a klystron amplifier package is the beam power supply. The power generated by the beam supply must go somewhere; part of it is converted to RF power; the remainder eventually shows up as heating somewhere in the klystron. The klystron cooling must be adequate to handle the entire beam power because, if no RF output is being generated (either due to low RF input power, or detuning of the klystron tube) then all of the beam power is dissipated in heat somewhere within the tube. As discussed previously, most of the electrons in the beam eventually end in the collector. When they strike the collector, their energy is dissipated and turned into heat. The small fraction of the beam lost to the drift tubes also generates heat. Klystron amplifiers are normally somewhere between 30 and 50 per cent efficient. It is obvious, therefore, that a tube rated at 10 kilowatts output must be designed to dissipate between 20 and 33 kilowatts depending upon its efficiency. A tube rated at 100 kilowatts must be able to get rid of about 250 kilowatts as heat. It is obvious therefore that very advanced cooling techniques are necessary. The power levels involved can melt a hole in the drift tube, or in the collector, in a small fraction of a second if the cooling system fails and adequate protective devices are not provided. There are other but smaller sources of heat in a klystron amplifier system. The heater must be hot to heat the cathode for electron emission. This heat will be conducted and radiated to the exterior surfaces of the electron gun assembly, and must be dissipated. Large tubes require a blower on the electron gun assembly to get rid of this heat. The electromagnet will generate a considerable amount of heat; the power generated by the focus coil power supply is all dissipated in the electromagnet. Large electromagnets are almost always liquid cooled. If the cooling liquid for the electromagnet fails for any reason, the focus coil power supply must be shut off quite soon or the magnet will burn out; the beam voltage must also be removed (preferably before turning off focus coil supply) to protect the tube against excessive body current, as discussed previously. Earlier in this discussion we described how electron currents oscillate back and forth in the metal walls of the resonant cavities. Although these cavities are made with very high-conductivity metal (usually copper), the metal does present a finite resistance to these oscillating currents; therefore, heat will be generated in the cavity walls. The amount of heat generated can be quite sizable in high-power, high-frequency tubes. For instance, consider the case of a 20-kilowatt CW, X-band klystron amplifier. In this tube, ap-
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 proximately 1 kilowatt of heat is generated by the circulating RF currents in the output cavity. Since the cavity is approximately a 1-inch cube, it is apparent that removing this kilowatt of heat is a formidable problem. Cooling the cavity tuners is particularly difficult. Tuners normally incorporate some type of metal bellows arrangement, to permit changing the cavity size and still maintain the vacuum envelope of the tube. Metal bellows are thin structures, normally more lossy than the remainder of the cavity walls; therefore, a large amount of heat is often generated in the tuner assembly. Removal of this heat is a serious problem in high power tubes, and water cooling is invariably necessary. Another problem associated with cavity heating is not immediately apparent. Remember that the resonant frequency of the cavity depends upon its physical size. The cavities are made of metal that expands as it gets hot; this effect tends to change the resonant frequency of the cavity and to detune the tube. As the tube detunes, the power output will drop; then the RF heating decreases and the tube will tend to come back “in tune.” If this problem was not considered in the initial tube design, it would be quite possible to design a tube that would never “settle down”; it would be continually unstable in its operation. This situation indeed exists in some tubes which use “external cavities.” These external cavities are cooled by air rather than by liquid, and the cavity tuning is seriously affected by the ambient air temperature. All high-power Varian klystrons are liquidcooled, including the cavities and the tuners. The cavities are maintained at a stable temperature by controlling the temperature of the cooling liquid, and “thermal-detuning” is no problem. Drift tube heating is a serious problem in very high-power klystrons, and in medium-power, high-frequency, klystrons. The drift tubes that are inside the vacuum envelope are physically quite small, and it is difficult to remove the heat by conduction to the region outside the vacuum envelope. In some high-power tubes, it is actually necessary to bring the cooling liquid inside the vacuum envelope, and around the drift tubes, to remove the heat from the drift tubes. In recent years it has become necessary to cool the waveguide in some high-power, high-frequency systems. RF currents circulate in a waveguide that is carrying power, just as in the cavity walls of the klystron. An X-band waveguide carrying 5 kilowatts CW becomes too hot to touch in normal ambient air. Fortunately, the waveguide can be cooled easily by soldering copper tubing along the sides of the waveguide and running cooling liquid through the tubing. Most klystron amplifiers have a dummy load to dissipate the RF power during adjustment and test (when it may be undesirable to radiate). All highpower dummy loads are cooled, usually by liquid. In many loads the RF energy is dissipated in the cooling liquid itself, since water, oil, and ethyleneglycol (the normal cooling liquids) are quite lossy at microwave frequencies. In other types of dummy loads the RF energy may be dissipated in a solid, lossy material. Some of these lossy-material loads can be cooled by air blasts (for low- and medium-power applications); higher power versions are liquid cooled. We have discussed the various sources of heat in a klystron amplifier system, to impress the reader with the fact that an expensive klystron can be destroyed in a matter of seconds if the cooling system fails. A well-designed system uses many protective devices to prevent this from happening. The moral is: Check the operation of these protective devices periodically, and never “short-circuit” the protective interlocks.
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Systems that use blowers for cooling will usually have an airflow switch. If the blower fails, the switch will open and remove power from the appropriate power supplies. Systems employing liquid cooling normally distribute the liquid into several paths, since the flow requirements are quite dissimilar. A well-designed amplifier system will have a low-flow interlock in each of the various paths. If one liquid-cooling circuit becomes plugged, the flow interlock will open and remove power from the system. Liquid-cooling systems also include pressure gauges and pressure switches, temperature gauges and over-temperature switches. Many systems have pressure or flow regulators. Some systems include devices that will sound an alarm before trouble actually occurs; sometimes the situation can be corrected without shutting down the equipment. In a liquid-cooled system, it is obviously necessary to pump the cooling liquid through the various parts of the tube, and the other equipment that is generating heat. The liquid becomes hotter as it is pumped through these channels, and it is then necessary to get rid of the heat that has gone into the liquid. Some type of heat exchanger is required. Most systems use a liquid-to-air heat exchanger, which consists of a radiator, and a blower which blows air through the radiator this system is very similar to that on an automobile. The hot liquid passes through the radiator and heats the radiator surface. The air blows across the radiator surface and removes the heat from the radiator. Therefore, the liquid that exits from the radiator is cooler than the liquid that entered the radiator. In some other systems, primarily those used on shipboard, a liquid-to-liquid heat exchanger may be used. In this device, two liquid cooling paths are involved. One path carries the coolant pumped through the klystron amplifier. The other path may carry sea water. Heat is transferred from the klystron cooling liquid to the sea water, and the sea water is dumped back into the ocean. Liquid-to-liquid heat exchangers are smaller than liquid-to-air heat exchangers, and they are also quieter, since no blower is required. Refer now to Figure 4.36, a diagram of a typical klystron amplifier liquidcooling system. The right half shows the method of distributing the cooling liquid to each of the individual channels. The cool liquid enters the “highpressure manifold” at the top of the drawing. From the high-pressure manifold, the liquid passes through valves used to adjust the flow (in each individual channel) to the desired level. The liquid then passes through the component of the system that requires cooling, such as the klystron collector, body, etc. Flow meters are normally incorporated in each of the individual channels to monitor the flow and to make it easy to adjust the flow to the desired level. Liquid flow-switches are placed in each individual channel, so that the appropriate power supplies will be turned off if the flow in that channel falls below the minimum required level. The liquid then goes through shutoff valves into the “low-pressure manifold.” Both manifolds are often fitted with temperature gauges and pressure gauges, over-temperature interlocks, and over-pressure interlocks.
RF Theory: Klystron Theory
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De-ionizer
By-pass Line
Heat Exchanger (Radiator)
P
Pump
M
Filter
Air-flow Alarm
M
Blower
Temperature Operated By-pass
Liquid Level Alarm
Nitrogen Tank
Power Supply
Misc. Cooling
Temperature Gauge
Shut-off Valves
Low-flow Interlocks
Flow Meters
Pressure Regulator
Pressure Gauge
Klystron Body
Over-temp Interlock
Pressure Gauge
Low-pressure Manifold
Klystron Collector
High-pressure Manifold
Over-temp Interlock
Flow Adjusting Valves
Temperature Gauge
Over-press Interlock
Waveguide
Magnet
Over-press Interlock
Dummy Load
Drain
Drain
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.36. Typical Liquid Cooling System for a Klystron Amplifier
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In Figure 4.36 we have shown the waveguide cooling in series with the magnet cooling. Individual system arrangements vary considerably. For instance, in some systems it is possible to put the klystron body cooling in series with the magnet cooling; in other systems the waveguide cooling may be in series with the RF dummy load, etc. The important thing to note is that each of the individual channels must have provision for adjustment and measurement of the amount of liquid flowing, and must be provided with low-flow interlock switches for protection. A pressure regulator is often installed somewhere in the system. In Figure 4.36 it is shown between the high-pressure and low-pressure manifolds. The middle of Figure 4.36 shows provisions for cooling the power supply, and any number of other things that may be liquid-cooled in a typical system. The hot liquid passes from the low pressure manifold, usually through a filter, and into the heat exchanger.Figure 4.36 shows a liquid-to-air heat exchanger consisting of a radiator and a blower. After the liquid has been cooled in the heat exchanger, it goes into the “coolant-storage and de-aerator tank.” The de-aerator tank deserves some discussion. Bubbles have a tendency to form in this type of liquid cooling system. Cool liquid will tend to pick up air bubbles if the system is open to the air at any point. As the liquid is heated, these bubbles tend to come out of solution. They will tend to collect in “high” parts of the system and may cause difficulty in filling the system in the first place. A fairly small bubble-content in the cooling liquid can seriously diminish the cooling efficiency, and may even cause damage to the equipment. Furthermore, air bubbles cause undesirable oxidation of the metal parts of the system. Fortunately, it is quite easy to remove the bubbles. The de-aerator tank is fitted with baffles. The liquid enters the tank and passes (rather slowly) around and through the baffles in the tank. The bubbles will be released and rise to the surface, rather than remaining in the liquid system. With a de-aerator of this type, it is quite easy to remove bubbles from the system and keep the liquid bubble-free. The air can be bled from the tank, and the tank refilled with liquid if necessary. A low-liquid level switch is normally used in the coolant storage tank. This switch can be connected to ring an alarm that alerts operating personnel to the situation; it may sometimes be connected to shut down the equipment in the event the liquid falls below the safe level. The liquid passes from the de-aerator tank into a de-ionizer. If free ions are allowed to build up in the cooling liquid, they may eventually cause damage to the collector and body channels in the klystron. The seriousness of this problem varies widely from system to system, and from tube to tube. Some systems require that ions be kept to a very low level. The de-ionizer, normally built with a replaceable cartridge, will remove ions from the liquid and prevent this problem from occurring. The liquid passes from the deionizer into the pump, where it is pumped into the high-pressure manifold. One or more filters are normally included in the cooling system to remove any accumulation of foreign material. In some klystrons the cooling passages are very small and can be plugged easily by dirt or sludge in the cooling system. Figure 4.36 shows a nitrogen tank that can be connected to “pressurize” the de-aerator tank. This requires some explanation. Some klystron amplifiers may be at a higher elevation than the heat exchanger and pump portion of the cooling system; a typical case would be a klystron amplifier on a large parabolic antenna, with the heat exchanger on the ground. These systems are quite hard to fill because it is difficult to remove the air from the
RF Theory: Klystron Theory
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 system initially. The problem can be solved with the pressurizing arrangement shown in Figure 4.36. The storage tank is first filled as full as possible. The drains on the manifolds are opened, and pressure is applied to the storage tank from the nitrogen bottle. The nitrogen pressure forces the liquid out of the tank and up to the manifolds. The air in the lines escapes through the drain valves. When the lines and manifolds are full, the drains are closed. The nitrogen tank is valved off and the storage tank may be filled to the top. The pump can then be started and the remaining air trapped in the system will normally be picked up by the coolant and delivered to the de-aerator tank. Figure 4.36 shows a bypass line around the heat exchanger radiator, and a temperature operated bypass valve. This is the system that is often used to control the temperature of the coolant liquid. In the previous discussion, we pointed out that klystron tuning may be changed somewhat by the temperature of the coolant, since this can change the physical size of the cavities. Fortunately, temperature control is done easily by bypassing some coolant around the radiator. In systems where the heat exchanger is fairly close to the klystron tube, the temperature valve may be simply a bimetal mechanism that senses the temperature of the liquid as it leaves the radiator. If the liquid is too hot, the valve closes partially and causes more of the total liquid to flow through the radiator. Conversely, if the temperature is too cold, the valve readjusts itself to cause more of the liquid to go around the radiator via the bypass line. If the klystron amplifier is a long distance from the radiator, a temperature sensor is normally placed at the high-pressure manifold. This temperature sensor operates the bypass valve. Since the temperature sensor is a long way from the temperature controller, and it takes an appreciable length of time for the liquid to travel this distance, a proportional controller may be necessary to keep the system from “hunting.” Some systems use motor-operated louvers (in the air stream between the blower and the radiator) for temperature control, rather than the bypass arrangement. Either system, of course, can only control the temperature to some point above ambient since the liquid leaving the radiator will always be somewhat hotter than the temperature of the air blowing through the radiator. Most systems are designed to control the liquid to a temperature between 10 and 20°F higher than the maximum-expected ambient temperature at the particular location. It is fairly easy to hold the liquid temperature within +5 °F. This close temperature control results in very stable klystron tuning. Some discussion of cooling liquids is appropriate here. Distilled water is the best all-round liquid for cooling klystron amplifiers. Some very-high-power amplifiers specify that only water can be used. Normal tap water usually has a large mineral content, and causes scaling of the klystron cooling surfaces. Scaling reduces heat transfer and may eventually completely close the cooling channels. If this occurs, the tube will be seriously damaged. Unfortunately, water freezes at an inconveniently high temperature. Many low- and medium-power klystrons permit the use of ethylene-glycol and water as the cooling liquid. The cooling efficiency of ethylene-glycol and water is not as good as pure water. Furthermore, ethylene-glycol reacts with certain types of metals and hoses that might be used in the system; therefore, special care must be taken in designing a system that is to use ethylene-glycol. Only nonferrous metals should be used in a cooling system for a klystron amplifier.
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RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Some very large tubes permit only water to be used for the coolant. This complicates the design of the cooling system, since care must be taken to protect it from freezing. This is no problem if the system is continually operated, but becomes a very serious problem if the system is shut down in cold weather. Some large systems are designed with immersion heaters in the coolant tank. If the klystron is shut down for any reason, these immersion heaters are turned on, and the pump is left running to keep the coolant circulating and prevent freezing. Additional information on klystron cooling is contained in Varian Application Engineering Bulletin No. 17.
11.5. RF Circuits
We have discussed the theory of klystron amplifier operation, power supply requirements, and cooling requirements. Now let us consider what is necessary to get RF into, and out of, the klystron amplifier tube. Figure 4.37 on Page 4-46 shows the RF components typically associated with a klystron power amplifier. We will not consider the RF exciter (or “driver”), since we are only discussing the amplifier portion of a complete transmitter. The RF input signal from the amplifier is derived, of course, from an RF exciter. This signal is shown on the left of Figure 4.37. The RF input signal normally goes through a ferrite isolator so that a constant RF load impedance is presented to the exciter. The input cavity of a well-designed klystron amplifier normally presents a low VSWR, to the input signal, at the resonant frequency of the cavity; but the VSWR increases very rapidly for frequencies slightly off the resonant frequency of the cavity. It is desirable to isolate these high VSWR's from the exciter; the ferrite isolator accomplishes this function. After the ferrite isolator, the input signal is normally applied to a variable attenuator. The attenuator is used to adjust the input signal level so that the amplifier may operate at saturation, or at lower-than-saturation levels if this should be desired. It may be desirable to monitor the amount of RF input power being applied to the tube. This is normally done with a directional coupler and some sort of RF power monitor. This monitor may be a simple crystal detector and meter, or it may be a thermistor and RF power bridge arrangement. The input coupler can also be used to help tune the first cavity of the klystron to resonance. Many klystrons have a coarse-tuning indicator that allows them to be set approximately to frequency. However, this is not true for all tubes, and it may be quite difficult to get them on frequency when they are first put into a system. The first cavity tuning can be done easily by using the input coupler as a “reflected-power” coupler. To do this, put the input monitor on the opposite arm from that shown in Figure 4.37. When this is done, the monitor will show the power that is “reflected” from the first cavity of the tube, back toward the RF exciter. We stated earlier that the cavity presents a low VSWR at its resonant frequency; but if it is not tuned to the frequency of the RF input signal, the input power will be mostly reflected from the first cavity. If we now set up to monitor this reflected power, and then tune the cavity, the reflected power will decrease and go through a null when the cavity is tuned to the frequency of the input signal. This simple procedure allows one to “find” the first cavity and tune it to resonance. After this is done, the RF input monitor is connected to again monitor “forward” input power.
RF Theory: Klystron Theory
4-45
RF Input
Ferrite Isolator
4-46
RF Input Monitor
Variable Attenuator Input Coupler Crystal Switch Klystron
To Beam Supply
Control Unit
Waveguide Arc Sensor
Back-Power Coupler
Back-Power Meter
Low-pass Filter
Harmonic Filter
RF Output Monitor
Thermistor
Low-pass Filter
Forward-Power Coupler
RF Sample
Sampling Coupler
Dummy Load
RF Switch
To Antenna
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Figure 4.37. Typical RF Circuitry for a Klystron Amplifier
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Between the input coupler and the first cavity, the diagram shows a crystal switch. This is a very fast-acting device that will insert between 20 and 30 dB attenuation in the RF input line. It is used primarily to remove RF input from the tube quickly in case of arcs in the output waveguide; this will be discussed in more detail later. The crystal switch is normally biased to have low RF attenuation. This completes the discussion of the components associated with the RF input to the tube. Let us now consider the RF output components. The first item normally found in the RF output circuit is a waveguide-arc sensor. Waveguide arcs are a troublesome problem in high power CW systems, particularly those at the higher frequencies where waveguide sizes are quite small. Typically, waveguide arcs will occur at power levels above 5 kilowatts at S-band, and above 1 kilowatt at X-band. Although the cause of this arcing is not completely understood, one of the most plausible theories is that ions build up due to thermal ionization from heating of contaminants within the waveguide, or from local area heating at small discontinuities within the waveguide. This ionization builds up in a CW system until the dielectric strength of the gas in the guide is sufficiently reduced to cause a sustained arc to form. Apparently, this buildup of ionization does not occur as readily in pulse systems because the ions have time to disperse during the interval between the pulses. In any event, once the arc is formed in a CW system, it almost invariably travels toward the source of RF power. If the arc is allowed to reach the output window of the klystron, local heating will occur and the window may be destroyed very quickly. Since the arc presents an effective short-circuit to the waveguide, a very high VSWR exists, and it is quite common to start a secondary arc in other points in the waveguide feedline or at the output window of the klystron. Experience with very high-power CW amplifiers at X-band indicates that the arc should be quenched in a few microseconds to prevent damaging the tube. This short time precludes removing the RF power by de-energizing the power supplies due to the long time lag of the relays and contactors involved in power supply shutoff. Removing RF drive is the fastest method of quenching the waveguide arc. This removes the RF output power and the arc disappears. Since the arc causes a bright light in the normally dark waveguide interior, a good way to sense the arc is to “look” into the waveguide with some light sensitive device, such as a photoelectric cell or a solar cell. Solar cells have proven superior to photo cells for this service, because they respond more quickly to the presence of light, and because they are less affected by temperature. When an arc occurs, the sensor will develop a voltage, which can be used (with follow-up control circuitry) to change the bias on the crystal switch. This inserts a large amount of attenuation in the RF input to the klystron; the RF output falls 20 or 30 dB, and the arc is extinguished. Additional circuits may be used to remove the beam voltage from the klystron if desired. The next component shown in the RF output circuitry is a “backpower” directional coupler. This coupler is connected to monitor power reflected from a mismatch in the antenna or the dummy load. Klystron amplifier specifications typically require the load VSWR to be below 1.2. Excessive reflected power causes high voltage gradients in the waveguide and excess heating in the tube. It may also tend to detune the last cavity and lower the output. Backpower monitoring is normally done with a directional coupler and some type of RF power meter. As shown in Figure 4.37, this may be a
RF Theory: Klystron Theory
4-47
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 simple crystal detector and meter, or it may be a thermistor and an RF power bridge. The backpower coupler can be arranged for redundant waveguide-arc protection. If an arc occurs between the coupler and the antenna, a very high VSWR will be present, and the reflected (back) power will rise suddenly. This can be sensed by the crystal detector, which can trigger the control unit of the waveguide arc-detector system. The control unit then changes the bias on the crystal switch, increases its attenuation, and removes the RF output. In this fashion, waveguide arcs occurring far from the light sensor can be detected. Again the beam power supply may be turned off if desired. A variable attenuator is often inserted between the backpower coupler and the detector to permit convenient adjustment to the desired operating level. The backpower meter is often of the type that includes upper-limit contacts. If the backpower slowly increases to an excessive level, these contacts will close and can be used to turn off the beam power supply. The next component in the RF output circuit is the “forward power” directional coupler. This coupler monitors the power being delivered to the antenna (or to the dummy load). The power indicating device can, again, be either a simple crystal detector and meter, or it may be a thermistor and an RF power bridge. A variable attenuator is normally included between the coupler and the power monitoring device to permit convenient adjustments. In some systems the power meter has both upper- and lower-limit contacts. These contacts can be arranged to ring alarms if the power output varies excessively. Many power amplifier systems use a third directional coupler to sample the RF output. Such a sample may be used to monitor noise performance of the equipment, to check distortion in the output signal, etc. The RF output is normally applied to an antenna. However, a dummy load is very useful for absorbing, and accurately measuring the power being generated. A dummy load is also handy for initial adjustment and tune-up when it may be undesirable to radiate. An RF switch is incorporated in some amplifiers to permit connecting the klystron easily either to the antenna or to the dummy load. This should be done only when the drive has been removed from the tube. A harmonic filter is sometimes included in the RF output circuit. Depending upon the type of tube and the operating conditions, the output of the klystron may be rich in harmonics. It is common for the second and third harmonics to be only 20 dB below the fundamental. In some situations radiation of this harmonic power causes objectionable interference. The harmonic energy can be removed by a harmonic filter in the system. This is simply a low-pass filter that absorbs the harmonic power, while passing the fundamental. Figure 4.37 shows low-pass filters in each of the directional coupler output arms. Directional couplers have the “unhappy” characteristic that they usually couple harmonics more strongly than the fundamental. An appreciable amount of harmonic energy in the RF output may cause incorrect readings in the RF power meters. Simple low power, low-pass filters remove this harmonic energy from the RF power monitors and prevent this inaccuracy. It is particularly important to have a low-pass filter in the forward power coupler, because the ratio of harmonic energy to fundamental energy is quite dependent upon the way the klystron tube is tuned. If one is watch-
4-48
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ing the forward power meter while tuning the tube, and the harmonics are not suppressed by a filter, it may be difficult to tune the tube correctly. This discussion has covered the microwave components usually used with a high-power klystron amplifier. The components discussed allow all of the important parameters to be monitored. In addition, some of these components provide features necessary to protect the tube and operating personnel.
11.6. Tuning
Tuning a klystron amplifier is very simple, when one understands the principles. Let us consider first the steps to tune the klystron to the synchronous-tuned condition. This is the simplest tuning adjustment. Remember that it results in highest-gain and narrowest-bandwidth operation of the tube. Many klystrons have a dial arrangement that allows adjustment of the cavities to approximately the right frequency before applying power to the tube. However, some tubes have no integral tuning indicators. Most tubes, when delivered, are tuned to some frequency, indicated by test data accompanying the tube. So, at least, one knows where the tube is tuned when new, and which direction to go for the desired frequency. The instructions also give the “direction” of tuner rotation to raise, or lower, the cavity frequency. All these things help tune the tube the first time. Let us consider the most difficult case, a tube with no built-in tuning indicators and no indication of the present tuning. In addition, the driver is fixed-frequency so that its frequency cannot be matched to that of the tube; one must tune the tube to the driver. The tube is installed in the transmitter; the exciter is operating and delivering power; the cooling has been turned on, and voltages are applied to the tube. Everything is working, but there is no power output, because the tube is not tuned to the frequency of the exciter. How can you get power from the tube? It is simple. First, you must adjust the first cavity frequency. When we were discussing Figure 4.37, we mentioned that you could find the first cavity tuning by reconnecting the “RF input” directional coupler to read the power “reflected” from the tube. This is done by moving the RF input power monitor to the “reflectedpower” arm of the coupler. (Don't forget to put the termination on the “forward power” arm of the coupler). You are now set up to monitor the power reflected from the first cavity of the amplifier. This power will be minimum when the first cavity is tuned to the driver frequency. When you start tuning a tube, it is a good idea to keep some mental notes on how far you've gone, so you can return to the starting point. A simple way to do this is “count turns” as you rotate the tuning tool. So, suppose you begin tuning the first cavity, rotating the tuner in a clockwise direction, and counting turns as you go. Look for a significant “dip” in the RF input monitor that is measuring the reflected power. You may find some small dips on the way, but look for the major one, which will be the correct tuning point. Most tubes are equipped with tuner stops to prevent damage to the tuner mechanism. Suppose you rotate the tuner clockwise and do not find any major dips in the reflected power all the way to the tuner stop. Then, return to the starting point, counting turns as you go. Then continue counterclockwise (again counting turns from the original position) until you find a significant dip in the reflected power. Minimize the reflected power by tuning. Now you can leave the first cavity alone for a while, since it will be almost on resonance. Reconnect the RF input monitor to read forward power.
RF Theory: Klystron Theory
4-49
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Next, tune the output cavity. Ignore any intermediate cavities for the moment. You may still have no measurable reading on the output meter. Increase the RF drive to the highest level you can get from the exciter. Now you are ready to tune the output cavity. Although you may have tuned the first cavity counterclockwise, it does not necessarily follow that the other tuners should be turned the same direction. Refer carefully to Operating Instructions or to markings on the tube itself. Start tuning the output cavity, counting turns as you go. With luck, you will soon bring the output cavity to resonance, and will see an indication on the RF output meter. Maximize this reading by tuning the last cavity of the tube. Once you see any reading on the RF output meter, the rest is simple. You know you have the first and last cavities in tune (or almost). If you have a three-cavity tube, now adjust the middle cavity. Determine the tuning direction from the instructions or tube markings, or carefully try one direction and then the other. Simply tune the cavity to maximize the power output reading. However, once you approach a sizable output, it is a good idea to reduce the RF drive to be sure that you do not inadvertently saturate the tube. Tune the middle cavity to maximize the power output reading. Now reduce the drive to a low level, so that the power output meter is far below full power (less than 30 per cent of full-power). Retune the input cavity to resonance, then retune the output cavity to resonance, then retune the middle cavity to resonance. The tube is now synchronouslytuned. Synchronous-tuning is always done with low RF input power. Now increase the RF drive until the tube saturates (or to whatever power level you may wish to use). The procedure for a four-cavity tube is very nearly the same. First, tune the input and output cavities to resonance, then the intermediate cavities one at a time. Cavities are often numbered, number 1 being the input cavity and the number 4 the output. For synchronously-tuning a four-cavity tube (after you have some power output), reduce the drive to a low level and tune in the sequence 1-4-3-2; i.e., first tune the input cavity, then the output cavity, then the next-to-the-output cavity, and then the next-to-the-input cavity. The tuning of the fourth cavity is normally quite “broad,” whereas the tuning of the first, second, and third cavities is quite “sharp.” The reason is that the output cavity is fairly “low-Q” compared with the other cavities. You have now learned to tune a klystron to the synchronous-tuned condition. This is always the first tuning condition even if you want to staggertune the tube later. There are several methods of stagger-tuning, two of which will be discussed briefly. Remember, in Theory of Operation we stated that stagger tuning could be used to obtain more power than synchronous-tuning. This is done simply by adjusting the third cavity to a higher frequency. The detailed steps are approximately as follows: First, tune the tube synchronously at low RF input; then increase the RF drive until the tube is saturated. Now, leave the drive alone and detune the third cavity in the high-frequency direction. The Operating Instructions for the tube will tell you whether this is clockwise or counterclockwise. As you detune the third cavity, the output will decrease, because more drive is needed for saturation. Continue detuning until the power output has dropped approximately 6 to 10 dB. Now increase the RF drive power until the tube again is operating at saturation. You will find that this “new” saturated output is higher than the output obtained with the tube synchronously-tuned. You may be able to “squeeze a little more out” of the tube, but probably not
4-50
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 much. You can detune the third cavity still farther and then increase the drive to see if you get more power output than before. The power output maximum is normally quite broad; you will be able to detune the third cavity considerably either side of this point without making an appreciable change in output. Eventually you may become limited by the amount of power available from the exciter. A second type of stagger-tuning should be mentioned. This is stagger-tuning to achieve a desired bandwidth characteristic over the passband. The problem is similar to broadbanding an IF amplifier by stagger-tuning. You will need an RF driver that can be swept in frequency rapidly (electronically-swept), and whose power output is constant during sweeping. You will need to sample the RF output with a crystal detector. Apply the crystal detector output to an oscilloscope so that you can see the passband of the tube. The X-axis of the oscilloscope sweep must be synchronized with the RF input sweep voltage. Again you will start with the synchronous-tuned condition and probably with the tube operating at saturation. To broadband the tube, it is usually best to detune the third cavity to the high frequency side, and to detune the second cavity to the low frequency side (assuming a four-cavity klystron). The first and fourth cavities are normally left tuned to the center of the passband. You may wish to adjust the RF input power periodically, as you detune the klystron, to keep the tube operating near saturation. You will find that the bandwidth of the tube is larger when you are operating at saturation than below saturation. The details of the broadband tuning that you may wish to accomplish are beyond the scope of this note. We have only indicated the equipment that is necessary and the general procedures to be followed.
11.7. Noise in Klystron Amplifiers
Volumes have been written about noise in microwave systems; obviously, we can only touch the very high points in this discussion. Noise is anything that causes the RF output signal to be different from the RF input signal. We have already mentioned that the output may contain harmonics. This is primarily because the RF output cavity is excited by “bunches” of electrons that come through the output gap once every cycle. These bunches essentially “kick” the output cavity and cause oscillating currents to flow in it. Since the driving force on the output cavity is not continuous, but rather occurs in quick kicks, it is intuitively evident that the output current may not be purely sinusoidal; therefore, it will contain harmonic components. This situation is quite analogous to a class-C triode amplifier in which the plate current flows in bursts, and sets up oscillating currents in the resonant plate circuit. Class-C amplifiers are also rich in harmonics for the same reason. In general, the harmonic output from klystron amplifiers is largest (percentage-wise with respect to the fundamental carrier power) when the tube is operating at saturation, or is being over-driven beyond saturation. Harmonic content decreases (percentage-wise) when the tube is operated below saturation. As discussed previously, harmonics can be reduced in the output by using harmonic filters. Another source of distortion is non-linearity of the klystron. If the RF input signal is amplitude-modulated, the RF output may not “perfectly” follow the RF input. This can result in distortion, becoming worse as the tube is driven closer to saturation on the peaks of the RF input signal. In general, klystron amplifiers should not be used to amplify amplitudemodulated signals if the RF output is driven higher than about 0.7 of the
RF Theory: Klystron Theory
4-51
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 saturated level. Between 70 and 100 per cent of saturation, considerable distortion can occur. A klystron amplifier will generate a certain amount of “white noise,” just as any other electron tube. White noise occurs primarily because an electron beam is never “perfectly” homogeneous. The number of electrons will vary slightly with time, primarily due to shot noise at the cathode surface; this variation shows up as random noise in the RF output. A certain amount of noise may also be generated by electrons striking the drift tubes. These are the electrons that create the body current. The body-current interception may be slightly random; this again will perturb the electron beam and cause a small amount of random noise to appear in the output. You should understand one interesting effect about klystron amplifiers. Intuitively, one would think that the output of an amplifier cannot possibly be “quieter” than the input signal. In certain cases, the klystron amplifier can, indeed, have an output that is quieter than its input. Consider a tube operated at saturation; this is the normal situation when the intelligence is being transmitted by frequency-modulation of the carrier. And suppose that the output of the RF exciter is fairly “noisy” with amplitude-modulation. The klystron amplifier has the desirable property that it will suppress amplitude-modulation of the input signal, if the tube is being operated at saturation. An examination of the output-vs.-input curves shown in Figure 4.34 on Page 4-32 will explain how this happens. It is obvious that, with the amplifier operating at saturation, rather large changes in the amplitude of the RF input signal will cause no change in the amplitude of the RF output signal. In some systems this effect is very noticeable, and it is not uncommon to find that the AM noise from the exciter can be suppressed by 10 to 20 dB, simply by operating the amplifier at saturation. Additional information on noise characteristics of klystrons is given in Varian Application Engineering Bulletins Numbers 11 and 18. Definitions of AM and FM noise and methods of measurement are discussed. Equations for computation of noise caused by power supply ripple are derived and examples are given for typical conditions.
11.8. Summary
4-52
This bulletin has attempted to familiarize you with the basic principles of klystron amplifiers and the equipment usually associated with these tubes. Precautionary measures and safety devices have been described in considerable detail in order to explain their importance, and to convince you that “cheating” them for expediency will very likely result in expensive damage to tubes, equipment, or personnel. Tuning procedures described are those used for Varian klystrons but are generally applicable to similar tubes. Too much cannot be said in favor of studying Operating Instructions for equipment and tubes thoroughly before applying power to your transmitter.
RF Theory: Klystron Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Five
Ion Chamber Theory
This chapter covers the rudimentary concepts of ion chamber characteristics and is written for an intended audience of engineers, test personnel, and manufacturing personnel, wishing to learn more about Varian's ion chamber.
Ion Chamber Theory
5-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents
1. Introduction:.................................................................................................................... 5-3 2. Present Configuration: ..................................................................................................... 5-3 3. Efficiency: ........................................................................................................................ 5-5 4. Upper Limit of Dose Range (Saturation): ........................................................................... 5-6 5. Applied Electric Field: ...................................................................................................... 5-7 6. Effects of Temperature and Pressure: ............................................................................... 5-7 7. Beam Opacity: ................................................................................................................. 5-8 8. Inverse Square Law: ......................................................................................................... 5-8 9. Insulation Materials: ........................................................................................................ 5-9 10. Pulse Shape: ................................................................................................................ 5-10 11. Concluding Remarks: ................................................................................................... 5-11 12. References: .................................................................................................................. 5-12
Table of Illustrations Figure 5.1. Basic Ion Chamber Components and Characteristics:......................................... 5-3 Figure 5.2. Simplified Dosimetry/Steering System Block Diagram: ....................................... 5-4 Figure 5.3. Geometric Layout of Electrode and Signal Plates:................................................ 5-4 Figure 5.4. Fraction of Ions Collected as a Function of Dimensionless Variable 1/Up: ........... 5-6 Figure 5.5. Upper Limit of Dose Range: ................................................................................ 5-6 Figure 5.6. The Different Regions of Operation of Gas-filled Detectors:.................................. 5-7 Figure 5.7. Diagram Illustrating the Inverse Square Law: ..................................................... 5-8 Figure 5.8. Diagram showing the Derivation of the Pulse Shape VR (t): ............................... 5-10 Figure 5.9. Output Pulse Shape: ........................................................................................ 5-11
5-2
Ion Chamber Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
Gas filled ion chambers are a type of radiation detector. The normal operation mode is based on collection of the charges through the application of an electric field. These charged particles are created by direct ionization of the gas within the ion chamber. After a neutral molecule is ionized, the resulting positive ion and free electron produce the basic electrical signal developed by the ion chamber. Figure 5.1 illustrates the basic principles of an elementary ion chamber. A volume of gas is enclosed within an electric field. At equilibrium, the current flowing in the external circuit will be equal to the ionization current collected at the electrodes, and a sensitive ammeter can be used to measure the ionization current. Notice on Figure 5.1 that current output is constant after the knee of the curve, especially at low dose rates.
Gas Enclosure I Electrodes V
I
High Irradiation Rate
Low Irradiation Rate
V Figure 5.1. Basic Ion Chamber Components and Characteristics
2. Present Configuration
The present monitoring ionization chamber of the high energy Clinac 1800 and Clinac 2100C is constructed of several plates or electrodes. The purpose of the ion chamber is twofold: 1. to monitor the beam position, and 2. to monitor beam intensity. These needs are satisfied as illustrated in Figure 5.2, a simplified diagram of the ion chamber, dosimetry, and beam steering systems. This ionization chamber consists of two collecting plates sandwiched between three polarizing plates. The two collecting plates are oriented 90° in relation to each other to allow inplane and crossplane beam symmetry monitoring. The collecting plates are divided into four sectors, each defining a distinct laminar collecting volume.
Ion Chamber Theory: Introduction
5-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Accelerator Guide Angle R Steering Coils
Position R Steering Coils
Buncher R Steering Coils
Buncher T Steering Coils
Position T Steering Coils
n Beam Electro
Angle T Steering Coils Target Dose Rate Meter A A 1 B A 2
A3 A4
F A 9 E A 10
A13
G A 11 H A 12 D A 5 C A 6 -500V P.S.
A14 A8
A+B
MU1 Integrator
A-B
(A - B) + (E - F)
SYM1
E-F
G-H
(C - D) + (G - H)
SYM2
C-D
A7
C+D
MU2 Integrator
Figure 5.2. Simplified Dosimetry/Steering System Block Diagram
Electrode Plate Signal Plate Figure 5.3. Geometric Layout of Electrode and Signal Plates
5-4
Ion Chamber Theory: Present Configuration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The two inner D-like sectors provide signals for both dosimetry and beam angle monitoring. The outer two arc-like sectors are for beam position monitoring only. The collecting plate closest to the target monitors the radial plane, the collecting plate closest to the patient monitors the transverse plane. A polarizing voltage of 500 volts is used with a plate spacing of 1mm. A good rule of thumb for dielectric breakdown is 24,000 V per in. So at 0.040" (mm) we should be able to hold off 1000 V.
3. Efficiency
Assume the plates to be a distance d apart and held at a potential difference v. The positive plate is on the left and the negative plate is on the right. The positive ions will occupy the space to the right as they are pulled to the negative plate, while the negative ions will occupy the space to the left as they are attracted to the positive plate. If no recombination occurs, the charge collected per second, i.e., the current, is: I = Qc × d × A
where:
(Equation 117)
Qc is a charge per unit volume per second. d is the plate separation. A is the area.
Recombination occurs when + and – ions meet and recombine before the charged ions make it to the collecting electrode. When recombination is taken into consideration, an efficiency factor must be multiplied to Equation 117. For pulsed radiation, whose duration is short compared with the collection time, the collection efficiency or fraction of charge collected for pulsed radiation, fp, may be expressed by: f p = 100 ⁄ U p × ln ( 1 + U p )
where:
Up is the dimensionless parameter: k 2 U p = -- × d × q′ v
where:
(Equation 118)
(Equation 119)
k is a gas constant. q is the charge in esu produced per cm3 per second. v is the voltage potential between the plates in volts. d is the separation of the plates.
The actual current or “charge collected per second” with recombination taken into consideration is Equation 117 multiplied by the efficiency (Equation 118). I = Qc × d × A × fp
(Equation 120)
From Equation 120 it can be seen that if the distance d is moving because of unstable signal plates, the effect on the current signal is enormous. Equation 120 also suggests that we want: a. small plate separation, and b. high voltage.
Ion Chamber Theory: Efficiency
5-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 10.0
10.0
Pulsed Radiation Fraction Collected fP
0.96
0.8
d V
P
0.7
0.6
0.5
±
±
N ±
+
+
+
–
–
–
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
–
–
–
±
±
±
+
+
+
0.4
X+
X
V+ 0.2 0.1
1.0
10
0.94
0.92
0.90
Fraction Collected – fP
0.98
0.9
0.88
X– V–
0.86 1000
100
Dimensionless Variable 1/UP
Figure 5.4. Fraction of Ions Collected as a Function of Dimensionless Variable 1/Up
4. Upper Limit of Dose Range (Saturation)
A constant dose sensitivity throughout the dose range provides a linear response (i.e., reading vs. dose, r vs. D). Saturation is manifested by a decrease in the dose sensitivity. If pushed to the limit, this function will become zero and then become a negative value. Figure 5.5 illustrates a double-valued dose response function resulting from a decrease in dosimeter sensitivity at high doses. One factor that can add to saturation in an ion chamber is recombination. An ion chamber is said to be saturated to the degree that ionic recombination is present. Saturation is minimized by ensuring that a large electric field exists everywhere within the ion chamber. Increasing the ion collecting potential generally helps, but is limited by electrical breakdown of insulators. For a more detailed explanation, refer to Attix1 page 281.
Reading
dr dD g = 0
Negative slope
Double-valued Function
r
dr Slope = dDg = sensitivity
Dg
Dose
Figure 5.5. Upper Limit of Dose Range
5-6
Ion Chamber Theory: Upper Limit of Dose Range (Saturation)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Applied Electric Field
The different regions of applied electric fields (volts per meter) are shown in Figure 5.6. Region I has very low values of applied voltage, 0 to 100 V for a plate spacing of 0.040". The electric field is insufficient to prevent recombination of the original ion pairs. Region II shows the normal mode of operation for ionization chambers. Region III is the region of true proportionality, and represents the mode of operation for proportional counters. Increasing the applied electric field even further introduces nonlinear effects as shown in region IV. If the applied electric field is increased even further, we enter the Geiger-Mueller region. The present ion chamber configuration uses an Acopian (or equivalent) power supply of 500V dc. Experimental data shows that the ion chamber output is constant from 300 – 600 volts. Good voltage regulation is essential, since HV fluctuations induce current to flow in the electrometer input circuit from the capacitive coupling. Also, we previously mentioned that reducing the plate separation increases the collection efficiency and signal to noise ratio.
Pulse Amplitude (log scale)
V GeigerMueller Region
IV Limited Proportional Region
2 MeV 1 MeV
II I
Ion Saturation
III Proportional Region
Applied Voltage Figure 5.6. The Different Regions of Operation of Gas-filled Detectors
6. Effects of Temperature and Pressure
Whenever absolute ionization measurements are made, corrections must be applied to account for the change in density of the gas (with pressure and temperature). The mass of a given volume of air at temperature T and pressure P is related to its mass at 0°C and 760mm Hg by: ( 273.2°K ) ( P mm ) m(T,P) = m(0°C, 760mm) ----------------------------------------------------------( 273.2°K + T ) ( 760mm )
Ion Chamber Theory: Applied Electric Field
(Equation 121)
5-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The first bracketed term corrects for the expansion of the gas with increased temperature. The second term corrects for changes due to pressure. If the ion chamber was calibrated at another temperature, the appropriate (absolute) temperature must be used. This correction formula assumes that the ion chamber cavity is not sealed and that pressure inside the chamber is atmospheric. This is not the case with any Clinac ion chamber; it is hermetically sealed with a weld. The changes in mass in a sealed ion chamber should be nearly zero. A worst case scenario would be an unsealed ion chamber. A 5°F change in temperature (2.7°C) represents a 0.9% change in mass. A 10°F change in temperature (5.6°C) represents a 2.0% change in mass. The effects of pressure changes are not as dramatic.
7. Beam Opacity
In dealing with a compound or a mixture of molecules, it is sometimes convenient to describe the mixture by an effective atomic number, Z. The concept is useful in dealing with ion chambers. The effective atomic number, Z, of a mixture may be defined by: Z =
where:
m
m
m
a 1 Z 1 + a 2 Z 2 + …a n Z n
m
(Equation 122)
a1 to an are the fractional numbers of electrons per gram belonging to materials of atomic numbers Z1 to Zn respectively. m has the experimental value of 3.5 for air.
8. Inverse Square Law
The ion chamber is not located at isocenter where one wishes to measure dose. The actual photon fluence, φ , (number of photons per unit area) seen by the ion chamber is quite high as illustrated below.
φ2
φ1
P
a
b
a b
f1
f2 Figure 5.7. Diagram Illustrating the Inverse Square Law φ φB
f
2
2 In general it follows that -----A- = φ B -----2
(Equation 123)
f1
This is a simple statement of the inverse square law. In Varian's High Energy Clinacs f2 = 100cm, f1 = 25cm. The calculated resulting ratio of these two distances squared is 16; which means that the photon fluence is 16
5-8
Ion Chamber Theory: Beam Opacity
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 times higher at the ion chamber than at isocenter. This is only approximate and does not take back scatter and secondary electron fluence into consideration. Experimental data shows that the photon fluence is 20 to 50 times higher at the ion chamber than at isocenter.
9. Insulation Materials
Polystyrene, polyethylene are excellent insulators for ion chambers. Teflon is more readily damaged by radiation and should be avoided. Charged-particle beams incident on a thick insulator will build up charge wherever the particle stops at the end of their path. When large insulating plastics are irradiated to high doses by electron beams, the charge buildup due to stopped electrons may cause electric fields strong enough to influence the paths of the primary electrons in the ion chamber.
9.1. Mica
Mica is the name given to a group of minerals of related similar physical properties characterized chiefly by perfect basal cleavage (they can split readily in one direction). The ASTM visual quality classifications for mica are as follows: V-1
clear
V-2
clear and slightly stained
V-3
fair stained
V-4
good stained
V-5
stained, A quality
V-6
stained, B quality
V-7
heavy stained
V-8
black dotted
V-9
black spotted
V-10 black stained In practice, first quality is equivalent to V-3 and second quality to V-4. Varian presently purchases V4 mica through Spruce Pine Mica Co. The micas are complex silicates of aluminum with potassium, magnesium, iron, sodium, lithium, fluorine and traces of other elements. The mica we use in our application is Muscovite, H2KAl3(SiO4)3. Other types of micas are: Phlogopite, Biotite, Lepidolite, Paragonite, Zinnwaldite. The mica thickness specification was dropped from 0.010" to 0.007" +0.003" in January 1988 because of the dwindling mica supply. The 0.003" reduction in thickness constitutes a 50% stiffness change. The unstable mica assemblies resulted in changes in symmetry and dose calibration. There is also suspicion that other changes occurred since we had instabilities even in chambers with selected thick mica. This phenomenon is pretty well understood now and engineering changes have been implemented to minimize the mica ion chamber problem, allowing manufacturing yields to go up from 40% to 90%. This is only a short term solution because the mica supply is still limited. The gold is silkscreened on with thicknesses ranging from 0.0001" to 0.0007". Shortly after this document was published, Varian switched to DuPont Kapton™ as the insulating material for all High Energy Clinacs except for the 2500 and 2500C.
Ion Chamber Theory: Insulation Materials
5-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Pulse Shape
The pulse shape depends on the configuration of the electric field and the position at which the ion pairs are formed. Assume a parallel plate ion chamber with electric field intensity Ef, voltage V across the electrodes and separated by a distance d. (Equation 124)
Ef = V ⁄ d
A further simplification assumes that all ion pairs are formed at an equal distance x from the positive electrode. This situation is sketched in Figure 5.8.
+
vt –
Vch
d
vt
C
R
VR
x
VO
Figure 5.8. Diagram showing the Derivation of the Pulse Shape VR (t) for the Ion Chamber Signal The pulse shape is most easily derived on arguments involving the conservation of energy. The energy required to move the charges from their origin must come from the energy originally stored across the capacitance C. This 2 energy is 1 ⁄ CV 0 where V0 is the applied voltage. After a time t, the ions will have drifted a distance v+ t toward the cathode, where v+ is the ion drift velocity. Similarly, the electrons will have moved a distance v– t toward the anode. Both of these motions represent the movement of charge to a region of lower potential (dV). This energy is equal to Q(dV) for both ions and electrons, where Q is the total charge and dV is the change in electric potential. The charge Q = noe, where no is the number of original ion pairs and e is the electron charge. Conservation of energy can be written: Original stored energy 1 ⁄ 2 CV o
2
Energy Energy Remaining = absorbed + absorbed + stored by ions by electrons energy =
–
n o eEv t
+
+
n o eEv t
+
1 ⁄ 2 CV ch
2
(Equation 125)
The signal voltage is measured across R in Figure 5.8 and will be denoted as VR. The following equation describes the initial portion of the signal pulse and predicts a linear rise with time: no e + – V R = -------- ( v + v )t dC
5-10
rising portion
(Equation 126)
Ion Chamber Theory: Pulse Shape
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The decaying portion of the signal pulse is dependent on the position “x” at which the electrons were originally formed within the chamber. The pulse reflects only the drift of the electrons and will have an amplitude given by: n o ex V D = ---------ed
decaying portion
(Equation 127)
If the collection circuit time constant were very large (RC>>t+) the maximum amplitude of the signal pulse would be: no e V max = ------c
maximum amplitude
(Equation 128)
VR
Vmax = no e c Velec
t–
t+
t
Figure 5.9. Output Pulse Shape Many of the details which are omitted in the preceding discussion can be found in the theoretical books on ionization chambers by Attix2 and Knoll3.
11. Concluding Remarks
The ion chamber converts flux (MU’s/minute) through the ion chamber into a more easily measurable quantity: a current signal. Factors affecting this conversion process include: !
collector and polarizing plate spacing
!
chamber saturation
!
voltage regulation of the polarizing plates
!
density of the plate substrate
!
temperature and pressure (although to a lesser degree than the above-mentioned factors).
Ion Chamber Theory: Concluding Remarks
5-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In Varian's application, the current signal generated is a pulsed signal. The basic equations for the pulse shape were presented (in terms of voltage instead of current, Thevenin's theorem was used). Discussions of manufacturing concerns include the effects of the inverse square law governing radiation attenuation. The photon fluence level is 20 to 50 times more intense at the Ion chamber versus isocenter (because of scatter and secondary electron fluence). Also, use of Teflon as a high voltage insulating material should be avoided. Again, the primary objective of this report is to inform. If you require more detailed information on any of the topics discussed, please consult the References list below, as well as the endnotes referenced in the text of this chapter.
12. References
H. Johns and J. Cunningham, The Physics of Radiology, Thomas, Illinois (1983). B. B. Rossi and H. H. Staub, Ionization Chambers and Counters, McGrawHill, New York (1949). D. H. Wilkinson, Ionization Chambers and Counters, Cambridge Univ. Press, Cambridge (1950).
1. F. Attix, Introduction to Radiological Physics and Radiation Dosimetry, John Wiley & Sons, New York (1949) 2. Ibid. 3. G. Knoll, Radiation Detection and Measurement, John Wiley & Sons, New York (1979)
5-12
Ion Chamber Theory: References
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Six
Vacuum Theory
In order to understand the Clinac vacuum systems, a basic knowledge of vacuum theory is required. This chapter will provide the reader with sufficient information to be able to perform service and maintenance on these vacuum systems.
Vacuum Theory
6-1
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Table of Contents 1. Introduction:.................................................................................................................... 6-3 2. The Nature of Vacuum: .................................................................................................... 6-3 2.1. What Is Vacuum?: .................................................................................................... 6-3 2.2. What About Pressure?: ............................................................................................. 6-4 2.3. How Is a Vacuum Produced?: ................................................................................... 6-4 2.4. Different Types of Vacuum:....................................................................................... 6-4 2.5. Where Is Vacuum Used?: .......................................................................................... 6-5 2.6. Why Is Vacuum Needed?: ......................................................................................... 6-5 3. Temperature: ................................................................................................................... 6-6 4. Pressure:.......................................................................................................................... 6-7 4.1. What is Gas?: ........................................................................................................... 6-7 4.2. Atmospheric Pressure: .............................................................................................. 6-7 4.3. Pressure Measurement: ............................................................................................ 6-8 4.4. Partial Pressure: ....................................................................................................... 6-9 4.5. Vapor Pressure: ...................................................................................................... 6-10 4.6. Effects of Pressure: ................................................................................................. 6-12 4.7. Pressure Ranges: .................................................................................................... 6-12 5. Gas Particles: ................................................................................................................. 6-13 6. Gas Laws: ...................................................................................................................... 6-13 6.1. Avogadro’s Law: ...................................................................................................... 6-13 6.2. Boyle’s Law:............................................................................................................ 6-14 6.3. Gas Expansion: ...................................................................................................... 6-14 6.4. Charles’ Law: .......................................................................................................... 6-15 6.5. Gay-Lussac’s Law: .................................................................................................. 6-16 6.6. General Gas Law: ................................................................................................... 6-16 7. Gas Flow: ....................................................................................................................... 6-17 7.1. Viscous Flow: ......................................................................................................... 6-17 7.2. Molecular Flow: ...................................................................................................... 6-17 7.3. Mean Free Path: ..................................................................................................... 6-18 8. Conductance:................................................................................................................. 6-18 8.1. Conductance in Viscous Flow: ................................................................................ 6-19 8.2. Conductance in Molecular Flow: ............................................................................. 6-20 9. Review of the Nature of Gases: ....................................................................................... 6-20 10. Ion Pump: .................................................................................................................... 6-21 10.1. Components: ........................................................................................................ 6-22 10.2. How the Pump Works: .......................................................................................... 6-22 10.3. Vacuum System Use: ............................................................................................ 6-26 10.4. Summary:............................................................................................................. 6-26 11. Vacuum Gauges:.......................................................................................................... 6-26 11.1. Thermocouple Gauge: ........................................................................................... 6-26
6-2
Vacuum Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
1. Introduction
In the first part of this chapter, we will introduce you to vacuum: !
What it is
!
How it relates to pressure
!
How it is produced
!
The different types of vacuum
!
Where it is used
!
Why we need it
You will also learn about temperature as a factor in vacuum work and the types of pressure and how it is measured. Finally, we will discuss some basic concepts used in vacuum work. These are: !
The effects of pressure
!
Pressure ranges in vacuum systems
!
Some basic laws about the behavior of gases
!
Some types of gas flow
!
How we measure the work done by vacuum systems
2. The Nature of Vacuum
The word vacuum comes from the Latin “vacua,” which means “empty.”
2.1. What Is Vacuum?
Actually, vacuum is only partially empty space. In a vacuum, some air and other gases have been removed from a contained volume. This volume is usually called the work chamber. It separates the vacuum from the outside world.
Vacuum Theory: Introduction
6-3
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 A more practical definition for vacuum is what exists in any contained volume where there is less gas than there is in the surrounding atmosphere. We will see that these gases exert a force on the surface area of the container. This force is called pressure. We can measure the pressure in the chamber by comparing it with the atmospheric pressure on the outside. In this way, we can find out how much gas is left in the vacuum.
2.2. What About Pressure?
Pressure is defined as force per unit area. Gases are composed of small particles. These gas particles are in constant motion. As these particles move around in space, they hit objects. When they hit something, they exert a force, or pressure. We can take a unit of area and measure the number and intensity of particle impacts on that surface. The result is a pressure measurement.
2.3. How Is a Vacuum Produced?
A vacuum is made by removing air and other gases from the work chamber. We remove the air and other gases by using special pumps, called vacuum pumps.
There are many, and very different, kinds of vacuum pumps. Some of them actually remove the gases. Other pumps trap the gases or change their form. In any case, the pump’s job is to take as many gases out of circulation as necessary.
2.4. Different Types of Vacuum
6-4
There are different degrees of vacuum, called rough vacuum, high vacuum, and ultrahigh vacuum. Which one is used depends on the application. As the chambers below show, the better (or higher) the vacuum is, the less air and gas are present.
Vacuum Theory: The Nature of Vacuum
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
2.5. Where Is Vacuum Used?
GOOD
BETTER
BEST
ROUGH VACUUM
HIGH VACUUM
ULTRA-HIGH VACUUM
Vacuum is used for many products and processes. Some of them are: Table 6.1. Uses of Vacuum Rough Vacuum
High Vacuum
Ultrahigh Vacuum
Food processing
Tube processing
Space research
Evaporation
Heat treating
Materials research
Freeze drying
Integrated circuit manufacture
Metallurgy
Distillation
Decorative coating
Physics research
Sputtering
Particle acceleration*
Surface analysis
Electrical conduction (neon lights)
Chemistry research
Molecular beam epitaxy
E-beam welding Vapor deposition Ion implantation Insulation (thermal)
*as in Varian Clinac® Linear Accelerators
2.6. Why Is Vacuum Needed?
We use a vacuum when we need a space that is very clean. It must be free of gases that can interfere with what we want to do. Let us take iron, for example. When iron is left out in air, it reacts with the gases in the air, and the result is rust. This would not happen in a vacuum. VACUUM SYSTEM WALL
RUSTY
Vacuum Theory: The Nature of Vacuum
CLEAN
6-5
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Another example is television. If gases are not removed from a TV tube, the electrons are blocked from reaching the screen — no picture! The easiest way to define “clean” is to say that everything is contaminated, or dirty, to some degree. It is a matter of how much contamination is present. The less contamination, the “cleaner” something is. Let us look at some of this contamination that we are trying to remove. Atmospheric air is a mixture of gases. Over 99% of atmosphere is nitrogen and oxygen. All other gases make up less than 1%. Table 6.2 Gas
Percent by Volume
Nitrogen
78.08
Oxygen
20.95
Argon
0.93
Carbon Dioxide
0.03
Neon
0.0018
Helium
0.0005
Krypton
0.0001
Hydrogen
0.00005
Xenon
0.0000087
Water vapor, another common gas, is not listed above because the amount changes with atmospheric pressure and temperature. Water vapor, which varies from 0.6% to 6% by volume, is one of the biggest sources of vacuum contamination, or “dirt.”
3. Temperature
We have mentioned temperature already in our discussion. Most of us are familiar with the Fahrenheit (°F) and the Celsius or Centigrade (°C) scales of temperature measurement. In the world of vacuum, we are also concerned with the absolute temperature as well. Temperature is a qualitative measurement of energy. The hotter something is, the more energy it contains. Or, if we want to get rid of gases, we could pump the energy out of them until they become frozen. That is, we have lowered the temperature of the gases. Calculations of heat and energy do not work well in the Celsius and Fahrenheit scales because of the negative numbers. This is where the absolute or Kelvin scale comes in. Let us compare some temperatures and conversion factors. Table 6.3. Temperature Scales
6-6
°F
°C
°K
Reference
212
100
373
Boiling point of water
32
0
273
Freezing point of water
321
196
77
LN2 temperature
437
261
12
Cold head temperature
459
273
0
Absolute zero
Vacuum Theory: Temperature
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Conversion factors: 5 °C = --- ( °F – 32 ) 9
°K = °C + 273
9 °F = ⎛⎝ --- °C⎞⎠ + 32 5
5 °K = --- ( °F – 32 ) + 273 9
Now let us discuss some information about gases.
4. Pressure
Earlier we defined pressure. Now, we will explain the kinds of pressure vacuum is concerned with. We will also describe how we measure pressure. First, let us look at what a gas is.
4.1. What is Gas?
What is a gas? It is a state of matter where the individual particles are free to move in any direction and tend to expand uniformly to fill the confines of a container. The gas particles are very small and freely moving. Some, like hydrogen and oxygen, are very reactive and easily form stable chemical compounds with other gases or elements. Other gases, such as helium and argon, are inert. These are sometimes known as the noble (inert) gases. They do not tend to form compounds. All gases have mass and are thus attracted to the earth by the force of gravity. This “ocean” of gas we call “air” has weight. This weight pushing on the earth’s surface is called atmospheric pressure. By definition, pressure (P) is the force (F) exerted on some particular area (A), such as a square inch, square foot, or square centimeter. Put into mathematical terms,
P = F --- (Pressure = Force per Unit Area) A
At 45° N latitude and at sea level, the average pressure exerted on the earth’s surface is 14.69 pounds per square inch (absolute), or 14.69 psia. When the temperature is 0°C, this 14.69 psia is called a standard atmosphere (1 std atm). Gas behavior is usually described with reference to “standard conditions” of temperature and pressure (stp).
4.2. Atmospheric Pressure
We use several different pressure scales. Here are four readings, all at standard conditions: 14.7 psia = 760 torr = 1 std atm = 101,325 pascal The average atmospheric pressure at sea level (45° N latitude) is 14.7 psia, 760 torr, or 101,325 Pa. Vacuum processes are usually done at pressures much lower than atmospheric pressure. Atmospheric pressure changes with distance above sea level (altitude) and changes in our weather.
Vacuum Theory: Pressure
6-7
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 6.4. Average Pressure at Various Altitudes Altitude (Ft)
Pressure (Torr)
Altitude (Ft)
Pressure (Torr)
–1,000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000
787.87 760.00 732.93 706.66 681.15 656.40 632.38 609.09 586.49 564.58 543.34 522.75
11,000 12,000 15,000 20,000 25,000 30,000 40,000 50,000 60,000 100,000 120,000 140,000
502.80 483.48 429.08 314.51 282.40 226.13 141.18 87.497 54.236 8.356 3.446 1.508
Source: U.S. Standard Atmosphere, 1962 (NASA)
A way to measure the force exerted by the atmosphere was developed in the mid-1600s by Evangelista Torricelli. It consisted of balancing a fluid of known weight against the weight of air. The first fluid used was water. Later, mercury was used. The measurement was made using an instrument called a barometer. We have named a pressure unit, torr, in Torricelli’s honor. Vacuum
3
1 in of water = 0.36 lb. Weight of water = 406.8 in.3 × 0.36 = 14.69 lb. Note: 33.9 ft 406.8 in
Mercury is 13.56 times heavier than water, so the mercury barometer will be 13.56 times shorter; i. e., 406.8 in/13.56 = 30 in.
Vacuum
1 Atm 760 mm. 30 in.
Water
1 Atm
Mercury
The Barometer
4.3. Pressure Measurement
There are several different scales for pressure measurement. Millimeters of mercury, torr, and microns are all commonly used. Pascal (Pa) is the metric unit for pressure measurement and is the international standard. The following table shows some of the common scales. The values for these scales are all listed at the same pressure one standard atmosphere (1 std atm).
6-8
Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Table 6.5. Pressure Equivalents Atmospheric Pressure (Standard) 0 14.7 760 760 760,000 101,325 1.013 1013
psig (gauge pressure) pounds per square inch (psia) mm of mercury torr millitorr or microns pascal bar millibar
Here is a table for the equivalent values for one torr and one millitorr (mtorr). Table 6.6 One Torr =
One Millitorr =
1/760 atmosphere
1/1000 TORR
1 mm of mercury
1/1000 mm of mercury
1000 microns or millitorr
10-3 torr or 1 millitor
103 microns or millitorr
0.001 torr
133 Pascal
0.133 Pascal
A conversion table and equivalents for the different measurement scales are provided in the Appendix.
4.4. Partial Pressure
The total pressure of a mixture of gases is the sum of each of the individual gas pressures in the mixture. This is known as Dalton’s Law of Partial Pressure. Each individual gas pressure in a mixture is called a partial pressure. At standard conditions (760 torr, 0°C), each gas exerts a pressure relative to its percent of the total volume: for example: N2 = 78% = 0.78 × 760 = 593 torr. Table 6.7. Partial Pressures of Gases Corresponding to Their Relative Volumes Gas (Air)
Symbol
Nitrogen
N2
Oxygen
O2
Argon
Ar
Carbon Dioxide
CO2
Neon Helium
Vacuum Theory: Pressure
Ne He
Percent by Volume
Partial Pressure Torr
Pascal
78
593
79,000
21
159
21,000
0.93
7.1
940
0.03
0.25
0.0018 0.0005
33 –2
1.8
–3
5.3 × 10
1.4 × 10 4.0 × 10
10–4
1.1 × 10–1
Krypton
Kr
0.0001
8.7 ×
Hydrogen
H2
0.00005
4.0 × 10–4
5.1 × 10–2
Xenon
Xe
0.0000087
6.6 × 10–5
8.8 × 10–3
Water
H 2O
(5 to 50 torr typically)
Variable
Variable
6-9
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.5. Vapor Pressure
When a liquid or solid becomes a gas, we call that process evaporation. The gas produced, we call a vapor. It, of course, exerts a pressure. This pressure we refer to as the vapor pressure for that particular material. The act of turning the gas back into a liquid, we call condensation. When a solid evaporates to a gas directly, we call that process sublimation. In general usage, vapors are gases that tend to condense back to the liquid state at moderate temperatures and pressures. All substances have a characteristic saturation vapor pressure that varies directly with temperature. The lower the temperature, the lower the vapor pressure. This is true for all substances. Water deserves special attention because of its behavior in the vacuum system. It is present in air as a gas in relatively large quantities. In the vacuum system, it is hard to remove condensed water vapor from surfaces at room temperatures. Table 6.8. Vapor Pressure of Water at Various Temperatures Temperature in °C
Pressure in Torr
100.0 (Boiling)
760
50.0
93
25.0
24
0.0 (Freezing)
4.8
40.0
0.1
78.5 (Dry Ice)
5.0 × 10–4
196.0 (LN2)
1.0 × 10–24
Table 6.9. Vapor Pressures of Some Liquids Liquid
Vapor Pressure in Torr @ 20°C (68°F)
Benzene
74.6
Ethyl Alcohol
43.9
Methyl Alcohol
96.0
Acetone Turpentine
184.8 4.4
Water
17.5
Carbon Tetrachloride
91.0
High Vacuum Pump Oil
1.0 × 0–7
Acetone has the highest vapor pressure of the liquids on this list. It evaporates the fastest of those substances on the list. It releases the most gas into the chamber in a given length of time. High vacuum pump oil is the least volatile liquid on the list. It will take the longest time to evaporate. When gases become cooled sufficiently, they liquefy and/or freeze. These curves give the vapor pressure for selected gases when they are liquids or solids. In the illustration below, curves to the right of the vertical dotted line (77°K, 196°C) indicate low vapor pressures at this temperature. Curves to the left show high vapor pressures at this temperature, which is the boiling point of liquid nitrogen.
6-10
Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 (BOILING POINT)
TEMPERATURE (°C) –260 –250 –200
–270
–272
–100
0 100
VAPOR PRESSURE (TORR)
3
10 2 10 1 10 100 10–1 –2 10 –3 10 10–4 –5 10 –6 10 –7 10 –8 10 –9 10 –10 10 10–11
He H2
Ne N2 O2 H2O CO2 NO
1
2
3 4 5
10 20 30 40 50 TEMPERATURE E (°K)
100
200
400
VAPOR PRESSURES OF COMMON GASES
Gases at the left side of the chart have high vapor pressures at extremely low temperatures. Note: Vapor pressure of all gases is the same at the boiling point in atmosphere (760 torr) even though they boil at different temperatures. All materials have a vapor pressure, even though it may be very small. Note that, for some of these materials, their vapor pressure may be high enough to be a problem in some vacuum systems. 3
101 Zn
10–1
Cd Mo
Pb
–3
10
Ti Cu
–5
10
W Fe
–7
10
–9
5000
4000
3000
2000
600
400
200
10–11
1500
10
800 1000
VAPOR PRESSURE (TORR)
10
ABSOLUTE TEMPERATURE (°K)
Vacuum Theory: Pressure
6-11
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
4.6. Effects of Pressure
Before the air and other gases are pumped from the work chamber, constant, high-speed motion makes the particles bump into each other and into the chamber walls. This activity develops a total actual (absolute) pressure of 14.7 pounds per square inch (psia). As we have already seen, 14.7 psia is the average atmospheric pressure at sea level. Therefore, the pressure is the same inside and outside the chamber.
As air is pumped out of the chamber, pressure drops. However, we can never remove all particles from the chamber.
After most of the free-moving gas (sometimes called the volume gas) is removed, there are still other sources of gas entering the system. Gases come off of surfaces in the vacuum system or out of the materials inside the work chamber. This is called desorption or outgassing. Vacuum systems can implode because of the external atmospheric pressure, causing the walls to collapse inward.
4.7. Pressure Ranges
These are the pressure ranges generally used in vacuum work: Rough (low) vacuum759 to 1 × 10–3 torr (approx.) High vacuum1 × 10–3 torr to 1 × 10–8 torr (approx.) Ultrahigh vacuumLess than 1 × 10–8 torr
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Vacuum Theory: Pressure
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
5. Gas Particles
Let’s talk about the nature of the gases that exert this pressure. They are made from naturally occurring chemical elements. These elements are the building blocks of earthly matter. The smallest identifiable part of an element is one of its atoms. An atom has a dense center portion known as the nucleus. This nucleus has particles called protons and neutrons. The protons have a positive electrical charge. Neutrons are neutral. The number of protons — and therefore the electrical charge in the nucleus — is different for each element. If the atom has more or less than its normal number of neutrons, it is called an isotope of the element and is unstable. Under normal conditions, the nucleus is surrounded by a number of electrons. Electrons have a negative electrical charge. The number of electrons balances the positive charge and this makes the atom electrically neutral. Neutrons and protons weigh approximately the same and make up the bulk of the atom. The atoms of the different elements have different numbers of protons and neutrons. They thus have different masses. This means they have different weights (masses). They are classified by their atomic mass or weight. We call this atomic mass units or amu. Molecules simply consist of one or more atoms joined together. with definite chemical and physical characteristics. Molecules are likewise classified by their molecular weight (or mass). This is simply the sum total of the individual atomic weights that make up the molecule. Some of the elements usually exist as gases. Some of these, like hydrogen, nitrogen and oxygen, travel as molecules with two or three atoms bound together. Some gases are composed of more than one element, such as water (H2O). For instance, the atomic weight of hydrogen (H) is 1 amu. Its molecule is made up of two hydrogen atoms (H2) so its Molecular weight or mass is 2 amu. The atomic weight of oxygen is 16 amu. Thus, the molecular weight of water (H2O) is 18 amu. That is the mass of two hydrogen atoms plus the mass of one oxygen atom (1 + 1 + 16). Under certain conditions, an atom or a molecule can become electrically charged. It is then referred to as an ion. This process will be considered in more detail in the discussion on Ionization.
6. Gas Laws
Let’s look at what happens to gases as we use them in our vacuum system. We first assume that gases are perfect — and in general, they are. So we can apply some “laws” to their behavior Let’s look at some of these laws.
6.1. Avogadro’s Law
Under the same conditions of pressure and temperature, equal volumes of all gases have the same number of particles (molecules, actually). We call this a mole. One mole of any gas has 6.023 × 1023 particles, under standard conditions (760 torr, 273°K), occupies 22.4 liters, and weighs one molecular weight. We know this as Avogadro’s Law. 1. How many particles would be in a standard liter? 23
6.023 × 10 particles- = 2.69 × 10 22 particles ⁄ l -------------------------------------------------22.4l
2. How many in a standard cubic centimeter? 23
–3
6.023 × 10 particles 10 -l = 2.69 × 10 25 particles/cc --------------------------------------------------- = -----------22.4l 1cc
Vacuum Theory: Gas Particles
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6.2. Boyle’s Law
Boyle’s Law, P1V1 = P2V2, or original pressure times original volume equals new pressure times new volume. Reduce the volume by half, the pressure is doubled. This equation predicts new pressure or new volume whenever the other is changed by any amount, providing that the temperature remains the same.
BOYLE’S LAW P1V1 = P2V2
VOLUME = 10 LITERS
VOLUME = 5 LITERS
PRESSURE = 50 TORR
PRESSURE = 100 TORR SAME NUMBER OF GAS MOLECULES
NOTE: TEMPERATURE HELD CONSTANT 6.3. Gas Expansion
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Gas expands tremendously under vacuum (from Boyle’s Law). This happens to gas absorbed in fingerprints and dirt in general.
Vacuum Theory: Gas Laws
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Water and solvents are also sources of large gas loads. The large volumes these materials produce are a major part of “outgassing.” Suppose you have a chamber which has a volume of 100l at a pressure of 1 × 10–4 torr. If 1 std cc of gas is suddenly added, what will be the pressure? Let’s use Boyle’s Law. Note that we are really calculating a new pressure, not a new volume. Also, the partial pressure of the gas we are adding will add to the gas pressure already there. P 1 V1 = P 2 V2 For the gas we are adding to the chamber: 760 torr × 1 cc = P2 × 100l Solving for P2 and converting cubic centimeters to liters: –3
760 torr × 1cc 10 l P 2 = ---------------------------------- × ------------100l 1cc –3
P 2 = 7.6 × 10 torr
Now the total pressure in the container is the sum of the pressure there (1 × 10–4 torr) plus the pressure from the gas we added (7.6 × 10–3 torr). P total = P chamber + P 2 –4
–3
= 1 × 10 torr + 7.6 × 10 torr –3
= 7.7 × 10 torr
We see that the 1 cc of gas at atmospheric pressure contributed much more to the pressure in the chamber than the gas already there!
6.4. Charles’ Law
Let’s look at what happens to the volume of gas as we change the temperature. As we cool a gas, its volume gets smaller. If we heat the gas, its volume increases. We call this Charles’ Law. The equation looks like this:
V V1 ------ = -----2T1 T2
Charles’ Law states that if the absolute temperature is doubled, the volume of gas is doubled providing that the pressure is unchanged.
Vacuum Theory: Gas Laws
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 CHARLES’ LAW V1 = V2 T1 = T2 1 ATM
1 ATM
TEMPERATURE = 100° K
TEMPERATURE = 200° K
VOLUME = 10 LITERS
VOLUME = 20 LITERS
SAME NUMBER OF GAS MOLECULES
HEAT
AMOUNT OF GAS DOES NOT CHANGE
NOTE: PRESSURE HELD CONSTANT
6.5. Gay-Lussac’s Law
If Charles’ Law is examined carefully, a more specific relationship develops: If the temperature of a volume of gas at 0°C is changed by 1°C, the volume will change (plus or minus) by 1/273 of its original value. This is Gay-Lussac’s Law. Thus: °C V = V o + ⎛ ---------⎞ × V o ⎝ 273⎠
Rearranging this equation gives us: °C V = V o ⎛ 1 + ---------⎞ ⎝ 273⎠
Lord Kelvin used this relationship to develop the absolute temperature scale.
6.6. General Gas Law
We can combine these laws to get a general gas law (Boyle’s and Charles’ combined): P2 V2 P1 V1 ------------ = -----------T1 T2
The general gas law combines pressure, volume, and temperature in a single equation. The temperature in Charles’ Law and the general gas law is expressed in the absolute scale, or degrees Kelvin; to convert from °C to °K add 273 to °C. Thus: 100°C + 273 = 373°K.
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Vacuum Theory: Gas Laws
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7. Gas Flow
Since we want to move gas molecules out of the vacuum chamber, we should know how gas flows. Of the many types of gas flow, we will discuss two kinds: viscous flow and molecular flow. Both types of flow have to do with how tightly molecules fill a space.
7.1. Viscous Flow
Generally, gas molecules occupying a space at a pressure greater than 1 × 10–2 torr act very much like a fluid, so this is called viscous flow. In the viscous flow range, the molecules are constantly bumping into each other. The molecules are so closely packed together that as our vacuum pump moves some of them out of the chamber. others will rush to fill up that empty space. In viscous flow conditions, molecular movement is predictable. When a molecule is hit or hits a surface, we can predict its movement after impact with reasonable accuracy. Because the molecules are tightly packed and move predictably, we can use smaller diameter hoses and tubulations for rough pumping operations. Viscous flow conditions will generally allow us to move great quantities of molecules per unit time from one place to another.
7.2. Molecular Flow
Molecular flow occurs when the molecules are so far apart that they no longer have any influence on each other. Their motion is strictly random. This occurs at low pressures where fewer molecules are present. Depending on the pressure, a gas molecule might travel inches, feet, or even miles before it strikes another molecule. This means we can’t depend on molecular interaction to push or start a flow pattern. In the molecular flow range, molecular movement is unpredictable. This is why we have such large inlets in high vacuum pumps. The use of large inlets increases the probability that one of these randomly moving molecules will move into the pump.
Vacuum Theory: Gas Flow
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 In molecular flow, the molecular motion is explained by kinetic theory, which uses statistics (chance) to describe the condition. The difference between viscous flow and molecular flow does not depend upon the pressure alone. It also depends upon the dimensions of the vacuum container (pipes, chamber, etc.). Basically, it depends upon the mean free path and whether it is longer or shorter than the container dimensions. Let’s take a look at what is meant by “mean free path.”
7.3. Mean Free Path
As we lower the pressure in the vacuum chamber, the amount of space between the gas particles increases. The particles bump into each other less frequently. The average distance a particle moves before it bumps another particle is the mean free path. Table 6.10. Molecular Density and Mean Free Path 7.6 × 102 Torr (atm)
1 × 10–3 Torr
1 × 10–9 Torr
# mol/cm3
3 × 1019 (30 million trillion)
4 × 1013 (40 trillion)
4 × 107 (40 million)
MFP
2 × 10–6 in.
2 in.
30 mi.
At atmosphere, the mean free path is extremely short, about two millionths of an inch. Under vacuum, fewer molecules remain, and the mean free path is longer. Its length depends on the number of molecules present, and therefore on the pressure. The mean free path for air can be estimated from the relationship:
–3
(----------------------------------------------------------------------------------------Mean Free Path = 5 × 10 torr cm )P torr
From this, we can see that as the pressure gets lower, the mean free path gets longer. Likewise, as the pressure gets lower, there are fewer molecules of gas present, so there is less chance of them running into each other. In 1 cc of gas at standard conditions (760 torr at 0°C), there are about 3 × 1019 gas molecules and the mean free path is about 2 × 10–6 cm (a few millionths of an inch). At 1 × 10–9 torr, there are about 4 × 107 molecules/cc, and the mean free path is about 30 miles or 50 kilometers. The number of molecules per unit volume (in this example cubic centimeters) is called the gas density.
8. Conductance
When we talk about moving a gas through a vacuum system, we use the term conductance. Conductance is the ability of an opening or pipe to allow a given volume of gas to pass through in a given time. It is expressed in such units as liters per second, cubic feet per minute or cubic meters per hour. In molecular flow, a good conductance path is wide and short. It has few turns, thus allowing free gas flow. In viscous flow, these conditions are not so important. This is because the molecules tend to push one another along under the influence of a pressure difference.
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Vacuum Theory: Conductance
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In the molecular flow range, a 1-in2 opening has a 75 l/sec conductance. The pump speed, in this case 400 l/sec, is really 75 l/sec as far as the chamber is concerned because the molecules must go through the hole before they can be pumped. To improve system performance, the conductance must first be improved. (Make the hole bigger!)
CHAMBER
2
1 IN. OPENING (75 l/SEC) 2 OR 11.6 l/SEC CM
400 l/SEC PUMP
MOLECULAR FLOW
To repeat: In the molecular flow range, a pump works only when molecules migrate into the pump by chance.
8.1. Conductance in Viscous Flow
The volume of gas that can flow per unit of time through a pipe under viscous flow conditions is related to the fourth power of the pipe diameter and is inversely related to the length of the pipe. For example, if you use a pipe with a diameter twice that of the pipe presently being used, it will allow 24 or sixteen times as much gas to flow through it, assuming that the length of the pipe is the same. Now let’s compare this to molecular flow conditions.
Vacuum Theory: Conductance
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8.2. Conductance in Molecular Flow
The volume of gas that can flow per unit of time through a pipe under molecular flow conditions is related to the cube of the diameter and inversely to the length of the pipe. Using the same pipe as in the viscous flow example, doubling the diameter of the pipe will, at most, allow 23 or eight times the flow for the same length of pipe. In either case of viscous flow or molecular flow, making the pipe shorter will increase the flow of gas through the pipe. Please note that these are gross statements that are subject to all kinds of qualifications. There is another region where we approach molecular flow, but the flow is not really viscous either. This region is called the transition range. There is another set of calculations to be used for the transition range, but we will not discuss them in this text.
9. Review of the Nature of Gases
Before continuing, a few basic facts about the atomic and molecular nature of gases should be reviewed. These facts will be useful for understanding how ion pumps operate. Let us review them briefly here. An atom is the smallest particle of matter that can exist and still retain the basic characteristics of the material or element from which it came. Molecules are simply one atom or two or more atoms joined together; many gases exist as molecules. Atoms and molecules normally have an equal number of protons (positively charged particles) and electrons (negatively charged particles). The neutrons in the nucleus contribute to the weight (mass) of the atom, but not to the charge. The atoms are thus neutral, or electrically balanced. If this balance is upset, useful work can be produced. If we remove electrons from the atom, we have made a positively charged atom or molecule we call an ion. This process of creating ions is called ionization. We can put these otherwise useless charged particles to work because we can direct their motion using a magnetic or electrical field.
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Vacuum Theory: Review of the Nature of Gases
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
10. Ion Pump
Let us now make our ion pump by connecting two electrodes to a high-voltage supply. Electron flow will be from cathode to anode as in this drawing. Ions will carry current from anode to cathode. Fewer ions than electrons will be produced so that we can say that the current through the pump is the “ion current.”
ELECTRON SOURCE
(–)
(+)
CATHODE
ANODE
(–)
POWER SUPPLY
(+)
In this drawing, a free electron is attracted to a positively charged anode. On the way to the anode, it collides with a neutral atom, ionizing it. Now two electrons are free to continue toward the anode, increasing the probability of still further ionization. The positively charged ions are then accelerated toward the negatively charged cathode. They may strike the cathode with such force that they stick to the cathode material, and are thereby pumped. As one gas molecule is driven into the cathode, one or more molecules of the cathode is usually released from this surface. This process is called sputtering. The ion pump is also a gas capture pump. It is not designed to pump heavy gas loads. For this reason, it is not generally used alone in high-production applications. Instead, it is more often used in research and analytical applications where there is no need to repeatedly and rapidly cycle the work chamber to atmosphere. When combined with a Titanium Sublimation Pump (TSP), it also provides adequate pumping for these applications. Ion pumps are clean operating devices. They are electronic devices which use no moving parts or oils. It is possible to achieve pressures in the 10–11 torr range, with overnight bakeout of the system. The bakeout process drives residual gas off walls. This gas is then pumped by the ion pumps. In research and analytical applications, the ion pump’s cleanliness, bakeability, low power consumption, vibration-free operation and long life make it the pump of choice for most ultra-high vacuum uses. Ion pumps come in various sizes. A small appendage ion pump is used not for pumping down, but for maintaining vacuum conditions in operating devices such as transmitting tubes. Larger pumps can be used to evacuate small chambers, or several can be connected in parallel with other ion pumps to pump down larger chambers.
Vacuum Theory: Ion Pump
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10.1. Components
10.2. How the Pump Works
A basic ion pump cell consists of two titanium cathodes and an anode. All are placed between the poles of a strong permanent magnet.
NEUTRAL GAS MOLECULE ANODE
CATHODE MAGNET ELECTRON
AN ELECTRON IS “BROKEN FREE” FROM A GAS MOLECULE. THE GAS MOLECULE THEN BECOMES A POSITIVE ION.
10.2.1. Pump Operation
The magnetic field forces the free electrons to travel in long helical paths instead of straight lines. This increases the probability of collision with molecules on their way to the positively charged anode. This, in turn, increases the ionization probability, and therefore the amount of useful pumping action that can be performed by the pump. Because of the action of the magnetic field, the electrons do not easily come in contact with the anode. As a result, a “cloud” of electrons is formed in the anode area. This electron cloud becomes fairly stable during pump operation. The electron density is high enough for efficient ionization of gas molecules. Therefore, a hot filament electron source is not needed. So, the name for this process is cold cathode discharge. The positively charged ions, which are relatively heavy particles, are accelerated into the negatively charged titanium cathodes. This impact causes sputtering, or chipping away of the titanium cathode material. Sputtered titanium deposits onto the internal structure of the pump. There it is available for chemical combination with gas molecules to convert them to solids. Thus we have the needed pumping action.
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Vacuum Theory: Ion Pump
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 TITANIUM CATHODES
POSITIVE ION ANODE
SPUTTERED ATOM FROM CATHODE
MAGNET In addition, a second pumping action takes place. Some of the ionized molecules strike the cathodes with enough force to become buried in them. This burial prevents them from recombining and becoming a free gas again.
(+) (–)
(–)
MAGNET Still another pumping process occurs in the case of hydrogen, which diffuses directly into and reacts with the cathode plate. Also, neutral molecules in the anode regions can literally be buried or “plastered over” by the sputtered cathode material. Complex molecules may also be split in the discharge to smaller, more readily pumped molecules.
Vacuum Theory: Ion Pump
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 There is a problem with the pump design we have described (also called a diode configuration). Some of the buried molecules can be released again into the vacuum system. This re-release can be caused by heating of the cathodes or reduction of cathode material due to sputtering. It can also be caused by a molecule or atom being physically separated from the sputtered film.
10.2.2. Pumping Characteristics of Different Configurations
Ion pumps are available in different design configurations. Each design has its own special pumping characteristics. In the diode pump, as we have seen, the ions strike the cathode plate and react with the sputtered titanium. The triode pump, which is a variation on the diode pump, improves inert or noble gas pumping. Titanium cathodes are in the form of grids. Ions sputter titanium onto the pump walls. This angled impact sputters more titanium than in the diode model and thus furnishes more material for argon or noble gas burial. Because of the electrical arrangement of the pump components, the glow discharge that happens in “starting” the diode pump is typically confined in the triode pump. As a result, the triode pump can be started at slightly higher pressure. TITANIUM CATHODE PLATES
CONTROL UNIT
CONTROL UNIT MAGNET N
S
N
S
MAGNET
MULTICELL ANODE PUMP WALL FORMS THIRD ELECTRODE IN TRIODE PUMP SPUTTER CATHODES TITANIUM VANES
MULTICELL ANODE
ION PUMP SCHEMATICS (A) DIODE ION PUMP
(B) TRIODE ION PUMP
VB
V+ ANODE + ARGON
SPUTTERING
MAGNETIC FIELD
TITANIUM ATOMS ENTRAPMENT OF BURIED ARGON IONS
MAGNETIC FIELD
ANODE +
SPUTTERED TITANIUM, ARGON ATOMS BURIED HERE
+
PUMP WALL TI SPUTTER CATHODE
OBLIQUE IMPACT CAUSES MAXIMUM SCATTERING
ARGON IONS
PUMPING MECHANISMS (A) DIODE ION PUMP
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(B) TRIODE ION PUMP
Vacuum Theory: Ion Pump
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10.2.3. Other Characteristics
The ion pump is self-regulating. At the higher pressures, where much ionization takes place, more current flows. At low pressures, less current flows. This characteristic current drain can be used to measure the pressure, or degree of vacuum achieved with the pump. This feature eliminates the need for an ion gauge on the system.
10–2
10–3
PRESSURE (TORR)
10–4
10–5
10–6
–7
10
10–8
10–9 1µA
10µA
100µA
1mA
10mA 100mA 1Amp
PUMP CURRENT
Ion pumps are long-lived; the lower the pressure, the longer the life. Once they begin pumping, they quickly lower the pressure to the long-life region. As long as they are not pumping against a leak, they will last for years. Ideally, ion pumps should be started at pressures approaching 10–5 torr. At higher pressures, the plasma discharge that is generated minimizes pumping speed and reduces cathode life. A more common and practical approach is to sorption rough the pump to less than 10–2 torr before applying the ion pump power. At very low pressures, the time taken to begin the ionization process may be excessively long. Table 6.11. Typical Diode Pump Service Life Pressure (Torr) –3
Life (Hours)
10
20
10–4
200
–5
2,000
–7
200,000 (over 20 years of constant operation)
10 10
Table 6.12. Life (Pumping N2 at 10–4 Torr)
Vacuum Theory: Ion Pump
Type
Life
Triode
35,000 hours — approx. 4 years
Diode
50,000 hours — approx. 6 years
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10.3. Vacuum System Use
Ion pumps are typically used in systems which demand ultra-clean, ultrahigh vacuum. This type of vacuum system is pumped to high vacuum or lower pressure and then kept in that condition for long periods of time. A load-lock chamber is often built on the system to allow access to the chamber without bringing the chamber back to air. Typical uses are for electron microscopes, mass spectrometers, and surface analysis, to mention a few. Very little maintenance can be performed on ion pumps other than an occasional bakeout. When pumping eventually deteriorates to the point where operating pressures can no longer be attained, pump replacement or sometimes anode/cathode assembly replacement is necessary.
10.4. Summary
We have discussed the pressure ranges of vacuum pumps and the major types of pumps in each range. By now, you should be familiar with the different types of vacuum pumps what their major components are and how they work. You have also learned how they are placed in vacuum systems and some general maintenance information. Let’s go on now to gauges. These are major vacuum components that tell you what is going on inside your vacuum system.
11. Vacuum Gauges
To transfer heat by convection, we need massive numbers of molecules flowing. Your hot-air furnace heats by convection. Some gauges use this principle between 760 torr and 2 torr but are generally less accurate in this pressure range. To transfer heat by radiation, we need light energy. Not the kind of light that you see, but typically infrared light. The heat you feel when standing in front of a fireplace is mostly the radiated heat. No gas molecules need be involved; that is, radiation is independent of the number of gas molecules present. Radiated heat is the only way to transfer heat inside of a vacuum system at high vacuum. There are insufficient molecules present to provide heat transfer by either conduction or convection. Now, let’s go on to discuss gauges that depend on heat transfer to work.
11.1. Thermocouple Gauge
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Vacuum Theory: Vacuum Gauges
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 The thermocouple, or TC, gauge is another rugged, simple instrument. It is used to measure pressures in the rough vacuum range. It does its work well under less than ideal conditions. The TC gauge measures temperature and converts it to a pressure reading. Many modern thermocouple gauges have been modified to use convection as well as conduction principles. This effectively extends their useful range to atmosphere. It is typically considered as a very approximate device. Let’s take a look at how it works.
11.1.1. How the Gauge Works
A thermocouple gauge consists of a gauge tube and control unit. Within the gauge tube is a heated filament. Spot welded to the filament is a thermocouple that measures the temperature of the hot wire. The meter is calibrated in pressure units, not in temperature. TO VACUUM SYSTEM TC GAUGE TUBE FILAMENT
CONTROL UNIT THERMOCOUPLE
At atmospheric pressure, there will be many molecular collisions with the heated filament. The gas molecules conduct heat away from the filament. The amount of heat removal can be related to the amount of gas in the chamber. At higher pressures, with lots of molecules, much heat will be conducted away from the wire. Therefore, the wire will be at a lower temperature (cooler). When we pump away the gas, there are fewer molecules to collide with the wire. The wire is therefore at a higher temperature (hotter). THERMOCOUPLE GAUGE PRINCIPLE FILAMENT
(+)
(–) THERMOCOUPLE
METER
Vacuum Theory: Vacuum Gauges
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 There is not a linear relationship between wire temperature and pressure, so the pressure scale on your TC gauge is not linear. The gauge stops responding at about 1 millitorr (10–3 torr) because the heat loss through radiation is now the largest factor. The heat lost through radiation is also constant. Therefore, the gauge reads “zero.” Compared to other gauges, the TC gauge has a slow response time. This is because the wire must have time to heat up or cool down as the pressure changes. Some newer gauges speed up the response time by operating the gauge at constant temperature and measuring the change in current required to hold the temperature constant.
11.1.2. Maintenance
If the sensing unit, or gauge head, gets dirty, it may be cleaned with an appropriate solvent. Most people will simply discard the TC gauge and install a new one in its place. Whenever you clean or replace a TC gauge, it should be adjusted to read the proper values. To do this, you expose the gauge head to a pressure of 10–4 torr or less and adjust the control unit to read zero on the pressure gauge. If for some reason you cannot obtain a pressure below 10–4 torr, then install a “good” gauge and set the system gauge to read the same pressure. Please check the operation manual for your particular unit for adjustment instructions, because they do vary in detail.
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Vacuum Theory: Vacuum Gauges
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Chapter Seven Glossary
This chapter is a glossary of terms commonly used in radiotherapy, and is intended for reference use only.
Glossary
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Table of Contents A:......................................................................................................................................... 7-3 B: ........................................................................................................................................ 7-5 C: ........................................................................................................................................ 7-8 D: ...................................................................................................................................... 7-12 E:....................................................................................................................................... 7-15 F: ....................................................................................................................................... 7-18 G: ...................................................................................................................................... 7-19 H: ...................................................................................................................................... 7-21 I:........................................................................................................................................ 7-22 J: ....................................................................................................................................... 7-24 K:....................................................................................................................................... 7-24 L: ....................................................................................................................................... 7-25 M: ...................................................................................................................................... 7-25 N: ...................................................................................................................................... 7-27 O: ...................................................................................................................................... 7-28 P: ....................................................................................................................................... 7-29 Q: ...................................................................................................................................... 7-31 R:....................................................................................................................................... 7-32 S:....................................................................................................................................... 7-35 T: ....................................................................................................................................... 7-38 U: ...................................................................................................................................... 7-40 V:....................................................................................................................................... 7-40 W:...................................................................................................................................... 7-41 X:....................................................................................................................................... 7-42
7-2
Glossary
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Absorbed Dose
See Dose, Absorbed Dose.
Absorbed Fraction
A term used in internal dosimetry. It is that fraction of the photon energy (emitted within a specified volume of material) which is absorbed by the volume. The absorbed fraction depends on the source distribution, the photon energy, and the size, shape, and composition of the volume.
Absorption
The process by which radiation imparts some or all of its energy to any material through which it passes. (See also Compton Effect, Pair Production and Photoelectric Effect.) Absorption Coefficient: Fractional decrease in the intensity of a beam of x or gamma radiation per unit thickness (linear absorption coefficient), per unit mass (mass absorption coefficient), or per atom (atomic absorption coefficient) of absorber, due to deposition of energy in the absorber. The total absorption coefficient is the sum of the individual energy absorption process (Compton effect, photoelectric effect, and pair production). Linear Absorption Coefficient: A factor expressing the fraction of a beam of x or gamma radiation absorbed in unit thickness of material. In the ex– µx pression I = I 0 e , I0 is the initial intensity, I the intensity of the material x, and µ is the linear absorption coefficient. Mass Absorption Coefficient: The linear absorption coefficient per cm. divided by the density of the absorber in grams per cu. cm. It is frequently expressed as µ/p, where µ is the linear absorption coefficient and p the absorber density. Self-Absorption: Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located; in particular, the absorption of radiation within a sample being assayed.
Accelerator
A machine that accelerates electrically charged atomic particles to high velocities. Electrons, protons, deuterons, and alpha particles can be accelerated to nearly the speed of light for use in nuclear research. Types of accelerators include the betatron, cyclotron, linear accelerator, and synchrotron.
Achromatic
A type of bend magnet in which particles of differing energies are brought to the same focus.
Actinic
A type of radiation that is capable of producing a chemical change.
Activation
The process of inducing radioactivity by irradiation.
Activity
The number of nuclear transformations occurring in a given quantity of material per unit time.
Adsorption
The adhesion of one substance to the surface of another.
Alpha Particle
A charged particle emitted from the nucleus of an atom having a mass and charge equal in magnitude to those of a helium nucleus, i.e., two protons and two neutrons. Aluminum Equivalent: The thickness of aluminum affording the same attenuation, under specified conditions, as the material in question.
Glossary: Absorbed Dose
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Alternating Current (A.C.)
Electric current that flows for a given length of time in one direction and immediately flows in the opposite direction for the same length of time. It usually consists of 60 complete cycles per second.
Alveoli
The terminal air sacs of the lung.
Ampere (A)
A unit of electrical current or rate of flow of electrons.
Amplification
As related to radiation detection instruments, the process (gas, electronic, or both) by which ionization effects are magnified to a degree suitable for their measurement.
Amplifier, Linear
A pulse amplifier in which the output pulse height is proportional to an input pulse height for a given pulse shape up to a point at which the amplifier overloads.
Amplifier, Pulse
An amplifier, designed specifically to amplify the intermittent signals of a nuclear detector, incorporating appropriate pulse-shaping characteristics.
Analyzer, Pulse Height
An electronic circuit which sorts and stores the pulses according to height.
Angstrom Unit (Å)
One angstrom unit equals 10–8 cm.
Anion
Negatively charged ion.
Annihilation (Electron)
An interaction between a positive and a negative electron in which they both disappear; their energy, including rest energy, being converted into electromagnetic radiation (called annihilation radiation).
Anode
Positive electrode; electrode to which negative ions are attracted.
Arc Therapy
Radiation therapy in which the source of radiation is moved through a limited arc about the patient during treatment. In this way, a larger dose is built up at the center of rotation within the patient’s body than on any area of the skin. Multiple arcs may be used. Synonymous with arc treatment and rotation therapy.
Ataxia
The inability to coordinate muscular movements.
Atom
Smallest particle of an element that is capable of entering into a chemical reaction.
Atomic Mass (u)
The mass of a neutral atom of a nuclide, usually expressed in terms of “atomic mass units.” The “atomic mass unit” is one twelfth the mass of one neutral atom of carbon 12; equivalent to 1.6604 × 10–24 gm.
Atomic Number (Z)
The number of protons in the numbers of a neutral atom of a nuclide. The “effective atomic number” is calculated from the composition and atomic numbers of a compound or mixture. An element of this atomic number would interact with photons in the same way as the compound or mixture.
Atomic Weight
The weighted mean of the masses of the neutral atoms of an element expressed in atomic mass units.
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Glossary: Alternating Current (A.C.)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Attenuation
The process by which a beam of radiation is reduced in intensity when passing through some material. It is the combination of absorption and scattering processes and leads to a decrease in flux density of the beam when projected through matter.
Attenuation Coefficient
A general term used to describe quantitatively the reduction in intensity of a beam of radiation as it passes through a particular material. Attenuation Coefficient, Compton: The fractional number of photons removed from a beam of radiation per unit thickness of a material through which it is passing as a result of Compton effect interactions. Attenuation Coefficient, Linear: The fractional number of photons removed from a beam of radiation per unit thickness of a material through which it is passing due to all absorption and scattering processes. Attenuation Coefficient, Pair Production: That fractional decrease in the intensity of a beam of ionizing radiation due to pair production in a medium through which it passes. Attenuation Coefficient, Photoelectric Effect: That fractional decrease in the intensity of a beam of ionizing radiation due to photoelectric effect in a medium through which it is passing.
Attenuation Factor
A measure of the opacity of a layer of material for radiation traversing it; the ratio of the incident intensity to the transmitted intensity. It is equal to I0/I, where I0 and I are the intensities of the incident and emergent radiation, respectively. In the usual sense of exponential absorption (I = I0e–µt), the attenuation factor is e–t where t is the thickness of the material and µ is the absorption coefficient.
Auger Effect
The emission of an electron from the extranuclear portion of an excited atom when the atom undergoes a transition to a less excited state.
Autoradiography
Record of radiation from radioactive material in an object, made by placing the object in close proximity to a photographic emulsion.
Autotransformer
A transformer with a single wrapping or winding of wire, with both ends of the wire attached to the primary alternating current.
Avalanche
The multiplicative process in which a single charged particle accelerated by a strong electric field produces additional charged particles through collision with neutral gas molecules. This cumulative increase of ions is also known as “Townsend ionization” or “Townsend avalanche.”
Average Life (Mean Life)
The average of the individual lives of all the atoms of a particular radioactive substance. It is 1.443 times the radioactive half-life.
Avogadro’s Number (Avogadro’s Constant) (NA)
Number of atoms in a gram atomic weight of any element; also the number of molecules in a gram molecular weight of any substance. It is numerically equal to 6.023 × 1023 on the unified mass scale.
Backplane
A printed circuit board with connectors for inserting other printed circuit boards.
Back Pointer
A linear accelerator accessory used to identify the central axis of the radiation beam.
Glossary: Attenuation
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Backscatter
The deflection of radiation by scattering processes through angles greater than 90 degrees with respect to the original direction of motion.
Barriers, Protective
Barriers of radiation-absorbing material, such as lead, concrete, and plaster, used to reduce radiation exposure. Barriers, Primary Protective: Barriers sufficient to attenuate the useful beam to the required degree. Barriers, Secondary Protective: Barriers sufficient to attenuate stray radiation to the required degree.
Beam
A unidirectional or approximately unidirectional flow of electromagnetic radiation or of particles. Useful Beam (Radiology): Radiation that passes through the aperture, cone, or other collimating device of the source housing; sometimes called “primary beam.”
Beam Axis
A geometric line from the target (or source) outward along the geometric center of the beam.
Beam Hardening
The process of eliminating the low-energy photons from a beam of x-rays. This process changes the quality of the beam in such a manner that the average energy of the beam increases.
Beam Quality
The spectral energy distribution of the radiation beam. Beam quality affects the penetration of the beam through tissue and the relative absorption of the energy in different types of tissue.
Beam Shaping
The use of special blocks, wedges, compensators, and other devices to create a treatment beam of the geometric proportions required for a treatment plan beyond the capabilities of the collimator.
Becquerel (Bq)
The new special unit of activity. One becquerel equals one nuclear disintegration per second.
Bend Magnet Assembly
A beam transport system for guiding the electron beam from the linear accelerator structure to the x-ray target or electron scattering foil.
Beta Particle
Charged particle emitted from the nucleus of an atom, with a mass and charge equal in magnitude to that of the electron.
Betatron
A magnetic induction accelerator which makes use of a varying magnetic field to accelerate electrons. Electrons are injected into a toroidal vacuum chamber which is between the poles of an iron-core magnet. The rate of change of the magnet flux and magnetic field at the orbit radius are related to maintain a constant radius for the accelerating electrons.
Biologic Effectiveness of Radiation
(See Relative Biologic Effectiveness.)
Blood Dyscrasia
Any persistent change from normal of one or more of the blood components.
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Glossary: Backscatter
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Bone Marrow
Soft material which fills the cavity in most bones; it manufactures most of the formed elements of the blood.
Bone Seeker
Any compound or ion which migrates in the body preferentially into bone.
Brachytherapy
Therapy at short distances with beta or gamma radiation. Implantation or placement therapy with needles, inserts, or other such applications containing radioactive materials. Useful in the treatment of various diseases.
Bragg-Gray Principle
The relationship between energy absorbed in a small gas-filled cavity in a medium to energy absorbed (in the medium) from ionizing radiation. The relationship is expressed as Em = W × Jg × Sgm, where Em = energy/mass absorbed in the medium, W = average energy needed to produce an ion pair in the gas, Jg = number of ion pairs/mass formed in the gas, and S = ratio of the stopping power for secondary particles in the medium to that in the gas.
Branching
The occurrence of two or more modes by which a radionuclide can undergo radioactive decay. For example, RaC can undergo α or ß– decay, 64Cu can undergo ß–, ß+, or electron capture decay. An individual atom of a nuclide exhibiting branching disintegrates by one mode only. The fraction disintegrating by a particular mode is the “branching fraction” for that mode. The “branching ratio” is the ratio of two specified branching fractions (also called multiple disintegration).
Bremsstrahlung
Secondary photon radiation produced by deceleration of charged particles passing through matter.
Buildup
The increase in absorbed dose with depth below the surface in a material irradiated by a beam of photons or particulate radiation. Buildup may be of two kinds: Electron Buildup: This is due to the production by the incident radiation of increasing numbers of forward-moving high-energy electrons increasing with depth until a maximum electron fluence rate has been reached. This effect gives rise to the phenomenon of “skin sparing” and is most marked for photon energies greater than about 400 keV. The effect is not noticeable for x-ray photons generated by potentials of less than 400 kV. For high-energy beams, this process is more important. Photon Buildup: Multiple photon scattering in the superficial layers of the phantom, which may lead to an increase in absorbed dose for a short distance. This effect is observed particularly with photons generated by potentials of 50 to 150 kV and large field sizes.
Buildup Factor
The ratio of the intensity of x or gamma radiation (both primary and scattered) at a point in an absorbing medium to the intensity of only the primary radiation. This factor has particular application for “broad beam” attenuation. “Intensity” may refer to energy flux, dose, or energy absorption.
Buncher
The input resonant cavity in a klystron or linear accelerator.
Burial Ground (Graveyard)
A place for burying unwanted radioactive objects to prevent escape of their radiations, the earth or water acting as a shield. Such objects must be placed in watertight, non-corrodible, containers so the radioactive material cannot leach out and invade underground water supplies.
Glossary: Bone Marrow
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Calibration
Determination of variation from standard, or accuracy, of a measuring instrument to ascertain necessary correction factors.
Calorie (cal)
Amount of heat necessary to raise the temperature of one gram of water 1°C (from 14.5 to 15.5°C).
Cancer
Any malignant neoplasm (Popular usage).
Capillary
A small, thin-walled blood vessel connecting an artery with a vein.
Capture, Electron
A mode of radioactive decay involving the capture of an orbital electron by its nucleus. Capture from a particular electron shell is designated as Kelectron capture, L-electron capture, etc.
Capture, K-Electron
Electron capture from the K shell by the nucleus of the atom. Also loosely used to designate any orbital electron process.
Capture, Radiative
The process by which a nucleus captures an incident particle and loses its excitation energy immediately by the emission of gamma radiation.
Capture, Resonance
An inelastic nuclear collision occurring when the nucleus exhibits a strong tendency to capture incident particles or photons of particular energies.
Carcinogenic
Capable of producing cancer
Carcinoma
Malignant neoplasm composed of epithelial cells, regardless of their derivation.
Card Cage
A chassis or frame that holds printed circuit boards.
Carrousel
An rotating assembly in the treatment head that places various elements, such as flattening filters and scattering foils, into the beam path.
Catalyst
A substance which alters the velocity of a chemical reaction (positive catalysts increase velocity) yet may be recovering practically unchanged after the reaction has occurred.
Cataract
A clouding of the crystalline lens of the eye which obstructs the passage of light.
Cathode
Negative electrode; electrode to which positive ions are attracted.
Cation
Positively charged ion.
Cell
(Biological) The fundamental unit of structure and function in organisms.
Cells, Somatic
Body cells, usually with two sets of chromosomes, as opposed to germ cells, which have only one set.
Central Axis
The straight line passing through the center of the target (or source) and the center of the final collimator. Concentric with the beam axis.
Chamber, Cloud
A device for observing the paths of ionizing particles. It is based on the principle that supersaturated vapor condenses more readily on ions than on neutral molecules.
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Glossary: Calibration
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Chamber, Ionization
An instrument designed to measure a quantity of ionizing radiation in terms of the charge of electricity associated with ions produced within a defined volume. (See also Condenser r-Meter.) Air-Wall Ionization Chamber: Ionization chamber in which the materials of the wall and electrodes are so selected as to produce ionization essentially equivalent to that in a free-air ionization chamber. This ionization is possible only over limited ranges of photon energies. Such a chamber is more appropriately termed an air-equivalent ionization chamber. Extrapolation Ionization Chamber: An ionization chamber with electrodes whose spacing can be adjusted and accurately determined to permit extrapolation of its reading to zero chamber volume. Free-Air Ionization Chamber: An ionization chamber in which a delimited beam of radiation passes between the electrodes without striking them or other internal parts of the equipment. The electric field is maintained perpendicular to the electrodes in the collecting region. As a result, the ionized volume can be accurately determined from the dimensions of the collecting electrode and the limiting diaphragm. This ionization chamber is the basic standard instrument for x-ray dosimetry within the range of 5 to 1400 kVp. Standard Ionization Chamber: A specially constructed ionization chamber from which other ionization chambers can be calibrated. Thimble Ionization Chamber: A small cylindrical or spherical ionization chamber, usually with walls of organic material. Tissue Equivalent Ionization Chamber: An ionization chamber in which the material of the walls, electrodes, and gas are so selected as to produce ionization essentially equivalent to that characteristic of the tissue under consideration. In some cases it is sufficient to have only tissue equivalent walls, and the gas may be air, provided the air volume is negligible. The essential point in this case is that the contribution to the ionization in the air made by ionizing particles originating in the air is negligible, compared to that produced by ionizing particles characteristic of the wall material.
Chamber, Pocket
A small, pocket-sized ionization chamber used for monitoring radiation exposure of personnel. Before use, it is given a charge, and the amount of discharge is a measure of the radiation exposure.
Charger-Reader
An auxiliary device used for establishing a particular voltage level in an ionization chamber and subsequently for evaluating that voltage level.
Charge, Space
The electric charge carried by a cloud or stream of electrons or ions in a vacuum or a region of low gas pressure, when the charge is sufficient to produce local changes in the potential distribution. It is of importance in thermionic tubes, photoelectric cells, ion accelerators, etc.
Cerenkov Radiation
Blue light emitted when a charged particle moves in a transparent medium with a speed greater than that of light in the same medium.
Circuit, Anticoincidence
A circuit with two input terminals which delivers an output pulse if one input terminal receives a pulse, but delivers no output pulse if pulses are received by both input terminals simultaneously or within an assignable time interval.
Circuit Breaker
A switch that automatically interrupts an electrical circuit upon sensing an abnormal flow of electrical current.
Glossary: Chamber, Ionization
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Circuit, Coincidence
An electron circuit that produces a usable output pulse only when each of two or more input circuits receives pulses simultaneously or within an assignable time interval.
Circuit, Integrating
An electron circuit which records the total number of ions or events collected for a given time from which an average value for the number of ions or events per unit time can be found.
Cladding (Clad)
An external layer of material applied directly to nuclear fuel or other material to provide protection from a chemically reactive environment, to provide containment of radioactive products produced during the irradiation of the composite, or to provide structural support.
Clinical
Pertaining to the observed symptoms and cause of disease.
Coincidence
The occurrence of counts in two or more detectors simultaneously or within an assignable time interval. A true coincidence is one that is due to the incidence of a single particle or of several genetically related particles. An accidental, chance, or random coincidence is one that is due to the accidental occurrence of unrelated counts in the separate detectors. An anticoincidence is the occurrence of a count in a specified detector unaccompanied simultaneously or within an assignable time interval by a count in other specified detectors. A delayed coincidence is the occurrence of a count in one detector at a short, but measurable, time after a count in another detector. The two counts are due to genetically related occurrence, such as successive events in the same nucleus.
Collimator
A device for confining the elements of a beam within an assigned solid angle. Sets of metal blocks, fixed and movable, in the treatment head that limit the treatment field to the desired size.
Collimator Rotation Readout
A display that indicates the degrees of rotation of the collimator about the central axis.
Collision
Encounter between two subatomic particles (including photons) which changes the existing momentum and energy conditions. The products of the collision need not be the same as the initial systems. Elastic Collision: A collision in which no change occurs either in the internal energy of each participating system or in the sum of their kinetic energies of translation. Inelastic Collision: A collision in which changes occur both in the internal energy of one or more of the colliding systems and in the sums of the kinetic energies of translation before and after the collision.
Compensator
A slab of material placed in the treatment beam to compensate for unevenness of machine output or body contour.
Compton Effect
An attenuation process observed for x or gamma radiation in which an incident photon interacts with an orbital electron of an atom to produce a recoil electron and a scattered photon of energy less than the incident photon. (See also Absorption, Pair Production, and Photoelectric Effect.)
Condenser rMeter
An instrument consisting of an “air-wall” ionization chamber together with auxiliary equipment for charging and measuring its voltage. It is used as an
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Glossary: Circuit, Coincidence
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 integrating instrument for measuring the exposure of x or gamma radiation in roentgens (R). (See also Chamber, Ionization) Console Group
The Clinac operator interface and system control units, typically including a keyboard, display terminal, control console, console computer, and printer.
Contactor
A heavy duty relay used to control high-power electrical circuits.
Contamination
A foreign substance dispersed where it is undesirable: for example, unwanted electrons in the photon beam or unwanted photons in an electron beam.
Contamination, Radioactive
Deposition of radioactive material in any place where it is not desired, particularly where its presence may be harmful. The harm may be in vitiating an experiment or a procedure, or in endangering personnel.
Control Console
The Clinac system control unit, which includes operator controls for starting and stopping an exposure.
Controlled Area
A defined area in which the occupational exposure of personnel (to radiation) is under the supervision of the Radiation Protection Supervisor.
Control System
A coordinated group of components designed to exert a directing influence on other components. A system of apparatus for automatically controlling an accelerator by a servo system that adjusts the control elements to maintain the flux level near a desired value.
Corpuscle
A blood cell.
Corpuscular Emission, Associated
The full complement of secondary charged particles (usually limited to electrons) associated with an x-ray or gamma ray beam in its passage through air. The full complement of electrons is obtained after the radiation has traversed sufficient air to bring about equilibrium between the primary photons and secondary electrons. Electronic equilibrium with the secondary photons is intentionally excluded.
Coulomb (C)
Unit of quantity in current electricity. A quantity afforded by 1 ampere of current in 1 second flowing against 1 Ohm of resistance with a force of 1 Volt.
Count (Radiation Measurements)
The external indication of a device designed to enumerate ionizing events. It may refer to a single detected event or to the total number registered in a given period of time. The term often is erroneously used to designate a disintegration, ionizing event, or voltage pulse. Spurious Count: In a radiation counting device, a count caused by any agency other than radiation.
Counter, Gas Flow
A device in which an appropriate atmosphere is maintained in the counter tube by allowing a suitable gas to flow slowly through the sensitive volume.
Counter, GeigerMuller
Highly sensitive, gas filled radiation measuring device. It operates at voltages sufficiently high to produce avalanche ionization.
Glossary: Console Group
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Counter, Proportional
Gas-filled radiation detection device; the pulse produced is proportional to the number of ions formed in the gas by the primary ionization particle.
Counter, Scintillation
The combination of scintillator, photomultiplier tube, and associated circuits for counting light emissions produced in the phosphors.
Counting, Coincidence
A technique in which particular types of events are distinguished from background events by coincidence circuits, which register coincidences caused by the type of events under consideration.
Counting Ratemeter
An instrument that gives a continuous indication of the average rate of ionizing events.
Cross-Sectional Area (of an x-ray beam)
An area in the plane of the beam perpendicular to its direction of travel.
Curie
The former special unit of activity. One curie equals 3.7 × 1010nuclear transformations per second. Several fractions of the curie are in common usage. (Abbreviated Ci) Microcurie: One-millionth of a curie (3.7 × 104 disintegrations per second). Abbreviated Ci. Millicurie: One-thousandth of a curie (3.7 × 107 disintegrations per second). Abbreviated mCi. Picocurie: One-millionth of a microcurie (3.7 × 10–2 disintegrations per second or 2.22 disintegrations per minute). Abbreviated pCi; replaces the term µµCi.
Cutie Pie
An ionization chamber device commonly used for detecting radiation exposure rate.
Cyclotron
A particle accelerator in which charged particles receive repeated synchronized accelerations or “kicks” by electrical fields as the particles spiral outward from their source. The particles are kept in the spiral by a powerful magnet.
Daughter
Synonym for Offspring. (See Decay Product.)
Deadman Switch
Synonymous with Motion enable switch.
Decay, Radioactive
Disintegration of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons.
Decay Constant
The fraction of the number of atoms of a radioactive nuclide which decay in unit time. Symbol: λ . (See also Decay Curve and Disintegration Constant.)
Decay Curve
A curve showing the relative amount of radioactive substance remaining after any time interval.
Decay Product
A nuclide resulting from the radioactive disintegration of a radionuclide, formed either directly or as the result of successive transformations in a radioactive series. A decay product may be either radioactive or stable.
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Glossary: Counter, Proportional
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Decrement Lines
Imaginary lines drawn through parts where the absorbed energy (radiation dose) is a certain percent of the energy absorbed at the same depth along the central axis of the radiation beam.
Delta Ray
Any secondary ionizing particle ejected by recoil when a primary ionizing particle passes through matter.
Densitometer
Instrument utilizing a photocell to determine the degree of darkening of developed photographic film.
Density (Physical)
The mass per unit volume of a substance. Usually kg/m–3 or g/cc–l. (Symbol: p)
Depth Dose
A radiation dose at some specified depth in tissue relative to the dose at a fixed reference point on the beam axis. It is usually expressed as a percentage of surface dose.
DeQing
A circuit in the modulator cabinet that regulates the size of dc voltage pulses delivered by the high-voltage power supply to the pulse forming network.
Detector
An instrument capable of registering the presence of radiation. The two common modes of operation for a detector are: Mean-Level or Integrating: The average effect of the radiation is cumulated over time. Pulse-Type: Individual radiation interactions are separated or resolved in time.
Detector, Radiation
Any device for converting radiant energy to a form more suitable for observation. An instrument used to determine the presence, and sometimes the amount, of radiation.
Deuterium (D)
A heavy isotope of hydrogen with one proton and one neutron in the nucleus.
Deuteron
An isotopic form of hydrogen in which the nucleus contains one proton and one neutron. When deuterons are substituted for the common form of hydrogen in the water molecule, the substance is known as “heavy” water.
Directly Ionizing Particles
Charged particles such as alpha or beta particles which cause ionization of an atom without any intermediate interaction taking place.
Disintegration, Nuclear
A spontaneous nuclear transformation (radioactivity) characterized by the emission of energy and/or mass from the nucleus. When numbers of nuclei are involved, the process is characterized by a definite half-life.
Disintegration Constant
The fraction of the number of atoms of a radioactive nuclide which decay in – λt unit time; λ in the equation N = N 0 e , in which N0 is the initial number of atoms present, and N is the number of atoms present after some time, t. (See also Decay Constant.)
Dose
A general form denoting the quantity of radiation or energy absorbed. For special purposes it must be appropriately qualified. If unqualified, it refers to absorbed dose. (See also Maximum Permissible Dose.)
Glossary: Decrement Lines
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Absorbed Dose: The energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The former unit of absorbed dose is the rad. One rad equals 100 ergs per gram. The new unit of absorbed dose is the gray. One gray equals 1 joule per kilogram. (See also Rad and Tissue Dose.) Cumulative Dose (Radiation): The total dose resulting from repeated exposures to radiation. Depth Dose: The radiation dose delivered at a particular depth beneath the surface of the body. It is usually expressed as a percentage of surface dose. Dose Distribution: The variation of dose in any region of an irradiated object. Dose Equivalent (H): A quantity used in radiation protection It expresses all radiations on a common scale for calculating the effective absorbed dose. It is defined as the product of the absorbed dose and certain modifying factors. (The former unit of dose equivalent is the rem. The new unit of dose equivalent is the Sievert [Sv].) Exit Dose: Dose of radiation at surface of body opposite to that on which the beam is incident. Integral Absorbed Dose (Volume Dose): A term used mainly in radiation biology to mean the total energy absorbed by an individual or other biological object or phantom during exposure to radiation. It is frequently obtained by integrating the absorbed dose with respect to mass throughout an irradiated region. It may be stated in joule or kilogram gray. Maximum Permissible Dose Equivalent (MPD): The greatest dose equivalent that a person or specified part thereof shall be allowed to receive in a given period of time. Median Lethal Dose (MLD): Dose of radiation that would be required to kill, within a specified period, 50% of the individuals in a large group of animals or organisms; also called LD50. Midline Absorbed Dose: The absorbed dose calculated or measured for a point in tissue and at the “midline” or “center” of the biological specimen, i.e., for the point lying equidistant from the exterior points on the specimen. The designation is for dosimetric purposes and implies no particular biological significance for the midline location. Percentage Depth Dose: The ratio expressed as a percentage, of the absorbed dose rate at a point, at depth along the beam axis, to the absorbed dose rate at a fixed reference point in the beam axis. Permissible Dose: The dose of radiation that an individual may receive within a specified period with expectation of no significantly harmful result. Skin Dose (Radiology): Absorbed dose at center of irradiation field on skin. It is the sum of the dose in air and scatter from body parts. Surface Dose: The absorbed dose delivered by a radiation beam anywhere the radiation passes through the superficial layer of the phantom or patient. Threshold Dose: The minimum absorbed dose that will produce a detectable degree of any given effect. Tissue Dose: Absorbed dose received by tissue in the region of interest, expressed in rads. (See also Absorbed Dose and Rad.)
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Glossary: Dose
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Dose Meter, Integrating Ionization chamber and measuring system designed for determining total radiation administered during an exposure. In medical radiology the chamber is usually designed to be placed on the patient’s skin. A device may be included to terminate the exposure when it has reached a desired value. Dose, Protraction
A method of administering radiation by delivering it continuously over a relatively long period at a low dose rate.
Dose Rate
Absorbed dose delivered per unit time.
Dose Ratemeter
Any instrument that measures radiation dose rate.
Dosimeter
Instrument to detect and measure accumulated radiation exposure. In common usage, a pencil-size ionization chamber with a self-reading electrometer, used for personnel monitoring.
Dosimetrist
An individual with training and knowledge in treatment planning. Under the supervision of a qualified radiological physicist, a dosimetrist makes dose calculations and assists in calibration and verification of dose distribution within the patient.
Dosimetry
The calculations, measurements, and other activities required for determining the radiation dose to be delivered.
Dosimetry Interlock
A machine condition is identified in which the ability of the Clinac to deliver or measure dose may be impaired.
Dosimetry, Photographic
Determination of cumulative radiation dose with photographic film and density measurement.
Drive Stand
The stationary unit on the Clinac that holds the gantry.
Dual Dosimetry
The use of two independent signals from the Clinac dosimeter proportional to the integrated dose. The primary channel is programmed to terminate the beam at the dose set by the operator. If the beam continues, the secondary channel is programmed to terminate the beam at a preset number of monitor units beyond the set dose.
Dyne
The unit of force which, when acting upon a mass of one gram, will produce an acceleration of one centimeter per second per second.
Eddy Current
An induced electric current circulating wholly within a mass of metal. Such currents are converted into heat, and thus cause serious waste.
Efficiency (Counters)
A measure of the probability that a count will be recorded when radiation is incident on a detector. Usage varies considerably, so it is well to ascertain which factors (e g., window transmission, sensitive volume, energy dependence) are included in a given case.
Electricity
One of the forces of nature developed by chemism, magnetism, or friction; also said to be electrons in motion.
Electrode
A conductor used to establish electric contact with a nonmetallic part of a circuit.
Glossary: Dose, Protraction
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Electrolyte
A substance capable of conducting an electric current and being decomposed by it.
Electromagnetic Interference (EMI)
Electromagnetic radiation that can degrade the performance of a radiotherapy accelerator or other nearby electronic equipment.
Electromagnetic Radiation
Transport of energy through space as a combination of an electric and magnetic field: for example, visible light and x-rays.
Electromagnetic wave
A wave produced by the oscillation of an electric charge.
Electrometer
Electrostatic instrument for measuring the difference in potential between two points. Used to measure change in electric potential of charged electrodes resulting from ionization produced by radiation.
Electromotive Force
Potential difference across electrodes tending to produce an electric current.
Electron
A stable elementary particle having an electric charge equal to –1.60 × 10– C and a rest mass equal to 9.1091 × 10–31 kg.
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Secondary Electron: An electron ejected from an atom, molecule, or surface as a result of an interaction with a charged particle or photon. Valence Electron: Electron that is gained lost, or shared in a chemical reaction. Electron Affinity
The tendency of a neutral atom to attract a free electron to itself.
Electron Beam Therapy
Treatment by electrons accelerated to high energies in a linear accelerator. Primarily used for lesions situated at or near the surface.
Electron Equilibrium
A condition established in a standard ionization chamber whereby the number of electrons entering a specified volume equals the number of electrons leaving that volume.
Electron Gun
A structure that injects electrons into the linear accelerator.
Electron Volt
A unit of energy equivalent to the energy gained by an electron in passing through a potential difference of 1 volt. Larger multiple units of the electron volt are frequently used: keV for thousand or kilo electron volts; MeV for million or mega electron volts. (Abbreviated: eV, 1 eV = 1.6 × 10–19 J.)
Electroscope
Instrument for detecting the presence of electric charges by the deflection of charged bodies. It has two metallic leaves hanging at the end of a very slender vane. When like charges are placed on the leaves, they move apart or repel. As the charge is reduced, the leaves move closer together until they are finally side by side when the charge has been reduced to zero.
Electrostatic Field
The region surrounding an electric charge in which another electric charge experiences a force.
Element
A category of atoms all of the same atomic number.
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Glossary: Electrolyte
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Emergency-off Circuit
An electrical circuit in the Clinac that causes all high power to be removed from the system whenever an emergency off button is pressed.
Energy
Capacity for doing work. “Potential energy” is the energy inherent in a mass because of its spatial relation to other masses. “Kinetic energy” is the energy possessed by a mass because of its motion; SI units: kg × m2 × S–2 or joules. Binding Energy: The energy represented by the difference in mass between the sum of the component parts and the actual mass of the nucleus. Excitation Energy: The energy required to change a system from its ground state to an excited state. Each different excited state has a different excitation energy. Ionizing Energy: The average energy lost by ionizing radiation in producing an ion pair in a gas.
Energy Dependence
The characteristic response of a radiation detector to a given range of radiation energies or wavelengths compared with the response of a standard free-air chamber.
Energy Fluence
The sum of the energies, exclusive of rest energies, of all particles passing through a unit cross-sectional area.
Energy Flux Density (Energy Fluence Rate)
The sum of the energies, exclusive of rest energies, of all particles passing through a unit cross-sectional area per unit time (energy fluence per unit of time).
Energy Imparted
See Dose, Integral Absorbed Dose.
Entrance Port
The area on the surface of a patient or a phantom on which a radiation beam is incident.
Enzyme
A biological catalyst of great specificity for a particular substance or a particular group of closely related substances which generally activates or accelerates a biochemical reaction.
Epidermis
The outermost layer of cells of the skin.
Epilation (Depilation)
The temporary or permanent removal or loss of hair.
Epithelium
A term applied to cell that line all canals and surfaces having communication with external air; also, cells specialized for secretion in certain glands as the liver, kidneys, etc.
Erg
Unit of work by a force of one dyne acting through a distance of one cm. Unit of energy which can exert a force of one dyne through a distance of one cm; ergs units: dyne-cm or gm-cm2/sec2.
Erythema
An abnormal redness of the skin due to distension of the capillaries with blood. It can be caused by many different agents: heat, drugs, ultraviolet rays, and ionizing radiation.
Erythrocyte
A red blood corpuscle.
Glossary: Emergency-off Circuit
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Eugenics
The science which deals with the influences that improve the hereditary qualities of a race or breed.
Excitation
The addition of energy to a system, thereby transferring it from its ground state to an excited state. Excitation of a nucleus, an atom, or a molecule can result from absorption of photons or from inelastic collisions with other particles.
Exit Dose
The absorbed dose at the point where the beam axis emerges from the patient.
Exposure
A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electric charges on all ions of one sign produced in air when all electrons liberated by photons in a volume element of air are completely stopped in air, divided by the mass of the air in the volume element. The former special unit of exposure is the roentgen. The new special unit of exposure is C · kg–1. Acute Exposure: Radiation exposure of short duration. Chronic Exposure: Radiation exposure of long duration.
Fallout
Radioactive debris from a nuclear detonation, which is airborne or has been deposited on the earth. Special forms of fallout are “Dry Fallout,” “Rainout,” and “Snowout.”
Field
A plane section of the beam perpendicular to the beam axis.
Field Block
A solid object of attenuating material used to shape a treatment beam.
Field Light
A light system that illuminates an area on the patient’s surface identifying the area of therapy beam entry.
Field Size
The size of an area irradiated by a given beam, usually measured by one of the following conventions: geometric field size, which measures the geometric projection on a plane perpendicular to the central axis, or physical field size, which measures the area included within the 50 percent maximum dose isodose curve at the depth of maximum dose.
Film Badge
A pack of photographic film that measures radiation exposure for personnel monitoring. The badge may contain two or three films of differing sensitivity and filters to shield parts of the film from certain types of radiation.
Film Ring
A film badge in the form of a finger ring.
Filter (Radiology)
Primary: A sheet of material, usually metal, placed in a beam of radiation to absorb preferentially the less penetrating components. Secondary: A sheet of material of low atomic number (relative to the primary filter) placed in the filtered beam of radiation produced by the primary filter.
Filtration, Inherent (Xrays)
The filter permanently in the useful beam; it includes the window of the xray tube and any permanent tube or source enclosure.
Fissile (Fissionable) Material
Any material readily fissioned by slow neutrons, for example, 235U, 239P.
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Glossary: Eugenics
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Fission
The splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy and frequently one or more neutrons. Fission can occur spontaneously, but usually it is caused by the absorption of gamma-ray photons, neutrons, or other particles.
Flattening Filter
A cone-shaped attenuator placed in the x-ray beam to achieve uniform intensity over the specific treatment field at a specific depth and a specific energy.
Fluence
The number of particles passing through a unit cross-sectional area.
Fluorescence
The emission of radiation of particular wavelengths by a substance as a result of absorption of radiation of shorter wavelength. This emission occurs essentially only during the irradiation.
Fluorescent Screen
A sheet of material coated with a substance (such as calcium tungstate or zinc sulfide) which will emit visible light when irradiated with ionizing radiation.
Fluorography (photofluorography)
Photography of image produced on fluorescent screen by x or gamma radiation.
Fluoroscope
A fluorescent screen, suitably mounted with respect to an x-ray tube for ease of observation and protection, used for indirect visualization (by xrays) of internal organs in the body or internal structures in apparatus or in masses of material.
Flux Density (fluence rate)
The number of particles passing through a unit cross-sectional area per unit of time. (Fluence per unit of time.)
Focal Spot (Xrays)
The part of the target of the x-ray tube struck by the main electron stream.
Fractionation
A technique of administering radiation therapy in multiple doses over a number of days or weeks to achieve a maximum therapeutic ratio.
Frequency
In harmonic motions, the number of cycles, revolutions, or vibrations completed in a unit of time. (See also Hertz.)
Function Key Assignments
A bar on the bottom line of the screen indicating the command currently assigned to each function key on the keyboard.
Fusion, Nuclear
Act of coalescing two or more atomic nuclei.
Gamete
Either of the two germ cells (sperm or ovum).
Gamma-ray
Short wavelength electromagnetic radiation of nuclear origin (range of energy from 10 keV to 9 MeV) emitted from the nucleus.
Gantry
The entire rotating unit of the Clinac that emits the treatment beam. The upper part of the gantry includes linear accelerator, and the lower part contains a counterweight or a retractable beam stopper.
Glossary: Fission
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Gantry Rotation Readout
A display that indicates the degrees of rotation of the gantry about the isocenter.
Gas Amplification
As applied to gas ionization radiation detecting instruments, the ratio of the charge collected to the charge produced by the initial ionizing event.
Geiger Region
In an ionization radiation detector, the operating voltage interval in which the charge collected per ionizing event is essentially independent of the number of primary ions produced in the initial ionizing event.
Geiger Threshold
The lowest voltage applied to a counter tube for which the number of pulses produced in the counter tube is essentially the same, regardless of a limited voltage increase.
Geiger Tube
An ionization type radiation detector with a very high sensitivity for photons in the energy range 10 to 1000 keV.
Gene
Fundamental unit of inheritance which determines and controls hereditary transmissible characteristics. Genes are arranged linearly at definite loci on chromosomes.
Genetics
The branch of biology dealing with the phenomena of heredity and variation.
Generator (“Cow”)
A device in which a daughter radionuclide is eluted from an ion exchange column containing a parent radionuclide long lived compared to the daughter.
Genetic Effect of Radiation
Inheritable change, chiefly mutations, produced by the absorption of ionizing radiations. On the basis of present knowledge these effects are purely additive; recovery does not occur.
Genetically Significant Dose
That absorbed dose equivalent which, if received by every member of the population, would be expected to produce the same total genetic injury to the population as the actual absorbed dose equivalent received by various individuals.
Genotype
The fundamental hereditary (genetic) constitution of an organism.
Germ Cells
The cells of an organism whose function is reproduction.
Given Dose
The applied dose delivered by one beam in a complete treatment or in a treatment session.
Glove Box
An enclosure used for working with radionuclides particularly those in the form of powders and volatile liquids.
Gonad
A gamete-producing organ in animals; testis or ovary.
Gray (Gy)
The new unit of absorbed dose equal to 1 joule per kilogram in any medium. (See Absorbed Dose.)
Grenz Rays
X-rays produced at voltages of 5 to 20 kVp, intended primarily for surface therapy.
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Glossary: Gantry Rotation Readout
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Ground State
The state of a nucleus, atom, or molecule at its lowest energy. All other states are “excited.”
Half-Life
A general term used to describe the time elapsed until some physical quantity has decreased to half of its original value. Here the concept of half-life will be applied to radionuclides. Half-Life, Biologic: The time required for the body to eliminate one-half of an administered dosage of any substance by regular processes of elimination. Approximately the same for both stable and radioactive isotopes of a particular element. Half-Life, Effective: Time required for a radioactive element in an animal body to be diminished 50% as a result of the combined action of radioactive decay and biologic elimination. Biological half-life × Radioactive half-life Effective half life = -----------------------------------------------------------------------------------------------------------------Biological half-life + Radioactive half-life Half-Life, Radioactive: Time required for a radioactive substance to lose 50% of its activity by decay. Each radionuclide has a unique half-life.
Half-Value Layer (Half Value Thickness) (HVL)
The thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduces the exposure rate by one-half.
Hand Pendant
A hand held control device that allows the operator to adjust the treatment couch, collimator, and gantry for a patient.
Hardness (Xrays)
A relative specification of the quality of penetrating power of x-rays. In general, the shorter the wavelength the harder the radiation.
Health, Radiologic
The art and science of protecting human beings from injury by radiation, and promoting better health through beneficial applications of radiation.
Heat Exchanger
A cooling device that uses city water to carry off the heat generated by certain Clinac systems.
Heel Effect
The cathode end of the x-ray tube has a slightly visible tendency to make a more dense image than has the anode end.
Heredity
Transmission of characters and traits from parent to offspring.
Hertz
Unit of frequency equal to 1 cycle per second. (See also Frequency.)
Heterogeneous Radiation
Beam of x-rays consisting of many x-rays of different wavelengths.
Highlight
A reverse-video bar shown on the monitor screen to identify a choice in a window or an input space.
Hysteresis
A lagging or retardation of the effect. The magnetization of a piece of iron or steel due to a magnetic field that is made to vary through a cycle of values; lags behind the field.
Glossary: Ground State
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Immunity
The power which a living organism possesses to resist and overcome infection.
Implant (Radiology)
Encapsulated radioactive material embedded in a tissue for therapy. It may be permanent (seed) or temporary (needle).
Indirectly Ionizing Particles
Particles that cause ionization to occur only after an intermediate interaction producing a charged particle has taken place.
Infrared Radiation
Invisible thermal radiation whose wavelength is longer than the red segment of the visible spectrum.
Input Space
A reverse-video box on the monitor screen that provides a space for the operator to enter input to the system.
Integral Dose
A measure of the total energy absorbed by a patient or object during exposure to radiation.
Integrating Circuit
An electronic circuit that records the total number of ions or events collected for a given time from which an average value for the number of ions or events per unit time can be found.
Intensification Factor
The quantity of intensification, expressed numerically as light energy, of the applied source of energy when it passes through the screen emulsion.
Intensifying Screen
Sheet of cardboard or other substance coated with fluorescent material, placed in contact with the film in radiography. The x or gamma rays excite the fluorescent substance. The light thus emitted adds to the radiation effect on the film and produces an image of greater density for a given exposure. Sheets of thin lead may be used in industrial radiography and radiation therapy with very high energy radiation. In this case, the increased effect is due largely to secondary electrons and x-rays emitted by the lead.
Intensity
Amount of energy per unit time passing through a unit area perpendicular to the line of propagation at the point in question.
Interlock
A electrical circuit or mechanical device to prevent operation or application of power until the circuit or device is placed in a certain state.
International Commission on Radiological Protection
(ICRP) An international organization, founded in 1928, and supported financially by the World Health Organization (WHO), the International Atomic Energy Agency (IAEA), the United Nations Environment Program the International Society of Radiology, and others, which operates under rules approved by the International Congress of Radiology. Members of ICRP are selected from nominations submitted to it by the National Delegations to the International Congress of Radiology and by the ICRP itself. The International Executive Committee of the Congress approves the selections.
International Commission on Radiation Units and Measurements
(ICRU) An international organization, founded in 1925, with the principle objective of development of internationally acceptable recommendations regarding: (l) quantities and units of radiation and radioactivity, (2) procedures suitable for measurement and application of these quantities in clinical radiology and radiobiology, and (3) physical data needed in the application of these procedures, the use of which, tends to assure uniformity in reporting. The ICRU works closely with the ICRP in its consider-
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Glossary: Immunity
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 ation of and recommendations for the radiation protection field. Financially, ICRU is supported by the United States National Institutes of Health and many national and international societies, foundations, and companies. International System of Units (SI)
The standard metric system of measurement adopted in 1975 for worldwide use. SI units commonly used in radiotherapy include the gray (measures absorbed dose), sievert (measures the dose equivalent), coulomb per kilogram (measures exposure), and the becquerel (measures the disintegration rate of a radionuclide).
Inverse Square Law
1. A rule relating two physical entities by a particular proportionality constant. This constant is one divided by the square of some other physical quantity, usually the distance between the two physical entities. 2. A formula for the relationship that the intensity of radiation is inversely proportional to the square of the distance from a point source.
Ion
Atomic particle, atom, or chemical radical bearing an electric charge, either negative or positive.
Ion Chamber
See Monitor ion chamber.
Ion Pair
Two particles of opposite charge, usually referring to the electron and positive atomic or molecular residue resulting after the interaction of ionizing radiation with the orbital electrons of atoms.
Ionization
The process by which a neutral atom or molecule acquires a positive or negative charge. Primary Ionization: (l) In collision theory: the ionization produced by the primary particles as contrasted to the “total ionization,” which includes the “secondary ionization” produced by delta rays. (2) In counter tubes: The total ionization produced by incident radiation without gas amplification. Secondary Ionization: Ionization produced by delta rays. Specific Ionization: Number of ion pairs per unit length of path of ionizing radiation in a medium, e.g., per centimeter of air or per micron of tissue. Total Ionization: The total electric charge of one sign on the ions produced by radiation in the process of losing its kinetic energy. For a given gas, the total ionization is closely proportional to the initial ionization and is nearly independent of the nature of the ionizing radiation. It is frequently used as a measure of radiation energy.
Ionization Density
Number of ion pairs per unit volume.
Ionization Path (Track)
The trail of ion pairs produced by an ionizing radiation in its passage through matter.
Ionization Potential
The potential necessary to separate one electron from an atom, resulting in the formation of an ion pair.
Ionizing Event
Any occurrence of a process in which an ion or group of ions is produced.
Ion Pump
A vacuum pump that maintains the high vacuum in the accelerator by removing gas molecules.
Glossary: International System of Units (SI)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Irradiation
Exposure to radiation.
Isobar
One of two or more chemical elements that have the same mass number but different atomic numbers.
Isocenter
The intersection of the gantry axis of rotation and the collimator bearing axis.
Isodose Curve
A line connecting points of equal radiation doses.
Isodose Chart
A graphic display containing a series of isodose curves that maps out the relative intensities of a radiation field in a phantom or patient.
Isomers
Nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. Commonly, the isomer of higher energy decays to one with lower energy by the process of isometric transition.
Isotones
Nuclides having the same number of neutrons in their nuclei.
Isotopes
Nuclides having the same number of protons in their nuclei, and hence the same atomic number, but differing in the number of neutrons, and therefore in the mass number. Almost identical chemical properties exist between isotopes of a particular element. The term should not be used as a synonym for nuclide. Stable Isotope: A nonradioactive isotope of an element.
Joule
The unit for work and energy, equal to 1 Newton expended along a distance of 1 meter (1J = 1N × 1M)
Kerma (kinetic energy released per unit mass)
The kinetic energy of charged ionizing particles liberated per unit mass of specified material by uncharged ionizing particles such as photons and neutrons. Kerma is measured in the same units as absorbed dose, joule per kilogram (J/kg-1) and its special name is gray (Gy). Kerma can be quoted for any specified material at a point in free space or in an absorbing medium. Since air kerma and tissue kerma differ by less than 10% over a wide range of photon energies, these two may be considered equal in magnitude for radiation protection purposes. In this respect, air kerma means air kerma in air. Kerma is independent of the complexities of geometry of the irradiated mass element, and permits, therefore, specification for photons or neutrons in free space or in an absorbing medium and hence has a wider applicability than exposure.
Keylock Switch or Keyswitch
A rotary switch that requires a special key to operate. Similar to an ignition switch on a car.
Kilo Electron Volt (keV)
One thousand electron volts, 103 eV.
Kilovolt (kV)
A unit of electric potential difference, equal to 1000 volts.
Kilovolt Constant (kVcp)
The value in kilovolts of the potential difference of a constant potential generator.
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Glossary: Irradiation
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Kilovolt Peak (kVp)
The maximum value in kilovolts of the potential difference of a pulsating potential generator. When only half the wave is used, the value refers to the useful half of the cycle.
Laboratory Monitor
(See Survey Meter.)
Laser
A device for transforming incoherent light of various frequencies into a very narrow, intense beam of coherent light.
Latent Image
Development occurring between the time of exposure of a film to radiation and the processing of that film.
Latent Period
The period or state of seeming inactivity between the time of exposure of tissue to an injurious agent and response.
LD50 (Radiation Dose)
Dose of radiation required to kill, within a specified period, 50 percent of the individuals in a large group of animals or organisms. Also called the Median Lethal Dose.
Lead Equivalent
The thickness of lead affording the same attenuation, under specified conditions, as the material in question.
Lesion
A hurt, wound, or local degeneration.
Leukemia
A disease in which there is great over-production of white blood cells, or a relative over-production of immature white cells, and great enlargement of the spleen. The disease is variable, at times running a more chronic course in adults that in children. It can be produced in some animals by long continued exposure to low doses of ionizing radiation.
Linear Accelerator Structure
A linear series of adjacent cylindrical microwave resonant cavities (called the guide) in which charged particles are accelerated by applying a highfrequency voltage during the particle transit inside the structure.
Linear Energy Transfer (LET)
The quotient of dE by dL, in which dL is the distance traversed by a particle and dE is the average energy loss in dL due to collisions with energy transfers less than some specific value. Simply, it is a conventional expression for energy deposition measured along the track of an ionizing particle. Gamma and x-ray photons generate low LET electron tracks. Natural alpha particles and fast neutrons and protons give high-LET tracks.
Localization Films
X-ray films taken with radiopaque markers to define the tumor position relative to external markings.
Major Interlock
A machine condition is identified that could damage the Clinac if not corrected.
Mass
The material equivalent of energy; different from weight in that it neither increases nor decreases with gravitational force.
Mass Numbers
The number of nucleons (protons and neutrons) in the nucleus of an atom. (Symbol: A)
Glossary: Kilovolt Peak (kVp)
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Maximum Permissible Dose (MPD)
(See Dose.)
Mean Free Path
The average distance that particles of a specified type travel before a specified type (or types) of interaction in a given medium. The mean free path may thus be specified for all interactions (i.e., total mean free path) or for particular types of interaction such as scattering, capture, or ionization.
Mean Life
The average lifetime for an atomic or nuclear system in a specified state. For an exponentially decaying system the average time for the number of atoms or nuclei in a specified state to decrease by a factor of e (2.718).
Mega Electron Volt (MeV)
One million electron volts, 106 eV.
Meson
One of a class of medium-mass, short lived elementary particles with a mass between that of the electron and that of the proton. Examples: Pi mesons (pions) and K mesons (kaons)
Metabolism
The sum of all physical and chemical processes by which living organized substance is produced and maintained and by which energy is made available foe the uses of the organism.
Metastasis
The transfer in the body of malignant neoplastic cells from the original or parent site to one more distant.
Micron (µ)
Unit of length equal to 10–6 meters.
Microwave
Radio waves in the frequency range of approximately 1000 megahertz and upward.
Milliampere
A unit of current. Generally the current flowing between the filament and anode of an x-ray tube is stated in this unit.
Milliroentgen (mR)
A submultiple of the Roentgen, equal to one one-thousandth of a Roentgen.
Minor Interlock
A machine condition is identified that prevents Clinac beam-on, but the condition is normally user-correctable.
Modulator
A system in the Clinac that generates a succession of short pulses of high current and voltage for operating the klystron.
Molecule
Smallest quantity of a compound which can exist by itself and retain all properties of the original substance.
Molybdenum Breakthrough
This term refers to the amount of parent nuclide, molybdenum, contained in an eluted sample of its offspring 99mTc. 37 kBq (1 µCi) of Mo is allowed per 37 MBq (l mCi) of 99mTc eluate. However, no more than 185 kBq (5 µCi) of Mo are allowed per patient dose.
Momentum
The product of the mass of a body and its velocity; SI units, kg × m · s–l.
Monitor
A cathode-ray tube device used to view data produced by a computer.
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Glossary: Maximum Permissible Dose (MPD)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Monitoring
Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in an occupied region. Area Monitoring: Routine monitoring of the radiation level or contamination of a particular area, building, room, or equipment. Some laboratories or operations distinguish between routine monitoring and survey activities. Personnel Monitoring: Monitoring any part of an individual, his breath, or excretions, or any part of his clothing.
Monitor Ion Chamber
A special radiation measuring device in which the collected electrical charge from ionization in a gas filled cavity is taken to be proportional to some parameter (for example, dose or exposure) or radiation field.
Monitor Unit (MU)
A unit of radiation exposure. A table for the conversion of monitor units into units of absorbed dose (gray or rad) can be generated by a dose calibration of the machine by a qualified physicist.
Monoenergetic
Having only one energy associated with it.
Monte Carlo Method
A method permitting the solution by means of a computer of problems of particle physics, such as those of neutron transport, by determining the history of a large number of elementary events by the application of the mathematical theory of random variables.
Motion Enable Switch
A safety switch that allows motion of certain motorized functions only so long as the operator continues to press the switch.
Multiple-port Treatment
Directing more than one radiation beam toward the tumor from different angles for the purpose of increasing the dose without irrevocably destroying normal tissue.
Mutation
Alteration of the usual hereditary pattern, usually sudden.
National Council on Radiation Protection and Measurements (NCRP)
This committee was granted a United States Congressional charter in 1964. It is operated as in independent organization financed by contributions from government, scientific societies, and manufacturing associations.
Natural (Napierian) Logarithms
A system of logarithms using the base e.
Negative Ion
Negative charged ion; commonly termed “anion.”
Neoplasm
Any new and abnormal growth, such as a tumor; “neoplastic disease” refers to any disease that forms tumors, whether malignant or benign.
Neutrino
A neutral particle of very small rest mass originally postulated to account for the continuous distribution of energy among particles in the beta decay process.
Neutron
An electrically neutral or uncharged particle of matter existing along with protons in the atoms of all elements except the mass 1 isotope of hydrogen. The isolated neutron is unstable and decays with a half-life of about 13 minutes into an electron, proton, and neutrino. Neutrons sustain the fission chain reaction in a nuclear reactor.
Glossary: Monitoring
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Epithermal Neutron: A neutron having an energy between 0.5 and 1O5 eV. (Sometimes the energy range is given as 0.5 to 100 eV and the energy range from 100 eV to 105 eV is called Intermediate.) Fast Neutron: A neutron having an energy above 0.1 MeV (105 eV). Thermal Neutron: A neutron having an energy of about 0.025 eV which corresponds to a velocity of 2200 m/s. Newton
The unit of force that, when applied to a 1 kilogram mass, will give it an acceleration of 1 meter per second per second. (1 N = 1 kg × 1m · 1s–2).
Nomogram
Conversion scale between two sets of units.
Nonionizing Radiation
Radiation that does not cause ionization when it interacts with matter.
Nuclear Reaction
See Reaction, Nuclear.
Nuclear Reactor
A device by means of which a fission chain reaction can be initiated, maintained, and controlled. Its essential component is a core with fissionable fuel. It usually has a moderator, a reflector, shielding, and control mechanisms. Thermal Nuclear Reactor: A nuclear reactor in which the fission chain reaction is sustained primarily by thermal neutrons. Most existing reactors are thermal reactors.
Nucleon
Common name for a constituent particle of the nucleus. Commonly applied to a proton or neutron.
Nucleus (Nuclear)
That part of an atom in which the total positive electric charge and most of the mass is concentrated.
Nuclide
A species of atom characterized by the constitution of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content; or, alteratively, by the atomic number (Z), mass number A = (N + Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable time. Thus nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not so considered.
Occupational Exposure
The exposure of an individual to ionizing radiation because the occupation of the individual includes duties or activities that necessarily involve the likelihood of exposure.
Ohm
The unit of electric resistance.
Oncology
Preferred name for tumor treatment. (See Therapy, Radiation Therapy.)
Offspring
Synonym for Daughter. (See Decay Product.)
Operating Software
The integrated collection of programs used by the Clinac system computer to interface the system with the operator and control the machine.
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Glossary: Newton
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Organ
Group of tissues which together perform one or more definite functions in a living body.
Osmosis
The passage of pure solvent from the lesser to the greater concentration when two solutions are separated by a membrane which selectively prevents the passage of solute molecules, but is permeable to the solvent.
Osmotic
Pertaining to osmosis.
Owner
A person or organization having title to or administrative control over one or more radiotherapy installations.
Ozone
A gas produced by an electrical discharge in ordinary oxygen or air. Pure ozone is an unstable, faintly bluish gas with a characteristically fresh, penetrating odor.
Pair Production
An absorption process for x and gamma radiation in which the incident photon is annihilated in the vicinity of the nucleus of the absorbing atom with subsequent production of an electron and positron pair. This reaction only occurs for incident photon energies exceeding 1.02 MeV. (See also Absorption, Compton Effect, and Photoelectric Effect.)
Parent
A radionuclide that, upon disintegration, yields a specific nuclide — either directly or as a later member of a radioactive series.
Password
A number or word used to gain entry to a certain program or a restricted part of a program.
Path, Mean Free
Average distance a particle travels between collisions.
Patient Support Assembly (PSA)
See Treatment couch.
Penumbra
The region, at the edge of a radiation beam, over which the absorbed dose rate changes rapidly as a function of distance from the axis. It may be defined geometrically and dosimetrically. Geometric Penumbra: That region in space which could be irradiated by primary photons or particles coming from part of the source only. By analogy, the transmission penumbra is the region irradiated by photons or particles which have traversed part of the thickness of the collimator, i.e., at its outer edge. Geometric Penumbra Width: The width of the geometric penumbra in a plane perpendicular to the beam axis at any distance of interest from the source. It is a geometrical concept only and is calculated from the expression W = c(SSD + d – SCD)/SCD in which c is the source diameter (or effective diameter), SSD + d is the distance from the source to point of interest, and SCD is the distance from the source to the edge of the collimator. Physical Penumbra: This is a dosimetric concept; the physical penumbra width is the lateral distance between two specified isodose curves at a specified depth.
Glossary: Organ
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Periodic Table
An arrangement of chemical elements in order of increasing atomic number. Elements of similar properties are placed one under the other, yielding groups and families of elements. Within each group, a gradation of chemical and physical properties exists but, in general, chemical behavior is similar. From group to group, however, a progressive shift of chemical behavior occurs from one end of the table to the other.
Permeable
Affording passage or penetration.
Personnel Monitor
A dosimeter (usually a film badge, thermoluminescent device, or ionization chamber) used for determining the exposure to an individual. Such monitoring is required for all persons who are radiation workers.
Phantom
A volume of material approximating as closely as possible the density and effective atomic number of tissue. Ideally a phantom should behave in respect to absorption of radiation in the same manner as tissue. Radiation dose measurements made within or on a phantom provide a means of determining the radiation dose within or on a body under similar exposure conditions. Some materials commonly used in phantoms are water, perspex polystyrene, Masonite, pressed wood, and beeswax.
Phosphorescence
Emission of radiation by a substance as a result of previous absorption of radiation of shorter wavelength. In contrast to fluorescence, the emission may continue for a considerable time after cessation of the exciting irradiation.
Photoelectric Effect
Process by which a photon ejects an electron from an atom. All energy of the photon is absorbed in ejecting the electron and in imparting kinetic energy to it. (See also Absorption, Compton Effect, and Pair Production.)
Photon
A quantity of electromagnetic energy (E) whose value in joules is the product of its frequency (v) in hertz and Planck’s constant (h). The equation is: E = hv. (See also Radiation.)
Photosynthesis
The production of carbohydrates by green plants in the presence of sunlight through the agency of chlorophyll.
Physics, Health
A science and profession devoted to the protection of man and his environment from unnecessary radiation exposure.
Pig
A lead-lined container used for storing radionuclides.
Planck’s Constant
A natural constant of proportionality (h) relating the frequency of a quantum of energy to the total energy of the quantum. – 34 h = E --- = 6.6256 × 10 J ⋅ s v
Plateau
As applied to radiation detector chambers, the level portion of the counting rate-voltage curve where changes in operating voltage introduce minimum changes in the counting rate.
Polycythemia
A disease characterized by overproduction of red blood cells.
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Glossary: Periodic Table
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Port Film Exposures
A radiograph taken with the patient interposed between the machine portal and an x-ray film. The purpose is to demonstrate that the treatment field on the patient adequately encompasses the treatment volume and at the same time avoids adjacent critical structures.
Positive Ion
Positively charged ion; commonly called cation.
Positron
Particle equal in mass to the electron and having an equal but positive charge.
Potential Difference
Work required to carry a unit positive charge from one point to another.
Potential Ionization
The potential necessary to separate one electron from an atom, resulting in the formation of an ion pair.
Power, Stopping
A measure of the effect of a substance upon the kinetic energy of a charged particle passing through it.
Printed Circuit Board (PCB)
A sandwich-like set of insulated boards onto which circuits are etched and various components are soldered.
Proportional Region
Voltage range in which the gas amplification is greater than one, and in which the charge collected is proportional to the charge produced by the initial ionizing event.
Proton
Elementary nuclear particle with a positive electric charge equal numerically to the charge of the electron and a rest mass of 1.67474 × 10–27 kg.
Pulse Forming Network (PFN)
An electrical circuit in the modulator that supplies accurately shaped pulses of high voltage necessary for klystron operation.
Pulse Height Selector
A circuit designed to select and pass voltage pulses in a certain range of amplitudes.
Pulse Repetition Frequency (PRF)
A signal applied to the thyratron tube that causes the capacitors in the pulse-forming network to discharge. This provides a nearly flat high-power dc pulse to the electron gun and the klystron of the accelerator.
Purpura
Large hemorrhagic spots in or under the skin or mucous tissues.
Quality (Radiology)
The characteristic spectral-energy distribution of x radiation. It is usually expressed in terms of effective wavelengths of half-value layers of a suitable material; e.g., up to 20 kV, cellophane; 20 to 120 kVp, aluminum; 120 to 400 kVp, copper; over 400 kVp, tin.
Quality Factor (Q)
The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that expresses, on a common scale for all ionizing radiations, the effectiveness of the absorbed dose.
Quantum
An observable quantity is said to be “quantized” when its magnitude is, in some or all of its range, restricted to a discrete set of values. If the magnitude of the quantity is always a multiple of a definite unit, that unit is called the quantum (of the quantity). For example, the quantum or unit of orbital angular momentum is h, and the quantum of energy of electromag-
Glossary: Port Film Exposures
7-31
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 netic radiation of frequency v is hv. In field theories, a field (or the field equations) is quantized by application of a proper quantum mechanical procedure. This quantization results in the existence of a fundamental field particle, which may be called the field quantum. Thus, the photon is a quantum of the electromagnetic field, and in nuclear field theories the meson is considered the quantum of the nuclear field. Quantum Theory
The concept that energy is radiated intermittently in units of definite magnitude called quanta, and absorbed in a like manner.
Quenching
The process of inhibiting continuous or multiple discharge in a counter tube which uses gas amplification.
Quenching Vapor
Polyatomic gas used in Geiger-Muller counters to quench or extinguish avalanche ionization.
Rad
The former unit of absorbed dose equal to 0.01 joule per kilogram in any medium. (See also Absorbed Dose and Tissue Dose)
Radiant Energy
The energy of electromagnetic radiation, such as radio waves, visible light, x and gamma rays.
Radiation
(l) The emission and propagation of energy through space or through a material medium in the form of waves; for instance, the emission and propagation of electromagnetic waves, or of sound and elastic waves. (2) The energy propagated through space or through a material medium as waves; for example, energy in the form of electromagnetic waves or elastic waves. The term radiation or radiant energy, when unqualified, usually refers to electromagnetic radiation. Such radiation commonly is classified, according to frequency, as hertzian, infrared, visible (light), ultraviolet and x-ray or gamma-ray. (See also Photon.) (3) By extension, corpuscular emissions, such as alpha and beta radiation, or rays of mixed or unknown type, as cosmic radiation. Annihilation Radiation: Photons produced when an electron and a positron unite and cease to exist. The annihilation of a positron-electron pair results in the production of two photons, each of 0.511 MeV energy. Background Radiation: Radiation arising from radioactive material other than the one directly under consideration. Background radiation due to cosmic rays and natural radioactivity is always present. Background radiation may also be due to the presence of radioactive substances in other parts of the building or in the building material itself. Characteristic (Discrete) Radiation: Radiation originating from an atom after removal of an electron or excitation of the nucleus. The wavelength of the emitted radiation is specific, depending only on the nuclide and particular energy levels involved. External Radiation: Radiation from a source outside the body — the radiation must penetrate the skin. Internal Radiation: Radiation from a source within the body (as a result of deposition of radionuclides in body tissues). Ionizing Radiation: Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter.
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Glossary: Quantum Theory
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Leakage (Direct) Radiation: All radiation coming from the source housing except the useful (primary) beam. Monochromatic Radiation: Electromagnetic radiation of a single wavelength, or radiation in which all the photons have the same energy. Monoenergetic Radiation: Radiation of a given type (e.g., alpha, beta, neutron, gamma) in which all particles or photons originate with and have the same energy. Primary Radiation: The useful beam of an x-ray tube. Scattered Radiation: Radiation that, during its passage through a substance, has been deviated in direction. It may also have been modified by a decrease in energy. Secondary Radiation: Radiation resulting from absorption of other radiation in matter. It may be either electromagnetic or particulate. Radiation Beam
The flow of therapeutically useful radiation energy through a defined area.
Radiation Protection Survey
An evaluation of the radiation hazards in and around an installation.
Radiation Oncologist
A physician who has received specific training and experience in therapeutic radiology.
Radiation Sickness
(Radiation Therapy): A self-limiting syndrome characterized by nausea, vomiting, diarrhea, and psychic depression, following exposure to appreciable doses of ionizing radiation, particularly to the abdominal region. It usually appears a few hours after irradiation and may subside within a day. It may be sufficiently severe to necessitate interrupting the treatment series or to incapacitate the patient. (General): The syndrome associated with intense acute exposure to ionizing radiations.
Radiation Therapist
An individual who has received specific training in radiation therapy technology and who is certified by a recognized specialty board as being competent in radiation therapy technology.
Radiation Therapy
Treatment of disease with any type of radiation.
Radioactivity
The property of certain nuclides of (l) spontaneously emitting particles or gamma radiation or (2) emitting x radiation following orbital electron capture or (3) undergoing spontaneous fission. Artificial Radioactivity: Man-made radioactivity produced by particle bombardment or electromagnetic irradiation, as opposed to natural radioactivity. Induced Radioactivity: Radioactivity produced in a substance after bombardment with neutrons or other particles. The resulting activity is “natural radioactivity” if formed by nuclear reactions occurring in nature, and “artificial radioactivity” if the reactions are caused by man. Natural Radioactivity: The property of radioactivity exhibited by more than 50 naturally occurring radionuclides.
Glossary: Radiation Beam
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Radiobiology
That branch of biology which deals with the effects of radiation on biological systems.
Radio Frequency (rf)
Any frequency at which coherent electromagnetic radiation of energy is possible. Usually considered to denote frequency above 150 kilohertz and extending up to the infrared range.
Radiography
The making of shadow images on photographic emulsion by the action of ionizing radiation. The image is the result of the differential attenuation of the radiation in its passage through the object being radiographed.
Radiological Physicist
An individual who devotes the majority of occupational time to the physics of radiology, including therapeutic radiological physics, diagnostic radiological physics, and medical nuclear physics.
Radiology
That branch of medicine which deals with the diagnostic and therapeutic applications of radiant energy, including x-rays and radionuclides.
Radionuclide
A nuclide that displays the property of radioactivity.
Radiopharmaceutical
A pharmaceutical compound that has been tagged with a radionuclide.
Radioresistance
Relative resistance of cells, tissues, organs, or organisms to the injurious action of radiation. The term may also apply to chemical compounds or to any substances.
Radiosensitivity
Relative susceptibility of cells, tissues, organs, organisms, or any living substances to the injurious action of radiation. Radioresistance and radiosensitivity are currently used in a comparative sense, rather than in an absolute one.
Radiotherapy Accelerator Service Technician
An individual with the following minimum qualifications: training equivalent to an associate’s degree in electronics, training in servicing and maintaining the machine, and a demonstrated understanding of the emergency and safety regulations adopted by the owner of the accelerator.
Range
The depth in any material measured from the entrance of an ionizing particle to the stopping position of that particle after it has lost all of its energy.
Rare Earth
Any of the series of very similar metals ranging in atomic numbers from 57 through 71.
Reaction (Nuclear)
An induced nuclear disintegration, i.e., a process occurring when a nucleus comes in contact with a photon, an elementary particle, or another nucleus. In many cases the reaction can be represented by the symbolic equation: X + a Y + b or, in abbreviated form, X(a,b) Y. X is the target nucleus, a is the incident particle or photon, b is an emitted particle or photon, and Y is the product nucleus.
Recombination
The return of an ionized atom or molecule to the neutral state.
Recovery Rate
The rate at which recovery takes place after radiation injury. It may proceed at different rates for different tissues. “Differential recovery rate:” Among tissues recovering at different rates, those having slower rates will ultimately suffer greater damage from a series of successive irradiations. This
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Glossary: Radiobiology
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 differential effect is considered in fractionated radiation therapy if the neoplastic tissues have a slower recovery rate than surrounding normal structures. Regenerative Process
The process by which damage or destroyed cells are replaced by new ones of the same type.
Relative Biologic Effectiveness (RBE)
The factor used to compare the biologic effectiveness of absorbed radiation doses (i.e., rads) due to different types of ionizing radiation; more specifically, it is the experimentally determined ratio of an absorbed dose of radiation in question to the absorbed dose of a reference radiation required to produce an identical biologic effect in a particular experimental organism or tissue. This term should not be used in radiation protection. (See also Quality Factor.)
Rem
A special unit of dose equivalent. The dose equivalent in rem is numerically equal to the absorbed dose in rad multiplied by the quality factor, the distribution factor, and any other necessary modifying factors.
Repair
The partial or complete restoration of functional integrity in cells following damage caused by radiation. Operationally, repair means that after irradiation a cell responds as though it had received a smaller dose than under conditions in which damage is more fully expressed. The ability to observe repair implies, therefore, that a comparison is made with a treatment of reference. Full repair indicates that cells respond as though they had not been previously irradiated. (Repair embraces processes sometimes referred to as bypassing of damage, shedding of damage, compensating for damage, elimination of damage, and/or the specific biochemical reversal of damage.)
Resolving Time, Counter
The minimum time interval between two distinct events which will permit both to be counted. It may refer to an electronic circuit, a mechanical indicating device, or a counter tube.
Rest Mass
The intrinsic mass of any physical entity; the mass possessed by that entity apart from any motion it may have.
Roentgen (R)
The special unit of exposure. One roentgen equals 2.58 × 10–4 coulomb per kilogram of air. (See also Exposure.)
Roentgenography
Radiography by means of x-rays.
Roentgenology
That part of radiology which pertains to x-rays.
Roentgen Rays
X-rays.
Scattering
Change of direction of subatomic particles or photons as a result of a collision or interaction. Coherent Scattering: Scattering of photons or particles in which definite phase relationships exist between the incoming and the scattered waves. Coherence manifests itself in the interference between the waves scattered
Glossary: Regenerative Process
7-35
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 by two or more scattering centers. An example is the Bragg scattering of xrays and neutrons by the regularly spaced atoms in a crystal, for which constructive interference occurs only at definite angles, called “Bragg angles.” Compton Scattering: The scattering of a photon by an electron. Part of the energy and momentum of the incident photon is transferred to the electron, and the remaining part is carried away by the scattered photon. Elastic Scattering: Scattering caused by elastic collisions and, therefore, conserving kinetic energy of the system. Rayleigh scattering is a form of elastic scattering. Incoherent Scattering: Scattering of photons or particles in which the scattering elements act independently of one another; no definite phase relationships exist among the different parts of the scattered beam. The intensity of the scattered radiation at any point is obtained by adding the intensities of the scattered radiation reaching this point from the independent scattering elements. Inelastic Scattering: The type of scattering that results in the nucleus being left in an excited state and the total kinetic energy being decreased. Scattering Coefficient, Compton
That fractional decrease in the energy of a beam of x or gamma radiation in an absorber due to the energy carried off by scattered photons in the Compton effect. (See also Compton Absorption Coefficient.)
Scattering Foil
In electron beam therapy, a thin metal plate used to disperse the electron beam before it passes through the collimator jaws. The function of the scattering foil is to flatten intensity over the field, analogous to the function of the flattening filter with x-rays.
Scintillation Counter
An instrument that detects and measures ionizing radiation by counting the light flashes (scintillations) induced by the radiation in certain materials.
Sealed Source
A radioactive source sealed in an impervious container which has sufficient mechanical strength to prevent contact with and dispersion of the radioactive material under the conditions of use and wear for which it was designed.
Series, Radioactive
A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results. The first member is called the “parent,” the intermediate members are called “daughters,” and the final stable member is called the “end product.”
Shield
A body of material used to prevent or reduce the passage of particles or radiation. A shield may be designated according to what it is intended to absorb (as a gamma-ray shield or neutron shield), or according to the kind of protection it is intended to give (as a background, biologic, or thermal shield). The shield of a nuclear reactor is a body of material surrounding the reactor to prevent the escape of neutrons and radiation into a protected area, which frequently is the entire space external to the reactor. It may be required for the safety of personnel or to reduce radiation enough to allow use of counting instruments for research or for locating contamination or airborne radioactivity.
7-36
Glossary: Scattering Coefficient, Compton
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Sievert
The new special unit of dose equivalent. The sievert equals the absorbed dose in gray times the quality factor for the radiation in question. (Symbol: Sv)
Sigmoid Curve
S-shaped curve, often characteristic of a dose-effect curve in radiobiological studies.
Simulation
Use of a simulator to determine the various treatment field outlines and orientations to be used during radiation therapy.
Simulation Films
X-ray films taken on a simulator with the same field size, target-to-skin distance, and orientation as a therapy beam.
Simulator
A radiation generator operating in the diagnostic x-ray range that can direct a radiation beam toward a patient with parameters imitating those proposed for therapy, and providing direct x-ray fluoroscopic visualization and roentgenographic images. The simulator x-rays do not contribute to the therapy dose required.
Skin Dose
Absorbed dose at the center of the irradiation field on skin. It is the sum of the dose in air and scatter from body parts.
Softness
A relative specification of the quality or penetrating power of x-rays. In general, the longer the wave length the softer the radiation.
Somatic Effects
Effects that may become evident in the irradiated individual.
Source
Synonymous with Target.
Source Axis Distance (SAD)
Synonymous with Target axis distance (TAD).
Source Film Distance (SFD)
Synonymous with Target-film distance (TFD).
Source Surface Distance (SSD)
The distance measured along the beam axis, from the front surface of the source to the surface of the irradiated object Synonymous with TSD.
Specific Activity
Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide.
Specific Gamma-ray Constant
For a nuclide emitting gamma radiation, the product of exposure rate at a given distance from a point source of that nuclide and the square of that distance divided by the activity of the source, neglecting attenuation.
Spectrum
A visual display, a photographic record, or a plot of the distribution of the intensity of radiation of a given kind as a function of its wavelength, energy, frequency, momentum, mass, or any related quantity.
Split Course
A course of radiotherapy delivered in two parts separated by a rest period of several weeks.
Standard
Something established as a measure; a model to which other similar things should conform.
Glossary: Sievert
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Standard, Radioactive: A sample of radioactive material, usually with a long half-life, in which the number and type of radioactive atoms at a definite reference time are known. It may be used as a radiation source for calibrating radiation measurement equipment. Sterility (Biological)
Temporary or permanent incapability to reproduce.
Sublethal Damage
Cellular damage, the accumulation of which, may result in lethality.
Survey, Radiologic
Evaluation of the radiation hazards incident to the production, use, or existence of radioactive materials or other sources of radiation under specific conditions. Such evaluation customarily includes a physical survey of the disposition of materials and equipment, measurements or estimates of the levels of radiation that may be involved, and sufficient knowledge of processes using or affecting these materials to predict hazards resulting from expected or possible changes in materials or equipment.
Survey Meter (Laboratory Monitor)
A detection instrument used to monitor an area for unsuspected radiation or to search for a lost radiation source or contamination.
Syndrome
The complex of symptoms associated with any disease.
Target
A metal plate placed in the beam of high-speed electrons to produce x-rays. For electron therapy, the target is retracted from the beam.
Target Axis Distance (TAD)
The distance measured along the central axis from the center of the front surface of the target to the isocenter.
Target Film Distance (TFD)
The distance measured along the central axis from the center of the front surface of the target to an x-ray film.
Target Skin Distance (TSD)
The distance measured along the central axis from the center of the front surface of the target to the surface of the irradiated object.
Target Theory (Hit Theory)
A theory explaining some biological effects of radiation on the basis that ionization, occurring in a discrete Volume (the target) within the cell, directly causes a lesion which subsequently results in a physical response to the damage at that location. One, two, or more “hits” (ionizing events within the target) may be necessary to elicit the response.
Tenth-Value Layer (TenthValue Thickness) (TVL)
The thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduce’s the kerma rate by ten.
Therapy
Medical treatment of a disease. Brachytherapy (therapy at short distances): The treatment of disease with sealed radioactive sources placed near, or inserted directly into, the diseased area. Contact Radiation Therapy: X-ray therapy with specially constructed tubes in which the target-skin distance is very short (less than 2 cm). The voltage is usually 40 to 60 kV.
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Glossary: Sterility (Biological)
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Radiation Therapy: Treatment of disease with any type of radiation. Rotation Therapy: Radiation therapy during which either the patient is rotated in front of the source of radiation or the source is revolved around the patient. In this way, a larger dose is built up at the center of rotation within the patient’s body than on any area of the skin. Teletherapy (therapy at long distance): The treatment of disease with gamma radiation at a distance from the patient.
Thermionic Emission
Release of electrons from the cathode filament by heat.
Thermoluminescent Dosimetry
A method of determining dose by exposing certain phosphoric materials to radiation, and then heating the materials and measuring the light emitted. The luminescence is proportional to the dosage delivered.
Threshold, Photoelectric
The quantum of energy hv0 that is just enough to release an electron from a given system in the photoelectric effect. The corresponding frequency, vO, and wavelength, 8O, are the threshold frequency and wavelength respectively. For example, in the surface photoelectric effect, the threshold hv0 for a particular surface is the energy of a photon which, when incident on the surface, causes the electron to emerge with zero kinetic energy.
Thyratron
A gas-filled electron tube in which the grid controls only the start of a continuous current, giving the tube a trigger action. A thyratron is used to discharge the pulse-forming network in the modulator.
Tissue Equivalent Material
Material made up of the same elements in the same proportions as they occur in a particular biologic tissue. In some cases, the equivalence may be approximated with sufficient accuracy on the basis of effective atomic number.
Townsend Avalanche
(See Avalanche.)
Track
Visual manifestation of the path of the ionizing particle in a chamber or photographic emulsion.
Transformer
An electrical device for increasing or decreasing the incoming voltage.
Transmutation
Any process in which a nuclide is transformed into a different nuclide, or more specifically, when transformed into a different element by a nuclear reaction.
Transport Group
This is any one of seven groups into which normal form radionuclides are classified according to their radiotoxicity and potential hazard in transportation.
Transport Index
The number to be placed on a package label to designate the degree of control to be exercised by the carrier during transportation and indicating the following: (l) the highest radiation absorbed dose equivalent rate in microSievert per hour at three feet from any accessible external surface of the package, or (2) for Fissile Class II packages only, the number calculated by
Glossary: Thermionic Emission
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 dividing the number “50” by the number of similar packages that may be transported together. Treatment Beam Parameters
The data required for complete specification of an individual treatment beam, including radiation energy, field size, use of wedges and blocks, orientation with respect to patient, and prescribed exposure time, dose, and distance.
Treatment Couch
The Clinac unit that supports the patient during therapy.
Treatment Field
A plane section of a beam, perpendicular to the beam axis, as defined by the collimator.
Treatment Head
The section of the Clinac gantry from which the treatment beam exits. The treatment head includes the carrousel, collimator, an ionization chamber, the range finder and field-defining light, and other supporting components.
Treatment Plan
An ensemble of radiation beams or sources designed to produce a prescribed dose pattern in and for the patient; includes spatial and temporal distributions.
Treatment Planning
A complex process carried out prior to the administration of radiation therapy. The planning process usually includes such items as tumor localization, treatment volume determination, contour preparation, and treatment dose determination to prescribe the dosage pattern required.
Treatment Room
An enclosed space specially designed and dedicated for patient treatment by a radiotherapy accelerator.
Treatment Type
Standard and specialized therapies available on the Clinac 2100C, including fixed x-ray, fixed electrons, arc x-ray, port film exposures, arc electrons, total body x-ray, total body electrons, and high dose rate total skin electrons.
Tritium (T)
The hydrogen isotope with one proton and two neutrons in the nucleus.
Tube, Photomultiplier
An electron multiplier tube in which the electrons initiating the cascade originate by photoelectric emission.
Umbra
The region within the beam receiving the full strength of the primary x-ray or gamma-ray photons.
Uncontrolled Area
An area not under the authority of the Radiation Protection Officer and not subject to restriction due to the presence of radiation.
Valence
Number representing the combining or displacing power of an atom; number of electrons lost, gained, or shared by an atom in a compound; number of hydrogen atoms with which an atom will combine, or which it will displace.
Van De Graaff Accelerator
An electrostatic machine in which electrical charge is carried into the high voltage terminal by a belt made of an insulating material moving at a high speed. The particles are then accelerated along a discharge path through a vacuum tube by the potential difference between the insulated terminal and the grounded end of the accelerator.
7-40
Glossary: Treatment Beam Parameters
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 Velocity
The ratio of displacement to the time required for this displacement. Time rate of motion in a given direction and sense. Average velocity equals the total distance passed over, divided by the whole time taken.
Video Display
See Monitor
Volt
The unit of electric pressure or electromotive force; the force necessary to cause 1 ampere of current to flow against 1 Ohm of resistance. –1 1V = 1J ⁄ C
Voltage
The potential difference, in volts, between two different points in an electric circuit or between two different electrodes. Voltage, Operating: As applied to radiation detection instruments, the voltage across the electrodes in the detecting chamber required for proper detection of an ionizing event. Voltage, Starting: For a counter tube, the minimum voltage that must be applied to obtain counts with the particular circuit with which it is associated.
Volume, Sensitive
That portion of a counter tube or ionization chamber which responds to a specific radiation.
Watt
The unit of power equal to 1 joule per second (1W = 1J/s).
Waveguide
Metal pipe of rectangular or circular cross section that transfers radio-frequency energy to the accelerator.
Wavelength
Distance between any two similar points of two consecutive waves ( λ ). For electromagnetic radiation, the wavelength is equal to the velocity of light (c) divided by the frequency of the wave (v), λ = c/v. The “effective wavelength” is the wavelength of monochromatic x-rays that would undergo the same percentage attenuation in a specified filter as the heterogeneous beam under consideration.
Wave Motion
The transmission of a periodic motion or vibration through a medium or empty space. Transverse: Wave motion in which the vibration is perpendicular to the direction of propagation. Longitudinal: Wave motion in which the vibration is parallel to the direction of propagation.
Wedge Filter
A tapered block of attenuating material, designed to produce wedge shaped isodose curves.
Window
(1) A plate, usually made of ceramic or glass, placed across the microwave waveguide to separate the air in the waveguide from the vacuum in the linear accelerator and the klystron. (2) A plate, made of thin aluminum or beryllium, through which electrons are extracted. (3) A rectangular portion of a monitor screen used for input from the operator or to display information to the operator.
Glossary: Velocity
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COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 X-rays
Penetrating electromagnetic radiations whose wavelengths are shorter than those of visible light. They are usually produced by bombarding a metallic target with fast electrons in a high vacuum. In nuclear reactions, it is customary to refer to photons originating in the nucleus as gamma-rays, and those originating in the extranuclear part of the atom as x-rays. These rays are sometimes called roentgen rays after their discoverer, W.C. Roentgen.
Some of these definitions are reproduced with permission of the U.S. Department of Health Education and Welfare Public Health Service Food and Drug Administration Bureau of Radiological Health. from the Radiological Health Handbook rev. ed. 1970.
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Glossary: X-rays
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Chapter Eight Index
This chapter contains an index of terms used in the C-series Clinac Accelerator System Basics manual.
Index
8-i
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
Index Numerics 3dB Quadrature Hybrid ................................................................ 4-21
A Advances in Linear Accelerator Design for Radiotherapy ............... 2-19
B Beam Trans port Magnet Systems ................................................. 2-44
C Circulators ................................................................................... 4-22
D Definitions, Intensity vs. Energy ..................................................... 2-3
E Electron Injection and Bunching ................................................... 2-18 Emergency and Safety .................................................................... 1-1 Emergency Off Button .................................................................. 1-10 Enabling emergency pendant ........................................................ 1-11
F Fill Time ....................................................................................... 2-16
G Gas Flow, defined ......................................................................... 6-17 Gas Laws ...................................................................................... 6-13 Gas, defined ................................................................................... 6-7
8-ii
Index
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7
I Injection Timing ............................................................................ 2-17 Ion Chamber Theory ....................................................................... 6-1 Ion Pump, defined ......................................................................... 6-21
K Karzmark, Dr. C. J. ....................................................................... 2-19 Kinetic Energy Relationships ........................................................... 2-3 Klystron Theory ............................................................................ 4-24
L Load Line Considerations .............................................................. 2-16
M Machine Physics ............................................................................. 2-1 Microtrons .................................................................................... 2-42 Microwave Accelerator Structures ................................................. 2-20 Modulator Theory ........................................................................... 3-1
N Nature of Vacuum ........................................................................... 6-3
P Pendant, emergency ...................................................................... 1-11 Pressure, defined ............................................................................ 6-7
R Resonant Circuits ......................................................................... 4-14 Rest Energy Relationships ............................................................... 2-4 RF Theory ....................................................................................... 4-1
Index
8-iii
COPYRIGHT © 2005 VARIAN MEDICAL SYSTEMS L FOR TRAINING PURPOSES ONLY 7 RF Transmission Theory ............................................................... 4-16
S Standing Wave Accelerator ............................................................. 2-9
T TE10 Mode ................................................................................... 4-18 Temperature, defined ...................................................................... 6-6 Thermocouple Gauge .................................................................... 6-26 Thyratron Theory .......................................................................... 3-19 Total Energy Relationships ............................................................. 2-4 Transmission Lines ........................................................................ 4-5
V Vacuum Gauges ........................................................................... 6-26 Vacuum, defined ............................................................................ 6-3 Vapor Pressure, defined ................................................................ 6-10
W Widely Variable Energy Linacs ...................................................... 2-33
8-iv
Index
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