1. Source Technologies and Magnets
Marco Carlone, PhD
BC Cancer Agency
Kelowna, BC, Canada
October, 27, 2017

2. Linac Beam Stability Marco Carlone, PhD BC Cancer Agency
Kelowna, BC, Canada
October, 27, 2017

3. About the presenter

4. Kelowna, BC, Canada 4

6. Some relevant quotes for this workshop
“A stable linac is a better linac”
Don Petzold

7. Some relevant quotes for this workshop
“A stable linac is a better linac”
Don Petzold
“But you never can tell with linacs”
If Winnie the Pooh had been a linac engineer

8. Electron Guns Create electrons.
Inject electrons into the accelerating waveguide (AWG).
Facilitate the acceleration of the e- to high energies.

10. Gun Types Diode Guns Cathode and Anode only
Current control is through temperature regulation
Triode Guns
Cathode, Anode and Grid
Current control is through activation of the grid
Photo Cathode RF Guns
Gate emission with laser
High Peak currents, low average currents
Cold Cathodes
Low peak currents
Emitter materials not yet optimized

11. Diode Guns Hot filament More electrons Cold filament Less electrons
Search YouTube: Elekta Linac

12. Diode Gun Failure
Filaments heating is greatest at the centre, causing failure at the filament centre

13. Triode Guns
Courtesy Varian Medical Systems

14. Accelerator Beam Current
Pierce type electron gun (space charge limited)
Courtesy Siemens Medical Solutions
iACC = k
( ) pV ⁄ 3 2
( )
Vgrid
Vcut
P = Perveance

15. Consequences of wandering emission spot from cathode
Radial position of electron beam wanders.
Affects electron bunching.
Affects the radial acceleration of electron bunches.
Affects beam spot on target.
Affects beam symmetry.

16. Binding (Attaching) electrons to Accelerator RF field
Electron must have sufficient energy to match to the phase velocity of the RF Field.
Energy spread of the electrons depends on the phase difference between the injected electron and RF field.

17. Injected Electron Energy
Karzmark, Electron Linear Accelerators

18. Phase dependence of Electron Acceleration
Humphreys, Principles of Charged Particle Accelerators

19. Energy Distribution as a function of phase acceptance
Humphreys, Principles of Charged Particle Accelerators

21. Diode Gun Control

22. Triode Gun Control

23. Holy Grail for linac electron sources
Synchronize the phase of electron injection with RF field in the linac first cavity.
Improves energy stability
Improves radial beam acceleration
Remove requirement for heating to eject electrons
Remove High Voltage requirement

24. Diamond Field emmiters
Cell geometry and field strength to control emission

25. Magnetic systems for linacs
Guide electrons towards the target.
Minimize electron beam defocussing.
Improve beam stability
For systems with a bend magnet

26. Bending Magnet is a Filter
Electron energy is matched with the bending magnet pass through energy

27. Bending Magnet is a Filter
Electron energy is too low for the bending magnet

28. Bending Magnet is a Filter
Electron energy is too high for bending magnet

29. Bending Magnet is Linear

30. Bending Magnets are designed to be achromatic
Karzmark, Medical Electron Accelerators

31. The Medical Physicists three best friends
The bending magnet

32. The Medical Physicists three best friends
The bending magnet

33. The Medical Physicists three best friends
The bending magnet

34. The Medical Physicists three best friends
The bending magnet
The bending magnet ensures energy stability
It is essentially linear in energy response

35. Focusing solenoids
Varian Training Materials

36. Focusing solenoids
Varian Training Materials

37. Focusing of electrons during the accelerating process
R.B. Palmer, PRISM Workshop, 2005

38. Defocusing Focusing e- direction v v B F Bz Bz B F B B v F F Br Br v v

39. Waveguide dependence on focus magnet
Required for all travelling waveguides.
Not required for 6 MeV standing short wave linac.
Not required for Siemens 8067 standing wave linac.
Not required for Therac-25
Required for Varian standing waveguide with energy switch.

40. Some design requirements
Linac electron source:
eliminate HV stage.
Eliminate filament heating.
Automatic phase synchronization with RF
Magnet systems
Simplified energy stability (re-think the 270 degree bend magnet).
Waveguides that do not need a focus magnet, and are stable with orientation in earth’s B field.