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

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

About the presenter

Kelowna, BC, Canada 4

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

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

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

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

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

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

Triode Guns Courtesy Varian Medical Systems

Accelerator Beam Current Pierce type electron gun (space charge limited) Courtesy Siemens Medical Solutions iACC = k ( ) pV ⁄ 3 2 ( 1 - ) Vgrid Vcut P = Perveance

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.

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.

Injected Electron Energy Karzmark, Electron Linear Accelerators

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

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

Diode Gun Control

Triode Gun Control

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

Diamond Field emmiters Cell geometry and field strength to control emission

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

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

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

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

Bending Magnet is Linear

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

The Medical Physicists three best friends The bending magnet

The Medical Physicists three best friends The bending magnet

The Medical Physicists three best friends The bending magnet

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

Focusing solenoids Varian Training Materials

Focusing solenoids Varian Training Materials

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

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

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.

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.