Compact Electron Linac for low energy, low dose applications

1. Compact Electron Linac for low energy, low dose applications Praveen Ambattu* (The Cockcroft Institute / Lancaster University) G. Burt, P. Corlett, K. Middleman, R. Smith, A. Goulden, P. McIntosh, C. White, T. Hartnet, A.Gallagher, I. Burrows, J. Smith, C. Lingwood, L. Nicolson, P. Goudket, P. Hindley, C. Hodgkinson Compact Particle Accelerators IoP Particles and Beams group meeting, The Cockcroft Institute, 18/04/2012

2. Linac based X-ray sources • X-ray sources play integral parts in radiography, radiotherapy and radiation processing in the industrial, scientific, medical and security sectors • Need of radiography for global security eg: cargo scanning is highly demanding • Tube based X-ray sources can’t be used as they are limited to ~ 450 keV which corresponds to a penetration of < 100 mm in steel • Here comes RF or linac based sources • Requirements: • 1. Energy > 1-8 MeV to penetrate at least 200 mm steel • 2. Beam power ~ 50-500 W for dose rate of 2-20 cGy / min at 1 meter • 3. Pulse rate: 50-500 Hz for high scan throughput and resolution

3. Compact Linac project • Low energy (1 MeV), low dose (2 cGy/min at 1 m) is identified to have potential use in Air cargo screening and mobile cargo screening • Compact structure would reduce overall cost of the system and enable mobilization of the scanner • Compactness requires linac and all sub-systems to be compact • Existing technology allows X-band frequency (8-12 GHz) for linac • Collaboration among STFC, Lancaster uni and uk Industries kicked off the project

4. 1 MeV Linac Design CST Microwave studioTM Single b = 1 cell • Beam dynamics modelled in ASTRA • Cavity modelled in CST MWS • Results verified with TechX VORPAL Q0=6500 Rsh=116 MW/m Es/Eacc=2.57 Hs/Eacc=0.003 TechX-uk, VORPAL 5 mm beampipe diameter 3.5 mm iris thickness 1 mm coupling cell thickness

5. Linac fabrication in the uk Single cell Cut view Frequency measurement Frequency measurement • Shakespeare Engineering, Ltd fabricated the linac • Diamond machining and vacuum brazing processes employed • Being the first experience, the required geometric tolerances of 5 m wasn’t achieved

6. Linac test area in DL Magnetron Cavity

7. Compact electron gun • * A 17 keV electron gun was specially designed from a TWT gun to fit the linac aperture • The gun gives 200 mA, 1mm spot size • The gun cathode was activated and tested at DL Cavity Gun • The CT used on the gun HV shows the cathode current ~ 100 mA (10 mV on scope) • Grid pulsing causes substantial ringing on the current pulse

8. Compact Magnetron Rating is 1.3 MW peak power, 4 us, 200 Hz pulse Achieved 1.2 MW at 2 us, 100 Hz Operation at high average power (P*Tp*fp) results in arcing within the circulator / magnetron

9. Magnetron tuning • Tuning stub on the magnetron can be remotely turned using a stepper motor arrangement • Range is 6.7 MHz around the centre frequency • Outside this range, manual turning of the stub is required

10. Linac testing • The cavity has resonance around 9.3 GHz with good matching • Magnetron was matched to this mode by tuning while monitoring the reflected power • When matched, the frequency is measured to be 9.2985 GHz

11. Energy measurement Target OUT, 1 us, 50 Hz Spectrometer magnet Faraday cup Radiation monitor • The peak energy measured on the spectrometer is 610 keV, the peak beam current is ~1 mA on a Faraday cup • This is because the gun heater/cathode degraded over time and could supply only 10 mA current and the cavity had incorrect e-m field pattern due to wrong tolerances • Replacement of the gun and linac will take place in coming months • Optimistic about achieving the required dose in the next go

12. Conclusion • ‘Compactness’ of the linac is very important for size, weight and cost reduction of the X-ray source • Level of linac ‘compactness’ is mainly based on available fabrication technology • For a multi cell linac, compactness increases machining complexity which in turn increases machining cost • X-band frequency of 9.3 GHz is a suitable choice as the technology is advanced and RF sources are available • Fabrication of the X-band linac was demonstrated for the first time in the uk by Shakespeare Eng • The commissioned linac system so far produced 610 keV electron beam at 1 mA • Next step is to replace the gun, cavity and circulator that will improve the linac performance to the expectation

13. Extra slides

14. p/2-mode cavity

15. RF bunching and focusing Solenoids are unacceptable for compact applications. Hence RF cavities themselves are used for focusing using the electric field in the injector section until an energy of nearly 1MeV (b=1) is reached. • The first cavity of the structure act as an electrostatic lens which capture the DC beam from the gun • The beam then sees an axial electric field increasing with time (-ve synchronous phase) for velocity modulation and bunching • The bunches then see a decreasing field (+ve synchronous phase) for radial focusing

16. Choice of cell-to-cell coupling is as important as the choice of p/2 mode for field stability Capacitive coupling through wall slot Capacitive coupling through beampipe Inductive coupling through wall slot Inductive side coupling Inductive coupling with high shunt impedance We chose the simplest geometry which is the capacitive coupling through beampipe

17. Biperiodic cavity • In a conventional p/2-mode cavity, the alternate cavities are contracted to occupy less space • Now we have two cells per p period, which must be independently resonant at the same frequency

18. Cavity optimisation A compromise among, • Available space to fit in the accelerator short, high gradient • Available power input low losses, high shunt impedance • Manufacturing cost simple geometry, low surface fields

19. Beadpull measurement of electric field Expected field at 9.3 GHz For 1 MeV acceleration, 30 MV/m peak field was identified Measured field at 9.283 GHz Because of the non-ideal field, the field has to be increased to 60 MV/m for 1 MeV