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
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
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
1 MeV Linac Design
CST Microwave studioTM
Parameter
Value
Energy
1 MeV
Frequency
9.3 GHz
Length
130 mm
Rsh max
116 M/m
Pin
433 kW
Single b = 1 cell
Q0=6500
Rsh=116 M/m
Es/Eacc=2.57
Hs/Eacc=0.003
5 mm beampipe diameter
3.5 mm iris thickness
1 mm coupling cell
thickness
TechX-uk, VORPAL
• Beam dynamics modelled
in ASTRA
• Cavity modelled in CST
MWS
• Results verified with TechX
VORPAL
Linac fabrication in the uk
• Shakespeare Engineering, Ltd fabricated the
linac
Single cell
• Diamond machining and vacuum brazing
processes employed
• Being the first experience, the required
geometric tolerances of 5 m wasn’t achieved
Cut view
Frequency
Frequency measurement
measurement
Linac test area in DL
Magnetron
Cavity
Compact electron gun
* A 17 keV electron gun was specially designed from a
TWT gun to fit the linac aperture
Cavity
Gun
• The gun gives 200 mA, 1mm spot size
• The gun cathode was activated and tested at DL
sigma_X,rms
sigma_Y,rms
emitt_X,rms
emitt_Y,rms
div_X, rms
div_Y, rms
0.242
0.235
0.141
0.133
19.85
19.32
mm
mm
p-mrad-mm
p-mrad-mm
mrad
mrad
• 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
Compact Magnetron
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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
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
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
Energy measurement
Target OUT, 1 us, 50 Hz
RF, 1 us, 50 Hz
Peak, kW
540
652
775
834
900
Target
OUT
IN
Fcup
p-p mV
40
31
37
23
Trip
Rad mon
p-p mV
682
800
Ekin
keV
300
350
600
610
Trip
Spectrometer magnet
Ekin
keV
610
610
• 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
Faraday cup
Radiation monitor
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
Extra slides
p/2-mode cavity
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.
1. The first cavity of the structure act as an electrostatic lens which capture the DC
beam from the gun
2. The beam then sees an axial electric field increasing with time (-ve synchronous
phase) for velocity modulation and bunching
3. The bunches then see a decreasing field (+ve synchronous phase) for radial
focusing
Choice of cell-to-cell coupling is as important as the choice of
p/2 mode for field stability
Capacitive coupling
through beampipe
Capacitive coupling
through wall slot
Inductive side coupling
Inductive coupling
through wall slot
Inductive coupling with high
shunt impedance
We chose the simplest
geometry which is the
capacitive coupling through
beampipe
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
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
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