Compact Radio Frequency Technology
for Applications in Cargo and Global
Security
Graeme Burt
Lancaster University, Cockcroft Institute,
Security Lancaster
Cargo Screening Accelerators
Luggage Scanning
requires a few tens to
hundreds keV. This
can be delivered by
traditional X-ray
tubes up to 450 keV.
Aircraft ULD or pallets are
too large for baggage
scanners and too small for
cargo scanners.
Currently searched by
hand.
Ideal energy is around 1-2
MeV but no current source
available.
Truck or shipping cargo is larger requires ~6
MeV. Industrial linacs can provide this.
CLASP Ph-I Collaboration Team
• Lancaster University:
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Graeme Burt (Project Leader)
Praveen Ambattu (Linac)
Chris Lingwood (Linac)
Tom Abram (Mechanical)
Mike Jenkins (Linac)
• STFC, ASTeC Daresbury Lab:
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Ian Burrows (Mechanical Eng.)
Clive Hill (Mechanical)
Peter Corlett (Project Manager)
Andrew Goulden (Cooling Sys.)
Paul Hindley (Installation)
Peter McIntosh (ASTeC PI)
Keith Middleman (Vacuum)
Rob Smith (Beam Diagnostics)
Chris White (Electrical Eng.)
Trevor Hartnett (Electrical)
Steve Griffith (electrical)
X-band Linac schematic
DC Electron Gun
e2V
X-ray Target
Buncher and Accelerating Structure (1 MeV)
CI Proposal Scope
Phase-I
Magnetron
e2V
(9.3 GHz, 1.2 MW, 100-400 Hz)
Dynamic switching of amplitude and
phase pulse-to-pulse)
CI-SAC Dec 2011
Automated Control System
(Energy, rep-rate, dose)
Why X-band?
• For a mobile linac mounted on
a robotic arm the weight of
the linac is critical.
• While the linac isn’t very big
or heavy the shielding is.
• X-band means that the
shielding diameter is much
less.
• Area of shielding is given by
• (2rcavtshield + tshield2)p
• Availability of 9.3 GHz
magnetrons
CLIC crab Cavity
• Lancaster has some experience
in X-band from the CLIC crab
cavities.
• A prototype of this cavity has
recently been manufactured
and tuned at CERN and is
awaiting a testing slot in XBox2.
Linac options
•
Side coupled pi/2-mode
• pi/2 >> frequency stability
especially in the presence of large
number of cells
• Shunt impedance comparable to
pi-mode
• Large transverse size due to
coupling cells
•
Bi-periodic pi/2-mode
• pi/2 >> frequency stability
especially in the presence of large
number of cells
• Shunt impedance less than sidecoupled structures pi-mode
• Hard to tune coupling cells and are
sensitive to brazing tolerances
1 MeV Linac Design
Parameter
Value
Energy
1 MeV
Frequency
9.3 GHz
Length
130 mm
Rsh max
116 M/m
Pin
433 kW
4 s
Pulse Length
5 mm beampipe diameter
3.5 mm iris thickness
1 mm coupling cell thickness
Pulse Rate
250 Hz
Peak Beam Current
70 mA
Average Beam Power
70 W
Gradient (MV/m)
E (MeV)
Ibeam (mA)
Spot Size (mm)
20 (nom)
1.08
70
1.6
+10 %
+11 %
-4 %
+58 %
-10%
-27 %
-33 %
-55 %
Voltage (kV)
E (MeV)
Ibeam (mA)
Spot Size (mm)
17 (nom)
1.08
70
1.6
+10 %
+0.8 %
-3.5 %
+48 %
-10%
-7 %
-20 %
-15 %
Beam Tracking Analysis
• Particle Tracking initially
performed in ASTRA.
• Collaboration with Tech-X
UK to verify Linac electron
beam capture and tracking.
• Using VORPAL code to
validate PIC transport.
• Good comparison was
found between both
methods.
http://www.txcorp.com/products/VORPAL/
Linac Fabrication
• Fabrication commissioned with
UK industry:
– Shakespeare Engineering, Ltd
• Geometric tolerances of 10 m
required.
• Diamond machining and vacuum
brazing processes employed.
http://www.shakespeareengineering.co.uk/
Cavity Tuning
0.0
-5.09.24
9.26
9.28
9.3
9.32
9.34
9.36
20
22
25
S11, dB
-10.0
-15.0
-20.0
-25.0
-30.0
-35.0
-40.0
Freq, GHz
water OFF
T=19 deg
Ideal profile
Structure was found to have poor
matching and field flatness.
Low beta cells were further off
frequency than could be tuned.
17 keV Electron Gun from E2v
A 17 keV electron gun was specially
designed for this project from a TWT gun.
The gun was modified to provide 200 mA
with a 1mm spot size.
Gun has been successfully tested at
Daresbury.
http://www.e2v.com/
Substantial ringing is found on the
ICT due to EM interference from
pulse operation.
Magnetron Testing
• E2V engineers acceptance tested the magnetron at
Daresbury.
• Maximum power achieved ~ 1.1 MW but not
sustainable due to arcs.
• Operating at long pulse lengths (4 us) and high
power (>1 MW) results in significant arcing within
the magnetron.
Diagnostics Line
• In order to fully diagnose the
beam from the linac we have a
diagnostics line fitted to the
output.
• We have a motorised section
which can either provide a slit, a
screen, a tungsten target or
vacuum.
Imaging and testing
• Conveyor system set up in
the linac area to perform
full system characterisation
in a realistic environment.
• Detector system developed
by a local scanning
company.
Good quality full scale imaging requires dose
of at least 0.03 Gy/min @ 1m @ 100Hz
Linac Testing
So far the linac has produced a 750 keV, 1 mA
beam as measured on the spectrometer and
Faraday cup/ICT at the end of the diagnostics
line (probably large beam loss prior to this).
This is limited by the cavity being slightly out
of tolerance affecting the fields.
Learning Curve
• Improvements to Mk II design
– Less rounding on equator to allow less stiffness, more tuning range
– Re-entrant first cell to reduce stiffness, increase tuning range
– Larger cell-to-cell coupling
– Longitudinal cooling pipes, more room for tuning pins
– Move coupler to centre cell
Modified structure
New
490 MHz wide
Mode 8
Contract placed with Comeb
Old
60 MHz wide
We have developed a new X-band structure with much
greater cell-to-cell coupling to increase tolerances.
Simple structure design with no slots to help tolerances
(low fields and low voltage make this acceptable)
Harmonic W-band Klystron
There is also interest in millimetre wave and THz scanning of personnel.
For this application we have been developing a 105 GHz Klystron.
To avoid issues with poor scaling of Klystrons to high frequencies we use a 3rd
harmonic output cavity.
105 GHz output
cavity
35 GHz input and intermediate cavities
Mm-wave upconverter
To increase the size of the
output cavity such that it
could be made from
conventional
machining
we use a higher order
mode cavity (TM020-like)
Input coupler 31GHz
Output coupler 94GHz
Harmonic W-band Klystron
•Structure casted in Silver
from a 3D printed hard
plastic mould.
•Allows better material
quality, tolerances and
surface roughness than
direct 3D printing in
metal.
•Initial prototypes are
promising.
Conclusion
• A strong UK collaborative team has been formulated to successfully
demonstrate a working system solution.
• Challenging requirement to develop and demonstrate a gun,
magnetron and 1 MeV linac system within 18 months.
• Major development successes:
– An optimised 17 keV, high peak current electron gun has been
designed, fabricated and activated.
– A highly compact combined buncher/accelerating structure has been
designed.
– High precision fabrication has been demonstrated for the complex
linac geometry.
– The linac system has been assembled and has so far produced 750 keV
electron beam at 1 mA.
• Also developing high frequency sources for imaging