Compact Radio Frequency Technology for Applications in Cargo and Global Security

Graeme BurtLancaster 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.

Truck or shipping cargo is larger requires ~6 MeV. Industrial linacs can provide this.

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.

• Lancaster University:– Graeme Burt (Project Leader)– Praveen Ambattu (Linac)– Chris Lingwood (Linac)– Tom Abram (Mechanical)– Mike Jenkins (Linac)

• STFC, ASTeC Daresbury Lab:– 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)

CLASP Ph-I Collaboration Team

CI-SAC Dec 2011

X-band Linac schematic

DC Electron Gun e2V

Buncher and Accelerating Structure (1 MeV)

Magnetron

e2V

(9.3 GHz, 1.2 MW, 100-400 Hz) Dynamic switching of amplitude and

phase pulse-to-pulse)

Automated Control System(Energy, rep-rate, dose)

CI Proposal ScopePhase-I

X-ray Target

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 side-coupled structures pi-mode

• Hard to tune coupling cells and are sensitive to brazing tolerances

1 MeV Linac Design

5 mm beampipe diameter3.5 mm iris thickness1 mm coupling cell thickness

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 %

Parameter Value

Energy 1 MeV

Frequency 9.3 GHz

Length 130 mm

Rsh max 116 M/m

Pin 433 kW

Pulse Length 4 s

Pulse Rate 250 Hz

Peak Beam Current 70 mA

Average Beam Power 70 W

• 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.

Beam Tracking Analysis

http://www.txcorp.com/products/VORPAL/

• Fabrication commissioned with UK industry:– Shakespeare Engineering, Ltd

• Geometric tolerances of 10 m required.

• Diamond machining and vacuum brazing processes employed.

Linac Fabrication

http://www.shakespeareengineering.co.uk/

Cavity Tuning

Structure was found to have poor matching and field flatness.Low beta cells were further off frequency than could be tuned.

9.24 9.26 9.28 9.3 9.32 9.34 9.36

-40.0

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

water OFF T=19 deg 2022 25

Freq, GHz

S11,

dB

Ideal profile

17 keV Electron Gun from E2v

http://www.e2v.com/

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.

Substantial ringing is found on the ICT due to EM interference from pulse operation.

• 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.

Magnetron Testing

• 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.

Diagnostics Line

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 TestingSo 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

New490 MHz wide

Old60 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)

Contract placed with ComebMode 8

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.

35 GHz input and intermediate cavities105 GHz output cavity

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)

Mm-wave upconverter

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