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. 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, Hz) Dynamic switching of amplitude and phase pulse-to-pulse) Automated Control System (Energy, rep-rate, dose) CI Proposal Scope Phase-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 (2r cav t shield + t shield 2 )  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 diameter 3.5 mm iris thickness 1 mm coupling cell thickness Gradient (MV/m)E (MeV)I beam (mA)Spot Size (mm) 20 (nom) %+11 %-4 %+58 % -10%-27 %-33 %-55 % Voltage (kV)E (MeV)I beam (mA)Spot Size (mm) 17 (nom) %+0.8 %-3.5 %+48 % -10%-7 %-20 %-15 % ParameterValue Energy1 MeV Frequency9.3 GHz Length130 mm R sh max 116 M  /m P in 433 kW Pulse Length 4  s Pulse Rate250 Hz Peak Beam Current70 mA Average Beam Power70 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

Fabrication commissioned with UK industry: – Shakespeare Engineering, Ltd Geometric tolerances of 10  m required. Diamond machining and vacuum brazing processes employed. Linac Fabrication

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

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. 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 Hz

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. 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 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) Contract placed with Comeb Mode 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 3 rd harmonic output cavity. 35 GHz input and intermediate cavities 105 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