Why do we require compact cavities? 400 MHz elliptical 800 MHz elliptical 400 MHz elliptical Using 800 MHz RF causes a S- shaped bunch which reduces luminosity hence a 400 MHz compact cavity is desired There is limited space for the crab cavities due to the opposing beamline. The cavity must within a 143 mm radius.
4R crab cavity – Cockcroft - Jlab To make suitable we increased the separation between the rods and made the rods wider. This meant they were close to the outer can.
Cavity manufacture CI SAC Meeting 4 th – 6 th November 2013 A Wheelhouse5 th November 2013 Lancaster and ASTeC are developing a comapct SRF crab cavity for the LHC upgrade The structure, manufactured by Niowave was the first compact crab cavity to be tested at high field and has achieved a gradient of 3.5 MV
First Tests The first test were performed March 2013. Cavity was not etched after bakeout hence had some surface contamination. FIRST EVER HIGH GRADIENT TEST OF A COMPACT SRF CRAB CAVITY! CI SAC Meeting 4 th – 6 th November 2013 A Wheelhouse5 th November 2013
After a quick buffered chemical polish and ultrapure water high pressure rinse the cavity meets the design gradient. 2 nd Test results
Stainless Steel FPC Copper Q 0 = 7.189E9 R s = 8.3 nOhms Stainless Steel Q 0 =2.737E9 R s = 22.0 nOhms Recent tests with new couplers confirm this We recently discovered that the couplers had accidentally been made from stainless steel instead of copper. To test the effect we simulated a cavity with no losses other than the couplers in frequency domain using the same methods as in the vertical test.
Cavity welds A mistake was made in machining the rods. To fix this Niowave cut it off and welded a new one on. The magnetic field has a longitudinal component on the weld which is not ideal.
Cryomodule ASTeC have been developing a flexible cryostat for the LHC crab cavity. The cryostat has to include a beampipe for the opposing beamline and fit in the limited space within LHC. The cryostat is also side loading to allow better access during testing and commissioning A prototype cryomodule is currently being developed to allow cavity testing in the SPS. This will be the worlds first test of a crab cavity on a hadron beam. CI SAC Meeting 4 th – 6 th November 2013 A Wheelhouse5 th November 2013
Prototypes The 1 st CLIC crab cavity prototype has been manufactured by Shakespeare Engineering in the UK. Tolerance and surface roughness on single parts have been measured. Structure is planned to be tested at SLAC in the near future. The structure being built for high gradient test at CERN has only a single feed as it will not see beam. Cavity is being machined at VDL along with main linac structure to allow comparison of gradients.
CI-SAC Dec 2011 Surface Fields PropertyValue Energy stored, J1 Q Cu 6395 R t /Q, Ohm54.65 v gr, %-2.92 E surf /E t 3.43 H surf /E t 0.0114 Sc (W/ m 2 ) 3.32 E surf H surf S surf =(ExH) surf Peak electric and magnetic fields of the dipole mode are located 90 degrees from each other on the iris Surface Poynting flux S surf is however at 45 deg to both E and H Location of the breakdown on the iris provides critical information about the role of magnetic field in breakdown. The cavity has a large Sc but relatively low E and H fields at the surface so this also provides an independent verification of new theories.
Compact Accelerators for Cargo scanning applications
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.
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
CI-SAC Dec 2011 1 MeV X-ray linac DC Electron Gun e2V collaboration Buncher and Accelerating Structure (1 MeV) Magnetron e2V collaboration (8-12 GHz, 1-2 MW, 100-200 Hz) Dynamic switching of amplitude and phase pulse-to-pulse) Automated Control System (Energy, rep-rate, dose) Proprietary Rapiscan Imaging and Data Analysis CI Proposal Scope Phase-I X-ray Target
1 MeV Linac Design 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 Power 70 W
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
Linac Testing So far the linac has produced a 750 keV, 1 mA beam as measured on the spectrometer and Faraday cup/ICT. This is limited by the cavity having the incorrect e-m field pattern.
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)
Cavity measurements Preliminary measurements of the new structure show good agreement with simulations. Simulation Measurement
Conclusion The 4R Crab cavity for LHC was designed that met all specifications, unfortunately it was not selected for LHC. CI is heavily involved in X-band structures and CLIC and are currently undertaking high gradient tests of X-band crab cavities for CLIC. Compact Linac work is nearing commercialisation and the new structure from Comeb is performing well in low power measurements.