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CLIC Crab Cavity Infrastructure Amos Dexter and Benjamin Woolley CERN 27th October 2011 October 2011 Crab cavity development for CLIC Main Collaborators Lancaster University SLAC CERN ASTeC Manchester University Graeme Burt, Praveen Ambattu Valery Dolgashev Alexej Grudiev, Walter Wuensch, Rogelio Tomas Peter McIntosh Roger Jones, Ian Shinton October 2011 Crab cavity action Without a correctly functioning crab cavity CLIC looses 90% of its luminosity The crab cavity system cannot be compromised. Detector quadrupoles quadrupoles 20 milli-rad crossing travelling wave crab cavity Bunches pass through deflecting cavities phased to give zero kick for bunch centres A deflecting cavity phased in this way is called a crab cavity For a bunch with length • The crab cavity kicks the bunch front to start rotating away from the other beamline • The crab cavity kicks the bunch rear to start rotating towards the other beamline Perfect alignment of bunches occurs only at the IP October 2011 Current work REMAINING EUCARD TASKS • Gradient tests of a cell design excited in a dipole mode • Manufacture a multi-cell un-damped cavity for high power tests at CERN • Complete phase measurement sampling electronics NEW CERN COLLABORATION • Development of a damped structure with elliptical cells (including a prototype for bench measurements) • Engineering design work to enable prototype cavities to be tested at CERN (cooling, vacuum, mounting, instrumentation etc.) • Experiments to understand stability of the RF distribution system • R&D as necessary to improve stability of RF distribution system • Design of components for active control of the RF distribution path length ALSO NEEDED INTEGRATION October 2011 Planned CLIC crab high power tests Travelling wave 11.9942 GHz phase advance 2p/3 TM110h mode Input power ~ 14 MW Test 1: Middle Cell Testing – Low field coupler, symmetrical cells Test 2: Coupler and cavity test – Final coupler design, polarised cells, no dampers. Test 3: Damped Cell Testing – Full system prototype Cavity length < 200 mm Cavity width ~ 200 mm CLIC TW Crab Cavity and Load Cooling ~ 100 W October 2011 Crab cavity optics Crab cavities are located near the final focus in linear colliders to get a large rotation for a given voltage (high R12). (15.2 metres from IP) Unfortunately this also means a large R34 the effect of wakes are magnified October 2011 Crab cavity issues • Wakefields - cause kicks and emittance growth • Poor Phase Stability - gives large horizontal kicks • Beam-loading - has large unpredictable fluctuations on a time scale of tens of ns Key required outcomes • Damp, measure and confirm the predicted wakes. • Establish feasible/achievable level of phase control performance. (Requirement looks beyond state of the art) • Need solution which is insensitive to beam-loading October 2011 RF solution at 12 GHz Wakefields Large irises Small number of cells Strong damping Phase and amplitude control Passive during 156 ns bunch train Beamloading compensation High energy flow through cavity * small number of cells * high group velocity * low efficiency Phase synchronisation Same klystron drives both cavities Active waveguide RF length adjustment Phase reference Mixing reflected pulses Optical interferometer Phase stability Thick cavity irises Strong cavity cooling No mode conversion in RF distribution Highly stable waveguide October 2011 Beam timing If cavities have perfect synchronism then timing issues on one beamline (here the positron line) can be understood by considering a long bunch symbolised by a dashed line through the actual bunch. Beam timing errors give head on collisions with a displaced IP and slight defocusing. cDterror t6 t5 Positron crab cavity position t3 Electron crab cavity position t2 Dxerror t3 t2 t1 qc t1 dip IP Position of positron bunch centre when electron bunch centre is at IP Position where bunches pass October 2011 Cavity synchronisation CLIC bunches ~ 45 nm horizontal by 0.9 nm vertical size at IP. Cavity to Cavity Phase synchronisation requirement Target max. luminosity loss fraction S 0.98 f (GHz) 12.0 x (nm) 45 720 x f cqc 1 S4rms qc (rads) frms (deg) 0.020 0.0188 1 degrees Dt (fs) Pulse Length (ms) 4.4 0.156 So need RF path lengths identical to better than c Dt = 1.3 microns October 2011 RF layout and procedure travelling wave cavity Laser interferometer Control Waveguide with micronlevel adjustment Magic Tee Waveguide with micronlevel adjustment LLRF Main beam outward pick up LLRF Phase Shifter From oscillator main beam outward pick up Pulsed Modulator 12 GHz Pulsed Klystron ( ~ 50 MW ) Control Vector modulation 12 GHz Oscillator Once the main beam arrives at the crab cavity there is insufficient time to correct beam to cavity errors. 0. 1 Send pre-pulse to cavities and use interferometer to measure difference in RF path length. (option1) 0. 2 Send off frequency pre-pulse and measure phase difference of reflections. (option2) 0. 3 Use measurement from last high power pulse. (option 3) 1. Perform waveguide length adjustment at micron scale 2. Measure phase difference between oscillator and outward going main beam 3. Adjust phase shifter in anticipation of round trip time and add offset for main beam departure time 4. Klystron output is controlled for constant amplitude and phase 5. Record phase difference between returning main beam and cavity 6. Alter correction table for next pulse October 2011 Distribution stability requirement • r.m.s. cavity to cavity synchronisation requirement is 4.4 fs • hence r.m.s klystron to cavity stability requirement is 3.1 fs (two paths) • Stability requirement for one transmission leg = 13.4 milli-degrees Control waveguide temperature to say 0.3oC. Copper expansivity 17 10-6 K-1. Width expansion example for circular TE11 (12 mm) A 12 mm radius could expand by 61 nm hence phase velocity changes by 1146 m/s For a 45 metre length phase change = 1.55 degrees = 115 times the allowance Width expansion example for circular TE10 (40 mm) A 40 mm radius could expand by 204 nm hence phase velocity changes by 281 m/s For a 45 metre length phase change = 0.52 degrees = 39 times the allowance Length expansion example for circular TE10 (40 mm) without expansion joints A 45 metre waveguide could vary in length by 230 mm. Waveguide wavelength ~ 27.0 mm Phase shift ~ 3.07 degrees which is 229 times the allowance! It is clear that the differential transmission length must be measured on each pulse and corrected. One would also seek to minimise disturbing influences. October 2011 Waveguide consideration Assume 45 m waveguide run from Klystron to each Crab cavity For copper s =5.8e7 S/m and at 11.994 GHz Attenuation Transmission Over moded Rectangular TE10 EIA90 (22.9 x 10.2 mm) 0.098 dB/m 36.2% no Rectangular TE10 special (24 x 14 mm) 0.073 dB/m 47.2% no Circular TE11 (r = 9.3 mm) 0.119 dB/m 29.3% no Circular TE11 (r = 12 mm) 0.055 dB/m 56.7% TM10 Circular TE01 (r = 40 mm) 0.010 dB/m 90.1% extremely Available klystron has nominal output of 50 MW Divide output for two beam lines = 25 MW For standard rectangular waveguide we have 9.1 MW available (OK for 12 cells) For special rectangular waveguide we have 11.8 MW available For circular 12mm TE11 waveguide we have 14.2 MW available (OK for 10 cells) (note that mode conversion from circular TE11 to circular TM10 is vanishingly small for properly designed bends) October 2011 Waveguide choice Probably 40 mm diameter over moded TE01 (as RF path length less dependent on diameter variation.) Waveguide needs isolation from influences (changing the RF path length by more than ~ 0.5 microns in 20 milli-seconds.) The dynamic range of the fast waveguide phase shifters is not likely to be more than 100 times the range adjustment required between pulses. Have a choice use slow waveguide phase shifter to compensate for long term temperature drifts (differential between cavities on timescales > hours) or temperature control. 40 mm waveguide Damping material with good thermal mass. October 2011 Civil engineering Machine By-Pass added Emergency escape tunnel Survey gallery added October 2011 Klystron position? Crab cavities Klystron October 2011 Waveguide routing? klystron in detector cavern preferred waveguide route in own bore then cavern detector beam tunnel crab cavity 15.2 m beam tunnel crab cavity awkward waveguide route in beam tunnel, then detector tunnel, then cavern October 2011 CLIC detector halls Crab cavity klystron? October 2011