ILC Crab Cavity Collaboration

1. ILC Crab Cavity Collaboration • Cockcroft Institute : • Graeme Burt (Lancaster University) • Richard Carter (Lancaster University) • Amos Dexter (Lancaster University) • Philippe Goudket (ASTeC) • Roger Jones (Manchester University) • Alex Kalinin (ASTeC) • Lili Ma (ASTeC) • Peter McIntosh (ASTeC) • Imran Tahir (Lancaster University) • FNAL • Leo Bellantoni • Mike Church • Tim Koeth • Timergali Khabiboulline • Nikolay Solyak • SLAC • Chris Adolphson • Kwok Ko • Zenghai Li • Cho Ng LC-ABD Plenary Durham 2006

2. Crab Cavity Function The crab cavity is a deflection cavity operated with a 90o phase shift. A particle at the centre of the bunch gets no transverse momentum kick and hence no deflection at the IP. A particle at the front gets a transverse momentum that is equal and opposite to a particle at the back. The quadrupoles change the rate of rotation of the bunch. LC-ABD Plenary Durham 2006

3. TM110 Dipole mode cavity vertical horizontal • Transverse magnetic and electric field components of the TM110 dipole mode combine to give the overall transverse momentum kick. • The net transverse momentum kick is phase dependent. • If the beam has an offset it can be accelerated or retarded by the longitudinal electric field and hence delivers or extracts power from the cavity. Electric Field Beam Magnetic field cross section LC-ABD Plenary Durham 2006

4. FNAL CKM 3.9 GHz Cavity CKM Cavity design parameters 3.9 GHz 13 cells length = 0.5 m Bmax = 80 mT Emax = 18.6 MV/m Leff = 0.5 m P = 5 M V/m Our recommendation to the GDE has been to develop a cavity based on a Fermi-lab design. To minimise wakefields for the short time structure of the ILC bunches, the number of cells must be optimised against overall length and new couplers designed. A 3.9 GHz cavity was favoured it is compact longitudinally and transversely. Courtesy: FNAL LC-ABD Plenary Durham 2006

5. Crab Cavity Parameters Current plan is to use 4 eight cell cavities LC-ABD Plenary Durham 2006

6. Work plan Status Goal 1 System design LC-ABD Plenary Durham 2006

7. Work plan Status Goal 2 Cavity and RF Design and development LC-ABD Plenary Durham 2006

8. Work plan Status Goal 3 Phase Stability Experiments LC-ABD Plenary Durham 2006

9. Beam Loading • CKM input coupler has Qe ~ 107 and 500 W CW power handling capability. • Predicted that crab cavity has to cope with up to  0.6 mm bunch transverse offset. • Dipole Beam loading is linearly proportional with bunch offset and is zero for beam on axis. • Recommend Qe is reduced to ~ 5 x 105 and power delivery increased to ~ 3.5 kW CW. • New input coupler needed for ILC! LC-ABD Plenary Durham 2006

10. Power Coupler • 1st iteration of improved coupler • 40 Ohm coaxial line, 27mm outer conductor • Shaped tip for higher coupling • Centre line is 40mm from cavity. • 3mm beampipe penetration • Simulation Qe=4.5x105 • Further work is required to improve the tip to allow a reduction in the beampipe penetration. Tests are planned on various tip shapes using normal conducting prototypes with a removable tip. LC-ABD Plenary Durham 2006

11. Calculation of Higher and Lower Order Modes Fundamental dipole πmode Trapped mode in the 5th dipole passband. R/Qs for 1st 250 dipole modes Loss parameter of the 1st monopole passband Fundamental Monopole 7π/9 mode LC-ABD Plenary Durham 2006

12. Vert. wake limited by unwanted polarisation of dipole mode, ILC threshold 0.7 nrad. Highly dependent on frequency separation. Horz. wakes lower than ILC threshold (10 nrad). Deflecting mode not included. External Q’s are estimated. Long Range Wakes (Transverse) Horizontal kick for 4 offset. Vertical kick for 4 offset. 9-cell 9-cell Graeme Burt (Cockcroft) Leo Bellantoni (FNAL) LC-ABD Plenary Durham 2006

13. Other polarisation of the dipole mode, Qe=5x104 Sum Wakefield Kicks X’ 1 sigma vertical displacement at the IP if 4 cavities hit the resonance If only 1 of the 4 cavities hits the resonance this is a sigma/4 kick (2% lum. loss) To have all wakes below tolerances, the dipole pi mode should be damped to Qe~104 (probably not feasible) Tolerance % variation in bunch separation LC-ABD Plenary Durham 2006

14. These couplers are difficult to fabricate at 3.9GHz. CKM cavity HOM couplers have shown problems in tests: high tuning sensitivity (~ 1.6 MHz/m) multipacting. New HOM coupler needed for ILC! 3 different couplers for mode extraction required: Higher Order Mode (HOM) Lower Order Mode (LOM) Same Order Mode (SOM) Crab Cavity Mode Couplers HOM Coupler SOM Coupler LOM Coupler LC-ABD Plenary Durham 2006

15. LOM Coupler Tuning |E|, 7/9 mode, F=2848.95MHz, e=1.15 tip_LOM. Timergali Khabiboullin, FNAL LC-ABD Plenary Durham 2006

16. Model will be used to evaluate: Mode frequencies Cavity coupling HOM, LOM and SOM Qe and R/Q Modular design allows evaluation of: Up to 13 cells. Including all mode couplers. Test Model Cavity Verifications /9 Dipole Philippe Goudket (ASTeC) Graeme Burt (Cockcroft) LC-ABD Plenary Durham 2006

17. Technique employed extensively on X-band structures at SLAC. Bench measurement provides characterisation of: mode frequencies kick factors loss factors Wire Measurements Technique Roger Jones (Cockcroft Institute) LC-ABD Plenary Durham 2006

18. RF Amplifier RF Amplifier RF Amplifier Feedbackloop Feedback loop Feedback loop Phase Control Phase Control Phase Control Anticipated RF system ~ 4m • Anticipated requirement for 20mrad crossing is 4  8 - cell cavities per linac • Need space for cryostat, input/output couplers, tuning mechanisms… to IP Cryostat Reference from beam Reference Phase Reference for crab cavities on other beam LC-ABD Plenary Durham 2006

19. Phase Control Development Cavity Vector modulation available to 4 GHz Digital phase detection in use will be upgraded using a mixer and digital IQ detection at an intermediate frequency Programmed basic control software in DSP and demonstrated phase locking of warm cavity to 0.2 degrees so far. Need to improve accuracy of measurement hence upgrade A/D and D/A from 12bit to fast 16bit Need to upgrade connectors, cabling and low noise amplifiers Vector Modulator Amp I Q D/A D/A 3.9 GHz Oscillator DSP Oscilloscope 10 MHz Reference A/D /3 Frequency Divider /3 Frequency Divider 1.3 GHz Digital Phase Detector Spectrum Analyzer LC-ABD Plenary Durham 2006

20. Vertical Cyrostat Phase Control Tests Local reference and controller Local reference and controller Phase measurement Line to synchronise the local references must have its length continuously measured to with an accuracy of a few microns LC-ABD Plenary Durham 2006

21. In 2007, the ERLP at Daresbury will allow for crab cavity phase control electronics evaluation using: 2 SRF accelerating cavities at 1.3 GHz. 2 NC dipole cavities at 3.9 GHz. Phase Control Beam Tests Beamline containing 2 NC dipole cavities plus diagnostics. LC-ABD Plenary Durham 2006

22. Horizontal FONT • If one can accurately measure the x displacement of the spend beam (after the IP) as it passes the crab cavity on the ingoing beam one can determine the phase error between the crab cavities. • The deflection can be cancelled either by changing the phase of one cavity or using a magnetic kicker Chris Adolphson (SLAC), Phil Burrows (Oxford) LC-ABD Plenary Durham 2006