ILC Crab Cavity Collaboration

1. ILC Crab Cavity Collaboration • Cockcroft Institute : • Richard Carter (Lancaster University) • Amos Dexter (Lancaster University) • Graeme Burt (Lancaster University) • Imran Tahir (Lancaster University) • Philippe Goudket (ASTeC) • Peter McIntosh (ASTeC) • Alex Kalinin (ASTeC) • Carl Beard (ASTeC) • Lili Ma (ASTeC) • Roger Jones (Manchester University) • FNAL • Leo Bellantoni • Mike Church • Tim Koeth • Timergali Khabiboulline • Nikolay Solyak • SLAC • Chris Adolphson • Kwok Ko • Zenghai Li • Cho Ng LC-ABD plenary April 2007

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 April 2007

3. RDR Crab Cavity Parameters For 500 GeV CM we might use 1 nine cell cavity or two 5 cell cavities LC-ABD plenary April 2007

4. Anticipated RF system spent beam BPM Cryostat • Minimum requirement for 14 mrad crossing is 1  9 cell or 2  5 - cell cavities per linac • 2  9 – cells would provide full redundancy in case of failure • Need space for cryostat, input/output couplers, tuning mechanisms… IP ~ 14 m RF Amplifier RF Amplifier kick reference from spent beam Feedbackloop Feedback loop luminosity reference from IP DSP Phase Control DSP Phase Control Linac timing Reference Phase 2 Reference Phase 1 References to and from symmetrically placed crab cavities on other beam LC-ABD plenary April 2007

5. Beam TM110 Dipole mode cavity Electric Field in red View from top Magnetic field in green • The electric and magnetic fields are 90o out of phase. • For a crab cavity the bunch centre is at the cell centre when E is max and B is zero. • A particle at the centre of the bunch sees no electric or magnetic field throughout . LC-ABD plenary April 2007

6. Beamloading • Longitudinal electric field on axis is zero for dipole mode • Beamloading loading is zero for on axis bunches • Bunches pass cavity centre when B transverse = 0 hence of axis E = maximum • Crab cavities are loaded by off axis bunches • Dipole deflection cavities are not loaded by off axis bunches • Power requirement for 9 cells (500 GeV CoM) ~ a few kW LC-ABD plenary April 2007

7. Using amplifier to extract cavity energy For the crab cavity the bunches can supply or remove energy. Whilst in principle the amplifier can be used to reduce cavity energy after shifting its phase by 180o this is undesirable when one is trying to control cavity phase. It is desirable to chose a low external Q so this never needs to happen. Bunch goes through cavity at time t =0 LC-ABD plenary April 2007

8. Modelling of cavity amplitude with microphonics Oscillatory bunch offset of 1 mm and random arrival phase of 1 degree Drive frequency in GHz = 3.9 GHz Centre cavity frequency in GHz = 3.9 GHz Cavity Q factor = 1.0E+09 External Q factor = 3.0E+06 Cavity R over Q (2xFNAL=53 per cell) = 53 ohms Energy point ILC crab~0.0284J per cell) = 0.0284 J Amplitude set point = 301670 V Maximum Amplifier Power per cell = 300 W Maximum voltage set point (no beam) = 617740 V Maximum beam offset = 1.0 mm Maximum bunch phase error = 1.0 deg Beam offset frequency = 2kHz Bunch charge (ILC=3.2e-9) = 3.2E-09 C RF cycles between bunches = 1200 Delay for control system in cycles = 3900 Bunch length = 1 ms Cavity frequency shift from microphonics = 600 Hz Cavity vibration frequency = 230 Hz Initial vibration phase (degrees) = 20 deg PI controller with 1 ms delay cpr = 2.5e-6  Qe cir = 1.0e-10  Qe cpi = 2.5e-6  Qe cii = 1.0e-10  Qe LC-ABD plenary April 2007

9. Modelling of cavity drive with microphonics follows microphonics follows beam offset LC-ABD plenary April 2007

10. Modelling of cavity drive power with microphonics • A single klystron can’t easily do this but solid state amplifiers can. • To work with a Klystron we must lower the external Q LC-ABD plenary April 2007

11. Modelling of cavity phase with microphonics Oscillatory bunch offset of 1 mm and random arrival phase of 1 degree Modelling suggests that we might be able to control the phase of the cavity to within 6 milli-degrees of the measured phase error with respect to a local reference. LC-ABD plenary April 2007

12. Wire Measurement Technique Technique employed extensively on X-band structures at SLAC. Bench measurement provides characterization of: - mode frequencies - kick factors - loss factors LC-ABD plenary April 2007

13. Wire Theory A wire through a uniform reference tube can be regarded as a transmission line characterised by Ro , Lo and Co A wire through the cavity under investigation is modelled with an additional series impedance Zll / l The impedance Zll is large close to each cavity mode . One measures the phase lag q of a wave passing along the wire for the cavity (DUT) with respect to the plain tube (Ref) then determines Zlland hence kloss from theequations opposite. As q is measured as a function of frequency one obtains a loss factor at each frequency where Zll is large i.e. for each mode. LC-ABD plenary April 2007

14. Impedance Measurements Measurements are underway to measure the loss factors of the dominant modes in the crab cavity. The coupling impedance has been calculated for a 3 cell cavity in order to investigate systematic errors in the measurements. LC-ABD plenary April 2007

15. Long Range Wakefields The long range wakes are found by summing over all modes and all bunches LC-ABD plenary April 2007

16. Prototype Coupler A prototype Coupler has been constructed with replaceable tips and variable cavity separation to measure the external Q for a variety of coupler configurations. A probe coupler will be matched to the ohmic Q of the cavity and calibrated using S11,with the loaded Q being measured using the bandwidth. Then the external Q can be measured from LC-ABD plenary April 2007

17. Placet The crab cavity has now been inserted into PLACET and bunches have been tracked through the final focus, with and without crab cavities. The results give good agreement with the previous analytical results, and show little emittance growth. LC-ABD plenary April 2007

18. Phase Control Development Digital Phase Detector Calibration Chart The phase control work at is focusing on the use of a 1.3GHz digital phase detector with fast 16 bit ADC and DAC conversion. The digital phase detector offers an absolute measurement without calibration. They may eventually be used alongside phase quadrature measurements with double balanced mixers at an intermediate frequency for enhanced resolution. Used in quadrature double balanced mixers give both phase and amplitude. A diode detector being used for amplitude measurements at the moment. Vector modulation available to 4 GHz Degrees per Volt of a digital phase detector can be amplified to overcome ADC noise. Ultimate resolution is 5 milli-degree rms for 100kHz bandwidth LC-ABD plenary April 2007

19. Phase Control Implementation for single Cavity A randomly vibrating tuning stub simulates microphonics in a warm cavity LC-ABD plenary April 2007

20. Development of 16 bit DAC & ADC Boards 16 bit ADC Sample Rate = 105MSPS, Latency = 130ns 16 bit DAC Sample Rate = 40MSPS, Settling time = 10ns Have potential to react to a phase errorof 5 mdeg in 2-3 ms. LC-ABD plenary April 2007

21. Status of measurement precision Precision of the measurement is determined by looking at the jitter on a DAC output using digital amplification inside DSP, when a controller in the DSP is used to phase lock a low Q cavity. Programmed basic control software in DSP and have demonstrated phase locking of warm cavity to better than 0.01 degrees rms within bandwidth of 50kHz. Have yet to implement FPGAs hence slow read & write. Read time by DSP CPU~300 ns Write time to DAC ~200 ns. Still using in built functions to get sine and cosine for vector modulator hence processing is a little slow. Steady State pk-pk phase jitter = +/- 15 mdeg LC-ABD plenary April 2007

22. Phase synchronisation I • If cavities are stabilized with respect to local references to ± 0.01 degrees we then seek synchronisation between local references to 0.06 degrees at 3.9 GHz = 43 fs • Optical systems have been developed elsewhere that achieve this. • Initial plans are to see what can be achieved with coax and s.o.a. RF components. simple synchronisation scheme LC-ABD plenary April 2007

23. divide or mix divide or mix to 1.3 GHz to 1.3 GHz 1.3 GHz master phase shifter oscillator 3.9 GHz master oscillator Phase Synchronisation II Cavity Control Cavity Control vector vector Note that the phase detectors for each system should be as close as possible to their cavity output couplers. mod. mod. digital phase digital phase Ü Ü Ü Ü D/A DSP A/D D/A DSP A/D detector detector synchronous synchronous digital phase output output digital pha se detector detector loop loop Load filter filter ~ 50 metre coax link phase phase shifter shifter 3 dB directional 3dB directional coupler coupler i nterferometer line p recision length adjustment reflector • managing reflection on the interferometer is the biggest challenge LC-ABD plenary April 2007

24. Vertical Cryostat 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 April 2007