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reported by Wolfgang Hofle CERN BE/RF/FB

Transverse feedback to cure electron-cloud induced single bunch vertical instability and TMC-Instability in the SPS Progress 2008- 2009. reported by Wolfgang Hofle CERN BE/RF/FB. Team Wolfram Fisher, Riccardo De Maria (BNL)

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reported by Wolfgang Hofle CERN BE/RF/FB

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  1. Transverse feedback to cure electron-cloud induced single bunch vertical instability and TMC-Instability in the SPS Progress 2008- 2009 reported by Wolfgang Hofle CERN BE/RF/FB Team Wolfram Fisher, Riccardo De Maria (BNL) John Byrd, M. Furman, Stefano De Santis, Jean-Luc Vay (LBL), Joel R. Thompson (Cornell) Gianluigi Arduini, Giovanni Rumolo, Urs Wehrle (CERN) TMCI: Elias Métral, Benoit Salvant (CERN) John D. Fox, Georges Ndabashimiye, Mauro Pivi, Claudio Rivetta (SLAC) Acknowledgement: T. Bohl, G. Kotzian, F. Caspers, T. Linnecar SPS Studies WG - November 17, 2009

  2. LARP Initiative for SPS e-cloud FB - single bunch TMCI Interest expressed in 2007 by J. Byrd (LBL, Berkeley) and J. Fox (SLAC) to collaborate on transverse feedbacks in the frame work of LARP now receiving increased support from LARP Joint MDs in 2008 and 2009 Preparation of instrumentation, pick-up response analysis (2008-2009) R. de Maria, U. Wehrle, W. Hofle (CERN) Simulations by Joel R. Thompson with head tail (2008) Simulations by Jean-Luc Vay et al. (LBL) Web meetings to follow-up collaborations organized by J. Fox (SLAC) Analysis of MD data of 2009: C. Rivetta (SLAC) Single bunch MDs: B. Salvant, E. Métral et al. Preparation of hardware for beam transfer function measurement W. Hofle, U. Wehrle, D. Valuch (CERN) SPS Studies WG - November 17, 2009

  3. Observation of electron cloud instability in the past From G. Arduini et al, Proceedings E-cloud 2004 • Observation of oscillations with strip line coupler (head tail monitor) along batch: • horizontal plane: coupled bunch behavior, strong correlation between bunches, • low frequency (~ 1 MHz), taken care of by the present transverse feedback system • vertical plane: some bunches oscillate with large amplitude, single bunch oscillations, • weak correlation between adjacent bunches, controlled by current transverse feedback and • high chromaticity, emittances at the limit for nominal LHC beam (blow-up at tail of batch) SPS Studies WG - November 17, 2009

  4. MD with LHC nominal beam in June 2008 5x72 bunches injected; 25 ns bunch spacing; machine well scrubbed with four batches shown are the last three bunches of the 5th batch, vertical pick-up, shortly after injection evidence of single bunch multi-100 MHz transverse instability, Instability at injection of 5th batch, analysis ongoing unstable bunch (D-signal) stable bunch (D-signal) unstable bunch (D-signal) S-signals always stable (yellow) 10 ns/div Considerable effort followed to understand signals and de-convolve coupler and cable response SPS Studies WG - November 17, 2009

  5. Exponential coupler Four such couplers installed in SPS (four electrodes at 45 degrees) can be used for MDs, hybrids and cables checked and their transfer functions measured in March 2008 (R. de Maria, Gerd Kotzian) Developed for SPS by T. Linnecar, Reference: CERN-SPS-ARF-SPS/78/17 phase response not linear frequency ! SPS Studies WG - November 17, 2009

  6. Pick-up 319.01 re-cabled with resistive combiners January 2009

  7. Single bunch signals after modification of pick-up connection (resistive combiners) (stable short bunch with ~5 x109 protons per bunch) 2.5 ns/div Injection, 26 GeV/c (longer bunch) 5 ns/div 450 GeV/c (shorter bunch) January 2009

  8. Reconstructing signal from pick-up response measurement with short bunch transverse longitudinal Reconstructed signal during debunching see DIPAC 2009 paper mopd17 (R. de Maria et al. ) SPS Studies WG - November 17, 2009

  9. Single bunch signals after modification of pick-up connection (stable bunch with ~5 x109 protons per bunch) January 2009

  10. Single bunch signals after modification of pick-up connection (stable bunch with ~5 x109 protons per bunch) averages of 20 measurements A and B time-aligned (A+17 ps) pick-up strips not exactly equal ! January 2009

  11. Measurements on all four exponential couplers in the SPS (data from November 4th, 2008) 319.01 321.01 (now) vertical BPWA, small diameter vertical BPWA, small diameter FC, resistive combiners 1st bunch of a train PU backwards installed FC, resistive combiners 1st bunch of a train PU forwards installed slow rise-time fast rise-time 319.31 horizontal BPW, large diameter 317.98 vertical BPWA, small diameter CCR, H-183 combiners 1st bunch of a train PU forwards installed CCR, H-183 combiners single bunch (semi-short) PU forwards installed fast rise-time fast rise-time saturation or cable? 1st bunch of bunch train oscillates -> jitter in measurement; single bunch -> no jitter January 2009

  12. Single bunch – spectral domain de-bunching, RF off Beam pipe diameter = 107 mm see DIPAC 2009 paper mopd17 (R. de Maria et al. ) SPS Studies WG - November 17, 2009

  13. Single Bunch and 25 ns bunch trains Transverse instability with captured beam single bunch instability (TMCI) low chromaticity E-cloud instability see DIPAC 2009 paper mopd17 (R. de Maria et al. ) SPS Studies WG - November 17, 2009

  14. Single bunch results Forrest of signals > 1.5 GHz from beam but enhanced by pick-up See PAC 2009 paper TH5RFP047 (B. Salvant et al. ) SPS Studies WG - November 17, 2009

  15. Simulation with headtail (e-cloud) no feedback, spectrum extends to ~ 1GHz SPS Studies WG - November 17, 2009

  16. Simulations with head-tail See PAC 2009 paper TH5RFP076 (J. R. Thompson et al. ) and AB report CERN-AB-2008-070 SPS Studies WG - November 17, 2009

  17. Courtesy J.D. Fox, simulations by team at LBNL (J.-L. Vay et al.) SPS Studies WG - November 17, 2009

  18. Courtesy J.D. Fox, analysis of 2009 MD data by C. Rivetta (SLAC) SPS Studies WG - November 17, 2009

  19. Pick-up 319.01 re-cabled with hybrids for excitation (up to ~200 W) Pick-up 321.01 used for observation January 2009

  20. Observed (transverse) spectrum with single bunch at 4xRF frequency (26 GeV/c) Same spectrum observed with and without excitation; signal disappears when beam is switched off  need to repeat spectrum measurement versus time with ramp down of chromaticity to catch onset of TMCI + excitation, NWA versus spectrum analyser January 2009

  21. Conclusions and outlook Instrumentation in the SPS improved and method developed to reconstruct time evolution of internal bunch motion, results for e-cloud and single bunch instability, should we build a new pick-up (precision-long coupler ?) Head-tail simulations with e-cloud and feedback showed that a minimum bandwidth of 300 MHz is necessary to damp the instability but unrealistic high gain is required; with larger bandwidth the necessary gain can be reduced, in practice with realistic gains up to 1 GHz bandwidth required Qualitatively the measured internal bunch motions agrees with simulation - strong head-tail tune shift observed as in simulation Single bunch measurements started as well (TMCI) Set-up prepared for beam transfer function measurements, not yet successful, need more time, possibly more RF power or a better sensitivity and a good technique for chromaticity control, first measurements  concentrate on single bunch J. D. Fox at SLAC is now looking into the technology available for ADCs, DACs, signal processing to cope with the required bandwidth SPS Studies WG - November 17, 2009

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