Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20091/23 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 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

Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20092/23 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 ( ) 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) LARP Initiative for SPS e-cloud FB - single bunch TMCI

Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20093/23 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)

unstable bunch (  -signal)  -signals always stable (yellow) unstable bunch (  -signal) stable bunch (  -signal) 10 ns/div 5x72 bunches injected; 25 ns bunch spacing; machine well scrubbed with four batches shown are the last three bunches of the 5 th batch, vertical pick-up, shortly after injection evidence of single bunch multi-100 MHz transverse instability, Instability at injection of 5 th batch, analysis ongoing Wolfgang Hofle BE/RF4/23SPS Studies WG - November 17, 2009 MD with LHC nominal beam in June 2008 Considerable effort followed to understand signals and de-convolve coupler and cable response

Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20095/23 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 !

Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20096/23 Evaluation of 7/8” cables used for exponential coupler Pulse response and response to a bunch of with 4  =1.7 ns Full non-linear phase included in red trace (response to bunch) and green trace with ripple (pulse response); the other two curves correspond to fits of the phase with a sqrt(f) term only total length 447 m

Wolfgang Hofle BE/RFSPS Studies WG - November 17, 20097/23 Evaluation of 7/8” cables with corrugation 100 m sample, computed pulse response from measured S 11 in short circuit Additional phase term (cubic) due to corrugation Taken on by IEC standardization organization (at initiative of F. Caspers) see PAC 2009 paper TH5RFP033 (G. Kotzian, F. Caspers, S. Federmann, W. Hofle, R. de Maria)

Pick-up re-cabled with resistive combiners Wolfgang Hofle BE/RF8/23January 2009

2.5 ns/div Injection, 26 GeV/c (longer bunch) Single bunch signals after modification of pick-up connection (resistive combiners) (stable short bunch with ~5 x10 9 protons per bunch) Wolfgang Hofle BE/RF9/23January ns/div 450 GeV/c (shorter bunch)

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Reconstructing signal from pick-up see DIPAC 2009 paper mopd17 (R. de Maria et al. ) response measurement with short bunch Reconstructed signal during debunching longitudinal transverse

Single bunch signals after modification of pick-up connection (stable bunch with ~5 x10 9 protons per bunch) Wolfgang Hofle BE/RF11/23January 2009

Single bunch signals after modification of pick-up connection (stable bunch with ~5 x10 9 protons per bunch) Wolfgang Hofle BE/RF12/23January 2009 averages of 20 measurements A and B time-aligned (A+17 ps) pick-up strips not exactly equal !

Measurements on all four exponential couplers in the SPS (data from November 4 th, 2008) Wolfgang Hofle BE/RF13/23January 2009 FC, resistive combiners 1 st bunch of a train PU backwards installed FC, resistive combiners 1 st bunch of a train PU forwards installed CCR, H-183 combiners single bunch (semi-short) PU forwards installed vertical BPWA, small diameter (now) vertical BPWA, small diameter horizontal BPW, large diameter vertical BPWA, small diameter CCR, H-183 combiners 1 st bunch of a train PU forwards installed 1 st bunch of bunch train oscillates -> jitter in measurement; single bunch -> no jitter slow rise-time fast rise-time saturation or cable?

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Single bunch – spectral domain see DIPAC 2009 paper mopd17 (R. de Maria et al. ) Beam pipe diameter = 107 mm de-bunching, RF off

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Single Bunch and 25 ns bunch trains Transverse instability with captured beam see DIPAC 2009 paper mopd17 (R. de Maria et al. ) single bunch instability (TMCI) low chromaticity E-cloud instability

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Single bunch results See PAC 2009 paper TH5RFP047 (B. Salvant et al. ) Forrest of signals > 1.5 GHz from beam but enhanced by pick-up

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Simulation with headtail (e-cloud) no feedback, spectrum extends to ~ 1GHz

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Simulations with head-tail See PAC 2009 paper TH5RFP076 (J. R. Thompson et al. ) and AB report CERN-AB

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Courtesy J.D. Fox, simulations by team at LBNL (J.-L. Vay et al.)

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 Courtesy J.D. Fox, analysis of 2009 MD data by C. Rivetta (SLAC)

Wolfgang Hofle BE/RF21/23January 2009 Pick-up re-cabled with hybrids for excitation (up to ~200 W) Pick-up used for observation

Wolfgang Hofle BE/RF22/23January 2009 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

Wolfgang Hofle BE/RFSPS Studies WG - November 17, /23 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