Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 1/23

reported by

Wolfgang HofleCERN BE/RF/FB

Transverse feedback to cure electron-cloud induced single bunch vertical

instability and TMC-Instability in the SPS

Progress 2008- 2009

TeamWolfram 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/RF SPS Studies WG - November 17, 2009 2/23

Interest expressed in 2007 by J. Byrd (LBL, Berkeley) and J. Fox (SLAC) to collaborate on transverse feedbacks in the frame work of LARPnow 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 measurementW. Hofle, U. Wehrle, D. Valuch (CERN)

LARP Initiative for SPS e-cloud FB- single bunch TMCI

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 3/23

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 batchesshown are the last three bunches of the 5th batch, vertical pick-up, shortly after injectionevidence of single bunch multi-100 MHz transverse instability,

Instability at injection of 5th batch, analysis ongoing

Wolfgang Hofle BE/RF 4/23SPS 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/RF SPS Studies WG - November 17, 2009 5/23

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 !

Pick-up 319.01 re-cabled with resistive combiners

Wolfgang Hofle BE/RF 8/23January 2009

2.5 ns/divInjection, 26 GeV/c (longer bunch)

Single bunch signals after modification of pick-up connection (resistive combiners)(stable short bunch with ~5 x109 protons per bunch)

Wolfgang Hofle BE/RF 9/23January 2009

5 ns/div450 GeV/c(shorter bunch)

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 10/23

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 x109 protons per bunch)

Wolfgang Hofle BE/RF 11/23January 2009

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

Wolfgang Hofle BE/RF 12/23January 2009

averages of 20 measurementsA 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 4th, 2008)

Wolfgang Hofle BE/RF 13/23January 2009

FC, resistive combiners1st bunch of a trainPU backwards installed

FC, resistive combiners1st bunch of a trainPU forwards installed

CCR, H-183 combinerssingle 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 combiners1st bunch of a train PU forwards installed

1st bunch of bunch train oscillates -> jitter in measurement; single bunch -> no jitter

slow rise-time fast rise-time

fast rise-timefast rise-time

saturation or cable?

319.01

317.98

321.01

319.31

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 14/23

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/RF SPS Studies WG - November 17, 2009 15/23

Single Bunch and 25 ns bunch trainsTransverse 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/RF SPS Studies WG - November 17, 2009 16/23

Single bunch results

See PAC 2009 paper TH5RFP047 (B. Salvant et al. )

Forrest of signals > 1.5 GHzfrom beam but enhanced by pick-up

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 17/23

Simulation with headtail(e-cloud)

no feedback, spectrum extends to ~ 1GHz

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 18/23

Simulations with head-tail

See PAC 2009 paper TH5RFP076 (J. R. Thompson et al. )and AB report CERN-AB-2008-070

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 19/23

Courtesy J.D. Fox, simulations by team at LBNL (J.-L. Vay et al.)

Wolfgang Hofle BE/RF SPS Studies WG - November 17, 2009 20/23

Courtesy J.D. Fox, analysis of 2009 MD data by C. Rivetta (SLAC)

Wolfgang Hofle BE/RF 21/23January 2009

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

Wolfgang Hofle BE/RF 22/23January 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/RF SPS Studies WG - November 17, 2009 23/23

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