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STATUS OF THE LHC INSTABILITIES

STATUS OF THE LHC INSTABILITIES. E. Métral , N. Mounet and B. Salvant W. Herr, E. Laface and S. Redaelli. Reminder => 2 types of (coherent) instabilities observed With only 1 bunch in a ring (no beam-beam) at high-energy without octupoles

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STATUS OF THE LHC INSTABILITIES

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  1. STATUS OF THE LHC INSTABILITIES E.Métral, N. Mounet and B. Salvant W. Herr, E. Laface and S. Redaelli • Reminder => 2 types of (coherent) instabilities observed • With only 1 bunch in a ring (no beam-beam) at high-energy without octupoles • Several bunches in stable-beam conditions (i.e. with beam-beam and also octupoles)

  2. 1st INSTABILITY • Benchmark Theory (Gaussian bunches) vs. HEADTAIL simulations for the instability rise-times at 450 GeV/c and 7 TeV/c => Scan in chromaticity • Evolution of the modes vs. time from the instability measurement performed on MO 17/05/2010 at 3.5 TeV/c • Evolution of the modes vs. time from HEADTAIL simulations in the case of the nominal beam at 7 TeV/c => Without and with beam losses (i.e. aperture) • Effect of the octupoles’ current on the single-bunch instability at 3.5 TeV/c with Q’x = + 6: HEADTAIL vs. theory • Conclusions and next steps

  3. Benchmark Theory (Gaussian bunches) vs. HEADTAIL simulations for the inst. rise-times at 450 GeV/c and 7 TeV/c 450 GeV/c 7TeV/c HEADTAIL simulations (2010) Impedance model to be checked Theory (Gaussian, 2006)

  4. Evolution of the modes vs. time from the instability measurement performed on MO 17/05/2010 at 3.5 TeV/c Every s from 22:44:00 to 22:46:00, i.e. during 120 s 120 s “Christmas tree” around 114 s => During losses Mainly m = - 1 still at 108 s

  5. Evolution of the modes vs. time from HEADTAIL simulations in the the case of the nominal beam at 7 TeV/c WITHOUT LOSSES WITH LOSSES A similar “Christmas tree” seems to be reproduced with the losses included

  6. Effect of the octupoles’ current on the single-bunch inst. at 3.5 TeV/c with Q’x = + 6: HEADTAIL vs. THEORY (1/2) MD settings 17 May 2010 – 3.5 TeV/c - 1.05 1011– x= 3.75 m • => Fully stable for +50 A < Ioct < +100A(i.e. +3 < K3F = K3D < +6) from HEADTAIL During the MD it was ~ 5 microm < x > < sigmax >

  7. Effect of the octupoles’ current on the single-bunch inst. at 3.5 TeV/c with Q’x = + 6: HEADTAIL vs. THEORY (2/2) • From THEORY • => Stable for Ioct ~ 85 A From Sacherer with 2010 impedance model => -1.2 10-4– j 6.5 10-6 (m = - 1)

  8. Conclusions and next steps (1/2) • A similar “Christmas tree” as the one observed in the CCC (when beam losses appear) seems also to be obtained from HEADTAIL simulations when the beam losses are included (i.e. introducing an aperture) => But the instability responsible for the beam losses should be a head-tail with m = - 1 • This can be checked by reproducing the MD done on MO 17/05/10 and looking at the signals inside the bunch, turn after turn (Headtail monitor) • HEADTAIL simulations confirmed that one should try and reduce the chromaticities as much as we can (still > 0 if no transverse feedback or slightly negative with a transverse feedback) • A good agreement is obtained between theoretical predictions and HEADTAIL simulations for the beam stabilization of the instability at 3.5 TeV/c • HEADTAIL simulations => Fully stable for +50 A < Ioct < +100 A (i.e. +3 < K3F = K3D < +6) • Theoretical predictions => ~ +85 A

  9. Conclusions and next steps (2/2) • Next steps: • Finish the scan in the octupoles’ current (60, 70, 80 and 90 A) for the HEADTAIL simulations • Redo the octupole analysis with negative gradients (as we suggested to use negative ones after discussion with StephaneF) => K3F = K3D = - 6 is currently used at 3.5 TeV/c • For more info on this subject => See for instance: https://impedance.web.cern.ch/impedance/documents/SBInstabilityStudiesInTheLHCAt3500GeV_LCU.pdf

  10. 2nd INSTABILITY • This is followed up by WH and EL (and also FS in the future) => 2 presentations by WH at the LHC Beam Commissioning Working Group • https://lhc-commissioning.web.cern.ch/lhc-commissioning/meetings/20100706/LHC-BC-WG-Min06July10.pdf • https://lhc-commissioning.web.cern.ch/lhc-commissioning/meetings/20100713/LHC-BC-WG-Min13July10.pdf • Some discussions on the observations made on FR 09/07/2010 • Preliminary conclusions and next steps

  11. Measurements on FR 09/07/2010 (1/8)

  12. Measurements on FR 09/07/2010 (2/8)

  13. Measurements on FR 09/07/2010 (3/8) 5 first bunches in the train of B1

  14. Measurements on FR 09/07/2010 (4/8)

  15. Measurements on FR 09/07/2010 (5/8) Measured instability rise-time ~ 13 s => Very close to the single-bunch instability meas. performed few weeks ago when reducing the octupoles strength (Y here instead of X last time, but similar rise-times predicted) 05:43:00 05:45:00 120 s Similar increase observedwhen performing HEADTAIL simulations with amplitude detuning from octupoles!

  16. 05:43:00 => Black 05:43:57 => Blue 05:43:00 => Black 05:44:09 => Green Measurements on FR 09/07/2010 (6/8) The vertical tune in collision (without BB) should be 0.32 BB tune shift? 05:43:00 => Black 05:44:33 => Orange 05:43:00 => Black 05:44:57 => Red - Qs?

  17. Measurements on FR 09/07/2010 (7/8)

  18. Measurements on FR 09/07/2010 (8/8) 05:45:00 05:43:00 Certainly a similar rise-time for the 4th step as for the 1st one 06:08:00 06:06:00

  19. When the beam (bunch) becomes unstable the instability rise-time (on the 2 cases analyzed!) seems very close to the one measured during a dedicated MD (and predicted from theory and simulations) with a single-bunch only (no beam-beam), reducing the octupoles’ strength => ~ 10 s of instability rise time • Is this rise-time similar in the other cases of beam losses? • What are the predicted rise-times from the coherent beam-beam modes? • We could imagine that something happens which leads to a loss of Landau damping: When Landau damping is lost, the single-bunch instability (due the machine impedance) could develop • We could for instance use the HEADTAL monitor to superimpose several consecutive traces and try and indentify the m = - 1 instability (as already proposed for the previous study). We could also look at the evolution of the spectrum to see the m = - 1 developing or not (as done in the previous MD) => To disentangle between coherent beam-beam modes and head-tail instability from the machine impedance • Many propositions by WH… More observations required… To be followed up Preliminary conclusions and next steps

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