1 / 22

SPS resistive-wall coupled-bunch instabilities – measurements and first simulations

SPS resistive-wall coupled-bunch instabilities – measurements and first simulations. N. Mounet and E. M é tral Acknowledgements: T. Bohl, W. Hofle, L. Jensen, G. Rumolo, B. Salvant !! Still preliminary results !!. Introduction.

bairn
Download Presentation

SPS resistive-wall coupled-bunch instabilities – measurements and first simulations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. SPS resistive-wall coupled-bunch instabilities – measurements and first simulations N. Mounet and E. Métral Acknowledgements: T. Bohl, W. Hofle, L. Jensen, G. Rumolo, B. Salvant !! Still preliminary results !! N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  2. Introduction • Several MDs performed (15-16 Sept. 2009, 3 Nov. 2009 & 22 July 2010) to measure the transverse coupled-bunch instability rise times with the 25ns LHC beam (one or several batches). The goal is to compare with multibunch HEADTAIL simulations (new code) using the SPS impedance model. • Idea: after injection oscillations are damped, switch off the transverse damper for typically 10 ms and observe (with the LHC-BPM) the growth of an instability. Try for several values of the chromaticity. • Here we present results on the last of these MD, where conditions were thought to be optimum: chromaticity and bunch length were measured on the same beam the same day, and LHC-BPM was fully functional. N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  3. v1 md off v2 md off v1 md on v2 md on damper (vertical here) v1 on v2 on v1 off v2 off magnets cycle 0 (inj) t1d t2d t (ms) -5 Initial chromaticity (x or y) (at inj. energy, before magnet ramp) Chromaticity profile after change t4c t1c t (ms) t2c t3c Timing of the MDs • Transverse dampers timing: • Chromaticity: Measurement between t1d and t2d, which are between t2c and t3c N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  4. Foreword: uncertainties • The instability rise times measured might well include some electron cloud contribution. A way to prevent this would be to use the CNGS beam (5ns spacing, almost full machine filled), but it is more difficult to simulate. • The rise times could also include contribution from single-bunch instabilities, in particular at negative chromaticities. • The horizontal chromaticity measured on the MD of 22/07/2011 (the MD analyzed in the following slides) is subject to caution: if we believe it, only strongly negative chromaticity allow single batch instability, whereas in 2009 we observed instabilities even with slightly positive chromaticity. N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  5. Parameters (MD 22/07/2011) • Qx=26.1289, Qy=26.1641. • Bunch length (total): between 2.5 and 3ns (depending on the bunch). • RF voltage: 3 MV. • Supercycle: LHC_4Inj_FB10860_FT835_Ext19415_2010_v1 • 1 batch of 72 bunches, 1.2 1011p+/bunch. • Timing: t1d=2000ms, t2d=2010ms (dampers), and t1c=1970ms, t2c=2000ms, t3c=2100ms, t4c=2150ms (chromaticity trim). • LHC-BPM delay set to 2016 or 2017 ms (additional delay of ~16ms is software related), 1000 turns acquired (usually). N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  6. Multibunch instabilities: horizontal • FBCT around the time when feedback is off: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  7. Example of instabilities: horizontal • LHC BPM vs. number of turns, for successive bunches: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  8. Example of instabilities: horizontal • LHC BPM along the bunch train, for successive turns: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  9. Example of instabilities: vertical (but triggered by horizontal plane) • LHC BPM vs. number of turns, for successive bunches : N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  10. Example of instabilities: vertical (but triggered by horizontal plane) • LHC BPM along the bunch train, for successive turns: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  11. Tunes along the bunch train Quite significant tune shift (up to 0.01), smaller with higher chromaticity N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  12. Instability rise times vs. chromaticity • Rise time measurements (on several sets of data) compared to first Headtailmultibunch simulations (with beam pipe resistive-wall as single source of impedance): → Order of magnitude “ok” but dependence in chromaticity totally out. → Problem of chromaticity offset in measurement ? Or trapped modes change completely the impedance picture ? Or something else (e.g. bug in code, e-cloud or single-bunch in meas.) ? N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  13. Instability rise times vs. chromaticity in horizontal • Rise time measurements (on several sets of data) compared to first Headtailmultibunch simulations, along the bunch train: → Measurements show a clear decrease of rise time when going toward the tail of the train. → On the contrary, simulations give flat rise times (first bunch as fast unstable as last bunch). N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  14. Preliminary conclusions for horizontal instability • The instability seems to be coupled-bunch. • Positive tune shift is seen along the bunch train → could it be due to electron cloud ? Or to the negative quadrupolar impedance in x ? • First Headtail multibunch simulations with the beam pipe resistive-wall impedance give correct order of magnitude for the rise times but wrong chromaticity dependence and wrong behaviour along the bunch train. • Wrong offset in chromaticity in the measurements ? N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  15. Multibunch instabilities: vertical • FBCT around the time when feedback is off: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  16. Example of instabilities: vertical • LHC BPM vs. number of turns, for successive bunches: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  17. Example of instabilities: vertical • LHC BPM along the bunch train, for successive turns: N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  18. Vertical tune along the bunch train Tune shift along the train is not clear (mainly noise, poor resolution) N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  19. Instability rise times vs. chromaticity in vertical • Rise time measurements (on several sets of data) compared to first Headtailmultibunch simulations (with beam pipe resistive-wall as single source of impedance): → Order of magnitude “ok” but dependence in chromaticity totally out. → Trapped modes change completely the impedance picture ? Or something else (e.g. bug in code, e-cloud or single-bunch in meas.) ? N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  20. Instability rise times vs. chromaticity in vertical • Rise time measurements (on several sets of data) compared to first Headtailmultibunch simulations, along the bunch train: → Measurements show a clear decrease of rise time when going toward the tail of the train. → Not so clear in simulations. N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  21. Preliminary conclusions for vertical instability • The instability seems to be coupled-bunch. • No clear positive tune shift is seen along the bunch train. • First Headtail multibunch simulations with the beam pipe resistive-wall impedance give correct order of magnitude for the rise times, but wrong chromaticity dependence and maybe wrong behaviour along the bunch train. • in vertical it is less likely that there is a problem of offset in chromaticity in the measurements (comparing to other MDs)… N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

  22. Future actions • Analyze in detail the rest of the data (same MD with three bunch trains + the two MDs of 2009). • Benchmark the new multibunch Headtail code with analytic formulae (e.g. Sacherer or Laclare) in simple cases, with dipolar impedances only. • Compare the role of single-bunch vs. coupled-bunch instability with the code, in particular for negative chromaticity. • Add other contributions to the impedance model (trapped modes, C magnets) and take into account all the “single-bunch oriented” impedance model in ZBASE (thanks to B. Salvant). • Maybe do other MDs (e.g. with CNGS beam). N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

More Related