Electron cloud in the CERN accelerators (PS, SPS, LHC)

1. Electron cloud in the CERN accelerators (PS, SPS, LHC) G. Rumolo, G. Iadarola in ECLOUD’12 Workshop, Elba island, 6 June 2012 Thanks to all those who contributed to these studies: H. Bartosik, O. Dominguez, G. Arduini, V. Baglin, P. Baudrenghien, G. Bregliozzi, S. Claudet, J. Esteban-Müller, S. Gilardoni, W. Höfle, G. Lanza, T. Mastoridis, H. Maury-Cuna, H. Neupert, G. Papotti, M. Pivi, F. Roncarolo, E. Shaposhnikova, M. Taborelli, L. Tavian, D. Valuch, C. Yin-Vallgren, F. Zimmermann, …

3. LHC beams in the PS Flat top Bunch shortening x 4 1st injection 2nd injection x 3 Bunch splittings 25ns bunch spacing 330ns

4. Electron cloud in the PSHistorical • First observations in 2001 • Shift of the baseline in pickups with 25ns beams • Beam instabilities excited on an artificially lengthened flat top with bunches shorter than 11ns and more intense than 6 x 1011 ppb • [1] R. Cappi, et al., Phys. Rev. ST Accel. Beams 5, 094401 (2002) • New appearance in 2006 • Bunches accidentally shortened to less than 11ns during adiabatic shortening led to instabilities before extraction • Problem was cured by keeping the bunches longer before the final step of bunch rotation • [2] E. Métral, R. Steerenberg in APC Meetings • Dedicated experimental set up for direct electron cloud measurements with shielded button pickup • Clearing electrode installed and tested with different voltages and in presence of different magnetic fields (2007 – 2008) • [3] E. Mahner, T. Kroyer and F. Caspers, Phys. Rev. ST Accel. Beams 11, 094401 (2008) • Systematic scans in bunch intensity and bunch length (25 and 50ns bunch spacing) & comparison with simulations + study of the instability (2011 – ongoing) • [4] F. Caspers, G. Iadarolaet al., WEPPR010 in IPAC 2012, New Orleans, US C. Bhat, “E-Cloud dependence on the Bunch Profile - An Experiment in the PS and an Extension to the LHC”

5. Electron cloud in the PSThe 2011 study 25ns 50ns

6. Simulation Electron cloud in the PSThe 2011 study Measurement 25ns

7. Electron cloud in the PS • The electron cloud is formed in the PS when producing 50 and 25ns LHC-type beams • It only makes a short appearance in the last few ms of the production cycleof these beams • With the present beam parameters, not long enough to render beam unstable or let incoherent effects develop • Very low electron doses on the chamber walls  no efficient scrubbing! • Bottleneck for the LHC Injector Upgrade (LIU) beams? • 50 and 25ns beams with higher bunch charges and lower transverse emittances • Causes stronger electron cloud build up (?) • Beam certainly more prone to suffer from coherent instabilities • Full simulation study needed to assess the margins!

9. LHC beams in the SPS 450 GeV Flat top Extraction 1st injection 2nd injection 3rd injection 4th injection 26 GeV 2ms 25ns bunch spacing 8ms (out of 23ms SPS circumference)

10. Electron cloud in the SPSHistorical (2000-2007) • Clearevidencewith 25ns beam operation from 2000 on • Shift of the baseline in pickups • Pressure rise especially localized in the arcs • [1]G. Arduini, K. Cornelis, M. Jimenez, et al. e.g. in ECLOUD’02, ECLOUD’04 • Effects on the beam • Coherent beam instabilities excited already at injection (26 GeV) and affecting the last bunches of long trains, could be cured with feedback (H) and high chromaticity (V) • Emittance growth, leading to beams out of specs @flat top (450 GeV) • [2] G. Arduini, K. Cornelis, E. Shaposhnikova, et al. • Dedicated experimental setups for direct electron cloud measurements and benchmark of data with simulations • Strip monitors to measure the electron cloud distribution • NEG & cold chamber (COLDEX) • Shielded pickup and rotatable sample for in-situ SEY measurements • [3] V. Baglin, B. Henrist, M. Jimenez, A. Rossi, D. Schulte, F. Zimmermann, et al.

11. Electron cloud in the SPSHistorical (2007-nowadays) • Electron cloud studies fully resumed in the framework of the SPS Upgrade program (currently LIU-SPS) • Mitigation/suppression techniques (coating, clearing electrodes, scrubbing) • Instability threshold scaling with beam energy • [1] G. Arduini, P. Costa-Pinto, GR, E. Shaposhnikova, M. Taborelli, et al. • Lots of Machine Development activities presently ongoing • Understand and qualify the scrubbing process in the different types of SPS chambers and for different beam parameters (e-cloud enhancement) • a-C coating efficiency and durability in the machine • Development of a high bandwidth feedback system • [1] + [2] H. Bartosik, F. Caspers, W. Höfle with LARP collaboration, S. Federmann, G. Iadarola, H. Neupert, C. Yin-Vallgren, et al. • Simulation studies to predict the impact of electron cloud on future beam scenarios (LIU beams) • Where do we expect the electron cloud to be now and with future beams? • How much electron cloud can we afford in the machine to run stably? • [3] H. Bartosik, G. Iadarola, K. Li, GR

12. Related ECLOUD’12 talks • SPS specific • P. Costa-Pinto,Carbon Coating of SPS Dipole Chambers • F. Caspers,SPS Dipole Multipactor Test and TE Wave Diagnostics • W. Höfle,Development of Transverse Feedbacks Against ECE at SPS and LHC • More general • G. Iadarola,PyECLOUDand Build Up Simulations at CERN • K. Li,Instability Simulations with Wideband Feedback Systems: CMAD, HEADTAIL, WARP • G. Franchetti,Incoherent beam effects • P. Costa-Pinto on behalf of M. Jimenez,Mitigation Strategy at CERN • J. Fox,Overview of EC-Instability Control Using Feedbacks

13. Simulations of the SPS chambers • Drift B • Drift A • Straight sections 90% of the whole SPS <βx> = 58.3m <βy> = 53.2m <βx> = 58.3m <βy> = 53.2m • MBB • MBA • Dipole magnets <βx> = 33.9m <βy> = 71.8m <βx> = 71.8m <βy> = 33.9m

14. Drift chambersSEY thresholds as function of ppb • Drift A • Drift B • SEY thresholds mostly decreasing with bunch current, but tend to change slope for 50ns beams with bunch populations above 2 x 1011 ppb • 50ns worse than 25ns in some regions • SEY thresholds becoming very low (close to 1.05) for 25ns beams in Drift B and with bunch currents above 2 x 1011 ppb

15. Dipole chambersSEY thresholds as function of ppb • MBA • MBB • SEY thresholds mostly increasing, or level, with bunch current • 50ns has thresholds above 2.0 in MBA chambers • SEY thresholds in general very low (around 1.2) for 25ns beams in MBB chambers

16. Electron cloud in the SPS • SEY threshold values below 1.3 – 1.4 (MBB; Drift B for high beam currents) • Beam induced scrubbing cannot be used as the only mitigation technique • Chambers/StSt samples extracted from the SPS never exhibited SEY below 1.5 & e-cloud never suppressed in MBB chambers with 25ns beams • Laboratory scrubbing shows saturation above 1.3 for StSt! • a-C coating of at least ~45% of the machine remains the baseline for the SPS upgrade

17. Electron cloud in the SPS • From 2003 to 2008 the SPS had regular scrubbing runs with 25ns beams at every start up after the Winter shutdown (3 to 7 days) plus several MD sessions with this type of beams • The performance with 25ns beams has been observed to be constantly improving over the years • In 2011, nominal 25ns beams with transverse emittances below 3mm were first produced and extracted • We believe that presently the electron cloud has weakened in most parts of the SPS and probably only survives in the MBBs for operation with nominal intensity 25ns beams • In these conditions, it seems to be efficiently kept under control • An Increase in bunch intensity may awaken the electron cloud in the Drifts (and MBAs, because the stripes move to unscrubbed regions) with the consequent detrimental effects & beam becomes more sensitive • And how much scrubbing again needed after the Long Shutdown 2013-2014?

19. Electron cloud in the LHCHistorical (2010) • Firstevidence with150ns beam operation • Pressure rise in common chambers with both beams in the machine • Suppressed with solenoids at some locations • [1]V. Baglin. G. Bregliozzi, M. Jimenez, G. Lanza, e.g. in IPAC’11 • Signs of strong electron cloud activity with 75 and 50ns beams • Pressure rise in non-NEG coated straight sections • Heat load on the cold beam screen in the arcs • Instability and emittance growth along the trains • Energy loss measured from the shift of the synchronous RF phase • [1] + [2] G. Arduini, P. Baudrenghien, S. Claudet, J. Esteban-Müller, F. Roncarolo, E. Shaposhnikova, L. Tavian, et al. • First mini-scrubbing with 50ns beams led to • Decreased heat load (vanished for one batch of 36 bunches @50ns) • More stable beam • Enough to resume operation in 2011 with 75ns beams • [3] G. Arduini, et al.

20. Electron cloud in the LHCHistorical (2011) Nb increasing towards 1.45 x 1011 ppb ex,y decreasing towards 1.1mm First 1380 bunches in LHC 21/02 13/03 05/04 12/04 28/06 18/07 30/09 30/10 Commissioning with beam Scrubbing run 50ns Physics run 50ns Nominal: 1.1 x 1011 ppb 2.5 mm 75ns physics run (nominal) Nominal 50ns beams

21. Electron cloud in the LHCHistorical (2011) • Focus of this talk  Results of the analysis of the 2011 electron cloud observations and measurements • 75ns operation No electron cloud observations in 2011 • 50ns operation • Electron cloud signatures during scrubbing • Physics operation with only residual electron cloud activity in ALICE common chambers (see G. Iadarola’s talk) • 25ns MDsAlways electron cloud, it allowed monitoring the evolution of dmax in the arcs 25ns MDs (nominal) 07-14-24/10 21/02 29/06 13/03 05/04 12/04 26/08 30/10 Commissioning with beam Scrubbing run 50ns Physics run 50ns Nominal: 1.1 x 1011 ppb 2.5 mm 75ns physics run (nominal)

22. Related ECLOUD’12 talks • LHC specific • V. Baglin,ECE'sat LHC: Vacuum and Heat Load • O. Domínguez ,Benchmarking at LHC: uncoated straight sections • H. Maury-Cuna,Build-up & Heat-load Simulations - Benchmarking for LHC • J. Esteban-Müller,Synchronous Phase Shift at LHC • H. Bartosik,Benchmarking of Instability Simulations at LHC • T. Demma, A Mapping Approach to the Electron Cloud for LHC • More general • G. Iadarola,PyECLOUDand Build Up Simulations at CERN • G. Franchetti,Incoherent beam effects • P. Costa-Pinto on behalf of M. Jimenez,Mitigation Strategy at CERN

23. Electron cloud in the LHCScrubbing run in 2011 • The scrubbing run took place in the week 5–12 April 2011 (~3d beam time due to technical issues + test ramps) • Nominal 50ns spaced beams with up to 1020 bunches per beam injected into the LHC and stored at 450 GeV/c • Very efficient machine cleaning • The dynamic vacuum decreased by one order of magnitude over 17h of effective beam time (i.e. 72h machine time) • The heat load on the beam screen in the arcs • significant at the beginning of the scrubbing run • within measurement resolution at the end • The average stable phasedecreased by one order of magnitude • Instabilities and emittance growth, visible during the first fills, disappeared later even with low chromaticity settings • After scrubbing, physics with 50ns and stable beams with 1380bunches per beam on 28 June 2011

24. Estimation of dmax in the arcs Beam 1 Beam 2 Energy 09/04 13/04 Before 50ns scrubbing After 50ns scrubbing Two snapshots before (09/04) and after (13/04) the scrubbing run to reproduce the measured heat load by means of simulations! Average heat load [x 10 mW/m/beam]

25. Estimation of dmax in the arcs Bunch-by-bunchintensity & length (B1) Bunch-by-bunchintensity & length (B2) Simulator PyECLOUD R0=0.7, scan in dmax Emax=330 eV Total simulated heat load Measured heat load

26. dmax in the arcs: results (50ns) Beam 1 Beam 2 Energy 09/04 13/04 Before 50ns scrubbing After 50ns scrubbing • dmax 2.28 2.2 50ns threshold@450 GeV 2.18 2.1 50ns threshold@3.5 TeV

27. 25ns experience in 2011 Beam 1 Beam 2 Energy 29/06 07/10 14/10 24-25/10 Scrubbing

28. dmax in the arcs: results (25ns) Beam 1 Beam 2 Energy 29/06 07/10 14/10 24-25/10 Six snapshots from the 25ns MDs to reproduce the measured heat load by simulations! Heat load averaged sector by sector [x 10 mW/m/beam]

29. dmax in the arcs: results (25ns) Beam 1 Beam 2 Energy 29/06 07/10 14/10 24-25/10 Three snapshots from the 25ns MDs to try disentangling aperture of Beam1 from Beam2

30. dmax in the arcs: results 29/06 07/10 14/10 24-25/10 50ns 2011 scrubbing history of LHC arcs dmaxhas decreased from the initial 2.1 to 1.52 in the arcs ! 25ns threshold @450 GeV 25ns threshold @3.5 TeV

31. Not only heat load… • Pressure rise • Beam energy loss • Beam instability • Slow emittancegrowthand beam loss with a pattern degrading at the tail(s) of the batches

32. Not only heat load… • Pressure rise  see V. Baglin and O. Domínguez’ talks • Beam energy loss • Beam instability • Slow emittancegrowthand beam loss with a pattern degrading at the tail(s) of the batches

33. Not only heat load… • Pressure rise • Beam energy loss • Beam instability • Slow emittancegrowthand beam loss with a pattern degrading at the tail(s) of the batches

34. Beam observables: energy loss Measurements  the energy loss per bunch is obtained from the stable phase shift Simulations − We use the test case the last fill on the 25 October See J. Esteban-Müller’stalk

35. Not only heat load… • Pressure rise • Beam energy loss • Beam instability  see H. Bartosik’s talk • Slow emittancegrowthand beam loss with a pattern degrading at the tail(s) of the batches

36. Not only heat load… • Pressure rise • Beam energy loss • Beam instability • Slow emittancegrowthand beam loss with a pattern degrading at the tail(s) of the batches

37. Beam observables: emittance growth

38. Electron cloud in the LHC • Concluding remarks • After the 25ns MDs, the LHC beam chambers have been cleaned to dmax values well below the build up threshold for nominal 50ns beams • Sincethe present level of machine conditioning was preserved, ‘ecloud-less’ operation of LHC with 50ns beams up to high intensitiesis currently taking place in 2012, even in absence of a new scrubbing run • 50ns physics operation has been serving the purpose to clean parts of the LHC open to air to the needed extent • 25ns beams are still affected by e-cloud, but scrubbing should be possible could with ~2 weeks machine time (including also test ramps) or alternative filling schemes (micro-batches) could be used

39. Electron cloud in the PS/SPS/LHCConcluding remarks • We have reached quite a deep knowledge of the electron cloud in the different CERN accelerators • EC not a limiting factor with the present operation parameters • PS it does not stay long enough as to affect the beam • SPS after years of scrubbing with nominal 25ns beams, it does not presently harm the beams (50ns, nominal 25ns) • LHC it does not have important adverse effects on operational 50ns beams, however it still affects the 25ns beams • It can still be responsible for isolated effects (e.g. sparking of ZS or kicker outgassing in the SPS, background in ALICE) • What about the 25ns beams in the LHC after LS 2013-2014? Potential bottleneck for the beams required by the LIU project?

40. Thank you for your attention

41. SPARE SLIDES

42. Beam observables: emittance growth • The benefits from scrubbing have been visible on the 25ns beam: • The effect of the electron cloud has gradually moved later later along the trains, in spite of the closer spacing! • First 1 – 2 trains seem to be hardly affected now • In general, improvement in vertical • Both beams are still unstable in the two planes, or anyway affected by emittance growth 14 October  batches injected with 3.6 ms spacing, Q’x,y=15 24-25 October  batches injected with 1 ms spacing, Q’x=3, Q’y=15

43. Beam observables: beam losses 24-25 October  first three batches injected of last three fills Beam 1 Beam 2 • At this point the behaviour of the two beams is very similar • This suggests similar electron cloud rise and saturation value • It is consistent with the dmax estimation made for beam 2 with the heat load data

44. Beam observables: beam losses 24 October  batches injected with 1 ms spacing Beam 1 Even weaker losses due to delayed injection + scrubbing from the injection of 1st batch (1.551.52) Weaker losses due to delayed injection Losses degrading batch by batch

45. Instability and emittance growth: • predictions • Calculated coherent ECI threshold for central density in dipoles is around re=1012 m-3 for nominal intensity and Q’=0 at 450 GeV (simulations were run assuming the whole LHC made of dipoles) • It can be stabilized with chromaticitiesQ’x,y>15, but emittance growth due to electron cloud + chromaticity remains! • Right plot shows that with 25ns beams stability could be achieved only for dmax≤ 1.5 50ns 25ns

46. Estimation of the scrubbing time • Curve of the decrease of dmax with the integrated electron dose deposited on the wall, d=JeDt [C/mm2] • Depends on material and electron energy, several measurements done in the past (two examples illustrated here) • If we use the 500eV curve (left plot) we end up with scrubbing times in the machine much lower than those measured  perhaps an indication that the real dmax in the machine are lower than we believe (R0=1.0 instead of 0.7?) • C. Yin-Vallgren, scrubbing of Cu measured with e- at 500eV Dose [C/mm2]

47. Bunch energy loss