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Special e -cloud bunch spacing: injectors

Special e -cloud bunch spacing: injectors. H. Bartosik, G. Iadarola , G. Rumolo , E. Shaposhnikova

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Special e -cloud bunch spacing: injectors

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  1. Special e-cloud bunch spacing: injectors H. Bartosik, G. Iadarola, G. Rumolo, E. Shaposhnikova Acknowledgements: G. Arduini, T. Argyropoulos, T. Bohl, S. Cettour Cave, H. Damerau, J. Esteban Muller,F. Follin, B. Goddard, S. Hancock, W. Höfle, L. Kopylov, C. Lazaridis, Y. Papaphilippou, M. Taborelli, H. Timko LBOC, 5. November 2013

  2. Outline • Motivation • Expectations from simulations • Experience with the doublet beam in the SPS • Comparison with simulations • Doublet beams for the LHC • Bunch splitting at SPS injection • Bunch splitting at SPS flat top • Bunch splitting at LHC injection • Expected beam parameters LBOC, 5. November 2013

  3. Motivation Scrubbing beam: • Lower SEY threshold in LHC dipoles • Should allow to further scrub at 450 GeV with high efficiency as far as the scrubbing curve of copper allows dipoles with scrubbing beam 25 ns scrubbing @450 GeV(2011 + 2012) scrubbing beam, hopefully … (2015) LBOC, 5. November 2013 2

  4. PyECLOUD simulations – 5 ns doublets • The 5 ns doublet beam shows a much lower multipacting threshold compared to the standard 25 ns beam LHC dipoles LBOC, 5. November 2013

  5. PyECLOUD simulations – 5 ns doublets • The 5 ns doublet beam shows a much lower multipacting threshold compared to the standard 25 ns beam • Efficient scrubbing with the doublet beam expected from e- energy spectrum for a wide range of intensities • Intensity larger than 0.8x1011p/b preferable for covering similar horizontal region as the standard 25 ns beam with nominal intensity LHC dipoles LHC dipoles LBOC, 5. November 2013

  6. PyECLOUD simulations – 2.5 ns doublets • The 2.5 ns doublet beam shows a lower multipacting threshold compared to the standard 25 ns beam, but higher threshold compared to 5 ns doublets LHC dipoles LBOC, 5. November 2013

  7. PyECLOUD simulations – 2.5 ns doublets • The 2.5 ns doublet beam shows a lower multipacting threshold compared to the standard 25 ns beam, but higher threshold compared to 5 ns doublets • Similar e- energy spectrum as with 5 ns doublets • E-cloud build-up is concentrated in central part of the chamber  less favorable compared to the 5 ns doublets LHC dipoles LHC dipoles LBOC, 5. November 2013

  8. Production of 5 ns doublet beam at SPS injection • Injection of long (~10 ns) bunches into the SPS with low RF voltage LBOC, 5. November 2013

  9. Production of 5 ns doublet beam at SPS injection • Injection of long (~10 ns) bunches into the SPS with low RF voltage • Fast voltage ramp in order to capture each bunch in two neighboring 200 MHz buckets 25 ns 25 ns 5 ns LBOC, 5. November 2013

  10. Tests of the 5 ns doublet beam in the SPS • First machine tests in the SPS at the end of 2012-13 run in order to • validate the doublet production scheme at SPS injection • obtain first indications about the e-cloud enhancement • The production scheme has been successfully tested • for a train of up to (2x)72 bunches with 1.7e11 p/doublet 4 3 1st inj. 200 MHz RF Voltage [MV] 2 1 0 4 -2 0 8 2 6 Time [ms] LBOC, 5. November 2013

  11. Tests of the 5 ns doublet beam in the SPS • First machine tests in the SPS at the end of 2012-13 run in order to • validate the doublet production scheme at SPS injection • obtain first indications about the e-cloud enhancement • The production scheme has been successfully tested • for a train of up to (2x)72 bunches with 1.7e11 p/doublet • injecting a second batch without degrading the circulating beam has been shown • Cycle included the start of acceleration to estimate capture losses (around 10%) 4 3 1st inj. 2nd inj. 200 MHz RF Voltage [MV] 2 1 0 0 3604 4 -2 0 8 3590 3592 3594 3596 3598 3600 2 3602 6 Time [ms] LBOC, 5. November 2013

  12. Experience with the 5 ns doublet beam in the SPS • Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the SPS arcs • Direct comparison of standard and doublet beam within the same supercycle 25ns std. (1.6e11p/bunch) the curves represent pressure gauges in the center of the SPS arcs (1.7e11p/doublet) 25ns “doublet” LBOC, 5. November 2013

  13. Experience with the 5 ns doublet beam in the SPS • Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the SPS arcs • Direct comparison of standard and doublet beam within the same supercycle • Clear enhancement observed also in the dedicated e-cloud monitors • Shown here for the MBB type chamber • Good agreement with PyECLOUD simulations • Build-up with doublet beam is concentrated in central region (SPS MBB chamber) PyECLOUD simulation Measurements LBOC, 5. November 2013

  14. Ways of producing doublets for the LHC (I) • “Long bunch splitting” at SPS injection (5 ns doublets) • Demonstrated in MDs (see previous slides) • Possible issues in the SPS • Transverse beam stability due to enhanced e-cloud  losses and emittance growth • Transverse damper (after LS1 can damp the common oscillation mode of doublets but not the pi mode) • Acceleration: RF power, longitudinal stability, LLRF (doublets treated as single bunch) • Beam quality at extraction (however not critical for scrubbing) • Possible issues in the LHC (to be treated in separate talks) • Transverse damper • Beam instrumentation in general, beam control and machine protection • Anything else? LBOC, 5. November 2013

  15. Ways of producing doublets for the LHC (II) • “Bunch splitting” at high energy in the SPS (5 ns doublets) • By sudden phase jump by 180° and recapturing each bunch in 2 neighboring buckets • A controlled phase jump will be possible with new module presently under development for operation with ions (to be tested in 2014) • Preferably done at intermediate energy for cleaning-up uncaptured beam before extraction • Not tested yet • Possible issues in the SPS • Transverse beam stability due to e-cloud • Transverse damper (during and after the splitting) • Acceleration of the needed high beam intensity: RF power, longitudinal stability • Splitting at high energy: LLRF, losses at high energy due to uncaptured beam, longitudinal stability after the splitting, e-cloud effects after the splitting • Beam quality at extraction • Possible issues in the LHC like in (I) LBOC, 5. November 2013

  16. Ways of producing doublets for the LHC (III) • “Long bunch splitting” at LHC injection (2.5 ns doublets) • Extracting long bunches (~5ns) from the SPS and capturing them in two neighboring LHC buckets  2.5 ns doublet spacing • Not tested yet • Possible issues in the SPS • Acceleration of the needed high beam intensity in the SPS: RF power, longitudinal stability • Transverse beam stability due to e-cloud • Possible issues in the LHC • Like in (I) and (II) • Several injections with RF voltage dips (feasible?) • Transverse damper (but should be easier than 5 ns doublets) • Losses due to uncaptured beam in the LHC during further injections LBOC, 5. November 2013

  17. SPS RF power during acceleration (I) • Possible ways to alleviate RF power limitations • Reduce ramp rate (example below for 3 times longer acceleration time Tacc) • Slightly less power needed in Q26, but other problems anticipated for high intensity (e.g. TMCI) • 1.6x1011p/doublet seems within reach0.8x1011p/b • however controlled long. emittance blow-up will be needed  to be checked in measurements Q26 Q20 LBOC, 5. November 2013

  18. SPS RF power during acceleration (II) • Possible ways to alleviate RF power limitations • Reduce ramp rate (example below for 3 times longer acceleration time Tacc) • Slightly less power needed in Q26, but other problems anticipated for high intensity (e.g. TMCI) • 2x1011p/doublet out of reach with present 200 MHz RF system Q26 Q20 LBOC, 5. November 2013

  19. Estimated beam parameters • 1.6x1011p/doublet within ~3 μm • Due to RF power limitation in the SPS • Assuming all benefits from larger longitudinal parameters at PS injection • Longitudinal emittance (first guess!) • ~0.45 eVs at LHC injection in case of injecting doublets from the SPS • >0.6eVs in case of injecting long bunches for splitting in the LHC • Refined estimations from simulations and instability considerations … LBOC, 5. November 2013

  20. Summary and Conclusions • 5 ns doublet beam is the most preferable option as scrubbing beam • Lowest SEY threshold in LHC dipoles • E-cloud covers the largest horizontal region • Production of 5 ns doublet beam at SPS injection demonstrated in MDs • Enhanced pressure rise • Higher e-cloud activity in strip monitors as predicted by simulations • Options for scrubbing beams for the LHC • 5 ns doublets at SPS injection  main complications expected in the SPS: RF power during acceleration and e-cloud along the cycle • 5 ns doublets at high energy in SPS  main complications expected in the SPS: RF power, losses at high energy (uncaptured beam), e-cloud effects • 2.5 ns doublets at LHC injection  main complications expected in the SPS: RF power,e-cloud effects • Other exotic ideas? (slip-stacking in the SPS or LHC, …) LBOC, 5. November 2013

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