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Longitudinal beam loss mechanisms for LIU and LAGUNA beams

Longitudinal beam loss mechanisms for LIU and LAGUNA beams. E. Shaposhnikova w ith input from T. Argyropoulos , T. Bohl , J. E. Muller, C. Lazaridis, H. Timko LIU-SPS collimation review CERN 21.11.2013. SPS Beam Performance.

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Longitudinal beam loss mechanisms for LIU and LAGUNA beams

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  1. Longitudinal beam loss mechanisms for LIU and LAGUNA beams E. Shaposhnikova with input from T. Argyropoulos, T. Bohl, J. E. Muller, C. Lazaridis, H. Timko LIU-SPS collimation review CERN 21.11.2013

  2. SPS Beam Performance *Feasibility including operational viability (especially in the PS) remains to be demonstrated LIU-SPS collimation review

  3. LHC beam LIU-SPS collimation review

  4. Distribution of losses in LHC cycle • Very large losses in the past, reduced with time (machine tuning, e-cloud scrubbing?) • Can be a serious limitation in future for very high intensity required by LIU for HL-LHC • Relative losses increase with intensity N, absolute ~ N2? • Losses occur • at capture (bunch shape) • on flat bottom (full bucket due to injected bunch shape) • during ramp above 120 GeV/c due to multi-bunch instabilities or/and controlled longitudinal emittance blow-up (1-2%) LIU-SPS collimation review

  5. Capture loss due to PS bunch shape(after rotation in longitudinal phase space) Shape can be improved by higher PS voltage:=> tails are less populated but losses are there! operation: 5% loss H. Timko et al., ESME simulations of realistic bunch distribution from PS tomography (no intensity effects included) best results: 2.5% loss LIU-SPS collimation review

  6. Capture losses: uncaptured beam after injection(200 MHz signal ) Injection at 26 GeV/c A few seconds later Uncaptured beam Uncapturedbeam is always moving to the left. Energy loss (dp/p < 0) due to resistive impedance? LIU-SPS collimation review

  7. Transmission of 25 ns beamin MDs with Q20 optics (2012) J. Esteban Muller et al. => Large losses and also increase with intensity LIU-SPS collimation review

  8. Losses Transmission (from BCT and Larger) ~ 85-89 % for single batch 90-92 % for 3 or 4 batches  continuous losses along flat bottom Single batch • larger inj. emittance • with similar intensities • scrapping on FB LIU-SPS collimation review T. Argyropoulos et al.

  9. Voltage programs for Q20 (MDs) • 200 MHz voltage program settings: • 2.5 to 4.5 MV - 4 dips at injections • 4.5 MV constant • 3.5 to 4.5 MV – 1st injection and 2.5 to 4.5 MV – for the rest • As in III + 500 kV at acceleration and flat top (avoid losses for higher intensities) • As in IV but no first dip optimal • Voltage increase during ramp required for higher intensities • Voltage close to the limit • For higher intensities • particle losses • or • increase length of the cycle • longer LHC filling time T. Argyropoulos et al. LIU-SPS collimation review

  10. Acceleration: voltage and power for nominal intensity in Q20 and Q26 Voltage Power => Power will beat the limit also during acceleration above nominal intensity! LIU-SPS collimation review

  11. High intensity LHC beam • High intensity: Np = 1.36x1011 p/b at FT • TWC 200 MHz voltage program: case III Controlled emittance blow-up is difficult to optimise for high intensity beam LIU-SPS collimation review

  12. High intensity FT (LBNO) beam LIU-SPS collimation review

  13. Main intensity limitations in the SPSfor CNGS-type beam (LBNO) • Equipment heating (MKE, HOM couplers, beam instrumentation…) • Beam losses • Transverse damper (40 MHz bandwidth) • RF voltage and power, beam control: • Beam stability and bucket area for (un)controlled emittance blow-up • Maximum available voltage in the 200 MHz RF system: • 7.5 (8) MV • Maximum available RF power in one 200 MHz TW cavity: • 700 kW for full SPS ring (CNGS-type beam) LIU-SPS collimation review

  14. LHC and CNGS-type beams in the SPS Nominal parameters of two main types of proton beam in the SPS • FT/CNGS beam from PS: • practically debunched beam • 5-turn extraction • no bunch-to-bucket transfer • injection below transition high intensity run LIU-SPS collimation review

  15. Particle losses during cycle • Capture • Beam structure from the PS: de-bunched beam with 200 MHz density modulation => no bunch-to-bucket transfer • Ramp • Uncontrolled emittance blow-updue to instabilities during transition crossing and at higher energies (Nth ~ 1/E) • Limited RF bucket area due to beam loading with RF power limited to 750 kW LIU-SPS collimation review

  16. Loss distribution during high intensity run (2004) • Injection losses - 5% • Losses on flat bottom ~ 2% • Particles in the kicker gap - losses at extraction ( 2%) => cleaning by trans. damper • Capture loss 3-4% • Beam losses at transition: 4% • Continuous losses after transition: 5% - 2% => early beam dump - main intensity limitation for the 2004 record of 5.3x1013 Critical losses LIU-SPS collimation review

  17. Bunch length along the batch during cycle for high intensity beam (5.6x1013 injected, 15% losses)(AB-Note-2005-034 RF, T. Bohl et al.) t=1.315 s t=1.534 s γ>γt t=3.286 s t=4.163 s LIU-SPS collimation review

  18. FT/CNGS acceleration cycle:voltage and power • at the moment maximum available voltage is used due to uncontrolled emittance blow–up during transition crossing - any voltage reduction leads to beam losses LIU-SPS collimation review

  19. MD withCNGS Beam in 2012 • Goals of 2012 measurements : • Study beam stability • Verify present intensity limitations MD data analysis was done by C. Lazaridis Obtained profiles CNGS Cycle 200MHz RF Voltage Program 2 PS batches after 2nd injection in SPS Beam momentum ~3600 bunches LIU-SPS collimation review

  20. Nominal CNGS Cycle • Injected beam is practically debunched • Bunches not well defined after injection • transition • instability at the end of the cycle Beam structure after injection a.u. Bunch Length Average and min-max for each frame Batch 1 Batch 2 LIU-SPS collimation review

  21. RF Voltage optimizationat injection • RF voltage at injection was varied in range 0.6 - 2.0 MV leading to changes in emittance • Trying to reduce capture losses => Nominal 0.9 MV close to optimal MV Nominal voltage 1.99 MV 0.6 MV ms Average Bunch Length ns SPS BCT Batch 1 - 0.6 MV/1.99 V Greater bunch spread More capture losses Nominal voltage 0.6 MV Batch 2 - 0.6 MV/1.99 V s LIU-SPS collimation review

  22. Transition crossing and after Relative Bunch Length Reference : 1450 ms Average Bunch Length Batch 1/Batch 2 Reference γtransition (1480 ms) Bunch number 1501 ms 1490 ms 1457 ms • Bunch oscillations start immediately after transition (1480.2 ms) LIU-SPS collimation review

  23. End of cycle Relative Bunch Length Reference : 1546 ms Average Bunch Length Batch 1/Batch 2 Reference Bunch number 4278 ms 1932 ms 2760 ms 3588 ms • Beam is very unstable at the end of ramp • Small losses observed LIU-SPS collimation review

  24. Reducing beam losses • Losses due to instabilities around 2.8 s in the cycle • Maximum 200 MHz RF voltage • Tried reducing voltage to improve stability • Constant bucket area • Increasing synchrotron frequency spread inside the bunch Nominal voltage Modified ms Beam Intensity For intensities ~3.7x1013 losses are 3.5% => 0.2% reduction in high-energy losses Proposed changes applied to CNGS cycle Bucket Area Nominal voltage Modified LIU-SPS collimation review

  25. Possible future improvements for beam loss reduction • 200 MHz power upgrade – limit will be still at 750 KW (due to full ring), but with 2 extra sections • LLRF upgrade of 200 MHz RF • Separate capture of each PS batch in the SPS would allow voltage capture modulation (e.g. 0.9 MV increased to 2.5 MV) • Variable gain of 1-turn-delay feedback • Upgrade of the frequency range of the feed-forward system (below 26 GeV/c) • Use of the 800 MHz RF system during cycle • Impedance reduction • Q20 optics(?) LIU-SPS collimation review

  26. The 200 MHz RF system Voltage available for acceleration with Pmax=0.7 MW 4200 bunches spaced by 5 ns N = 4.8x1013 (Irf = 0.73 A) -> V=7.5 MV N = 7x1013 (Irf = 1.06 A) -> V = 9 MV • Presently both voltage and power are at the limit: 7.5 MV used after transition crossing (due to uncontrolled longitudinal emittance blow-up) • Improvement after power upgrade with 6 cavities (18 sections) LIU-SPS collimation review

  27. Voltage during FT/CNGS cycle for two optics Q26 Q20 limit => after transition crossing some bunches have emittance > 0.6 eVs => voltage above present limit of 7.5 MV even for 0.4 eVs LIU-SPS collimation review

  28. Can new optics help to reduce uncontrolled emittance blow-up? Voltage program for 0.6 eVs Slip factor (~ beam stability) LIU-SPS collimation review

  29. Summary • LHC beam • Capture losses are determined by S-shape of injected bunches • Better stability for larger PS emittance, but more losses as well • Flat bottom losses are defined by full bucket • High energy losses come from beam instabilities and controlled emittance blow-up in conjunction with limited RF voltage (power) • FT high intensity beam • Absence of bunch-to-bucket transfer will be always a source of capture loss • Beam control during transition crossing is difficult - a source of losses • Only small increase in available voltage can be expected after the 200 MHz power upgrade -> limited voltage (bucket) at high energies • Relative losses increase with intensity -> high absolute losses can be expected during HL-LHC era (and LBNO) in the SPS at high energies unless beam instabilities are eliminated at source (impedance) LIU-SPS collimation review

  30. LIU-SPS upgrades • Main difference between the two beams: • injection at 14 GeV/c and transition crossing => different beam control (LLRF) • CNGS beam fills whole SPS ring and LHC beam – only half => different requirements for beam power (continuous and pulsed regimes) • bunch spacing => multi-bunch effects (instabilities, heating) • CNGS-type beam will profit from planned SPS upgrades: • impedance reduction (shielding of MKE kickers, …) • e-cloud mitigation • 200 MHz and 800 MHz RF upgrade • beam instrumentation • low γt (transition energy) optics? LIU-SPS collimation review

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