Longitudinal Beam Dynamics of LHC Beam in the
Longitudinal Beam Dynamics of LHC Beam in the SPS: Losses and Instabilities Markus Schwarz Acknowledgements: H. Bartosik, P. Baudrenghien, T. Bohl, A. Farricker, B. Goddard, K. Iliakis, I. Karpov, P. Kramer, A. Lasheen, J. Repond, E. Shaposhnikova, H. Timko, G. Papotti, BLon. D dev team, LIU-SPS BD & Injection Losses working groups,
HL-LHC and LIU 1/36
LIU Goal for the SPS • LHC Injectors Upgrade (LIU) → implemented during the Long Shutdown 2 (LS 2) • LIU target for the SPS: • capture 4 x 72 bunches with 2. 6 e 11 p/b (double intensity) and losses no larger than 10%! Accelerating the beam in the SPS… … pre-LIU … post-LIU 2/36
High-Intensity Beam in pre-LS 2 SPS • Measured losses and bunch lengths, 1 x 48 bunches at 2. 2 e 11 p/b (~60% of LIU intensity): instability 30% losses after 1 s for some bunches! Bunches become unstable 3/36
Outline • • • LIU goal and pre-LS 2 situation Capture losses Flat-bottom instability Flat-top instability SPS cycle for LHC beam: injection flat bottom ramp flat top 4/36
Outline • • • LIU goal and pre-LS 2 situation Capture losses Flat-bottom instability Flat-top instability 5/36
Measured Losses in the SPS • Beam Current Transformer → number of all particles circulating in the machine (e. g. displayed on SPS page 1) • Wall current monitor + fast oscilloscope → bunch profiles capture losses injection flat-b oflat bottom losse s energy Uncaptured protons ramp circulating in SPS losses at start of acceleration 6/36
Longitudinal Beam Dynamics Code • Longitudinal tracking code BLon. D [blond. web. cern. ch] • Flexible modular code allows modelling of • multiple RF systems • intensity effects • cavity feedback loops • beam feedback loops • Used in simulation of PSB, PS, SPS, LHC • Fast tracking of more than 0. 5 Billion macro-particles 7/36
Injected Bunch in SPS RF-Bucket • Non-linear bunch rotation in PS creates halo particles: particle density / a. u. Particles in halo outside of RF-bucket → lost RF-bucket: particles inside are captured Bunch core inside RF-bucket 8/36
Simulating capture losses • 1 x 72 bunches, 1. 7 e 11 p/b, V 200=2. 0 MV X no intensity effects X no feedback • → simulated losses only due to halo particles 9/36
Intensity Effects • Wake fields → induced voltage • increase with higher beam intensity • decrease RF-bucket area wake fields bunch [Zotter, Khheifets, Impedances and Wakes, 1998] 10/36
SPS Impedance Model • Main 200 MHz Travelling Wave Cavities (TWC) and Higher Order Modes (HOM) (BE-Seminar by N. Nasresfahani) • 800 MHz TWC • Flanges, kickers, … Impedance reduction campaign during long shutdown 2 (LS 2): • reduced sections per 200 MHz TWC • HOM damping (BE-Seminar by P. Kramer) • shielding of vacuum flanges 200 MHz TWC 800 MHz TWC flanges HOM 11/36
Simulated capture losses: intensity effects • 1 x 72 bunches, 1. 7 e 11 p/b, V 200=2. 0 MV ü with intensity effects X no feedback • →uncompensated beam loading leads to large particle losses 12/36
Beam Dynamics Effects of SPS 1 -turn Delay Feedback 13/36
Simulated capture losses: feedback • 1 x 72 bunches, 1. 7 e 11 p/b, V 200=2. 0 MV ü with intensity effects ü with feedback • → reduced beam loading reduces initial particle losses 14/36
Simulated capture losses: feedback & phase loop • 1 x 72 bunches, 1. 7 e 11 p/b, V 200=2. 0 MV ü with intensity effects ü with feedback and phase loop • Simulation and measurements agree! 15/36
Simulated and Measured Capture Losses • Similar intensity curves for all amounts of halo • Absolute amount of losses strongly depends on initial halo! • Measured and simulated bunch-bybunch losses ü modulation reproduced when shape reproduced in simulations X including injected bunch length modulation variation 16/36
Simulated Capture Losses for LIU-Intensity Beams • Without mitigation measures (blue) • After mitigation measures of LS 2: • reduced injected emittance • impedance reduction • feedback • improved phase loop • Capture losses significantly reduced! 1 x 72 bunches, 2. 6 e 11 p/b 17/36
RF-Bucket Area and Capture Losses • Increase voltage of main cavity → larger bare RF-bucket area → less capture losses • Higher beam intensity → more beam loading → higher losses • Feedforward (FF) system → less beam loading → less losses • FF on → more RF noise → higher loss rate FF on FF off 18/36
Outline • • • LIU goal and pre-LS 2 situation Capture losses Flat-bottom instability Flat-top instability 19/36
Flat-Bottom Losses • Increased RF-bucket area: + less capture losses - higher losses at flat bottom decreased RF-bucket area: increased RF-bucket area: 20/36
Flat-Bottom Losses: Momentum Aperture • Nominal optics (Q 20) • Optics with larger momentum aperture (Q 22) → less flat-bottom losses • One physical aperture limitation identified Q 20 Q 22 21/36
Outline • • • LIU goal and pre-LS 2 situation Capture losses Flat-bottom instability Flat-top instability unstable beam start of instability 22/36
Measured Flat-Bottom Instability: 1 x 12 bunches • Feedback on → beam stable → instability driven by 200 MHz TWC 23/36
Benchmark Simulations: 1 x 12 bunches • Same conditions (Q 20 optics, feedback off, phase loop on) • Threshold (~1. 2 e 11 p/b) reproduced (with fundamental frequency shift of 200 MHz TWC due to HOM couplers) measurement simulation 24/36
Flat-Bottom Instability: LIU-Intensity Beam in post-LS 2 SPS • Simulation of 1 x 48 bunches at 2. 6 e 11 p/b in post-LS 2 SPS • some bunches become unstable • beam with double RF system (V 800/V 200=0. 1) single RF system double RF system 25/36
Outline • • • LIU goal and pre-LS 2 situation Capture losses Flat-bottom instability Flat-top instability 26/36
915 MHz HOM 27/36
Flat-Top Instability Threshold: 1 x 72 bunches • V 200=10 MV, double RF (V 800/V 200=0. 1), post-LS 2 impedance model 28/36
Flat-Top Instability Threshold: 4 x 72 bunches • Reduced threshold for 4 x 72 bunches because last batches become unstable 29/36
Four Batches with damped 915 MHz HOM • Single batch threshold recovered → multi-batch instability driven by 915 MHz HOM 30/36
Flat-top Instability Threshold: 4 x 72 bunches • Increasing voltage ratio of 800 MHz TWC to V 800/V 200=0. 16 (was 10% previously) increases threshold… • … but power limitation not included! 31/36
Power Limitation 32/36
Flat-Top Instability Threshold: 4 x 72 bunches 33/36
Summary • LIU target for SPS is to capture 4 x 72 bunches at 2. 6 e 11 p/b and a loss budget of 10% • Capture losses occur at the PS bunch to SPS RF-bucket transfer: • caused by halo particles outside and close to the SPS RF bucket and increase with intensity due to increased beam loading and larger halo • reproduced in BLon. D simulations that include bunch rotation in PS, SPS intensity effects, cavity feedback system, and beam phase loop • simulations with mitigation measures in post-LS 2 SPS give significantly reduced capture losses • Flat-bottom losses caused by • Limited momentum aperture → one physical aperture limitation will be removed in LS 2 • RF noise → potentially less noise due to new digital LLRF system 34/36
Summary 35/36
Outlook • Continue investigation on the effect of 915 MHz HOM on multi-batch instability • Simulation of entire SPS cycle • four injections of 72 bunches • ramp with emittance blow-up Thank you for your attention! 36/36
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