BeamBeam and Luminosity studies meeting Friday 1 July

Beam-Beam and Luminosity studies meeting Friday, 1 July The indico page link with the draft agenda can be found at: https: //indico. cern. ch/event/547555/ “BCMS potential performance in the LHC” , Elias Metral and Yannis Papaphilippou “BSRT status”, Georges Trad “Modelling and measurements of bunch profiles at the LHC flat bottom” , Stefania Papadopoulou

“BCMS potential performance in the LHC” , Elias Metral • Potential transverse beam stability issues: Past predictions: impedance + scaling from 2012 instabilities assuming they were due to impedance • 2 questions since 2012 => Why do we need (at high energy) to use high chromaticities (~ +15 units, and even more recently in stable beam)? and max current in the Landau octupoles, i. e. much more (factor ~ 5) than predicted from impedance only? • 2015 -2016 news: -Confirmation of good agreement between predictions and measurements for impedance-induced transverse beam instabilities model (for Q’ > ~ 2). -Detrimental effect of e-cloud at high energy => Led to a factor ~ 5 more Landau octupole current needed to stabilize the beam. -Detrimental effect of linear coupling (linked to the tune separation) => Can also lead to a factor ~ 4 -5 more Landau octupole current -Detrimental effect of e-cloud in stable beam => Could be cured by increasing the vertical chromaticity to ~ 20 -25 (Proposed mechanism to be confirmed ) • Predicted BCMS (2 × 48 b) effects from e-cloud, concerning heat load and beam stability. • Measurements with 2 × 48 b (but not BCMS yet) , concerning heat load and beam stability. Discussion (questions, comments, etc) -check the potential. of injectors. -want to have the bbb tune measurement (that is demanding). -want to see tune shift growth during SB. -news: some bunches are unstable during Adjust. For the Adjust we see that emit. ~4 -5 mm, check if it comes from calibration. -we can go to 165 W/hc (hc=half cell) heat load for 1 -2 h, so that for example to reduce bunch length.

“BCMS potential performance in the LHC” , Yannis Papaphilippou • Luminosity performance of BCMS beams -BCMS exercise. • IBS at FB, 30 min at injection, for 1 ns and 1. 2 ns bunch length • Luminosity evolution for 1. 2 ns and 1. 0 ns bunch length • Bunch parameters evolution for the 1. 2 ns and 1. 0 ns case • Integrated lumi for 20 h and 10 h @ SB: different parameters comparison • Integrated luminosity gain @ 20 h and 10 h • Beam-beam DA for lower emittance and crossing angles Discussion (questions, comments, etc) -do 0. 9 ns levelling -smaller brightness, smaller burn off, it is better to keep longer the beam -to explore-> chromaticity limit, 8 pole limit without pushing brightness. -shorter fills, dumping, etc to optimize -turnaround time less sensitive for long fills. -the more levelling the less sensitivity in peak luminosity.

“BSRT status”, Georges Trad • BSRT – Calibrating: -Why? Mainly for improving the accuracy of the measurement. Sophisticated Calibration process was developed Contrarily to Run I where it relied on both BPMs and WS, Run II only WS needed. Reached now < 5% error on b-by-b size measurement. -How? -Results? Beam sizes VARIABLE: LHC. BSRT. 5 R 4. B 1: LSF_H VARIABLE: LHC. BSRT. 5 L 4. B 2: LSF_H Timestamp (UTC_TIME) Value Great data quality during calibration->Great Results Value 0. 303 VARIABLE: LHC. BSRT. 5 R 4. B 1: LSF_V 0. 299 VARIABLE: LHC. BSRT. 5 L 4. B 2: LSF_V Timestamp (UTC_TIME) 51: 59. 8 Value 0. 294 51: 59. 8 Emit. error Value 0. 299 Calibration done twice->Repeatability better than 2% Achieved accuracy at FT beam size of <5 % • BSRT logging (extra): -Special requests of bunch profile logging can be satisfied, however still not a viable option storage wise. Delicate treatment for FT profiles is required, since for small beam sizes, the measured profile is dominated by the Line Spread Function (strongly non Gaussian). Binary files are saved containing all the profiles File Description and Data Viewer are also available. • Planned improvements: Porting BSRT to FESA 3, Testing digital cameras, Commissioning BSRI – BSRC, R&D Slit Scanner on B 1 Discussion (questions, comments, etc) -B 1 is always more noisy because of its camera (some error). -they obtain 30 profiles/sec (not of the some bunch). -we can have 1 profile/sec, without asking someone to acquire them, Yannis said this is enough for our beam profiles studies.

“Modelling and measurements of bunch profiles at the LHC flat bottom” , Stefania Papadopoulou • • The q Gaussian distribution function. One beam profile example at LHC FB, (low frequency excitation MD) SIRE code (inputs, computing IBS and outputs) Comparing SIRE with B-M (MADX) when plotting the resulted emittance and energy spread growth for 30 min at FB, both for nominal and Hi. Lumi parameters. -using the full lattice (twiss file) makes some days to run and the reduced lattice (with 92 points) makes <2 h to run. -keeping 5 mp/cell and scanning #cells in the longitudinal & #macroparticles. -SIRE runs many times (cause there are random generators) for the optimal parameters, then the mean values are compared with the B-M result from MADX. For the Hi. Lumi optimal case the #cells and the #macroparticles are larger compared to the nominal case, this is logical when considering how stronger is the effect for the Hi. Lumi parameters. -Having initially Gaussian and q Gaussian distributions in SIRE, plots that show the growths and profiles change. Discussion (questions, comments, etc) -run SIRE for longer time (more than 30 min). -run it for q gaussians with heavier tails (more than the q=1. 15 that is used now). -compare the code with Piwinski also. -it was suggested to get beam profiles from beam gas data. -next steps: 1. Acquire beam profiles more regularly. 2. See the evolution of the profiles during the fills. 3. Interplay of IBS and SR at FT where profiles arrive with clearly non-Gaussian shape.