Outcome of beam dynamics simulations - Scenarios, requirements and expected gains s LIU-SPS Coordination meeting 26/08/2015 A. Lasheen, E. Shaposhnikova, T. Argyropoulos, T. Bohl, J. Varela Thanks to: S. Hancock, summer students, impedance team, MD coordination team, operation team

Introduction and motivation 2  To be sure that we can achieve the requirements for HL-LHC, we need to cure the longitudinal instabilities.  A lot of work was done to identify the impedance sources in the SPS responsible for the instabilities.  The current impedance model is being tested by comparing measurements and simulations, for single bunch and multi-bunch.  The following simulations were done to check the effect of the vacuum flanges in the various present conditions.

Outlook 3  Benchmarking simulations with measurements  Single bunch  Different voltage configurations (1RF – 2RF, 2 voltage programs)  Impedance reduction simulations  Main sources of impedances  Instability thresholds with full impedance for different cycles and voltage programs  Impedance reduction of the flanges  Multi-bunch instabilities  At flat top

Benchmarking simulations with measurements

SPS Impedance model 5  Latest impedance model (J. Varela, B. Salvant, C. Zannini, D. Bazyl, P. Kramer):  Travelling wave cavities (and HOMs)  Kickers  Vacuum flanges  Unshielded pumping ports  BPMs  Y chambers  Beam scrapers  Resistive wall  Space charge (flat bottom)  ZS/MSE

Fast ramp programs 6  Fast ramp (LHC pilot cycle)  Two RF voltage settings  Constant bucket area CBA (acceptance 0.5eVs)  High voltage HV (~7.2MV)  800MHz voltage set to 10% of the main RF CBAHV Voltage (V) Time (s) Momentum (eV)

Single RF: instability threshold vs. energy 7 Single RF – Constant bucket area Single RF – High voltage

Double RF: instability threshold vs. energy 8  CBA : instability threshold is close, but at higher intensities the energy threshold is not reproduced…  HV : the instability threshold is much higher in simulations than in measurements  800MHz parameters ? Double RF – Constant bucket area Double RF – High voltage

Double RF calibration 9 90% 25%

Double RF phase scan (flat bottom) 10  The effective voltage in measurements wrt simulations appears to be ~6% instead of 10%  The phase offset between measurements and simulations is 31° at flat bottom and 42° at flat top Tilt (s) Flat top Tilt (s) Flat bottom

Double RF simulations 11  CBA : the picture didn’t change much  HV : the simulations are now closer to measurements, but still slightly more stable  More investigation on the 800MHz effective voltage and phase ongoing Double RF – Constant bucket area Double RF – High voltage

Conclusions 12

Impedance reduction simulations: single bunch

Studied parameters space 14  Different parameters to scan:  Intensity  Emittance  Different RF programs (and double RF)  Different cycles (fast and long ramp)  Different distributions (binomial)  Number of bunches  Impedance tested in this study:  Flanges (group 1)  Flanges (group 2)  Other impedance that may change:  TWC 200 main impedance (after LS2)  TWC 800 main with feedback  TWC HOMs (with HOM couplers)  Kickers (change in the number of kickers)

Vacuum flanges groups 15  Vacuum Flanges Group 1:  QD-QD enameled flanges (99)  QD-QD closed flanges (75)  Shielded pumping ports – Long QD Bellows (71)  Shielded pumping ports VVSA – Long QD Bellows (17)  BPV – QD flanges  Vacuum Flanges Group 2:  QF – QF closed flange no bellows (18+2)  QF – QF closed flange (22+1+3)  BPH – QF flanges (25+12)  MBA – MBA flanges (12+2)  QF – MBA flanges (78+2)

16 CBAHV 1RF 2RF

17 CBAHV 1RF 2RF Instability at flat top

18 CBAHV 1RF 2RF

19 CBAHV 1RF 2RF

20 CBAHV 1RF 2RF

Conclusions for the fast ramp cycle 21

Slow ramp parameters 22  Slow ramp (8.6s nominal LHC cycle)  Two RF voltage settings  Constant bucket area CBA (acceptance 0.6eVs)  High voltage HV (~6.6MV, different than in fast ramp)  Second RF voltage set to 10% of the main RF CBAHV Voltage (V) Time (s)

23 CBAHV 1RF 2RF

24 CBAHV 1RF 2RF Instability at flat top

25 CBAHV 1RF 2RF

26 CBAHV 1RF 2RF

27 CBAHV 1RF 2RF

Conclusions for the slow ramp cycle 28

Impedance reduction simulations: 12 bunches

Multi-bunch instability thresholds 30  Scan started by T. Argyropoulos for 12 bunches at flat top

Conclusions 31

Further steps and ideas 32  Multi-bunch  12 bunches simulations in nominal ramp  code optimization for simulations > 12 bunches  Low-level RF  Phase loop  Feedback/feedforward  Interplay between impedances  Higher emittances (usually obtained with blow-up during ramp)  Future RF configuration (after LS2)  Effect of other impedances (HOMs, Kickers, …)