Beam-beam simulations in FY07-FY08

1. Beam-beam simulationsin FY07-FY08 Tanaji Sen FNAL

2. LARP beam-beam simulations • FNAL – H.J. Kim and T. Sen (BBSIM) Weak-strong simulations used for SPS and RHIC wire experiments • LBL – J. Qiang (Beambeam3D) Strong-strong simulations used for RHIC and LHC emittance growth • SLAC – A. Kabel (PlibB) Weak-strong simulations used for RHIC wire experiments

3. FNAL – Dynamic Aperture in RHIC BBSIM • Base tune (0.220,0.231) at RHIC Au collision energy. • Wire current 50A, separations 6-9σ . • DA is smaller in the plane of beam-wire separation (y) • Tune scan indicates that nominal tune is close to optimal. • A sharper drop in dynamic aperture is observed near at 5th and 4th order resonances.

4. FNAL - diffusion in RHIC w/wo wire BBSIM No wire • Diffusion measurements were done in 2002 with different tunes, machine parameters. Large error bars in action measurement • The wire increases diffusion by ~ 2 orders of magnitude at all amplitudes Dxx at constant y Wire: 50A, 6σ BBSIM vs Measurement (EPAC2002)

5. FNAL – RHIC BTF, losses BTF comparison at 100 GeV BBSIM Loss rates with wire at Injection ρ(ν) Wire off • RHIC simulations appear to be approaching reality Loss rates with wire at 100 GeV ρ(ν) Wire on

6. LBL – RHIC emittance growth Beambeam3D W/wo tune modulation Wire off W/wo chromaticity Emittance growth with wire at 5A, 50A

7. LBL – LHC emittance growth Horizontal separation at IP Beambeam3D 10% mismatch in beam size No parasitics 60 lumped parasitics

8. FY07 Summary • RHIC simulations were done for different conditions: injection, collision, w/wo wire compensator. • Main focus was on beam-wire experiments performed in 2007 with gold beams. • Dynamic apertures, emittance growth, diffusion coefficients, BTFs, loss rates have been simulated. • Agreement between simulations and measurements is improving. • Preliminary strong-strong simulations of LHC show that parasitics have a major influence on the emittance growth.

9. FY08 tasks • RHIC wire compensation • LHC wire compensation, tolerances • Simulations for different IR layouts (1) Early separation (2) Large crossing angle • Rf noise and impact with beam-beam • Crab cavity impact with beam-beam • Additional physics in the codes: resistive wall wakefields, IBS, …. • Diffusion solver; test of diffusion model • Strong-strong effects, potential loss of Landau damping

10. RHIC wire compensation Wire compensation experiments in FY08 • Further evaluation of wire compensator effect on single beam • Experimental test of the wire compensation principle with 1 parasitic interaction per beam. Simulation effort • Model the experimental conditions closely and calculate observed beam parameters such as loss rates, BTFs, emittance growth, orbit and tune shifts etc.

11. stronger triplet magnets D0 dipole small-angle crab cavity ultimate bunches + near head-on collision LHC IR - Early separation scheme • Bunch intensity ~ 1.7x1011, 25nsec spacing • β* = 10cm • Crab cavities for ~zero crossing angle. Impact on emittance growth • 4 parasitic interactions at 4-5σ • Large off-momentum β beating.

12. wire compensator LHC IR - Large crossing angle • Longer and more intense bunches (~4-fold), bunch spacing 50nsec • β* = 25cm • Wire compensation • Large crossing angle – unproven regime, synchro-betatron resonances

13. LHC IR - Crossing schemes • Nominal scheme: H-V crossing. Reduces tune spread • H-H crossing – larger tune spread. Weaker resonances? • Simulations by K. Ohmi show H-H scheme creates less halo growth with long bunches. Test this prediction

14. Impact of rf noise • Several strong coherent lines at 50Hz and multiples • Simulations of only longitudinal dynamics show (1) 50Hz lines cause slight emittance blow-up during ramp (2) During a store these lines do not have much impact • These lines at ~ 2νs drive amplitude oscillations and may have an impact with beam-beam interactions included

15. Crab cavities • Phase jitter between cavities will lead to offsets at the IPs. • Cavities induce a z-dependent orbit distortion. • How do these distortions couple with the parasitics – enhanced offsets, changes in tunes, chromaticities? • Beam-beam interactions may place tight tolerances on crab cavity errors. Evidence from KEK. Simulations would be useful.

16. Summary • RHIC experiments in 2007 were useful In benchmarking the simulation codes. • In 2008 the codes will be used to test wire-based compensation and compare with RHIC measurements. • Beam-beam simulations for different IR layouts will help develop their designs. • Noise tolerances on accelerating and crab cavity will be studied. • Plan to enrich the physics content of the codes in FY2008.