# Presentation: 725864

## Beam-beam simulationsin FY07-FY08

- Tanaji Sen
- FNAL

## 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

## FNAL – Dynamic Aperture in RHIC

- 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.

- BBSIM

## FNAL - diffusion in RHIC w/wo 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

- No wire

- Wire: 50A, 6σ

- Dxx at constant y

-  BBSIM

- BBSIM vs Measurement (EPAC2002)

## RHIC simulations appear to be approaching reality

- BBSIM

- Loss rates with wire at Injection

- BTF comparison at 100 GeV

- Loss rates with wire at 100 GeV

- Wire off

- Wire on

- ρ(ν)

- ρ(ν)

## LBL – RHIC emittance growth 

- W/wo tune modulation

- W/wo chromaticity

- Wire off

- Emittance growth with wire at 5A, 50A

- Beambeam3D

## LBL – LHC emittance growth

- Horizontal separation at IP

- 10% mismatch in beam size

- No parasitics

- 60 lumped parasitics

- Beambeam3D

## 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.

## 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

## 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.

## 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 4-5__σ_
- Large off-momentum β beating.

## 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_

## 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_

## 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

## 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.

## 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.