# Presentation: 188358

## Overview of Beam-beam simulations

- Tanaji Sen
- FNAL
- US LARP Collaboration Meeting
- April 23, 2008

## Outline

- Measurements with the wire compensator
- Beam transfer function comparisons
- Emittance growth from diffusion coefficients
- Physics additions to codes
- Future plans

## Beam-beam codes

- FNAL: BBSIM  (H.J. Kim and T. Sen)
-   with contributions from
-         V. Boocha, B. Erdelyi, V. Ranjbar
- LBL: BeamBeam3D (J. Qiang)
- SLAC: PlibB (A. Kabel)

## Loss rates: RHIC studies in FY08

- Deuteron-gold store in January 2008. (d: Blue, Au: Yellow ring)
- 2 head-on collisions and no long-range interactions
- Wire current 50A, separation changed from 26mm-50mm
- Losses of deuteron beam were measured as a function of the
-     separation

- BBSIM simulations of loss rate vs measurement. Onset of sharp losses well reproduced.

## Beam transfer function 

- BTF modules have been added to all 3 codes.
- Useful partial validation of codes.
- Measurements of deuteron BTFs in d-Au stores in 2008 provide cleaner signals than in Au-Au stores of 2007.

## BTF simulations : FNAL

- Deuteron-gold store in January 2008
- Tune shift and BTF features with wire need to agree better

- Horizontal

- Vertical

**Wire On**

**Wire On**

**Wire Off**

**Wire Off**

## BTF simulations: LBL

- Gold-gold store of April 25, 2007
- Difference of ~0.002 between horizontal  tunes with wire on.
- Simulated tune shift agrees with analytical tune shift.

- Without wire

- With wire

## BTF simulations: SLAC

- Gold-gold run of 2007
- Reasonably good agreement in amplitude & phase

- BTF amplitude with wire

- BTF phase with wire

## Emittance growth from diffusion coefficients

- Emittance growth may be driven by diffusive processes in the beam core. Dynamics in the tails which determines lifetime may not be diffusive
- Calculate diffusion coefficients from tracking code (BBSIM) and use as input to an independent diffusion equation solver
- Evolve the density and the moments to find emittance growth and lifetime over length of the store – several hours

## Diffusion equation

- Diffusion equation in 2D action space
- Tracking with BBSIM in 6D phase space to calculate diffusion coefficients.
- Compared numerical diffusion coefficients with available measurements of diffusion coefficients at RHIC (R. Fliller’s thesis)

## Emittance growth: d-Au store

- Measured horizontal emittance growth is much larger than in simulations.
- Agreement with vertical emittance growth is significantly better.

- Horizontal plane

- Vertical plane

- Deuterons: Store 9572

## Emittance growth: d-Au stores

- The pattern is consistent in several stores. Horizontal emittance growth is underestimated, vertical emittance growth is more accurate.

- Horizontal

- Vertical

- Horizontal

- Vertical

## Physics additions to codes

-  BBSIM (FNAL)
- Coupling (Edwards-Teng) – no change to RHIC dynamic aperture
- Resistive wall wakefields
- Symplectic synchro-beam map
-  Beambeam3D (LBL)
- Crab cavities – first results, impact of phase noise
-  PlibB (SLAC)
- IBS – application to RHIC
- Electron Lens

## Simulation results summary

- Loss rates with wire:
-   - Onset of loss rates with beam-wire separations is closely reproduced in BBSIM.
-   - Similar results obtained in FY07 with earlier RHIC measurements.
- BTF simulations:
-   - Reasonable agreement without wire between meas. & sim.
-   - Some unresolved questions with measured BTFs with the wire
-   - Effect of coupling with wire not well understood.
- Emittance growth from diffusion in d-Au stores
-   - Significantly underestimate hor. plane. Missing IBS
-   - Good agreement in ver. plane
-   - Diffusion model does not predict the lifetime accurately, as expected.
- First results from crab cavity (Beambeam3D). Need to be better understood.

## Forthcoming studies

- Evaluate wire compensation in LHC and benefit to luminosity.
- Simulate wire compensation in RHIC and compare with measurements in FY09
- Improve the diffusion model: add other sources of diffusion in tracking, use diffusion equation in 3D action space. Extend the comparison to other stores.
- Evaluate the impact of crab cavities in the LHC with weak-strong and strong-strong simulations. Set tolerances on noise parameters.