# Presentation: 091395

## Beam-beam EffectandElectron-Cloud Effect

- Miguel Furman
- LBNL
- LARPAC mtg.
- FNAL, 17-18 June 2002

## Participants

- BNL
- P. Cameron
- W. Fischer
- F. Pilat
- V. Ptitsyn
- M. Blaskiewicz
- FNAL
- T. Sen
- V. Shiltsev
- M. Syphers
- LBNL
- M. Furman
- M. Pivi
- J. Qiang
- W. Turner

- UNM
- M. Vogt
- CERN: close contact with
- SL/AP group
- LHC Vacuum group
- particularly:
- G. Arduini
- I. Collins
- O. Gröbner
- W. Herr
- N. Hilleret
- J. M. Jiménez
- J. P. Koutchouk
- F. Zimmermann

## Beam-beam issues for LHC

- New regime for proton collider:
-  0>0.003
- many parasitic collisions (>100 on either side of IP)
- many bunches (~3000)
- many gaps in the beam (=> pacman bunches)
- each bunch has its own closed orbit
- crossing angle (150 rad)
- requires smooth operation for many hours at a time
- LBNL luminosity monitor
- sweep one beam about the other
- => luminosity modulation signal
- => bunch-by-bunch closed orbit correction

## Beam-beam issues for LHC (contd.)

- Potential limitations in strong-strong mode (BNL, FNAL, CERN, LBNL, UNM)
- tune choice
- coherent modes
- consequences of long-range beam-beam interactions:
- lifetime vs. *d/*** and *N**p* (Tevatron experience)
- synchrobetatron resonances
- instability coupling via beam-beam, etc.
- bring separated beams into collision
- Beam-beam compensation with electron lens (FNAL)
- bunch-by-bunch tune shift compensation
- investigate feasibility of TEL-like device for LHC

## Beam-beam issues for LHC (contd.)

- Beam-beam compensation with a wire (FNAL & CERN)
- needs ~80 A-m integrated current & LHC nominal operation
- if DC => compensate average LR tune shift
- if pulsed => compensate bunch-by-bunch LR tune shift
- SPS experiment 2002-2003
- will model wire with symplectic maps, then compare with experiment
- Sweeping luminosity detector (LBNL)
- emittance growth (sweeping vs. steady collisions)
- bunch-to-bunch signal strength
- may need bunch-by-bunch closed orbits (already calculated by W. Herr & co.)

## Beam-beam issues for LHC (contd.)

- Continue development of strong-strong simulation programs (BNL, FNAL, LBNL, UNM, CERN).
- desired features:
- head-on plus long-range collisions
- finite bunch length
- crossing angle
- 3-D dynamics
- multi-bunch beams
- independent beam parameters for the two rings
- arbitrary beam separation

## Beam-beam effect at RHIC: coherent modes

- Recently observed at RHIC/protons
- a first for hadron colliders
- simulations in good agreement (W. Fischer, M. Vogt)

## Beam-beam effects: coherent modes

- Simulated LHC beam-beam tune spectrum
- parallel PIC simulation code (J. Qiang, LBNL; code run at NERSC)

- LHC beam-beam
- simulation
- x1=x2= y1=y2=0.31,
- 0=–0.0034

-  mode

-  mode

- (-0)/|0|

-  split=1.21

- incoherent spectrum

- parallel PIC code
- 128x128 grid
- 10**6 particles/bunch
- 1 kick
- 16 processors
- 1 bunch/beam

## LBNL sweeping detector schematics

- stationary, accidentally
- displaced beam

- rotating, deliberately
- displaced beam

## Simulated LHC luminosity signal (sweeping detector)0.2/0.6, T=1000 turns

- LHC beam-beam
- simulation
- x1=x2= y1=y2=0.31,
- 0=–0.0034

- parallel PIC code
- 128x128 grid
- 10**6 particles/bunch
- 1 kick
- 16 processors
- 1 bunch/beam

## Simulated beam size vs. time (sweeping detector)

- LHC beam-beam
- simulation
- x1=x2= y1=y2=0.31,
- 0=–0.0034

- parallel PIC code
- 128x128 grid
- 10**6 particles/bunch
- 1 kick
- 16 processors
- 1 bunch/beam

## Electron cloud effects

- Consequence of coupling between beam and chamber.
- main concern for LHC: power deposition in beam screen
- also: beam stability, emittance dilution (not expected to be a problem)
- First seen at ISR (1977) as beam-induced multipacting
- Observed and studied at: PF, PSR, APS, BEPC, CESR, PEP-II, KEKB, PS, SPS
- Possibly seen at RHIC

## Electron-cloud LHC-ARP goals

- Main goal: accurate prediction of power deposition
- Validation of simulation codes
- reproduce SPS measurements (electron analyzer, WAMPAC, COLDEX)
- also reproduce APS, PSR, PEP-II observations
- cross-check with CERN simulations
- work in close contact with LHC personnel (LHC Vacuum Group and SL/AP)
- ECE at RHIC
- simulate and verify effect
- evaluate remedies
- Improve simulation codes
- parallelize for speed
- self-consistent calculation of single-bunch effects
- better data on secondary emission yield and secondary energy spectrum
- these are essential inputs to simulations

## Simulation example: Electron Cloud in an LHC Arc Dipole  Power deposition vs. time: sensitivity to secondary electron emission spectrumat fixed peak yield max=2.1

## Conclusions

- We have essential AP expertise of direct relevance to LHC
- RHIC and Tevatron beam-beam experience
- electron-cloud studies at LBNL for LHC and other machines
- good numerical tools
- Strong ongoing collaborations
- among BNL, FNAL and LBNL
- with CERN
- with ANL and LANL
- Work leveraged by other efforts/labs/programs:
- SciDAC program (particularly beam-beam work at LBNL)
- NERSC (LBNL) (supercomputer support)
- PSR (LANL) (electron-cloud measurements)
- SNS (ORNL) (electron-cloud simulations)
- HIF-VNL (?) (electron-cloud measurements and simulations)

## Additional information

- Important websites:
- http://lhc-beam-beam.web.cern.ch/lhc-beam-beam/
- http://wwwslap.cern.ch/collective/electron-cloud/electron-cloud.html
- http://www.aps.anl.gov/asd/physics/ecloud/ecloud.html

## Simulation example: Electron Cloud in an LHC Arc Dipole  Effective SEY vs. time: sensitivity to secondary electron emission spectrum

## ECE: Simulation Code Calibration at the APS(e+)electron current at the chamber wall vs. bunch spacing