# Presentation: 064581

## Electron Cloud Update

- Miguel A. Furman (LBNL)
- _[mafurman@lbl.gov](mailto:mafurman@lbl.gov)_
*LARP Collaboration Mtg.*
*Pheasant Run (Illinois) Oct. 5-6 2005*

***bnl - fnal- lbnl - slac***

_**US LHC Accelerator Research Program**_

## Recent activities

- BNL
- CERN e– detectors for IP12 (not LARP funded, but important)
- Two dipole magnets, B0.2 T, room temperature (one detector/dipole)
- J. Miguel Jiménez to come this month to BNL for installation
- Testing and calibration during 2006 run (A. Drees)
- Ping He now working on ecloud
- LBNL
- Summer student (V. Chaplin, July 2005)
- Updated simulations of ecloud power deposition for LHC arc dipoles (next viewgraph)
- Renewed RHIC simulations; goal: map out parameter space
- Calibrate against measurements
- Explain phase transitions
- Augmented diagnostic capability of POSINST code
- Quantify effects from various components of the electron-emission spectrum
- 3D self-consistent code (WARP/POSINST)
- Not LARP-funded in FY05
- Initial results for one LHC arc FODO cell in early 2005 (LARP mtg, Apr. 05)
- Will take up in FY06 (Jean-Luc Vay)

## Simulated LHC arc dipole power depositionbunch spacing: tb=25 ns

- * “LTC40”: LHC Tech. Committee. Mtg #40, April 2005 (CERN simulations, presented by F. Zimmermann)

- Aver. power deposition vs. bunch intensity
- for a given peak value of the SEY
- (POSINST and ECLOUD codes)

## Same as previous (tb=25 ns) but no rediffused electrons(*)

- Motivation: POSINST model w/o rediffused ≈ ECLOUD model

- This is “good agreement” by the standards of the trade (IMHO)

- (*) We set r=0 and simultaneously
- increased e and ts by a common
- factor such that tot remained
- the same

## Bunch spacing: tb=75 ns (POSINST code)

## Updated LHC dipole simulations: conclusions

- No problem for tb=75 ns, even up to Nb=1.6x1011 and max=2
- In qualitative agreement with CERN results
- If rediffused electrons ignored, good agreement with CERN simulations
- As expected (similarity of models)
- No problem up to max≈1.4 (for Nb=1x1011)
- But rediffused electrons are there
- Our model is based on bench measurements of emission spectrum for Cu
- Maximum acceptable max≈1.3 (for Nb=1x1011)
- Caveats:
- Power depos. estimates above are based on 1 batch (=72 bunches + gap)
- Steady-state estimates are higher by ~30-40%
- (0) varies in 0.3-0.5 depending on max; we have not assessed sensitivity to (0) separately from max

## Goals for FY05-06 (list from LARP mtg. April 05)

- LHC heat-load estimate: POSINST-ECLOUD benchmarking (*) essentially done
- 3D beam-ecloud self-consistent simulations (*) ongoing
- Electrons, gas, ions, ...
- Main goal: understand effects from ecloud on beam
- Analyze June 2004 SPS data (*) ongoing
- Especially e– energy spectrum
- Constrain SEY model for better predictions for LHC
- Benchmark CERN calculations
- z dependence
- Help define optimal LHC conditioning scenario (*) not started
- Define optimal fill pattern during first two (?) years of LHC beam
- Apply Iriso-Peggs maps to LHC (*) not started
- Understand physics of map simulation technique
- Understand global e-cloud parameter space, phase transitions
- Simulate e-cloud for RHIC detectors (**) just begun
- Calibrate code
- Then predict BBB tune shift
- Simulate e-cloud for LHC IR4 “pilot diagnostic bench” not started
- What to expect when high-N, low-sb beam turns on

- (*) endorsed by CERN AP group
- (**) endorsed by CERN vacuum group

## Additional material

## Calibration of POSINST at APS (e+ beam)

- (Furman, Pivi, Harkay,
- Rosenberg, PAC01)

- time-averaged e– flux at wall vs. bunch spacing

## Calibrating POSINST at PSR

- bunch length ~60 m
- a portion the EC phase space is in resonance with the “bounce frequency”
- “trailing edge multipacting” (Macek; Blaskiewicz, Danilov, Alexandrov,...)

- ED42Y electron detector signal 8C/pulse beam

- 435 A/cm2

- (simulation input)

- electron signal

- measured (R. Macek)

- simulated (M. Pivi)

-  (max=2.05)

- (ROAB003; RPPB035)

## Three components of secondary emission:sample spectrum at E0=300 eV

- from M. F. and M. Pivi,
- PRST-AB 5, 124404 (2002)

## Electron-wall collision energycomparison w/wo rediffused electrons

- Four successive bunches in a 25-ns batch

- ~5 ns after bunch passage: 1st wave of electrons hits the wall (were kicked by the beam)
- ~5 ns later: second wave of electrons hits the wall; these were mostly rediffused electrons created when the 1st wave hit the wall
- NB: the 2nd wave is absent in the “NR” case (“no rediffused”)

## Effective SEYcomparison w/wo rediffused electrons

- The 2nd wave leads to a higher effective SEY (eff) than in the “NR” case...
- [definition: eff= (no. of emitted electrons)/(no. of incident electrons) averaged over all electron-wall collisions anywhere on the chamber wall, over any given time interval]

## Average electron line densitycomparison w/wo rediffused electrons

- ...which leads to ~twice the number of electrons...

## Average power depositioncomparison w/wo rediffused electrons

- ...which, in turn, leads to ~twice the power deposition.
- Most of the power deposition comes from the 1st-wave electrons. The factor ~2 is mostly because there are ~twice the number of electrons.
- The 2nd wave contributes an additional ~5-10% of “direct” power deposition (small bump ~10 ns after the bunch passage)

## The electron-cloud effect in LHC

-  Beam synchrotron radiation is important
- provides source of photo-electrons
-  Secondary emission yield (SEY) (E) is important
- characterized by peak value max
- determines overall e– density
-  e– reflectivity (0) is important
- determines survival time of e–
-  Bunch intensity Nb and beam fill pattern are important
-  Main concern: power deposition by electrons