# Presentation: 271236

## Electron Cloud - Status and Plans

- Miguel A. Furman
- LBNL
- _[mafurman@lbl.gov](mailto:mafurman@lbl.gov)_
*LARP Collaboration Mtg.*
*LBNL, April 26-28, 2006*

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

_**US LHC Accelerator Research Program**_

## Recent activities

- Completed updated simulations of ecloud power deposition in LHC dipoles
- M. Furman and V. Chaplin, PRST-AB **9**, 034403 (March 20, 2006)
- Tedious exploration of parameter space with 2D code POSINST (see next slide)
- Peak SEY**max now constrained to be <~1.2 for nominal intensity & bunch spacing
- 3D self-consistent code (WARP/POSINST)
- Jean-Luc Vay (LBNL) now 20% LARP funded (starting in FY06)
- Initial qualitative results for one bunch in one FODO cell (LARP mtg, Apr. 05)
- New results for a train of 5 bunches with more detailed model (see p. 4)
- See Jean-Luc’s talk _Friday_
- RHIC studies
- Feb. 2006: two CERN e– detectors installed (not LARP funded, but important)
- Common pipe region in IP10, warm section
- Polarized proton beams for this run
- Ping He (BNL) in charge of RHIC simulations
- No useful results yet

## Sample simulation: ecloud power deposited in LHC dipoles(from Furman and Chaplin, PRST-AB 9, 034403 (2006)) 

- ecloud power deposition
- POSINST code
- LHC arc dipole magnet
- key parameters: *N*b, *t*b, **max
- current result: **maxmust be <~1.2
- conditioning scenario needs to take account of this

**: cooling capacity available for EC power deposition**

## Sample 3D self-consistent simulations (WARP/POSINST)(courtesy J.-L. Vay. More details in J-L’s talk this Friday)

- WARP/POSINST-3D
- T = 4.65s

- experiment
- simulation

**Oscillations**

**Electrons bunching**

**Beam ions hit end plate**

- WARP/POSINST-3D
- T = 0.3s

- LHC FODO cell
-  can now follow batch of bunches with photo-e– and secondary e–
-  snapshot from run with 5 bunches:

-  Benchmark code against HCX experiment (LBNL)
-  expt. and sim. agree quantitatively on characteristics of e– oscillations observed in magnetic quadrupole flooded with electrons:

**Notes:**
On the picture, the color coding is: yellow for the bunches, 
rainbow blue-to-red for electrons according to local density.

## Simulation code progress

- POSINST
- CVS repository for controlled updates and easy distribution
- Stand-alone mode
- Uses publicly available (free) subroutine library
- WARP/POSINST
- New particle pusher
- follow electrons in *B*-fields with large time steps w/o loss of acuracy (done)
- Field solver with arbitrary vacuum chamber geometry (done)
- Multi-bunch launching for LHC beam (done)
- with mesh refinement around each bunch (very soon)
- Dynamically adjustable independent time-steps (in progress)
- Add “QuickPIC” mode (see _J.-L. Vay’s talk Friday_) (soon)
- this is a hybrid 2D/3D computation of fields from ecloud
- much faster than fully 3D calculation, but sufficiently accurate

## Future Plans

- ecloud effects on the LHC beam
- Ultimately: 3D self-consistent simulations
- But simpler approximations first (_J.-L. Vay’s talk Friday_)
- Nontrivial exercise
- Main new concern: slow emittance growth (Benedetto et. al. PRSTAB **8** 124402 (2005) et. seq.)
- SPS: “/=0.1 in 106 turns”
- Summer student at LBNL for 12 weeks this summer
- Simulate optimal conditioning scenario for LHC
- Understand details from SPS measurements
- RHIC:
- Collect data with CERN detectors
- Continue and complete simulations (Ping He in charge of simulations)
- Understand ecloud and correlate it to other observables
- LHC upgrade (this is a new task requested by CERN)
- ecloud power deposition for *t**b*=12.5 ns (“burning issue”)
- “...therefore I think an urgent task will be to repeat ecloud simulations for the “ultimate”, "short bunch", and "longer bunch" upgrade scenarios, with and without rediffused electrons, and report your results at the CARE-HHH workshop in Valencia, Spain, 16-20 October 2006.” (F. Ruggiero, 20 April 2006)

## Related developments (outside LARP scope)

- New R&D effort at LBNL to evaluate ecloud for the FNAL MI upgrade
- Indirect evidence for ecloud at MI and Tevatron exists
- Proposed proton driver to replace booster
- Increase *N**b* in MI by large factor (~5), change RF frequency (now 53 MHz) to ??? (40-325 MHz)
- R&D at LBNL to be funded by FNAL (“check’s in the mail, almost...”)
- Post-doc to start soon (we hope)
- LBNL trying to augment ecloud R&D effort for ILC DR’s
- especially ecloud effects on beam
- HHH Workshop “LHC-LUMI-06” (Valencia, Oct. 12-16, 2006)
- Roadmap for upgrade of LHC and GSI complex
- Define *t**b* and total LHC beam intensity (and other matters)

## Goals for FY06+NB: this is a full wish list. Task completion not guaranteed within FY06. Need to re-prioritize.

- LHC heat-load estimate and POSINST-ECLOUD benchmarking (*) done
- 3D beam-ecloud self-consistent simulations (*) continuing
- Understand effects from ecloud on beam stability and  growth
- Effects of ionized gas on ecloud (*) (n) not started
- Analyze June 2004 SPS data (*) ongoing
- Further constrain SEY model, and benchmark CERN calculations
***z* dependence
- Help define optimal LHC conditioning scenario (*) not started
- Power deposition for *t**b*=12.5 ns (*) (n) not started
- Apply Iriso-Peggs maps to LHC (–) not started
- Understand global e-cloud parameter space, phase transitions
- Simulate e-cloud for RHIC detectors (**) continuing
- Calibrate code against data collected
- Then predict BBB tune shift
- Simulate e-cloud for LHC IR4 “pilot diagnostic bench” not started
- What to expect when high-*N**b*, 25 ns, beam turns on

- (–) no longer endorsed by CERN AP group
- (n) new goal

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

## Additional material

## EC at FNAL: indirect evidence

- At present: indirect evidence for an EC exists
- But no direct electron measurements yet
- Tevatron:
- Fast pressure rise (X. Zhang, Dec. ‘02 and May ‘05)
-  ***P* seen at some of the warm straight sections (ion pump measurements)
- Threshold ~4e10 p/bunch for 30 consecutive bunches
- No good way to measure *P* in cold regions
- Fast emittance growth (flying wire technique)
*d****/dt*~28 mm-mr/hr  (95%, normalized, vertical, averaged over 30 bunches)
- this is for *E*=150 GeV and *N*=82e10 in 30 bunches
- this is *much* faster growth than estimated IBS growth rate
*d****/dt* sensitive to *N* above threshold
- Unfortunately, no BBB measurements (yet)

## EC at FNAL: indirect evidence

- Main Injector:
- Fast pressure rise (R. Zwaska, Jan. 06)
- 82 bunches of ~9e10 p/bunch, or 418 bunches of ~5e10 p/bunch
-  ***P* seen at 24 of 523 pumps
-  ***P/P* typically 5-50%
- but reached 600%-700% at 2 pumps: uncoated ceramic chamber
- NB: ceramic has a high SEY, therefore high ***P/P* is consistent with e-cloud hypothesis
- Maximum effect at transition (short ***z*)

## EC at FNAL: what next

- 14-week shutdown started 28 Feb. 2006
- Will install two electron detectors in straight sections
- 1 in the MI, 1 in the Tevatron
- RFA-type detectors (ANL design): can measure e– intensity and e– energy spectrum
- Special “ecloud section” at Tevatron, *L*~1 m
- with adjustable 20-G solenoidal field (to control the EC)
- late ‘06 or early ‘07: add NEG-coated clearing electrode (for further EC studies)
- Measure BBB tunes
- Instrumentation in place, but software needed; “high priority”
- Carry out systematic simulations
- Vacuum chamber materials: st. st. and ceramic
- We don’t have accurate SEY parameters, esp. for ceramic
-  assume range of input parameters for simulations
- Calibrate against measurements after shutdown

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

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

- Time-averaged e– flux at wall vs. bunch spacing
- (e+ beam, 10-bunch train, field-free region)

## 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