electron cloud - status and plans

LARP-FNAL-Apr-2007 Electron Cloud - M. Furman1

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Electron Cloud - Status and Plans

Miguel A. FurmanLBNL

[email protected]

Collaboration Mtg.FNAL, April 18-20, 2007

bnl - fnal- lbnl - slac

US LHC Accelerator Research Program





are needed to see this picture.Summary

Assessment of ecloud for upgraded LHC and injectors

• With build-up code POSINST (no effects on the beam)

Effects on the beam • With code WARP/POSINST

Quasi-static mode “QSM” Lorentz-boosted frame method

• Progress towards full self-consistency

RHIC measurements Status and future goals






Assessment of ecloud buildup for LHC upgradeand injectors

Initial results for injectors presented at LUMI06 Substantial improvements since then:

• Numerical convergence required in some cases up to 500 kicks/bunch (integration time step t=3x10–11 s)

ecloud much more benign than initial estimate

Limited investigation:

• Bunch spacings: tb=12.5, 25, 50, 75 ns

• Dipoles only

• Eb, Nb, z fixed according to FZ’s files “psplusetcparameters” and “lhcupgradeparameters

Example:

•PS2 (Eb=50 GeV) vs PS+ (Eb=75 GeV)

20

15

10

5

0

W/m

1.81.61.41.2delta_max

PS2, Eb=50 GeV tb=25 ns tb=50 ns tb=75 ns

PS+, Eb=75 GeV

tb=25 ns tb=50 ns tb=75 ns

tb [ns] 25 50 75

Nb [1011] 4 5.4 6.6

Nb depends on tb:






Assessment of ecloud buildup for LHC upgrade and injectors: conclusions

Heat load depends inversely with tb both for LHC and injectors:• tb=75 ns is best, closely followed by 50 ns• tb=50 ns much better than 25 ns• tb=12.5 ns is terrible

Cu (or Cu-coated) chamber much better than St.St.• But this conclusion is premised on a particular set of measurements of the

SE energy spectrum for St.St.• Need to re-measure energy spectrum in order to verify this conclusion

Not much difference in heat load between gaussian vs. flat longitudinal bunch profile for the LHC, at least for tb=50 ns

Not much difference between PS2 and PS+, nor between SPS50 and SPS+a50, except at high max for tb=25 ns

See my LUMI06 proceedings paper (not my PPT file) A full, definitive report is forthcoming Caveats:

• So far, heat load in the dipole bends only• z, Nb, … not independently exercised






Ecloud effects on the LHC beam:code WARP, QSM approx.

Recent example of emittance growth Assume e=1014 m–3

E=450 GeV, Nb=1.1x1011, single bunch• Code WARP, parallel, 3D calc.

Quasi-static approx. mode (QSM) AMR, parallel 8 processors

• Beam transfer maps from EC station to next Up to 6000 stations

• Actual LHC chamber shape• Constant focusing approx.• Conclusion: need to resolve to

reach convergence, as expected (ie., no. of EC stations > tune)

More studies to come• Actual optics, vary e, vary Nb, etc• Look at instability, not simply emitt.

gr., multibunch, more self-consistency• Benchmark against CERN results!

0.01

0.1

1

10

100

fractional emittance growth per turn

101

2 3 4 5 6

102

2 3 4 5 6

103

2 3 4 5 6

104

no. ecloud stations

Horiz. Vert.

WARP/QSM: LHC emittance growthconstant focusing, C=27 km, E=450 GeV,Nb=1e11, (nux,nuy)=(64.28,59.31)

ecloud density=1014 m

-3





are needed to see this picture.RHIC studies

Two CERN e– detectors installed in RHIC common-pipe region >1 yr ago• Inside a weak adjustable dipole magnet

Electron detectors & installation was not a LARP-funded effort (M. Jiménez, CERN VAC)

But LARP funds simulation & benchmarking activity• Simulations carried out thus far have been problematic

Partly due to code problems (now believed fixed) Partly due to complexity of two coexisting beams in common-pipe region

Detectors now interfaced to the RHIC control system (Eric Blum, BNL)

Improved power supply for the magnets No useful results at present

• I look forward to results in the near future






Towards “full self-consistency”: Lorentz-boosted frame method

“Fully self-consistency” (FSC)• Beam and ecloud affect each other• Beam-gas ionization, secondary electrons, lost protons striking wall, etc

This is a formidable problem• We’ll approach it step by step• In the end, probably use FSC only as spot-checks on simpler, faster calculations• But all necessary “modules” already in code WARP

Essential computational problem in ecloud: wide disparities of time scales needed to resolve e– motion, proton motion and lattice (eg., betatron wavelength)

Found that self-consistent calculation has similar cost than quasi-static mode if done in a Lorentz-boosted frame (with >>1), thanks to relativistic contraction/dilation bridging space/time scales disparities (J. L. Vay, with partial LARP support)

Computational complexity is not a Lorentz invariant (for certain problems)





are needed to see this picture.Boosted frame calculation sample:

proton bunch through a given e– cloudHose instability of a proton bunch

•b=500 in Lab• L= 5 km, continuous focusing

• Mag. field: B=kr • No chamber• Nb=1012

• e=1013 m–3

electron streamlinesbeam

proton bunch radius vs. zCPU time:

• lab frame: >2 weeks• frame with 2=512: <30 min

CPU time:• lab frame: >2 weeks• frame with 2=512: <30 min

Speedup x1000J.-L. Vay, PRL 98, 130405 (2007)






QuickTime™ and a decompressor


Courtesy J.-L. Vay

Boosted frame calculation sample:proton bunch through a given e– cloud






• Added complications:• moving boundary conditions• non-rectilinear moving frame (in curved trajectories)• sort out simultaneity of events for useful Lab frame diagnostics• …

• Need to be understood and implemented• Real-life simulation case not yet available• But clear indications of breakthrough in self-consistent simulations

Lorentz-boosted method: my concerns






Related developments (outside LARP scope, but synergistic)

ecloud at the FNAL MI upgrade (HINS effort)• Direct e– measurements with RFA at the MI (R. Zwaska)

Interesting effects at transition (shortest z)

• Also ecloud evidence at the TEVATRON (X. Zhang)

• In parallel, simulations at LBNL of: 1. ecloud build-up at MI: code POSINST

• Extensive (but still ongoing) studies Qualitative agreement w/ measurements Quantitative: there’s a fly in the ointment (to be understood)

2. Effects on the beam (emittance growth, single-bunch and multibunch inst.): code WARP

3. Microwave transmission technique (F. Caspers & T. Kroyer): code VORPAL• Hopefully new experiments at PEP-II will help to calibrate





are needed to see this picture.More related stuff

ECLOUD07 (Korea, last week)• Significant new effort reported from KEKB: dedicated SEY equipment

• SEY of metals condition down to max≈1 (Cu, StSt, TiN, TiZrV) When bombarded with 5 keV e– beam (dose: 0.01–1 C/cm2) “Graphitization” of the surface (XPS analysis) Also tested 500 eV e– beam bombardment; will redo at 100 eV Also measured ecloud conditioning by ecloud in the e+ beam

• Results slightly less favorable Synchrotron radiation spoils graphitization (consistent with PEP-II tests)

Some of these new results seem at odds with previous from CERN & SLAC Why do existing machines still have an ecloud problem?

• (they would not if max≈1)





are needed to see this picture.Status summary and future goals

1. Nominal LHC heat-load estimate and POSINST-ECLOUD benchmarking: (*) done

2. Upgraded LHC heat load: (*) done

3. Upgraded injector upgrade heat load: (*) done, but beam parameters not independently exercised (only certain SEY-related parameters varied)

4. Effects from ecloud on beam: (*) recent initial results, after substantial code development • 3D beam-ecloud self-consistent simulations: continuing• AMR, QSM, adaptive time stepping, parallelization: implemented in code• Development of Lorentz boosted frame method: proof of principle exists; needs more

developments for realistic applications

5. Effects of ionized gas on heat load and beam: not started

6. Analyze SPS data, esp. measured heat load and e– spectrum: (*) first set of results; need to benchmark against expts.

7. Help define optimal LHC conditioning scenario: (*) not started; delayed in favor of 2, 3 and 4. This is our intended next task this year in the area of ecloud buildup.

8. Apply Iriso-Peggs maps to LHC: (–) ongoing at low level; should it be deleted from LARP list?• Quick understanding of global ecloud parameter space, phase transitions

9. Simulate e-cloud for RHIC detectors and benchmark against measurements: (**) continuing

10. Simulate ecloud for LHC IR4 “pilot diagnostic bench:” not started

(–) no longer endorsed by CERN AP group

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





are needed to see this picture.Additional material





are needed to see this picture.Quasi-static mode (“QSM”)

1. 2-D slab of electrons (macroparticles) is stepped backward (with small time steps) through the frozen beam field • 2-D electron fields are stacked in a 3-D

array, 2. push 3-D proton beam (with large time steps)

using • maps - “WARP-QSM” - as in HEADTAIL (CERN) or • Leap-Frog - “WARP-QSL” - as in QUICKPIC

(UCLA/USC).

benddrift driftquad s0

lattice

2-D slab of electrons

3-D beams






Time (ms)

Em

ittan

ces

X/Y

(-m

m-m

rad

)

2 stations/turn

Benchmarking WARP-QSM vs. HEADTAIL

Time (ms)

Em

ittan

ces

X/Y

(-m

m-m

rad

)

1 station/turn

WARP-QSM X,YHEADTAIL X,Y

WARP-QSM X,YHEADTAIL X,Y

CERN code benchmarking website

electron cloud - status and plans

Documents

lhc beam

tb ns255075nb

cern results

ps2 eb

code problems

ns dipoles

3x1011 s ecloud

rhic commonpipe region