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Electron Cloud - Status and Plans

Electron Cloud - Status and Plans

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Electron Cloud - Status and Plans

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  1. US LHC Accelerator Research Program bnl - fnal- lbnl - slac Electron Cloud - Status and Plans Miguel A. Furman LBNL Collaboration Mtg. FNAL, April 18-20, 2007 Electron Cloud - M. Furman

  2. 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 Electron Cloud - M. Furman

  3. 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 Dt=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, sz fixed according to FZ’s files “psplusetcparameters” and “lhcupgradeparameters • Example: • PS2 (Eb=50 GeV) vs PS+ (Eb=75 GeV) Nb depends on tb: Electron Cloud - M. Furman

  4. 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 dmax 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 • sz, Nb, … not independently exercised Electron Cloud - M. Furman

  5. Ecloud effects on the LHC beam:code WARP, QSM approx. • Recent example of emittance growth • Assume re=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 lb to reach convergence, as expected (ie., no. of EC stations > tune) • More studies to come • Actual optics, vary re, vary Nb, etc • Look at instability, not simply emitt. gr., multibunch, more self-consistency • Benchmark against CERN results! Electron Cloud - M. Furman

  6. 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 Electron Cloud - M. Furman

  7. 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) Electron Cloud - M. Furman

  8. Boosted frame calculation sample:proton bunch through a given e– cloud • Hose instability of a proton bunch • gb=500 in Lab • L= 5 km, continuous focusing • Mag. field: Bq=kr • No chamber • Nb=1012 • re=1013 m–3 electron streamlines beam proton bunch radius vs. z CPU time: • lab frame: >2 weeks • frame with 2=512: <30 min Speedup x1000 J.-L. Vay, PRL 98, 130405 (2007) Electron Cloud - M. Furman

  9. Boosted frame calculation sample:proton bunch through a given e– cloud Courtesy J.-L. Vay Electron Cloud - M. Furman

  10. Lorentz-boosted method: my concerns • 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 Electron Cloud - M. Furman

  11. 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 sz) • Also ecloud evidence at the TEVATRON (X. Zhang) • In parallel, simulations at LBNL of: • 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) • Effects on the beam (emittance growth, single-bunch and multibunch inst.): code WARP • Microwave transmission technique (F. Caspers & T. Kroyer): code VORPAL • Hopefully new experiments at PEP-II will help to calibrate Electron Cloud - M. Furman

  12. More related stuff • ECLOUD07 (Korea, last week) • Significant new effort reported from KEKB: dedicated SEY equipment • SEY of metals condition down to dmax≈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 dmax≈1) Electron Cloud - M. Furman

  13. Status summary and future goals • Nominal LHC heat-load estimate and POSINST-ECLOUD benchmarking: (*) done • Upgraded LHC heat load: (*) done • Upgraded injector upgrade heat load: (*) done, but beam parameters not independently exercised (only certain SEY-related parameters varied) • 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 • Effects of ionized gas on heat load and beam: not started • Analyze SPS data, esp. measured heat load and e– spectrum: (*) first set of results; need to benchmark against expts. • 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. • 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 • Simulate e-cloud for RHIC detectors and benchmark against measurements: (**) continuing • Simulate ecloud for LHC IR4 “pilot diagnostic bench:” not started (*) endorsed by CERN AP group (**) endorsed by CERN vacuum group (–) no longer endorsed by CERN AP group Electron Cloud - M. Furman

  14. Additional material Electron Cloud - M. Furman

  15. quad drift bend drift Quasi-static mode (“QSM”) 2-D slab of electrons 3-D beam s lattice s0 • 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, • 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). Electron Cloud - M. Furman

  16. WARP-QSM X,Y HEADTAIL X,Y WARP-QSM X,Y HEADTAIL X,Y Benchmarking WARP-QSM vs. HEADTAIL CERN code benchmarking website 1 station/turn Emittances X/Y (-mm-mrad) Time (ms) 2 stations/turn Emittances X/Y (-mm-mrad) Time (ms) Electron Cloud - M. Furman