Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 LHC electron-cloud update Frank Zimmermann, AB/ABP cooling capacity & heat load instabilities & incoherent blow up vacuum pressure scrubbing Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 can e- cloud be problem for s.c. hadron ring? RHIC At 108 ns bunch spacing, electron cloud and electron-ion interactions cause transverse instabilities, emittance growth, and beam loss, along with vacuum pressure rise and background increase. Electron cloud effects occur both in the warm (30% of length) and cold (about 70%) regions. vacuum pressure rise measured e-flux warm cold evolution of longitudinal profile during beam loss near g t J. Wei Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 does scrubbing always help? can we predict e-cloud effects reliably? DAFNE 1997 prediction of e-cloud in DAFNE no evidence until 2003; DAFNE reached 2.5 A e+ current w/o problem since 2003, e+ current limited to 1.2 A in collision by strong instability (~10 ms rise time) with e-cloud signature (pressure rise, positive tune shift in e+ ring) w/o collision instability at 500 mA sensitive to orbit, bunch current, injection conditions, transverse emittance instability strongly increases along train rise time faster than synchrotron period grow-damp measurement 90 consecutive bunches + 20 bucket gap beam current = 500 mA bunches at the train end:75, 80, 85,90 A. Drago M. Zobov C. Vaccarezza Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 updated heat load & cooling capacity (1) “revised” cooling capacity at 4.6-20 K [L. Tavian, LTC 02.06.04; private communication 26.05.05] increase in cooling capacity at high current (>100%) (2) corrected dependence emax(q) in ECLOUD code [G. Bellodi / RAL, in preparation for HHH-2004] about 40% reduction in simulated heat load (3) SPS benchmarking: lowered e- reflectivity from R~1.0 to ~0.5 [D. Schulte, AB-ABP-RLC meeting 14.01.2005] about 50% additional reduction in heat load Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 (1) cooling capacity for e- cloud L. Tavian B. Jeanneret injection w/o contingency local limitation at top energy low luminosity w/o contingency high luminosity w 25% contingency old curve: February 2004 nominal gas pressure, separate cryoplant for rf cavities Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 (2) simulation model of emax (q) revision triggered by G. Bellodi / RAL for HHH-2004 bug not much & no clear data to support either model no certified dependence exists Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

dependence of secondary emission yield on impact angle q data from SLAC: R.E. Kirby, F.K. King, “Secondary Emission Yields from PEP-II Accelerator Materials”, NIM A 469, 2001 old model Copper - different surface finish and surface chemistry - large variation in behavior, CERN data not available new model Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 (3) SPS benchmarking, strip detector simulation Daniel Schulte two different bunch train spacings pressures (40 ntorr and 4 ntorr) surface conditions (dmax, R) and detector properties are uncertain constrain parameters by benchmarking multiple measurements change distance between trains & use relative measurements Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 flux: (1) ratio 1&2 trains, (2) two spacings, (3) absolute Daniel Schulte note: results sensitive to pressure, chamber geometry, etc., variation: dmax~1.4-1.3 R~0.1-0.7 three curves intersect at dmax=1.35, R=0.3; flux at later times (F=0.3 mA) dmax=1.2 was reached Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 example: average arc-cell heat load per meter for dmax=1.3, emax=235 eV LHC design report, R=1.0 corrected emax(q) BS cooling capacity R=1.0 0.5 “best 2005 estimate” February 2004 bunch intensity Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 arc heat load vs. intensity, 25 ns spacing, ‘best’ model R=0.5 dmax=1.7 dmax=1.5 dmax=1.3 dmax=1.1 dmax=1.3-1.4 suffices calculation for 1 train computational challenge! higher heat load for quadrupoles in 2nd train under study Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 heat load in COLDEX (model of LHC vacuum chamber) V. Baglin heat load - constant !? (possibly consistent with conditioned state) estimated SEY simulated heat load threshold at ~7x1010 p/bunch favored interpretation: very fast conditioning Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 instabilities & emittance growth multibunch instability – not expected to be a problem single-bunch instability – threshold r0~4x1011 m-3 at inj. incoherent emittance growth new understanding! (CERN-GSI collaboration) 2 mechanisms: periodic crossing of resonance due to e- tune shift and synchrotron motion (similar to halo generation from space charge) periodic crossing of linearly unstable region also due to synchrotron motion and strong focusing from electron cloud (for large tune shifts at bunch center) Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

effects of electron cloud on the beam emittance growth below & above e- density threshold “Transverse Mode Coupling Instability (TMCI)” for e- cloud (r > rthresh) re = 3 x 1011 m-3 Long term emittance growth (r < rthresh) re = 2 x 1011 m-3 re = 1 x 1011 m-3 LHC, Q’=0, at injection E. Benedetto Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 e- central density vs. Nb, 25 ns spacing, ‘best’ model R=0.5 dmax=1.7 dmax=1.5 typical “TMCI” instability threshold at injection dmax=1.3 dmax=1.1 calculation for 1 train challenge: how to go from dmax=1.7 to 1.3? scrubbing should be done at nominal Nb (stripes) Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 electrons are “pinched” during bunch passage high local density, high tune shift varying with x,y,z density (solid) tune shift (dashed) p rotation in e- phase space LHC at injection, r0=1012 m-3 E. Benedetto Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

incoherent ‘slow’ emittance growth two mechanisms: resonance crossing and trapping → halo growth linear motion may become unstable → core growth initial final halo growth Ts core growth Horizontal invariant vs. time of a proton at large synchrotron amplitude E. Benedetto, G. Franchetti CERN-GSI collaboration Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 poor lifetime in the SPS after scrubbing Courtesy G.Arduini at 26 GeV/c lifetime 10-20 minutes, decreasing along bunch train not a problem per se in SPS, but it would be in LHC at injection origin not understood Poor beam lifetime with LHC beam in the SPS on August 13, 2003 (can it be explained by electron cloud?) Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 poor lifetime in the SPS after scrubbing, cont’d bunch intensity in store e-cloud on shielded pick up J.M. Laurent, J.M. Jimenez, ~2002 E. Shaposhnikova et al., 11/11/04 two nominal batches at 26 GeV/c, 225 ns spacing between batches; both patterns are similar and show similar dependence on batch spacing Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 vacuum pressure with electron cloud e- flux@wall vs. intensity, 25 ns spacing, ‘best’ model dmax=1.7 dmax=1.5 dmax=1.3 dmax=1.1 R=0.5 17 hr running at 3 mA/m gives CO pressure corresponding to 100-hr beam lifetime (N. Hilleret, LHC MAC December 2004) Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 calculation for 1 batch

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 is “scrubbing” needed in LHC? still lacking experimental data, e.g., on emax(q) uncertainty in heat load prediction of factor ~2 also incomplete understanding of scrubbing (COLDEX data vs. prediction, RHIC, DAFNE) if dmax~1.3 reached in commissioning, no scrubbing is needed for heat load and fast instabilities pressure should be ok too according to N. Hilleret one concern: long-term emittance growth and poor lifetime (observed in SPS after scrubbing) we still believe we need to prepare a scrubbing strategy in case it turns out to be necessary to go to dmax~1.3 (e.g., tailor train spacings & train lengths at nominal bunch intensity) Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 stability limit at injection nominal filling pattern top energy dmax=1.7 nominal Nb dmax=1.5 dmax=1.3 dmax=1.1 the challenge is to decrease dmax to 1.3 with a stable beam Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005 summary updated cooling efficiency, modified simulations, & parameters from SPS benchmarking all lead to more optimistic heat load prediction; dmax~1.3 sufficient to reach nominal & ultimate (dmax~1.3 was obtained in SPS after ~1-2 days at 25-ns spacing) fast instabilities also under control for dmax~1.3 uncertainties: (1) LHC vacuum chamber is different from SPS; COLDEX either shows no conditioning or it conditions too fast to notice (2) RHIC experience (3) poor lifetime in SPS resembling e-cloud build up pattern (4) dynamic vacuum & detector background in LHC incoherent slow emittance growth remains concern we identified two mechanisms causing halo or core blow up: periodic crossing of resonance or unstable region may explain Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005

thanks to Gianluigi Arduini, Vincent Baglin, Giulia Bellodi, Elena Benedetto, Giuliano Franchetti, Noel Hilleret, Bernard Jeanneret, Miguel Jimenez, Laurent Tavian, Kazuhito Ohmi, Francesco Ruggiero, Daniel Schulte, Elena Shaposhnikova, and Jie Wei for important contributions & discussions & help Frank Zimmermann, Electron Cloud, LHC MAC 10. June 2005