Overview of LHC Electron-Cloud Effects

Overview of LHC Electron-Cloud Effects
& Present Understanding Frank Zimmermann introduction electron build up pressure rise heat load & scrubbing instabilities incoherent effects simulation needs Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
introduction CERN ISR (70s) & KEK PF (late 80s) experience → 1997: 1st LHC ECLOUD simulation, crash program 1999: e- cloud seen with LHC beam in SPS, PS & even PS-SPS transfer line 1999: e- cloud at both B factories ~2002: e- cloud at RHIC & Tevatron → observed in all proton rings with LHC-like parameters (though for 1/5 LHC bunch charge or 10x bunch spacing) 2004: DAFNE, 2006: CESR ?: SNS & J-PARC truly astonishing if this problem will not occur in LHC Frank Zimmermann, LHC Electron Cloud, GSI Meeting

blue: e-cloud effect observed red: planned accelerators Frank Zimmermann, LHC Electron Cloud, GSI Meeting

e- build up Frank Zimmermann, LHC Electron Cloud, GSI Meeting

schematic of e- cloud build up in LHC arc beam pipe, due to photoemission and secondary emission [F. Ruggiero] Frank Zimmermann, LHC Electron Cloud, GSI Meeting

LHC strategy against electron cloud 1) warm sections (20% of circumference) coated by TiZrV getter developed at CERN; low secondary emission; if cloud occurs, ionization by electrons (high cross section ~400 Mbarn) aids in pumping & pressure will even improve 2) outer wall of beam screen (at 4-20 K, inside 1.9-K cold bore) will have a sawtooth surface (30 mm over 500 mm) to reduce photon reflectivity to ~2% so that photoelectrons are only emitted from outer wall & confined by dipole field 3) pumping slots in beam screen are shielded to prevent electron impact on cold magnet bore unique vacuum system! 4) rely on surface conditioning (‘scrubbing’); commissioning strategy; as a last resort doubling or tripling bunch spacing suppresses e-cloud heat load Frank Zimmermann, LHC Electron Cloud, GSI Meeting

R. Cimino, I. Collins, 2003; CERN-AB yield probability of elastic electron reflection seems to approach 1 for zero incident energy and is independent of d*max Frank Zimmermann, LHC Electron Cloud, GSI Meeting

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 Copper - different surface finish and surface chemistry - large variation in behavior, CERN data not available model Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Present Model of Secondary Emission Yield secondary electrons consist of true secondaries and elastically reflected; since 2003 we assume that elastic reflection is independent of Q (no data) true secondaries: [M. Furman, 1997] [Kirby, 2001; Henrist, 2002; Furman, 1997] elastic reflection: [Cimino, Collins, et al., 2003] this quantum-mechanical formula fits the data well for E0~150 eV M. Furman includes rediffused electrons and finds that they increase the heat load by 100% Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Illustration of present secondary-yield model R=1, R=1, Frank Zimmermann, LHC Electron Cloud, GSI Meeting

e- cloud diagnostics @ SPS
variable aperture strip detector cold strip detector Motor Moving plate RF contacts Collecting strips Beam “pipe” (< 30 K) Thermal shielding (80 K) quadrupole strip detector COLDEX shielded pick ups in-site dmax + WAMPAC1-4 + pick-up calor. + SD1-2 + RGAs… J.M.Jimenez, V. Baglin, N. Hilleret et al. Frank Zimmermann, LHC Electron Cloud, GSI Meeting

benchmarking ECLOUD code with SPS measurements Daniel Schulte two different bunch train spacings pressures (40 ntorr and 4 ntorr) ECLOUD simulation surface conditions (dmax, R) and detector properties are uncertain constrain parameters by benchmarking multiple measurements change distance between trains & use relative measurements Frank Zimmermann, LHC Electron Cloud, GSI Meeting

flux: (1) ratio 1&2 trains, (2) two spacings, (3) absolute Daniel Schulte ECLOUD simulation note: results sensitive to pressure, chamber geometry, etc., variation: dmax~ R~ 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, LHC Electron Cloud, GSI Meeting

pressure rise Frank Zimmermann, LHC Electron Cloud, GSI Meeting

pressure rise observations RHIC SPS vacuum pressure rise warm cold measured e-flux no field dipole field TEVATRON threshold ~4x1010 ppb vacuum increase in most straights Frank Zimmermann, LHC Electron Cloud, GSI Meeting

vacuum pressure with electron cloud e- vs. intensity, 25 ns spacing, ‘best’ model ECLOUD simulation 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, LHC Electron Cloud, GSI Meeting calculation for 1 batch

Vincent Baglin; see W. Turner, PAC93 desorption yield “recycling desorption yield”, varies with surface coverage, pressure, sticking coefficient strongly bound molecules, varies with e- dose!, cleaning rate is a function of material, cleanliness, temperature usually in equilibrium: hole pumping BS pumping speed Vincent Baglin surface coverage e- flux Frank Zimmermann, LHC Electron Cloud, GSI Meeting

pressure effects 17 hours 1 hour 1 min Information from Vincent Baglin
AT-VAC (V.Baglin, N.H.) has simulated the LHC pressure evolution. According to Noel’s lab measurement, for E> 30 eV, the e- recycling yield is large. Therefore, under electron bombardment the BS will have a bare surface without any monolayers. Monolayers will be only on the cold bore. 1 monolayer = 1E15 molecules/cm2 for 2E16 e/m/s i.e. 3 mA/m dmax~1.3 N. Hilleret, LHC MAC Dec 2004 17 hours 1 hour 1 min Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Information from Vincent Baglin Pressure increase due to e-cloud. Level is a linear function of the electron flux. It depends only on the electron dose for 2E16 e/m/s i.e. 3 mA/m 100-hr lifetime H2 100-hr lifetime CO2 e- flux dose h and cleaning rate a depend on the e- energy; if the energy decreases from 300 eV down to 100 eV, the eta decreases by a factor 3, similarly, the cleaning rate decrease as well. V.B. expects the pressure of Noel's plot will be about the same for 300 eV or 100 eV. 1 hour N. Hilleret, LHC MAC Dec 2004 1 min 17 hours (assuming 2 stripes of 3 mm each) Frank Zimmermann, LHC Electron Cloud, GSI Meeting shortest lifetime ~ 10 hr

heat load Frank Zimmermann, LHC Electron Cloud, GSI Meeting

arc heat load vs. intensity, 25 ns spacing, ‘best’ model R=0.5 ECLOUD simulation dmax=1.7 dmax=1.5 BS cooling capacity injection low luminosity dmax=1.3 high luminosity dmax=1.1 dmax= suffices calculation for 1 train computational challenge! higher heat load for quadrupoles in 2nd train under study Frank Zimmermann, LHC Electron Cloud, GSI Meeting

heat load in COLDEX (prototype LHC vacuum chamber in the SPS) 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, LHC Electron Cloud, GSI Meeting

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, LHC Electron Cloud, GSI Meeting

stability limit at injection nominal filling pattern top energy dmax=1.7 nominal Nb dmax=1.5 dmax=1.3 ECLOUD simulation dmax=1.1 the challenge is to decrease dmax to 1.3 with a stable beam Frank Zimmermann, LHC Electron Cloud, GSI Meeting

instabilities Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Argonne ZGS,1965 INP Novosibirsk, 1965 BNL AGS, 1965 Bevatron, 1971 ISR, ~1972 PSR, 1988 AGS Booster, 1998/99 CERN SPS, 2000 KEKB, 2000 Frank Zimmermann, LHC Electron Cloud, GSI Meeting

coupled-bunch instability
electrons protons extrapolating instability threshold from SPS to LHC SPS: 26 GeV/c, b~40 m; LHC: 450 GeV/c, b~100 m → CBI is ~7 times weaker in LHC Frank Zimmermann, LHC Electron Cloud, GSI Meeting

single-bunch “TMC” instability
fast e growth above e- density threshold; slower e growth below “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, LHC Electron Cloud, GSI Meeting

TMCI e- threshold density estimate of threshold density pinch enhancement assume only vertical pinch second term is much larger → synchrotron tune changes if sz and e|| are held constant → Frank Zimmermann, LHC Electron Cloud, GSI Meeting

attempt at extrapolating TMCI threshold from SPS to LHC using analytical estimate SPS: C~6900 m, sz~0.3 m, b~40 m, 26 GeV/c, ac~1.8x10-3 LHC: C~26700 m, sz~0.011 m, b~100 m, 450 GeV/c, ac~3.2x10-4 → threshold LHC ~ 1/3 threshold SPS without pinch enhancement factor H: → threshold LHC ~ 1/2 threshold SPS Frank Zimmermann, LHC Electron Cloud, GSI Meeting

simulated emittance growth vs. electron density HEADTAIL simulations E. Benedetto fast growth slow growth no field no field rise time ~1/s rise time ~1/s SPS 26 GeV/c LHC 450 GeV/c dipole dipole rise time ~1/s rise time ~1/s threshold LHC ~ SPS Frank Zimmermann, LHC Electron Cloud, GSI Meeting

incoherent effects Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Experimental Indications KEKB: Emittance increase with current below threshold & reduced luminosity w/o instability sidebands RHIC: transverse instabilities, emittance growth, and beam loss, especially near gt SPS: Poor beam lifetime & bunch-length shrinking after scrubbing TEVATRON: Fast beam emittance growth and short beam lifetime observed simultaneously with the ECE pressure rise. But little coherent motion seen on Schottky monitor. Longitudinal quadrupole oscillation. Frank Zimmermann, LHC Electron Cloud, GSI Meeting

KEKB e+ beam blow up, 2000 (H. Fukuma, et al.) IP spot size threshold of fast vertical blow up slow growth below threshold? beam current Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Fourier power spectrum of KEKB BPM data
V. Tune Sideband Peak LER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing Solenoids off: beam size increased from 60 mm ->283 mm at 400 mA No excitation J. Flanagan et al. Frank Zimmermann, LHC Electron Cloud, GSI Meeting

KEKB Sidebands and Spec. Lum.
Sideband Peak Height Sidebands disappear at around a bunch current of 0.8 mA. Specific luminosity of 2-bucket and 4-bucket spacing bunches do not merge at that point, however. Possible indication of the presence of an incoherent component below the sideband threshold (non-linear focusing by cloud leading to non-Gaussian beam tails, e.g.) Sideband Threshold Specific Luminosity 4-bucket spacing 2-bucket spacing J. Flanagan et al. Frank Zimmermann, LHC Electron Cloud, GSI Meeting

RHIC beam loss at transition evolution of longitudinal profile during beam loss near g t J. Wei Frank Zimmermann, LHC Electron Cloud, GSI Meeting

TEVATRON vertical emittance vs. time vertical Schottky power vs. time Schottky power -12 dBm (normal instability signals between 0 and 10 dBm) emittance growth >34 p mm mrad/hr (> 100% hr); beam lifetime ~24 hr (normally ~1000 hr) X.L. Zhang et al. Frank Zimmermann, LHC Electron Cloud, GSI Meeting

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, LHC Electron Cloud, GSI Meeting

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, LHC Electron Cloud, GSI Meeting

e- central density vs. Nb, 25 ns spacing R=0.5 ECLOUD simulation 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, LHC Electron Cloud, GSI Meeting

simulated e- density evolution during a bunch passage in an LHC field-free region on log scale HEADTAIL code bunch tail E. Benedetto Frank Zimmermann, LHC Electron Cloud, GSI Meeting

e- density on horizontal axis at different time steps during a bunch passage, for the LHC high local density, high tune shift, varying with x,y,z E. Benedetto Frank Zimmermann, LHC Electron Cloud, GSI Meeting

average e- density inside circle of variable radius high local density, high tune shift, varying with x,y,z HEADTAIL code p rotation in e- phase space bunch tail E. Benedetto Frank Zimmermann, LHC Electron Cloud, GSI Meeting

tune footprint obtained by tracking through a frozen e- potential at z=+2sz by a frequency-map analysis of HEADTAIL simulation E. Benedetto, Y. Papaphilippou Frank Zimmermann, LHC Electron Cloud, GSI Meeting

e- distribution in dipole measured by SPS strip detector approximation for HEADTAIL code E. Benedetto Frank Zimmermann, LHC Electron Cloud, GSI Meeting

e- density evolution in a dipole field x SPS LHC y Frank Zimmermann, LHC Electron Cloud, GSI Meeting E. Benedetto

evolution of on-axis e- density for the SPS r/r0 Frank Zimmermann, LHC Electron Cloud, GSI Meeting E. Benedetto

simulated emittance growth for 1 and 10 e-beam Interaction points per turn with & w/o synchrotron motion HEADTAIL code r=2x1011 m-3 E. Benedetto Frank Zimmermann, LHC Electron Cloud, GSI Meeting

horizontal invariant of a proton vs. turn number HEADTAIL code G. Franchetti, E. Benedetto Ts two mechanisms: resonance crossing and trapping → halo growth linear motion may become unstable → core growth Frank Zimmermann, LHC Electron Cloud, GSI Meeting

incoherent emittance growth due to e- cloud simulated either by HEADTAIL (weak-strong mode) or by analytical field model G. Franchetti, E. Benedetto → emittance growth is not a numerical artifact → analytical model allows accessing longer time scale Frank Zimmermann, LHC Electron Cloud, GSI Meeting

vertical phase space and frequency spectrum of particle motion at different z positions along the bunch E. Benedetto, G. Franchetti single interaction point r=1014 m-3 linear instability, hyperbolic fix point chaotic motion Frank Zimmermann, LHC Electron Cloud, GSI Meeting

simulation needs Frank Zimmermann, LHC Electron Cloud, GSI Meeting

ion desorption, ionization regular instability code vacuum code ion code electron desorption, scrubbing, “e- pumping” beam- beam code ionization E(x,t) self-consistent code e-cloud build up code re e-cloud SB/ CB instability code s.c. code beam motion, losses E(x,t), B(x,t) beam sizes apertures, B fields, … ‘ecloud wake’, generalized impedance cloud density, local growth rates, around the ring or ‘from DR to IP’ (M. Pivi), “ECLOUD TWISS TABLE”, incl. 3D e- motion wake/impedance code, e.g., HFSS, MAFIA, GdfidL optics code e.g., MADX future ‘complete’ e-cloud simulation? CARE-HHH-2004 Frank Zimmermann, LHC Electron Cloud, GSI Meeting

summary & conclusions simulations based on SPS benchmarking lead to 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 r~5x1011 m-3, but slow e growth <1%/s !?! uncertainties: (1) LHC vacuum chamber is different from SPS; COLDEX either shows no conditioning or it conditions too fast to notice (2) RHIC, Tevatron & KEKB 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, LHC Electron Cloud, GSI Meeting

thanks to Gianluigi Arduini, Vincent Baglin, Giulia Bellodi,
Elena Benedetto, Giuliano Franchetti, Noel Hilleret, Bernard Jeanneret, Miguel Jimenez, Laurent Tavian, Kazuhito Ohmi, Francesco Ruggiero, Giovanni Rumolo, Daniel Schulte, Elena Shaposhnikova, Jie Wei, and Xiaolong Zhang for important contributions & discussions & help Frank Zimmermann, LHC Electron Cloud, GSI Meeting

Overview of LHC Electron-Cloud Effects

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