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Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
Frank Zimmermann
introduction electron build up pressure rise heat load & scrubbing instabilities incoherent effects simulation needs
Overview of LHC Electron-Cloud Effects & Present Understanding
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 30.03.2006
blue: e-cloud effect observedred: planned accelerators
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- build up
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
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 30.03.2006
LHC strategy against electron cloud
1) warm sections (20% of circumference) coated by TiZrVgetter developed at CERN; low secondary emission; if cloud occurs, ionization by electrons (high cross section ~400 Mbarn) aids in pumping & pressure will even improve2) outer wall of beam screen (at 4-20 K, inside 1.9-K cold bore) will have a sawtooth surface (30 m over 500 m) 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 preventelectron impact on cold magnet bore4) rely on surface conditioning (‘scrubbing’); commissioning strategy; as a last resort doubling or triplingbunch spacing suppresses e-cloud heat load
uniquevacuum system!
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
R. Cimino, I. Collins, 2003; CERN-AB-2004-012
probability of elastic electron reflection seems to approach 1 forzero incident energy and is independent of *max
yield
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
data from SLAC: R.E. Kirby, F.K. King, “Secondary Emission Yields from PEP-II Accelerator Materials”, NIM A 469, 2001
dependence of secondary emission yield on impact angle
Copper -different surface
finish andsurface chemistry - large variation
in behavior, CERN data not
available
model
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
[M. Furman, 1997]
cos17.01
cos121exp
maxmax
maxmax
EE
2
0
0
EEEEEE
Eelastic
Present Model of Secondary Emission Yield ,,,, prediffusedpelasticptrueptot EEREE
secondary electrons consist of true secondaries and elastically reflected;since 2003 we assume that elastic reflection is independent of (no data)
sp
pptrue EEs
EEsE
)(/1/
,max
maxmax
[Kirby, 2001;Henrist, 2002;Furman, 1997]
true secondaries:
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 theheat load by 100%
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
R=1, R=1,
Illustration of present secondary-yield model
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- cloud diagnostics @ SPS
MBA chamber
Copper strips
Holes (transparency 7%)
Beam
B field
Collecting stripsBeam “pipe” (< 30 K)Thermal shielding (80 K)
cold strip detector Motor
Moving plate
RF contacts
variable aperture strip detector
shielded pick ups quadrupole strip detector COLDEX
+ WAMPAC1-4+ pick-up calor.+ SD1-2 + RGAs… J.M.Jimenez, V. Baglin,
N. Hilleret et al.
IcAIsA
M
e- GUN
e-
e-
e-
BeamSPS chamber
=Ic+Is
IcIbeam
= Ic
in-situ max
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
benchmarking ECLOUD code with SPS measurements
surface conditions (max, R) and detector properties are uncertain constrain parameters by benchmarking multiple measurements change distance between trains & use relative measurements
two differentbunch train spacings
two differentpressures(40 ntorrand 4 ntorr)
Daniel Schulte
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
three curves intersect at max=1.35, R=0.3;flux at later times (=0.3 mA) max=1.2 was reached
flux: (1) ratio 1&2 trains, (2) two spacings, (3) absolute
Daniel Schulte
note:resultssensitiveto pressure,chambergeometry,etc.,variation: max~1.4-1.3R~0.1-0.7
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
pressure rise
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
vacuum pressure rise
warm
cold
measured e-flux
pressure rise observationsRHIC
0
2
4
6
8
10
0.0E+00 2.0E+10 4.0E+10 6.0E+10 8.0E+10 1.0E+11Protons bunch intensity
P/P
Duty cycle ~ 45 %after 17h integratedtime of LHC beam
0
2
4
6
8
10
12
14
0.E+00 1.E+10 2.E+10 3.E+10 4.E+10 5.E+10 6.E+10 7.E+10 8.E+10 9.E+10 1.E+11Proton bunch intensity
P/P
Duty cycle ~ 45 %after 17h integratedtime of LHC beam
SPS
dipole field
no field
TEVATRON
threshold ~4x1010 ppbvacuum increase in most straights
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- flux@wall vs. intensity, 25 ns spacing, ‘best’ model
R=0.5
calculation for 1 batch
max=1.7
max=1.5
max=1.3max=1.1
vacuum pressure with electron cloud
17 hr running at 3 mA/m gives CO pressure corresponding to100-hr beam lifetime (N. Hilleret, LHC MAC December 2004)
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
' tot desorption yield
strongly bound molecules,varies with e- dose!, cleaning rate is a function ofmaterial, cleanliness, temperature
“recycling desorption yield”, varieswith surface coverage, pressure,sticking coefficient
' usually
'
'
SndtdA
SnCndtdnV
BS pumping speed
hole pumping
surfacecoverage
e- fluxVincent Baglin
Cn
in equilibrium:
Vincent Baglin;see W. Turner,PAC93
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
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.
BEAM SCREEN COVERAGE VERSUS ELECTRON DOSE
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
1.E+13 1.E+14 1.E+15 1.E+16 1.E+17 1.E+18 1.E+19 1.E+20 1.E+21
ELECTRON DOSE ( e -/cm2)
CO
VE
RA
GE
(mol
./cm
2 )
H2 COVCO COVCO2 COVCH4 COVH2 EQ COVCO EQ COVCO2 EQ COVCH4 EQ COV
300.01.0
E moy (eV):P (W/m/ap.):
1 monolayer = 1E15 molecules/cm2
N. Hilleret,LHC MAC Dec 2004
Information from Vincent Baglinpressure effects
1 min 17 hours
for 2E16 e/m/s i.e. 3 mA/m
max~1.3
1 hour
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
Pressure increase due to e-cloud. Level is a linear function of the electron flux. It depends only on the electron dose
GAS DENSITY VERSUS ELECTRON DOSE
1.E+12
1.E+13
1.E+14
1.E+15
1.E+16
1.E+17
1.E+13 1.E+14 1.E+15 1.E+16 1.E+17 1.E+18 1.E+19 1.E+20ELECTRON DOSE (e-/cm2)
GA
S D
EN
SIT
Y (
m-3
)
H2
CO
CO2
CH4
H2 EQ
CO EQ
CO2 EQ
CH4 EQ
300.01.0
E moy (eV):P (W/m/ap.):
for 2E16 e/m/s i.e. 3 mA/m
1 min 17 hours (assuming 2 stripes of 3 mm each)
N. Hilleret,LHC MAC Dec 2004
Information from Vincent Baglin
e- fluxdose
100-hr lifetime H2
100-hr lifetime CO2
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.
SDD
SP
a1
00
1 hour
shortest lifetime ~ 10 hr
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
heat load
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
arc heat load vs. intensity, 25 ns spacing, ‘best’ model
calculation for 1 train
R=0.5
computational challenge! higher heat load for quadrupolesin 2nd train under study
max=1.7
max=1.5
max=1.3max=1.1
max=1.3-1.4 suffices
BS cooling capacityinjection
low luminosityhigh luminosity
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
heat load in COLDEX (prototype LHC vacuum chamberin the SPS)
heat load - constant !?(possibly consistent with conditioned state)
threshold at ~7x1010
p/bunch
V. Baglin
favored interpretation:very fast conditioning (?)
estimated SEY
simulatedheat load
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
is “scrubbing” needed in LHC? still lacking experimental data, e.g., on max
uncertainty in heat load prediction of factor ~2 also incomplete understanding of scrubbing
(COLDEX data vs. prediction, RHIC, DAFNE) if max~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 necessaryto go to max~1.3 (e.g., tailor train spacings & train lengths at nominal bunch intensity)
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
max=1.7
max=1.5
max=1.3max=1.1
nominal Nb
stability limitat injection
the challenge is to decrease max to 1.3 with a stable beam
nominal filling patterntop energy
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
instabilities
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
INP Novosibirsk, 1965 Argonne ZGS,1965 BNL AGS, 1965
Bevatron, 1971 ISR, ~1972 PSR, 1988
AGS Booster, 1998/99 KEKB, 2000 CERN SPS, 2000
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
coupled-bunch instability
extrapolating instability threshold from SPS to LHC
electrons protons
crp
CB 2 SPS: 26 GeV/c, ~40 m; LHC: 450 GeV/c, ~100 m
→ CBI is ~7 times weaker in LHC
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
single-bunch “TMC” instabilityfast growth above e- densitythreshold; slower growth below
= 1 x 1011 m-3= 2 x 1011 m-3
= 3 x 1011 m-3
“Transverse Mode Coupling Instability (TMCI)” for e- cloud ( > thresh)
Long term emittance growth ( < thresh)
E. Benedetto LHC, Q’=0,at injection
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
HCrQ
py
sthre
12,
Nyy
zeb
Nyy
zebyezy
rNrNc
H,,
,
24
24141
ebzy
Ny
p
sthre rNCr
Q
,,
22
estimate ofthreshold density
pinch enhancement
assume only vertical pinchsecond term is much larger
→synchrotron tunechanges if z and ||
are held constant
→
TMCI e- threshold density
2
,||,2
2
1
zrev
Nrmsc
s mfQ
12
4
1,
22
,||,2
,ebzy
Ny
pzrev
Nrmse
thre rNCrmf
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
attempt at extrapolating TMCI threshold from SPS to LHC using analytical estimate
SPS: C~6900 m, z~0.3 m, ~40 m, 26 GeV/c, c~1.8x10-3
LHC: C~26700 m, z~0.011 m, ~100 m, 450 GeV/c, c~3.2x10-4
2/12/12/5
2/1
z
cthr C
→ threshold LHC ~ 1/3 threshold SPS
→ threshold LHC ~ 1/2 threshold SPSwithout pinch enhancement factor H:
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
simulated emittance growth vs. electron density
no field
dipole
no field
dipole
SPS 26 GeV/c LHC 450 GeV/c
rise time ~1/s rise time ~1/s
rise time ~1/s rise time ~1/s
threshold LHC ~ SPS
E. Benedetto
fast growthslow growth
HEADTAILsimulations
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
incoherent effects
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
KEKB: Emittance increase with current below threshold & reduced luminosity w/o instability sidebands
RHIC: transverse instabilities, emittance growth, and beam loss, especially near t
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.
Experimental Indications
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
KEKB e+ beam blow up, 2000(H. Fukuma, et al.)
threshold of fastvertical blow up
slow growthbelow threshold?
beam current
IP spot size
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
Fourier power spectrum of KEKB BPM data
• LER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing• Solenoids off: beam size increased from 60 m ->283 m at 400 mA• No excitation
V. Tune Sideband Peak
J. Flanagan et al.
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
KEKB Sidebands and Spec. Lum.
• 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 Peak Height
Specific Luminosity
4-bucket spacing
2-bucket spacing
SidebandThreshold
J. Flanagan et al.
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
evolution of longitudinal profile during beam loss near t
RHIC beam loss at transition
J. Wei
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
TEVATRON
emittance growth >34 mm mrad/hr(> 100% hr); beam lifetime ~24 hr(normally ~1000 hr)
vertical emittance vs. time vertical Schottky power vs. time
Schottky power -12 dBm(normal instability signals between0 and 10 dBm)
X.L. Zhang et al.
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
poor lifetime in the SPS after scrubbing
Poor beam lifetime with LHC beam in the SPS on August 13, 2003 (can it be explained by electron cloud?)
Courtesy G.Arduini
at 26 GeV/clifetime 10-20 minutes, decreasingalong bunch train
not a problem per sein SPS, but it would bein LHC at injection
origin not understood
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
poor lifetime in the SPS after scrubbing, cont’d
two nominal batches at 26 GeV/c,225 ns spacing between batches;
bunch intensity in store e- cloud signal
LHC beam signal
e-cloud on shielded pick up
J.M. Laurent, J.M. Jimenez, ~2002
E. Shaposhnikova et al., 11/11/04
both patterns are similar and show similar dependence on batch spacing
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
typical“TMCI”instabilitythresholdat injection
e- central density vs. Nb, 25 ns spacing
R=0.5
calculation for 1 train
max=1.7
max=1.5
max=1.3 max=1.1
challenge: how to go from max=1.7 to 1.3?scrubbing should be done at nominal Nb (stripes)
ECLOUDsimulation
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
simulated e- density evolution during a bunch passage in an LHC field-free region on log scale
E. Benedetto
HEADTAILcode
bunch tail
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- density on horizontal axis at different time stepsduring a bunch passage, for the LHC
E. Benedetto
high local density, high tune shift, varying with x,y,z
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
average e- density inside circle of variable radius
E. Benedetto
HEADTAILcode
bunch tail
high local density, high tune shift, varying with x,y,z
rotationin e- phasespace
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
tune footprint obtained by tracking through a frozen e- potential at z=+2z by a frequency-map analysis of HEADTAIL simulation
E. Benedetto,Y. Papaphilippou
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- distribution in dipole measured by SPS strip detector
approximation forHEADTAIL code
E. Benedetto
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e- density evolution in a dipole field
SPS LHC
x
y
E. Benedetto
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
evolution of on-axis e- density for the SPS
E. Benedetto
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
simulated emittance growth for 1 and 10 e-beam Interaction points per turn with & w/o synchrotron motion
E. Benedetto
HEADTAILcode
=2x1011 m-3
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
horizontal invariant of a proton vs. turn number
G. Franchetti,E. Benedetto
HEADTAILcode
two mechanisms: • resonance crossing and trapping → halo growth• linear motion may become unstable → core growth
Ts
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
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 30.03.2006
vertical phase space and frequency spectrum of particlemotion at different z positions along the bunch
E. Benedetto,G. Franchetti
singleinteractionpoint=1014 m-3 linear
instability,hyperbolicfix point
chaoticmotion
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
simulation needs
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
e-cloud build up code
e-cloud SB/CB instabilitycode
self-consistent code
optics codee.g., MADX
beam sizesapertures, B fields, … cloud density,
local growth rates,around the ring or‘from DR to IP’ (M. Pivi),“ECLOUD TWISS TABLE”, incl. 3D e- motion
wake/impedancecode, e.g., HFSS,MAFIA, GdfidL
E(x,t), B(x,t) ‘ecloud wake’,generalizedimpedance
ion codevacuum code
ionizationE(x,t)
ion desorption,ionization
electrondesorption,scrubbing,“e- pumping”
regularinstabilitycode beam-
beamcode
s.c.code
e
beam motion,losses
future ‘complete’ e-cloud simulation?CARE-HHH-2004
Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006
summary & conclusions simulations based on SPS benchmarking lead to optimistic heat load prediction; max~1.3 sufficient to reach nominal & ultimate (max~1.3 was obtained in SPS after ~1-2 days at 25-ns spacing) fast instabilities also under control for max~1.3
~5x1011 m-3, but slow 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 concernwe 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 30.03.2006
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