View
18
Download
2
Category
Preview:
DESCRIPTION
Electron-Cloud Build-up in the FNAL Main Injector and the LHC Complex. Miguel Furman LBNL ECLOUD07 Daegu, April 9-12, 2007. Outline. Motivation POSINST code features Initial results Ongoing work Conclusions. My gratitude to: - PowerPoint PPT Presentation
Citation preview
M. Furman, “ecloud at the MI and LHC” p. 1ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Electron-Cloud Build-up in the FNAL Main Injector and the LHC Complex
Miguel Furman
LBNL
ECLOUD07
Daegu, April 9-12, 2007
M. Furman, “ecloud at the MI and LHC” p. 2ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Outline
• Motivation• POSINST code features• Initial results• Ongoing work• Conclusions
My gratitude to:
A. Adelmann, G. Arduini, V. Baglin, M. Blaskiewicz, O. Brüning, Y. H. Cai, C. Celata, R. Cimino, R. Cohen, I. Collins, F. J. Decker, A. Friedman, O. Gröbner, K. Harkay, P. He, S. Heifets, N. Hilleret, U. Iriso, J. M. Jiménez, R. Kirby, M. Kireef-Covo, G. Lambertson, R. Macek, A. Molvik, K. Ohmi, S. Peggs, M. Pivi, C. Prior, A. Rossi, G. Rumolo, D. Schulte, K. Sonnad, P. Stoltz, J.-L. Vay, M. Venturini, S. Y. Zhang, X. Zhang, A. Zholents, F. Zimmermann and R. Zwaska.
M. Furman, “ecloud at the MI and LHC” p. 3ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.ecloud at FNAL: background
• Proposed High Intensity Neutrino Source (HINS)— MI upgrade:
• Increase bunch intensity Nb from ~6e10 to ~3e11
• RFA electron detectors installed (one in the MI and one in the Tevatron)
— See R. Zwaska’s talk (session B)
• We’ve been simulating ecloud effects at the MI for >~1 yr— Goal: assess ecloud effects on the operation
— ecloud build-up (this talk)
— ecloud effects on the beam
— simulations of microwave transmission through ecloud (Caspers-Kroyer diagnostic technique)
— see Kiran Sonnad’s talks (sessions D & E)
M. Furman, “ecloud at the MI and LHC” p. 4ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.“POSINST” simulation code features
• Code development started ~1994 (PEP-II design stage)—essential contributions by M. Pivi since 2000—this is a “build-up type” code
• Formation of an ecloud by a prescribed (non-dynamical) beam
—Based on Ohmi’s original simulation approach—Similar to other codes (e.g., “ECLOUD”, …)—2D—incorporates a detailed model of SEE
• both SE yield (E0) and SE emission energy spectrum d/dE
—incorporates approximate models of primary electron emission —validated against measurements at APS and PSR (~2000)
• good agreement with RFA measurements • required peak SEY ~2 both for PSR and APS
• SEY is an essential ingredient in most cases; however:— many SEY parameters not well known— can trade off one for another
M. Furman, “ecloud at the MI and LHC” p. 5ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Initial results
• Preliminary assessment for MI upgrade: —Uniform fill (504 bunches out of 588
buckets)—Injection energy (K.E.=8 GeV)
—Bunch population Nb=(6–30)x1010
—Elliptical chamber cross-section (~2:1)—Field-free or dipole bending magnet
• Conclusions:— Sharp threshold at Nb~1.25x1011 for max=1.3
— above threshold: EC ~neutralizes beam
— ~ 0.06 (assuming uniform EC density around the ring)
• The assumed value max=1.3 was a first step
Nb below thr.
Nb above thr.
M. Furman, LBNL-57634/FERMILAB-PUB-05-258-AD
M. Furman, “ecloud at the MI and LHC” p. 6ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Initial results: z dependence
• Lower de for shorter bunches
• Possibly due to higher electron-wall impact energy
aver. de
1- de
e– flux at wall
e– energy
SEY
M. Furman, “ecloud at the MI and LHC” p. 7ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Recent simulations at RFA location
• MI ramp: KEb=8120 GeV in ~0.9 s (~100,000 turns)• Transition at t~0.2 s (KEb~20 GeV)
• train=(82 H) + 5x(82 L) + gaps,
Nb=10.3x1010 for H
Nb=5.7x1010 for L
• RFA detector location: field-free region • We typically simulate only one turn• CPU~3.3 hrs (Mac G5, 1.8 GHz)
0.0020
0.0015
0.0010
0.0005
0.0000
nC/m
11x10-6109876543210
time [s]
av. line density beamsignal (arb. units)
MI, K=20 GeV, Tb=1 ns, 1 revolutionpeak SEY=1.3
(a)
max=1.3KEb=20 GeV
line density vs. time
M. Furman, “ecloud at the MI and LHC” p. 8ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Recent simulations: 1-turn averages
• From Bob Zwaska’s e– detector observations, infer e– flux ~1 A/m2 at transition— this assumes 30% area efficiency and 100% e– energy efficiency
• Then these simulations imply max >~ 1.3–1.4• But direct measurements of chamber samples by R. Kirby show max~ 2 (R.
Zwaska, session B)• Caveats:
— Several variables not yet adequately investigated — Ongoing work; need to reconcile simulations and measurements
1.0x1012
0.8
0.6
0.4
0.2
0.0
m**-3
2.01.81.61.41.21.0
peak SEY
K=8 GeV, Tb=8 ns K=8 GeV, Tb=6 ns K=20 GeV, Tb=1 ns K=20 GeV, Tb=0.75 ns K=30 GeV, Tb=1.8 ns K=30 GeV, Tb=1.5 ns
aver. beam neutr.=6e11 m**-3
(a)
0.20
0.15
0.10
0.05
0.00
A/m**2
2.01.81.61.41.21.0
peak SEY
K=8 GeV, Tb=8 ns K=8 GeV, Tb=6 ns K=20 GeV, Tb=1 ns K=20 GeV, Tb=0.75 ns K=30 GeV, Tb=1.8 ns K=30 GeV, Tb=1.5 ns
(b)e– density vs. maxe– wall flux vs. max
M. Furman, “ecloud at the MI and LHC” p. 9ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Discussion
• Other simulation exercises carried out:—Time development of ecloud—Dependence on z, Nb and max but not in all combinations—Sensitivity to SE energy spectrum—Dependence on transverse beam size —Simulation parameters (e.g., t=1.4x10–11 s, # of
macroparticles=20,000,…)• Incidentally, find empirical relation between e– flux at the wall Je
and e– aver. line density e: — Je=e, where =6x107 m–1 s–1
• Fairly robust (independent of max, z and Eb; even valid during the build-up stage, but not tested against all possible parameter variations)
M. Furman, “ecloud at the MI and LHC” p. 10ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Conclusions
• Extensive (but still ongoing) build-up simulations of the MI• If interpret RFA measurements with these simulations, conclude that max~1.3–1.4; then
de~(1–10)x1010 m–3
• Even if RFA detector is seeing only 10% of the incident electrons, would conclude that max~1.4–1.5
• But direct chamber sample measurements show max~2— This is a significant discrepancy!— Need to reconcile simulations and measurements
• Simulations results qualitatively stable against several simulation conditions— eg., Emax, SE spectrum composition, no. of macroparticles, t,…
• Not yet done, or partially done:— Sensitivity to (0) (thus far, assumed (0)=0.3xmax)
• NB: if (0) is assumed higher, then would conclude that max is lower
— Further sensitivity to SE spectrum composition (elastics, rediffused, true secondaries)
— Clarify simulation issues at high max:• appearance of “virtual cathodes” near the wall• dependence of SEY on space-charge forces (no such dependence in POSINST)
• Ultimate goal: assess effects on the beam (see K. Sonnad’s talk session E)
M. Furman, “ecloud at the MI and LHC” p. 11ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.References
M. A. Furman, "A preliminary assessment of the electron cloud effect for the FNAL main injector upgrade," LBNL-57634/CBP-Note-712/FERMILAB-PUB-05-258-AD, June 28, 2005. Revised: June 26, 2006. An abbreviated version is published in: New Journal of Physics Focus Issue: Accelerator and Beam Physics, New J. Phys. 8 (2006) 279, http://stacks.iop.org/1367-2630/8/279
M. A. Furman, "Studies of e-cloud build up for the FNAL main injector and for the LHC," LBNL-60512/CBP Note-736, June 15, 2006, Proc. 39th ICFA Advanced Beam Dynamics Workshop on High Intensity High Brightness Hadron Beams "HB2006" (Tsukuba, Japan, May 29-June 2nd, 2006), paper TUAX05. http://hb2006.kek.jp/
M. A. Furman, "HINS R&D Collaboration on Electron Cloud Effects: Midyear Progress Report," CBP-Technote-364/FERMILAB-TM-2369-AD, 22 September 2006.
M. A. Furman, K. Sonnad and J.-L. Vay, "HINS R&D Collaboration on Electron Cloud Effects: Midyear Report," LBNL-61921/CBP-761/FERMILAB-TM-2370-AD, Nov. 7, 2006.
M. A. Furman, "HINS R&D Collaboration on Electron Cloud Effects: MI ecloud build-up simulations at the electron detector location," CBP Technote-367, Dec. 5, 2006.
Kiran G. Sonnad, Miguel A. Furman and Jean-Luc Vay, "A preliminary report on electron cloud effects on beam dynamics for the FNAL main injector upgrade," CBP Technote-369, January 16, 2007.
M. Furman, “ecloud at the MI and LHC” p. 12ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Backup material
M. Furman, “ecloud at the MI and LHC” p. 13ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Electron-wall energy spectrum
0.004
0.003
0.002
0.001
0.000
[A/(m**2*eV)]
5004003002001000
electron-wall impact energy [eV]
wcek0h=(1/sarea)*dIwall/dE0
MI, field free
max=1.7, KE=20 GeV, z=0.06 m
M. Furman, “ecloud at the MI and LHC” p. 14ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Three components of secondary emission:sample spectrum at E0=300 eV
from M. F. and M. Pivi, PRST-AB 5, 124404 (2002)
E0
E
M. Furman, “ecloud at the MI and LHC” p. 15ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Secondary emission spectrum
• Depends on material and state of conditioning
—St. St. sample, E0=300 eV, normal incidence, (Kirby-King,
NIMPR A469, 1 (2001))
0.08
0.06
0.04
0.02
0.00300250200150100500
Secondary electron energy [eV]
Secondary energy spectrum St. St., E0=300 eV, normal incidence
true secondaries(area[0,50]=1.17)
backscattered(area[295,305]=0.12)
rediffused(area[50,295]=0.75)
st. steel sample= 2.04e = 6%r = 37%ts =57%
e+r =43%
– Hilleret’s group CERN: Baglin et al, CERN-LHC-PR 472. – Other measurements: Cimino and Collins, 2003)
Cu sample= 2.05e = 1%r = 9%ts =90%
e+r =10%
M. Furman, “ecloud at the MI and LHC” p. 16ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
6
5
4
3
2
1
0
heat load [W/m]
2.0x10111.61.20.80.40.0
Nb
dmax=1.3, NR; LTC40 dmax=1.5, NR; LTC40 dmax=1.7, NR; LTC40 ACC at high L w 25% cont. ACC at low L w/o cont.
Sample simulated LHC heat load vs. Nbarc dipole, nominal beam energy
Code POSINST (M. Furman, LUMI06 wkshp. et. seq.)NB: ACC calculation has been recently revised. See LUMI06 proc.
max=1.7
max=1.5
max=1.3
solid: CERN simulations(code ECLOUD)
dotted: available cooling capacity for ecloud (ACC)
• We don’t know what peak SEY max will be at start-up
– but need to stay within cryogenic cooling capacity• Simulation gives an idea of where the LHC will be able to operate during run-in• Also: excellent agreement between LBNL and CERN simulations
dashed: LBNL simulations(codePOSINST)
M. Furman, “ecloud at the MI and LHC” p. 17ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Sample assessment of two PS upgrade options:heat load vs. peak SEY max
• PS2: Eb=50 GeV
• PS+: Eb=75 GeV
• Bunch spacings: tb=25, 50, 75 ns
• Conclusion:—PS2 and PS+ comparable—75 ns slightly better than 50 ns—50 ns much better than 25 ns
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:
(Similar assessments carried out for SPS and LHC upgrades)
M. Furman, “ecloud at the MI and LHC” p. 18ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Sample simulated heat load vs. max
LHC and upgraded injectors: Cu vs. St.St.
• Effect of different emission spectra:— Smaller rediffused component in SE energy spectrum— Subtle mechanism; explained in detail in Sec. IV-B of
http://prst-ab.aps.org/pdf/PRSTAB/v9/i3/e034403• Caveat: Cu and StSt emission parameters need to be re-measured
to confirm Cu advantage!
120-150 W/m for St.St.
“PS2”, tb=25 ns
“PS2”, tb=50 ns LHC nom., tb=25 ns
SPS nom., tb=25 ns
“SPS+”, tb=25 ns
M. Furman, “ecloud at the MI and LHC” p. 19ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Conditioning
• Peak SEY max vs e– dose:
max~1 when D~1 C/cm2
—under vacuum and steady e– current
• ECE is a self-conditioning effect
—Beam conditioning observed at SPS, PSR, PEP-II, RHIC…
max vs. dose for TiN/AlKirby & King, NIMPR A469, 1 (2001)
max vs. dose for CuHilleret, 2stream2001 (KEK) 1 C/cm2
~1 C/cm2
M. Furman, “ecloud at the MI and LHC” p. 20ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.EC detectors installed recently
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
RFA e– detectors (ANL design; Rosenberg-Harkay) measure flux and energy spectrum
Main Injector Tevatron
RFA
ion gauge
ion pump
beam separator
M. Furman, “ecloud at the MI and LHC” p. 21ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.What is the ECE
• Step 1: beam produces primary electrons— Photoelectrons, ionization of residual gas, stray beam particles striking the
chamber, …
• Step 2: electrons get rattled around the chamber— Amplification by secondary electron emission
• Particularly intense for positively-charged beams• Possible consequences:
— dipole multibunch instability— emittance blowup— gas desorption from chamber walls— excessive energy deposition on the chamber walls (important for
superconducting machines, eg. LHC)— particle losses, interference with diagnostics,…
• The ECE is a consequence of the interplay between the beam and the vacuum chamber— beam intensity, bunch shape, fill pattern, photoelectric yield, photon
reflectivity, secondary emission yield (SEY), vac. chamber size and geometry, …
M. Furman, “ecloud at the MI and LHC” p. 22ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Importance
• PEP-II and KEKB:—controlling the EC was essential to achieve luminosity performance
• ECE limits performance of PSR at high current• RHIC: vacuum pressure instability a high current
• Possibly serious in future machines:• LHC: potentially large energy deposition from electrons
— need to dissipate it• otherwise, less-than-nominal performance
• ILC DR’s: potential for instability and/or emittance growth— main concern: wiggler regions
• MI upgrade: — Nbx5; recently begun to investigate
M. Furman, “ecloud at the MI and LHC” p. 23ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Observations
• ECE has been observed at many machines:— PF, PEP-II, KEKB, BEPC, PS, SPS, APS, PSR, RHIC, Tevatron(?),
MI(?), SNS(?)• undesirable effects on performance, and/or• dedicated experiments
• “Old” effects:— two-stream instabilities (BINP, mid 60’s)— beam-induced multipacting (ISR, mid 70’s)
• multibunch effect– pressure rise instability
— trailing-edge multipacting (PSR, since mid 80’s)• single-long-bunch effect
– beam loss and instability
M. Furman, “ecloud at the MI and LHC” p. 24ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.Controlling the ECE
• Add weak solenoidal fields (~20 G)— confines electrons near the chamber, away from the beam
• used in PEP-II and KEKB• RHIC tests
• Tailor the bunch fill pattern (gaps in train)— used at PEP-II for a while, before solenoids
• Modify vacuum chamber geometry— antechamber (eg., PEP-II)
— antigrazing ridges (tests at RHIC)— grooves (LHC arcs; tests at SLAC)
• Lower the SEY— coatings (TiN, TiZrV,…)
• PEP-II, LHC, SNS, RHIC, …
— conditioning
M. Furman, “ecloud at the MI and LHC” p. 25ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.EC at FNAL: background
• Proposed proton driver to replace booster
• Proposed MI upgrade:
— Increase bunch intensity from present 6e10 to 3e11
— New RF system• fRF not yet chosen (range considered=40-325 MHz), vs. 53 MHz at present
• Bunch intensity and bunch frequency are essential ingredients for EC
• Parameter regime has high potential for a significant EC
M. Furman, “ecloud at the MI and LHC” p. 26ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.EC at FNAL: indirect evidence
• At present: indirect evidence for an EC exists
— But no direct electron measurements yet
• Tevatron:
• Fast pressure rise (X. Zhang, Dec. 02; May 05)
— P seen at some of the warm straight sections (ion pump measurements)
— Threshold ~4e10 p/bunch for 30 consecutive bunches
— No good way to measure P in cold regions
• Fast emittance growth (flying wire technique)
— d/dt~28 mm-mr/hr (95%, normalized, vertical, averaged over 30 bunches)• this is for E=150 GeV and N=82e10 in 30 bunches • this is much faster growth than estimated IBS growth rate
— d/dt sensitive to N above threshold
— Unfortunately, no BBB measurements
M. Furman, “ecloud at the MI and LHC” p. 27ECLOUD07
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.EC at FNAL: indirect evidence
• Main Injector:
• Fast pressure rise (R. Zwaska, Jan. 06)
— 82 bunches of ~9e10 p/bunch, or 418 bunches of ~5e10 p/bunch
— P seen at 24 of 523 pumps• P/P typically 5-50%• but reached 600%-700% at 2 pumps: uncoated ceramic chamber
– NB: ceramic has a high SEY, therefore high P/P is consistent with e-cloud hypothesis
• Maximum effect at transition (short z)
Recommended