Electron-Cloud Build-up in the FNAL Main Injector and the LHC Complex

M. Furman, “ecloud at the MI and LHC” p. 1ECLOUD07

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Electron-Cloud Build-up in the FNAL Main Injector and the LHC Complex

Miguel Furman

LBNL

ECLOUD07

Daegu, April 9-12, 2007



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.



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)



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




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



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



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



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



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)



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)



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.



are needed to see this picture.Backup material



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




Three components of secondary emission:sample spectrum at E0=300 eV

from M. F. and M. Pivi, PRST-AB 5, 124404 (2002)

E0

E



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%




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)




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)



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



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



are needed to see this picture.EC detectors installed recently





RFA e– detectors (ANL design; Rosenberg-Harkay) measure flux and energy spectrum

Main Injector Tevatron

RFA

ion gauge

ion pump

beam separator



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, …



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



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



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



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



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



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)

Electron-Cloud Build-up in the FNAL Main Injector and the LHC Complex

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