Issues in the Formation and Dissipation of the Electron Cloud Miguel A. Furman, LBNL

M. A. Furman, BNL, Dec. 8-12, 2003, “Electron Cloud ...” p. 1

Issues in the Formation and Dissipationof the Electron Cloud

Miguel A. Furman, [email protected]

13th ICFA Beam Dynamics Mini Workshop“Beam-Induced Pressure Rise in Rings”

BNL, Dec. 8–12, 2003

Lawrence Berkeley National Laboratory

My gratitude to:

A. Adelmann, G. Arduini, M. Blaskiewicz, O. Brüning, Y. H. Cai, R. Cimino, I. Collins, O. Gröbner, K. Harkay, S. Heifets, N. Hilleret, J. M. Jiménez, R. Kirby,G. Lambertson, R. Macek, K. Ohmi, M. Pivi, G. Rumolo, F. Zimmermann.


Summary• Motivation: better understand electron cloud (EC) dynamics

– in particular: effect of secondary electron process

• Tools:– simulations (mostly code POSINST – Furman and Pivi); other codes by Ohmi, Zimmermann,

Rumolo, Blaskiewicz, Adelmann,... also take SE into account

– electron detectors (APS, SPS, PSR, RHIC – Harkay, Jiménez, Macek, Browman, Zhang,...)

• EC formation– primary processes: photoelectrons, residual gas ionization, beam-particle losses

– secondary electron emission (SEY): may lead to beam-induced multipatcing (BIM)

– examples:

• sensitivity to secondary emission yield (E0) (E0=incident electron energy)

• secondary emission spectrum d/dE (E=emitted electron energy)

• EC dissipation– focus: mostly PSR, also APS and SPS: role of (0)

• Scrubbing effect and conclusions



Tools• Simulation

– detailed model for and d/dE– input data: measurements by R. Kirby, N. Hilleret, R. Cimino, I. Collins and

others• St. St., Cu, Al, TiN

– electron cloud is dynamical– beam is a prescribed function of time, space

• Electron detectors– RFA (Harkay and Rosenberg, NIMPR A453, 507 (2000); PRSTAB 6, 034402)

• installed at APS, PSR, BEPC, ANL IPNS RCS

• measure Iew and d/dE at chamber wall (“prompt” electrons)

– “sweeping detector” at PSR (Browman, Macek)• installed at PSR• measure EC density in the bulk (“swept” electrons)

– strip detector at SPS, COLDEX, PUs• (Jiménez et al., PAC03)• strip detector in an adjustable B field


(ROAB003; ROPA007)

(ROAB003)

(TOPC003; TPPB054)

PAC03 refs. in blue


EC formation: basics

• Electron charge conservation in a given chamber section– assuming no antechamber, no net end-losses

– assumes 3 primary processes: • photoelectrons

• residual gas ionization

• beam-particle losses

Assume: =beam line density Z=beam particle chargep=chamber x-section perimeter

Iew=e– flux at wall [A/m2]

=primary production rate [m–1]

per beam particleLawrence Berkeley National Laboratory

(M. Blaskiewicz)


EC formation: primary e– rate of creation


vb = beam speed

Yeff = eff. quantum efficiency (e– yield per )

i = ioniz. cross-section per beam particle

pvac = vac. pressure

T = temperature

eff = eff. e– yield per (beam particle)-wall collision

n'bpl = beam particle loss rate per unit length per beam particle

• Electron production rate per beam particle per unit length of beam trajectory:


Secondary e– emission

Simulation (Furman-Pivi, PRSTAB 5, 124404):– event=one electron-wall collision– instantaneous generation of n secondaries (or absorption)– include E0 and 0 dependence– detailed phenomenological model for and d/dE

Three main components of emitted electrons:

elastics:

rediffused:

true secondaries:

NB: d/dE is different for e, r and ts!!!



Two sample measurements of the SEY

2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

measured data (R. Kirby) model fit (Furman-Pivi)

E0ts=0E0tspk=310dtspk=1.22powts=1.813P1epk=0.5P1einf=0.07E0epk=0powe=0.9E0w=100P1rinf=0.74Ecr=40qr=1

Stainless steel sample (data R. Kirby) 2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

fit (Furman-Pivi) measured data

E0tspk=276.812dtspk=1.8848powts=1.54033E0ts=0P1epk=0.496229P1einf=0.02E0epk=0powe=1E0w=60.8614P1rinf=0.2Ecr=0.0409225qr=0.104045

Copper sample (Hilleret data)


Cu St. steel

• caveat: samples not fully conditioned!

(N. Hilleret; R. Kirby)


PSR simulation: sensitivity to max


• stainless steel chamber, field-free region, • dominant primary process: proton losses:

beam signal(arb. units)

0.1

1

10

100

1000

line density [nC/m]

2.0x10-6

1.81.61.41.21.00.80.60.40.20.0

runtime [s]

beam line density EC line density (deltamax=1.5) EC line density (deltamax=1.7)

PSR simulation, field-free regionnsteps=1000 or 2000, macrop=500, nkicks=1001prot. loss rate=4.44e-8, yield=100

aver. beam neutralization

max=1.5, (0)=0.36

max=1.7, (0) = 0.4

aver. electron line density vs. time

(see also RPPB035)




beam signal(arb. units)

e– flux at the wall vs. time

101

2

3

4

5

6

789102

2

3

4

5

6

789103

2

3

4

5

6

789104

electron wall current [micro-A/cm**2]

2.0x10-6

1.81.61.41.21.00.80.60.40.20.0

tsm [s]

electron-wall current (dpk=1.5) electron-wall current (dpk=1.7) beam signal (arb. units)


max=1.7, (0) = 0.4

max=1.5, (0)=0.36



300

250

200

150

100

50

0

electron energy at wall [eV]

2.0x10-6

1.81.61.41.21.00.80.60.40.20.0

tsm [s]

Ek0_sm15 (dpk=1.5) Ek0_sm17 (dpk=1.7) beam signal (arb. units)



electron-wall collision energy vs. time


PSR simulation-contd.

8000

6000

4000

2000

0

beam potential [V]

2.0x10-6

1.81.61.41.21.00.80.60.40.20.0

timekick [s]



beam potential well vs. time


Sample spectrum: d/dE

• 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%


Sensitivity to relative ratios of e, r and ts: LHC


• LHC simulation max fixed at 2.05;

• dominated by photoelectrons; electron line density vs. time (LHC arc dipole)

7

6

5

4

3

2

1

0

aver. electron line density [nC/m]

1.4x10-61.21.00.80.60.40.20.0

timeW [s]

aver. beam neutralization level

beam signal (arb. units) Copper, true sec. only Copper Stainless st.

LHC arc dipole simulation average line density

photoelectrons: outer edge only

n'e() =6.3 -4 / ,e e m max=2.05

e+r = 43%

e+r = 10%

e+r = 0

(Furman-PiviEPAC02)

max=2.05

(see also: TPPB054)



2000

1500

1000

500

0

electron-wall collision energy [eV]

1.4x10-61.21.00.80.60.40.20.0

time_sm [s]

beam signal (arb. units) Copper Stainless st. Copper, true sec. only

LHC arc dipole simulation electron-wall collision energy


n'e() =6.3 -4 / ,e e m max=2.05


e–-wall collision energy vs. time (LHC arc dipole)

e+r = 43%

e+r = 10%

e+r = 0



2.0

1.5

1.0

0.5

0.0

effective SEY

1.4x10-61.21.00.80.60.40.20.0

time_sm [s]

beam signal (arb. units) Copper Stainless Copper, true sec. only

LHC arc dipole simulation effective SEY


n'e() =6.3 -4 / ,e e m max=2.05


effective SEY vs. time (LHC arc dipole)

e+r = 43%

e+r = 10%

e+r = 0


800

600

400

200

0

aver. power deposition [W/m]

1.4x10-6

1.21.00.80.60.40.20.0

time_sm [s]

LHC arc dipole simulation: electron-cloud power deposition


n'e() =6.3 -4 / ,e e m max=2.05

( . )beam signal arb unitsCopper Stainless steel , .Copper true sec only

. 0.5< <1.2Aver power deposition in t μs

:11 /copper W m. .:152 /st st W m

, :2.1 / .copper TS only W m



power deposition vs. time (LHC arc dipole)

e+r = 10%

800

600

400

200

01.060x10

-61.0501.0401.0301.020

time_sm [s]

e+r = 0

e+r = 43%


EC formation: beam-induced multipacting (BIM)


• train of short bunches, each of charge Q=NZe, separated by sb

• t = e– chamber traversal time

• b = chamber radius (or half-height if rectangular)

The parameter defines 3 regimes:

If G = 1 and eff > 1, EC can grow dramatically (O. Gröbner, ISR; 1977)

e−

e−

e−

e−

+ + + + + +

γ or p

(also for solenoidal fieldif T/2=sb/c: WOAA004)


BIM in the APS

120

100

80

60

40

20

0

aver. electron-wall current [nA/cm

2]

35302520151050

bunch spacing sB [RF buckets]

measured simulated

APS, positron beam

Detector Current vs. Bunch Spacing

(10 bunches, 2 mA/bunch in all cases; measurements courtesy K. Harkay, ANL)

region of BIM

sB=d2/(reN), b<d<a


(Furman, Pivi, Harkay, Rosenberg, PAC01)

time-averaged e– flux at wall vs. bunch spacing

measuredsimulated

• e+ beam, 10-bunch train, field-free region

(see also: RPPG002)


BIM for long bunches: case of PSR• bunch length >> t

– a portion the EC phase space is in resonance with the “bounce frequency”

– “trailing edge multipacting” (Macek; Blaskiewicz, Danilov, Alexandrov,…)


ED42Y electron detector signal 8μC/pulse beam

435 μA/cm2

(simulation input)

electron signal

measured (R. Macek) simulated (M. Pivi)

(max=2.05)

(ROAB003; RPPB035)


BIM for long bunches: case of PSR-contd.

head

truncated bunch(nominal charge)

nominalbunch

tail

L=150 ns

• simulated “experiment” in trailing edge multipacting: — truncate bunch tail at fixed bunch charge


• suppresses the resonance • hard to put into practice! (M. Pivi)

bunch profile

aver. e– line density

(RPPG024)


EC dissipation - simplest analysis


N

N’2b

If not monoenergetic and not along a straight line, then

• beam has been extracted, or gap between bunches• field-free region, or constant B field • assume monoenergetic blob of electrons• neglect space-charge forces

where K=f(angles)≈1.1–1.2

simulations show that this formulaworks to within ~20%

and = dissipation time


EC dissipation in PSR after beam extraction

• “Sweeping e– detector”– measures electrons in the bulk ≈ 200 ns eff ≈ 0.5 if E = 2–4 eV

– since eff ≈ (0), you infer (0)

– well supported by simulations (see next slide)

(Macek and Browman)


(RPPB035)

M. A. Furman, BNL, Dec. 8-12, 2003, “Electron Cloud ...” p. 23Lawrence Berkeley National Laboratory

EC dissipation after beam extraction: PSR simulation

0.01

0.1

1

10

100

1000

line density [nC/m]

2.0x10-61.81.61.41.21.00.80.60.40.20.0

time [s]

EC line density beam line density

exponential decay(slope=2e-07 s)

PSRdissip3

aver. neutralization level

PSR simulationfield-free section, N=5e13

p loss rate=4e-6/m, yield=100 e/pNB: primary e– rateis 100 x nominal

input SEY:

max = 1.7 (0) = 0.4

EC line density vs. time (field-free region)

slope = 200 ns


EC dissipation after beam extraction: PSR simulation

0.1

1

10

100

1000

electron energy [eV]

2.0x10-6

1.81.61.41.21.00.80.60.40.20.0

tsm [s]

collision energy per electron absorbed energy per electron beam signal (arb. units)

PSR simulationfield-free section, N=5e13

p loss rate=4e-6/m, yield=100 e/p

PSRdissip3

e–-wall collision energy vs. time (field-free region)



EC dissipation after beam extraction: SPS simulation

0.01

0.1

1

10

line density [nC/m]

2.4x10-62.22.01.81.61.41.21.00.80.60.40.20.0

time [s]

EC line density beam line density

exponential decayslope=1.7e-07 [s]

SPS_P1e_4_nb72a.dir

av. beam neutralization level

SPS simulationP=1e-4 Torr, B=0.2 T, N=8e10,

rect. chamber (a,b)=(7.7,2.25) cm

NB: pvac is>> nominal

• stainless steel chamber, dipole magnet, B = 0.2 T, • dominant primary process: residual gas ionization;

slope = 170 ns

input SEY:

max = 1.9 (0) = 0.5


EC dissipation after beam extraction: SPS simulation

0.1

1

10

100

1000

electron energy [eV]

2.4x10-6

2.22.01.81.61.41.21.00.80.60.40.20.0

tsm [s]

collision energy per electron absorbed energy per electron

SPS simulationP=1e-4 Torr, B=0.2 T, N=8e10,

rect. chamber (a,b)=(7.7,2.25) cm SPS_P1e_4_nb72a.dir

e–-wall collision energy vs. time (B-field region)


Conditioning effects: beam scrubbing

• Decrease of SEY by e– bombardment– self-conditioning effect for a storage ring: “beam scrubbing”

• SPS ECE studies (M. Jiménez; F. Zimmermann):– 3+ years of dedicated EC studies with dedicated instrumentation

– scrubbing very efficient; favorable effects seen in:• vacuum pressure

• in-situ SEY measurements

• electron flux at wall

– e– energy distribution in good agreement with simulations above 30 eV

– TiZrV coating fully suppresses multipacting after activation


(see also: MOPA001; TPPB054)

(TOPC003)


Conditioning effects: beam scrubbing

• PSR “prompt” e– signal (BIM) is subject to conditioning: (R. Macek)– signal is stronger for st.st. than for TiN

– sensitive to location and N

– signal does not saturate as N increases up to ~8x1013

– conditioning: down by factor ~5 in sector 4 after few weeks (low current)

• PSR “swept” e– signal is not:– signal saturates beyond N~5x1013

– ≈ 200 ns, independent of:

• N

• location

• conditioning state

• st. st. or TiN

• Tentative conclusion: beam scrubbing conditions max but leaves (0) unchanged


(ROAB003; RPPB035)


Conditioning effects–contd.

• consistent with bench results for Cu found at CERN!

– the result (0)≈1 seems unconventional

– if validated, it could have a significant unfavorable effect on the EC power deposition in the LHC

• because electrons survive longer in between bunches


(R. Cimino and I. Collins, proc. ASTEC2003, Daresbury Jan. 03)

Copper SEY (CERN)


Conclusions

• A consistent picture of the ECE is emerging for– low-energy machines (long bunch, intense beam)

– high-energy machines (short, well separated bunches)

– methodical measurements and simulation benchmarks at APS, PSR and SPS are paying off

– some interesting surprises along the way

• Quantitative predictions are becoming more reliable– we are growing older but wiser



Additional material



2.5

2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

delta_SS (Kirby data) delta_e delta_r delta_ts delta_er (=delta_e+delta_r) delta_tot delta_tsp


SEY for stainless steel, normal incidence(data courtesy R. Kirby, SLAC standard 304 rolled sheet,chemically etched and passivated but not conditioned)


2.5

2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

delta_e delta_r delta_ts delta_er (=delta_e+delta_r) delta_tot delta_tsp deltaCuhilleret


SEY for Cu, normal incidence (Data courtesy N. Hilleret for chemically cleaned but not in-situ vacuum baked samples) (macro hilleret_fit_mauro)



2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

fit (Furman-Pivi) measured data


Copper sample (Hilleret data)



2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

measured data (R. Kirby) model fit (Furman-Pivi)


Stainless steel sample (data R. Kirby)



0.08

0.06

0.04

0.02

0.00350300250200150100500

Esec [eV]

dde_300_ss_abs (Kirby data, renormalized to delta(300)=2.04489) ddeRV_totp_bin

dele=0.0916988delr=0.739591delts=1.21947deltot=2.05076int_ddeRV_tot=2.06258int_ddeRV_totp=1.83298int_ddeRV_totp_bin=2.0676delout=2.05075delpout=1.81494delpbinout=2.05076deltsout=1.21947deltspout=0.983654

maxsec=10E0=300 eVpr=0.4sige=-1 eVsigee=1.88287

pnpar[1]=1.6pnpar[2]=2pnpar[3]=1.8pnpar[4]=4.7pnpar[5]=1.8pnpar[6]=2.4pnpar[7]=1.8pnpar[8]=1.8pnpar[9]=2.3pnpar[10]=1.8

enpar[1]=3.9enpar[2]=6.2enpar[3]=13enpar[4]=8.8enpar[5]=6.25enpar[6]=2.25enpar[7]=9.2enpar[8]=5.3enpar[9]=17.8enpar[10]=10

Emission energy spectrum, E0=300 eVstainless steel, normal incidence(data courtesy R. Kirby, SLAC standard 304 rolled sheet,chemically etched and passivated but not conditioned)

NOTE: rediffused+backscattered~50%(assuming low-energy cutoff=50 eV)


NOTE: rediffused+backscattered~5%(assuming low-energy cutoff=50 eV)



Current parameter values from fits to data


Q: is the electron emitted spectrum Maxwellian? A: only approximately.

Fits to data, however, imply pn~1.8–5, depending on n and material


definition of Maxwellian spectrum:

dN

d3p∝ exp−E kT( ), E =

p2

2me

⇒dNdE

∝ E1/2 exp−E kT( )≡Epn−1exp−E εn( )

⇒ pn =3 2


2.5

2.0

1.5

1.0

0.5

0.010009008007006005004003002001000

E0 [eV]

delta_e delta_r delta_ts delta_er (=delta_e+delta_r) delta_tot delta_tsp deltaCuhilleret

E0tspk=276.812dtspk=2.1powts=1.54033E0ts=0P1epk=0P1einf=0E0epk=0powe=1E0w=60.8614P1rinf=0Ecr=0.0409225qr=0.104045

SEY for Cu, normal incidence (data courtesy N. Hilleret)

true secondaries only

(macro hilleret_fit_mauro_TS_only)


backscattered and rediffused electronsartificially suppressed (true secondaries only)


BIM for long bunches: case of PSR

• bunch length >> t


ED02X electron detector signal 8μC/pulse beam

ED42Y electron detector signal 8μC/pulse beam

145 μA/cm2 435 μA/cm2


Effect of bunch shortening (PSR simulation; M. Pivi)• truncate the bunch tail to reduce trailing-edge multipacting

truncated bunch(nominal charge)

nominal bunch

head

tail


Effect of bunch shortening (PSR simulation; M. Pivi) – contd.

L=254 ns (nom..)

L=200 ns

L=180 ns

L=150 ns


100

80

60

40

20

0

W/m

350x10-9

300250200150100500

time_sm [s]

curr (beam current, arb. units) avPD_sm_Cu (Copper) avPD_sm_SS (Stainless) avPD_sm_Cu_ts (Copper, true sec. only)

LHC arc dipole simulation average power deposition

time-averaged power deposition:Copper: 0.59 W/mStainless: 5.7 W/mCopper, TS: 0.01 W/m



2.0

1.5

1.0

0.5

0.0

nC/m

350x10-9

300250200150100500

timeW [s]

curr (beam current, arb. units) avlineden_Cu (Copper) avlineden_SS (Stainless) avlineden_Cu_ts (Copper, true sec. only)

LHC arc dipole simulation average line density

(Y'=0.05; phels. produced at outer edge only)



2.0

1.5

1.0

0.5

0.0

W/m

350x10-9

300250200150100500

time_sm [s]

curr (beam current, arb. units) avPD_sm_Cu (Copper) avPD_sm_Cu_ts (Copper, true sec. only)

LHC arc dipole simulationpower deposition

(Y'=0.05; phels. produced at outer edge only)

time-averaged power deposition:Copper: 0.59 W/mCopper, TS: 0.01 W/m


(detailed view for copper only)


800

600

400

200

0

eV

350x10-9

300250200150100500

time_sm [s]

curr (beam current, arb. units) E0_sm_Cu (Copper) E0_sm_SS (Stainless) E0_sm_Cu_ts (Copper, true sec. only)

LHC arc dipole simulation electron-wall collision energy

Issues in the Formation and Dissipation of the Electron Cloud Miguel A. Furman, LBNL

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Transcript of Issues in the Formation and Dissipation of the Electron Cloud Miguel A. Furman, LBNL