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Electron clouds and vacuum pressure rise in RHIC

Wolfram Fischer

Thanks to

M. Blaskiewicz, H. Huang, H.C. Hseuh, U. Iriso, S. Peggs,G. Rumolo, D. Trbojevic, J. Wei, S.Y. Zhang

ECLOUD’04, Napa, California19 April 2004

Wolfram Fischer 2

Abstract

Electron clouds and vacuum pressure rise in RHIC

The luminosity in RHIC is limited by a vacuum pressure rise in the warm regions, observed with high intensity beams of all species (Au, p, d). At injection, the pressure rise could be linked to the existence of electron clouds. In addition, a pressure rise in the experimental regions may be caused by electron clouds, and leads to increased backgrounds. We review the existing observation, comparisons with simulations, as well as corrective measures taken and planned.

Wolfram Fischer 3

Contents

• History of pressure rise problems at RHIC

• Run-4 pressure problems– Blue ring sector 8 [unbaked collimators]

– Interaction region 10 [long Beryllium pipe]

• Counter measures

• Summary

Wolfram Fischer 4

Pressure rise observations

1st fill with 110 Au79+ bunches N=0.50·109 Oct. 2001

next fill N=0.44·109

10-7 Torr abort limit

Beam lossesduring acceleration

Wolfram Fischer 5

RHIC Pressure rise observation to date

Au79+ d+ p+

Pressure rise locations only in warm beam pipes

InjectionPressure rise observed Yes Yes Yes

E-clouds observed directly Yes Yes Yes

TransitionPressure rise observed Yes Yes N/A

E-clouds observed directly Yeswith large losses

No N/A

StorePressure rise observed Yes No No

E-clouds observed directly No No No

Pressure rise observed Yes = pressure rise 1 decadeE-clouds observed directly = observed with electron detector

Wolfram Fischer 6

Pressure rise mechanisms

Pressure rise mechanisms considered so far• Electron cloud confirmed

– Coherent tune shift in bunch train

– Electron detectors

• Ion desorption small

– Rest gas ionization, acceleration through beam

– Ion energies ~10eV

– Effect too small to explain pressure rise at injection

• Beam loss induced desorption under investigation

– No reliable desorption coefficients

– Need to have beam losses in all locations with pressure rise

[W. Fischer et al., “Vacuum pressure rise with intense ion beams in RHIC”, EPAC’02]

Wolfram Fischer 7

Electron cloud observation at injection (1)

Q2.5·10-3

(1) From measured tuneshift, the e-cloud density is estimated to be 0.2 – 2.0 nC·m-1

(2) E-cloud density can bereproduced in simulationwith slightly higher chargeand 110 bunches (CSEC by M. Blaskiewicz)

Indirect observation – coherent tune shift along bunch train

33·1011 p+ total, 0.3·1011 p+/bunch, 110 bunches, 108 ns spacing (2002)

[W. Fischer, J.M. Brennan, M. Blaskiewicz, and T. Satogata, “Electron cloud measurements andobservations for the Brookhaven Relativistic Heavy Ion Collider”, PRSTAB 124401 (2002).]

Wolfram Fischer 8

Electron cloud observation at injection (2)

[U. Iriso-Ariz et al. “Electron cloud and pressure rise simulations for RHIC”, PAC’03.]

U. Iriso-ArizObservation: 88·1011 p+ total 0.8·1011 p+/bunch 110 bunches 108 ns spacing

Simulation: Variation of SEYmax: 1.7 to 2.1 Keep R=0.6 (reflectivity for zero energy)

Good fit for SEYmax = 1.8 and R=0.6

Code: CSEC by M. Blaskiewicz

bunches with lower intensity

Direct observation – electron detectors

Wolfram Fischer 9

Electron cloud observation at injection (3)

86·1011 p+ total, 0.78·1011 p+/bunch, 110 bunches, 108 ns spacing U. Iriso-Ariz

[U. Iriso-Ariz et al. “Electron cloud observations at RHIC during FY2003”, in preparation.]

Electron cloud and pressure rise

12 min

e-cloud and pressure

total beam intensity

Clear connectionbetween e-cloudand pressure atinjection

Estimate for e

assuming pressurecaused by e-cloud:

0.001-0.02(large error from multiple sources)

Wolfram Fischer 10

RHIC Location of limiting pressure rise problems Run-4

Blue sector 8: Unbaked collimator

Yellow sector 4: Unbaked stochastic cooling kicker

IP10: PHOBOS(common Be beam pipe)

Run-4 Au-AuNov. 2003 to Apr. 2004

No of bunches: 61, 56, 45Ions per bunch: 0.5-1.1109

Wolfram Fischer 11

RHIC Blue pressure rise sector 8

Wolfram Fischer 12

RHIC Blue pressure rise sector 8

Injection with different bunch spacing

Wolfram Fischer 13

RHIC Blue pressure rise sector 8

Additional losses at pressure rise location

Collimator movement lead toloss of 7·107 Au ions in 5sec No pressure rise observed

J. Wei, D. Trbojevic, W. Fischer

Wolfram Fischer 14

RHIC Blue pressure rise sector 8

Are electron clouds the source of the pressure rise?

• No electron detectors in sector 8

• Intensity dependent

• Bunch spacing dependent

• Bunch length dependent

• Not dependent on additional beam loss

• Not dependent on beam energy

Characteristics of electron cloudsUnsolved problem: Why is pressure rise exponential?

Wolfram Fischer 15

RHIC Pressure rise IR10

PHOBOS background increase after rebucketing, drops after minutes to 2 hours(most severe luminosity limit in Run-4)

intensity

vacuum

background

Rebucketing, bunch length reduced to 50%

[Some thoughts on switch-off: U. Iriso and S. Peggs, “Electron cloud phase transitions”,BNL C-A/AP/147 (2004). Can e-cloud codes create 1st order phase transitions?]

Wolfram Fischer 16

RHIC IR10 pressure rise history (1)

Average bunch intensity at rebucketing/pressure drop, and duration of increased pressure sorted by bunch patterns

Wolfram Fischer 17

RHIC IR10 pressure rise history (2)

Run-4 physics stores

Pressure before and after rebucketing (50% bunch length reduction)

Did not find narrow range that triggers problem for• average bunch intensity• peak bunch intensity• pressure before rebucketing No good correlation with any parameter and duration either

Wolfram Fischer 18

Be pipe

Considered 2 cases:At IP: nominal bunch spacing (~216ns) and double intensity

At end of the beryllium pipe: normal intensity, spacing of 40ns then 176ns

12m ~ 40ns

RHIC IR10 pressure rise simulations (1) G. Rumolo, GSI

[G. Rumolo and W. Fischer, “Observation on background in PHOBOS and related electroncloud simulations”, BNL C-A/AP/146 (2004).]

Wolfram Fischer 19

RHIC IR10 pressure rise simulations (2) G. Rumolo, GSI

Can calibrate Be surface parameters:• No e-cloud before rebucketing (10ns bunch length)• E-cloud after rebucketing (5ns bunch length)

N. Hilleret, LHC-VACTechnical Note 00-10

Modified to match observation

Wolfram Fischer 20

Center of Be pipe

RHIC IR10 pressure rise simulations (2) G. Rumolo, GSI

Important result: After surface parameter calibration find e-clouds at end of 12m Be pipe, but not in center May be sufficient to suppress e-cloud at ends

Emax=400 eV and max=2.5

End of Be pipe

Wolfram Fischer 21

Counter measures

• In-situ baking (>95% of 700m/ring warm pipes baked)

Occasionally installation schedules too tight

• Solenoids Tested last year, this year

• NEG coated pipes Installed 60m last shut-down for test, about 200m next shut-down

• Bunch patterns Tested last year, used this year

• Scrubbing Tested last year

Wolfram Fischer 22

Counter measures: solenoids (1)

• 50m of solenoids– Maximum field: 6.8 mT [68 G]

• Close to e-detectors and pressure gauges• Solenoidal fields generally reduce e-cloud

– Often with only 0.1 mT [10 G]

– Not in all cases completely– In some cases increasing fields increase pressure

• Solenoids have operational difficulties(routinely used in B-factories)

– Many power supplies– Highest field (6.8 mT) not always best

Wolfram Fischer 23

Counter measures: solenoids (2)

[U. Iriso-Ariz et al., “Electron cloud observations at RHIC during FY2003”, BNL C-A/AP note in preparation (2003)]

U. Iriso-Ariz

beam intensity

solenoid currents

pressure

pressure increase with increasing solenoid fields

Wolfram Fischer 24

Counter measures: NEG coated pipes (1)

• Installed 60 m of NEG coated pipes in selected warm regions for evaluation

• NEG coated beam pipes– Coating done by SAES Getters, Milan, Italy– ~1m sputtered TiZrV layer (30%–30%–40%)

– Activated with 2 hrs baking at 250C(can be done with 24 hrs at 180C)

– Expected speed of 300 ls-1m-1 with load of 1e-5 Torrlcm-2 (based on CERN data)

– Expected SEY of 1.4 (after activation) to 1.7 (saturation)

H.C. Hseuh

NEG coating setupat SAES Getters

Generally found lower pressure near NEG pipes No excessive pressure rise when hit with beam [H. Huang, S.Y. Zhang et al.] Installation of about 200m NEG coated pipes next shut-down

Wolfram Fischer 25

Counter measures: bunch pattern (1)

• Question: How should one distribute n bunches along the circumference to minimize pressure? ( larger n possible with optimum distribution)

• Extreme distributions:– Long bunch trains with long gaps– Most uniform along the circumference

Wolfram Fischer 26

Counter measures: bunch pattern (2)

Beam test of 3 different bunch patterns (6 trains with 16, 12 or 14 bunches – ring not completely filled)

e-clouds detectable

Wolfram Fischer 27

Counter measures: bunch pattern (3)

Shorter trains (with 3 bucket spacing) give more luminosity with comparable vacuum performance(in limited data set)

Longer bunchesand larger intensity variations

Wolfram Fischer 28

Counter measures: bunch pattern (4)

Assuming e-cloud induced pressure rise, test bunch patternsin simulation, and observe e-cloud densities. U. Iriso-Ariz

5 cases tested with 68 bunches (20% more than Run-3),all with same parameters close to e-cloud threshold (except pattern)

4 turns 4 turns

1 turn1 turn

Code: CSEC by M. Blaskiewicz

Wolfram Fischer 29

Counter measures: bunch pattern (5)

If pressure correlates with either maximum or average line density of an e-cloud, most uniform bunch patter is preferable (in line with KEKB observations, and PEP-II as long as e-clouds are the dominant luminosity limit)

Successfully used to mitigate IR10 pressure rise problem temporarily

3 long trains, 3 long gaps

most uniform

[W. Fischer and U. Iriso-Ariz, “Bunch pattern and pressure rise in RHIC”, BNL C-A/AP/118 (2003)]

Wolfram Fischer 30

Counter measures: scrubbing (1)

High intensity beam tests scrubbing visible(~1.5e11 p/bunch, up to 112 bunches possible)

S.Y. ZhangH. Huang

10% more intensityafter 20 min scrubbing

poor beam lifetime(large losses)

Wolfram Fischer 31

Counter measures: scrubbing (2)

• Scrubbing effect more pronounced at locations with high pressures removes bottle necks successively

• Based on observation, need hours – days of scrubbing,depending on intended beam intensity

• High intensity tests damaged BPM electronics in tunnel need to move BPM electronics into alcoves before further scrubbing (1/2 done)

[S.Y. Zhang, W. Fischer, H. Huang and T. Roser, “Beam Scrubbing for RHIC Polarized Proton Run”,BNL C-A/AP/123 note in preparation (2003)]

Wolfram Fischer 32

Summary

• Electron cloud driven pressure rise observed in RHIC(no other e-cloud driven problems so far)– With all species (Au79+, d+, p+), – In warm region only– At injection

• Limits intensity

– At store• Limits intensity (after rebucketing)• Causes experimental background

• Counter measures– Complete baking of all elements– NEG coated pipes tested successfully, will install ~200m for next Run

– Bunch patterns most uniform distributions used

– Solenoids work, no wide scale application for now (NEG preferred)

– Scrubbing works, but need to remove remaining electronics from tunnel

Wolfram Fischer 33

Additional material Run-4 Au-Au pressure rise in Blue sector 8 (unbaked collimator)

Wolfram Fischer 34

Additional material Run-4 Au-Au IR6 pressure rise history