Electron clouds and vacuum pressure rise in RHIC

1
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
• ECLOUD04, Napa, California19 April 2004

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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.

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

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Pressure rise observations
1st fill with 110 Au79 bunches N0.50109 Oct.
2001
Beam lossesduring acceleration
next fill N0.44109
10-7 Torr abort limit
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RHIC

Pressure rise observation to date
Pressure rise observed Yes pressure rise ? 1
decade E-clouds observed directly observed with
electron detector
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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, EPAC02
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Electron cloud observation at injection (1)
Indirect observation coherent tune shift along
bunch train
331011 p total, 0.31011 p/bunch, 110 bunches,
108 ns spacing (2002)
(1) From measured tuneshift, the e-cloud density
is estimated to be 0.2 2.0 nCm-1 (2)
E-cloud density can bereproduced in
simulationwith slightly higher chargeand 110
bunches (CSEC by M. Blaskiewicz)
DQ?2.510-3
W. Fischer, J.M. Brennan, M. Blaskiewicz, and T.
Satogata, Electron cloud measurements
andobservations for the Brookhaven Relativistic
Heavy Ion Collider, PRSTAB 124401 (2002).
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Electron cloud observation at injection (2)
U. Iriso-Ariz
Direct observation electron detectors
Observation 881011 p total 0.81011
p/bunch 110 bunches 108 ns
spacing Simulation Variation of SEYmax 1.7
to 2.1 Keep R0.6 (reflectivity for zero
energy) Good fit for SEYmax 1.8 and
R0.6 Code CSEC by M. Blaskiewicz
bunches with lower intensity
U. Iriso-Ariz et al. Electron cloud and
pressure rise simulations for RHIC, PAC03.
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Electron cloud observation at injection (3)
Electron cloud and pressure rise
U. Iriso-Ariz
861011 p total, 0.781011 p/bunch, 110
bunches, 108 ns spacing
e-cloud and pressure
Clear connectionbetween e-cloudand pressure
atinjection
Estimate for heassuming pressurecaused by
e-cloud 0.001-0.02 (large error from
multiple sources)
12 min
total beam intensity
U. Iriso-Ariz et al. Electron cloud
observations at RHIC during FY2003, in
preparation.
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RHIC
Location of
limiting pressure rise problems Run-4
IP10 PHOBOS(common Be beam pipe)
Run-4 Au-Au Nov. 2003 to Apr. 2004 No of
bunches 61, 56, 45Ions per bunch 0.5-1.1?109
Yellow sector 4 Unbaked stochastic cooling
kicker
Blue sector 8 Unbaked collimator
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RHIC

Blue pressure rise sector 8
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RHIC

Blue pressure rise sector 8
Injection with different bunch spacing
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RHIC

Blue pressure rise sector 8
Additional losses at pressure rise location
Collimator movement lead toloss of 7107 Au ions
in 5sec? No pressure rise observed
J. Wei, D. Trbojevic, W. Fischer
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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 clouds
• Unsolved problem Why is pressure rise
  exponential?

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RHIC

Pressure rise
IR10
PHOBOS background increase after rebucketing,
drops after minutes to 2 hours(most severe
luminosity limit in Run-4)
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?
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RHIC

IR10 pressure rise history (1)
Average bunch intensity at rebucketing/pressure
drop, and duration of increased pressure sorted
by bunch patterns
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RHIC

IR10 pressure rise history (2)
Pressure before and after rebucketing (50 bunch
length reduction)
Run-4 physics stores

• 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

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RHIC
IR10 pressure rise
simulations (1) G. Rumolo, GSI
12m 40ns
Be pipe
Considered 2 casesAt IP nominal bunch
spacing (216ns) and double intensity At end of
the beryllium pipe normal intensity, spacing of
40ns then 176ns
G. Rumolo and W. Fischer, Observation on
background in PHOBOS and related electroncloud
simulations, BNL C-A/AP/146 (2004).
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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
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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
Emax400 eV and dmax2.5
Center of Be pipe
End of Be pipe
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Counter measures

• In-situ baking (gt95 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

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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

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Counter measures solenoids (2)
pressure
beam intensity
solenoid currents
U. Iriso-Ariz
pressure increase with increasing solenoid fields
U. Iriso-Ariz et al., Electron cloud
observations at RHIC during FY2003, BNL C-A/AP
note in preparation (2003)
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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
• 1mm sputtered TiZrV layer (303040)
• Activated with 2 hrs baking at 250?C(can be done
  with 24 hrs at 180?C)
• Expected speed of 300 l?s-1m-1 with load of 1e-5
  Torr?l?cm-2 (based on CERN data)
• Expected SEY of 1.4 (after activation) to 1.7
  (saturation)

H.C. Hseuh

• 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

NEG coating setupat SAES Getters
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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

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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
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Counter measures bunch pattern (3)
Longer bunchesand larger intensity variations
? Shorter trains (with 3 bucket spacing) give
more luminosity with comparable vacuum
performance(in limited data set)
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Counter measures bunch pattern (4)
Assuming e-cloud induced pressure rise, test
bunch patternsin simulation, and observe e-cloud
densities. U. Iriso-Ariz5 cases
tested with 68 bunches (20 more than Run-3),all
with same parameters close to e-cloud threshold
(except pattern)
1 turn
1 turn
4 turns
4 turns
Code CSEC by M. Blaskiewicz
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Counter measures bunch pattern (5)
3 long trains, 3 long gaps
most uniform
? 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
W. Fischer and U. Iriso-Ariz, Bunch pattern and
pressure rise in RHIC, BNL C-A/AP/118 (2003)
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Counter measures scrubbing (1)
High intensity beam tests ? scrubbing
visible(1.5e11 p/bunch, up to 112 bunches
possible)
10 more intensityafter 20 min scrubbing
poor beam lifetime(large losses)
S.Y. ZhangH. Huang
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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)
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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

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

Run-4 Au-Au IR6 pressure rise history