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Electron clouds and vacuum pressure rise in RHIC

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, California 19 April 2004. Abstract. Electron clouds and vacuum pressure rise in RHIC

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Electron clouds and vacuum pressure rise in RHIC

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  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 ECLOUD’04, Napa, California19 April 2004

  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.

  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

  4. Pressure rise observations 1st fill with 110 Au79+ bunches N=0.50·109 Oct. 2001 Beam lossesduring acceleration next fill N=0.44·109 10-7 Torr abort limit

  5. RHIC Pressure rise observation to date Pressure rise observed Yes = pressure rise  1 decade E-clouds observed directly = observed with electron detector

  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]

  7. Electron cloud observation at injection (1) Indirect observation – coherent tune shift along bunch train 33·1011 p+ total, 0.3·1011 p+/bunch, 110 bunches, 108 ns spacing (2002) (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) DQ2.5·10-3 [W. Fischer, J.M. Brennan, M. Blaskiewicz, and T. Satogata, “Electron cloud measurements andobservations for the Brookhaven Relativistic Heavy Ion Collider”, PRSTAB 124401 (2002).]

  8. Electron cloud observation at injection (2) Direct observation – electron detectors U. Iriso-Ariz Observation: 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 [U. Iriso-Ariz et al. “Electron cloud and pressure rise simulations for RHIC”, PAC’03.]

  9. Electron cloud observation at injection (3) Electron cloud and pressure rise U. Iriso-Ariz 86·1011 p+ total, 0.78·1011 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.]

  10. 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

  11. RHIC Blue pressure rise sector 8

  12. RHIC Blue pressure rise sector 8 Injection with different bunch spacing

  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

  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 clouds Unsolved problem: Why is pressure rise exponential?

  15. Rebucketing, bunch length reduced to 50% intensity vacuum background 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?]

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

  17. 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

  18. RHIC IR10 pressure rise simulations (1) G. Rumolo, GSI 12m ~ 40ns 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 [G. Rumolo and W. Fischer, “Observation on background in PHOBOS and related electroncloud simulations”, BNL C-A/AP/146 (2004).]

  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

  20. 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 dmax=2.5 Center of Be pipe End of Be pipe

  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

  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

  23. Counter measures: solenoids (2) pressure beam intensity solenoid currents pressure increase with increasing solenoid fields U. Iriso-Ariz [U. Iriso-Ariz et al., “Electron cloud observations at RHIC during FY2003”, BNL C-A/AP note in preparation (2003)]

  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 • ~1mm sputtered TiZrV layer (30%–30%–40%) • 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

  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

  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

  27. 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)

  28. 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

  29. 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)]

  30. 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

  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)]

  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

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

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

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