1. 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, California 19 April 2004

2. 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.

3. 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

4. Wolfram Fischer 4 Pressure rise observations 1 st fill with 110 Au 79+ bunches N=0.50·10 9 Oct. 2001 next fill N=0.44·10 9 10 -7 Torr abort limit Beam losses during acceleration

5. Wolfram Fischer 5 RHIC Pressure rise observation to date Au 79+ d+d+ p+p+ Pressure rise locationsonly in warm beam pipes Injection Pressure rise observedYes E-clouds observed directlyYes Transition Pressure rise observedYes N/A E-clouds observed directlyYes with large losses NoN/A Store Pressure rise observedYesNo E-clouds observed directlyNo Pressure rise observed Yes = pressure rise  1 decade E-clouds observed directly = observed with electron detector

6. 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]

7. Wolfram Fischer 7 Electron cloud observation at injection (1)  Q  2.5·10 -3 (1) From measured tune shift, the e-cloud density is estimated to be 0.2 – 2.0 nC·m -1 (2) E-cloud density can be reproduced in simulation with slightly higher charge and 110 bunches (CSEC by M. Blaskiewicz) Indirect observation – coherent tune shift along bunch train 33·10 11 p + total, 0.3·10 11 p + /bunch, 110 bunches, 108 ns spacing (2002) [W. Fischer, J.M. Brennan, M. Blaskiewicz, and T. Satogata, “Electron cloud measurements and observations for the Brookhaven Relativistic Heavy Ion Collider”, PRSTAB 124401 (2002).]

8. 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-Ariz Observation: 88·10 11 p + total 0.8·10 11 p + /bunch 110 bunches 108 ns spacing Simulation: Variation of SEY max : 1.7 to 2.1 Keep R=0.6 (reflectivity for zero energy) Good fit for SEY max = 1.8 and R=0.6 Code: CSEC by M. Blaskiewicz bunches with lower intensity Direct observation – electron detectors

9. Wolfram Fischer 9 Electron cloud observation at injection (3) 86·10 11 p + total, 0.78·10 11 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 connection between e-cloud and pressure at injection Estimate for  e assuming pressure caused by e-cloud: 0.001-0.02 (large error from multiple sources)

10. 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-Au Nov. 2003 to Apr. 2004 No of bunches: 61, 56, 45 Ions per bunch: 0.5-1.1  10 9

11. Wolfram Fischer 11 RHIC Blue pressure rise sector 8

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

13. Wolfram Fischer 13 RHIC Blue pressure rise sector 8 Additional losses at pressure rise location Collimator movement lead to loss of 7·10 7 Au ions in 5sec  No pressure rise observed J. Wei, D. Trbojevic, W. Fischer

14. 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 clouds Unsolved problem: Why is pressure rise exponential?

15. 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 1 st order phase transitions?]

16. 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

17. 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

18. 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 electron cloud simulations”, BNL C-A/AP/146 (2004).]

19. 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-VAC Technical Note 00-10 Modified to match observation

20. 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 E max =400 eV and  max =2.5 End of Be pipe

21. 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

22. 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

23. 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

24. 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 –~1  m 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 -1 m -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 NEG coating setup at 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

25. 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

26. 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

27. 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 bunches and larger intensity variations

28. Wolfram Fischer 28 Counter measures: bunch pattern (4) Assuming e-cloud induced pressure rise, test bunch patterns in 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 1 turn Code: CSEC by M. Blaskiewicz

29. 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)]

30. Wolfram Fischer 30 Counter measures: scrubbing (1) High intensity beam tests  scrubbing visible (~1.5e11 p/bunch, up to 112 bunches possible) S.Y. Zhang H. Huang 10% more intensity after 20 min scrubbing poor beam lifetime (large losses)

31. 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)]

32. Wolfram Fischer 32 Summary Electron cloud driven pressure rise observed in RHIC (no other e-cloud driven problems so far) –With all species (Au 79+, 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. Wolfram Fischer 33 Additional material Run-4 Au-Au pressure rise in Blue sector 8 (unbaked collimator)

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