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e - cloud studies for different LHC upgrade scenarios ( HL-LHC and (V)HE-LHC )

e - cloud studies for different LHC upgrade scenarios ( HL-LHC and (V)HE-LHC ). C. Octavio Dom ínguez. Thanks to Stephane Fartoukh, G. Rumolo, G. Iadarola, F. Zimmerman. 8 April 2013 - e - cloud meeting. Outline. HL-LHC 1.1 Motivation 1.2 Microbatches 1.3 Reduced bunch spacing

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e - cloud studies for different LHC upgrade scenarios ( HL-LHC and (V)HE-LHC )

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  1. e-cloud studies for different LHC upgrade scenarios (HL-LHC and (V)HE-LHC) C. Octavio Domínguez Thanks to Stephane Fartoukh, G. Rumolo, G. Iadarola, F. Zimmerman 8 April 2013 - e-cloud meeting

  2. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  3. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  4. HL-LHC: Microbatches • Luminosity goal: ~250 fb-1/year • Potential stoppers: • LRBB interactions • Excessive pile-up in the detectors • e-cloud • Different filling schemes have been suggested (S. Fartoukh) to minimize the first two: • Microbatches • Reduced bunch spacing • Bunch “doublets” • It is necessary to check whether e-cloud activity would prevent these schemes to be used 8 April 2013 - e-cloud meeting

  5. HL-LHC: Microbatches (Idea by S. Fartoukh, presented at the HL-LIU Brainstorming, 30.03.2012) • PACMAN bunches: First and last bunches from a train  They experience less LRBBI • This can affect dynamic behaviour and beam life time  Direct impact of Luminosity • The effect will scale up for the HL-LHC due to: • a longer inner triplet, i.e. more LR encounters: • typically 20-23 instead of 15 for 25 ns • a higher bunch charge (stronger kicks): • typically 2E11 instead of 1.15 for 25 ns • Removing properly the central bunches and separating • the trains sufficiently would cancel this effect • If so all the bunches will see strictly the same number of LRBBIs, of course not at the same location, but the integrated kick would be very similar for all bunches • Assuming 23 LRBBI for the longer HL-LHC triplet (most pessimistic case with 100 T/m NbTi), one gets the most symmetric (25 ns) micro-batch filling scheme: • 10 x ( 7 x (24b+24e) + 12e ) +84e • (corresponding to 10 LHC injections and 1680 bunches) Illustration for 50 ns (courtesy G. Papotti) 8 April 2013 - e-cloud meeting

  6. HL-LHC: Microbatches * Stretched parameters ** injection/top energy *** sinel = 85 mb (COMPETE scaling law) **** Needed to achieve 250 fb-1 in 150 days • The reduction in the number of bunches (40%) obliges machine availability to increase up to 99% 8 April 2013 - e-cloud meeting

  7. HL-LHC: Microbatches Injection Top energy dmax, thres. MB dmax, thres. MB dmax, thres. 25ns dmax, thres. 25ns • There is a clear reduction of the e-cloud activity with respect to the 25-ns scheme • Nevertheless still far away from 50 ns (well outside the plots) • If the 25-ns filling scheme can’t be implemented due to e-cloud issues, this scheme could be another fallback option since it offers a very similar performance to the 50-ns scheme 8 April 2013 - e-cloud meeting

  8. HL-LHC: Reduced bs • Reducing bunch spacing  Straightforward way to reduce pile-up in the detectors • Bunch spacings explored: 12.5 ns, 5 ns, 2.5 ns and 1.25 ns Difficult Not possible (a priori) * Stretched parameters ** injection/top energy *** sinel = 85 mb (COMPETE scaling law) **** Needed to achieve 250 fb-1 in 150 days 8 April 2013 - e-cloud meeting

  9. HL-LHC: Reduced bs Injection Top energy • Lowest threshold  12.5 ns • Peak activity  5ns. Similar for 2.5 – 12.5 ns. Behaviour changes slightly with energy • Sharper behaviour at the threshold for shorter bunch spacings (especially at top energy) • 1.25 ns marks the backward tendence for e-cloud activity for further reduced bunch spacings • 5-ns scheme presents an almost identical behaviour at both energies (above threshold, which is the same) • All these schemes must be discarded for operation 8 April 2013 - e-cloud meeting

  10. HL-LHC: Reduced bs What about scrubbing? • A 12.5 ns scheme would scrub efficiently almost the entire region for 25 ns • A 5 ns scheme would scrub efficiently only the central part of the chamber (due to reduced bunch charge), but: • + We could use steering to displace the beam horizontally and scrub more • - Steering takes more scrubbing time (a higher machine availability needed) 8 April 2013 - e-cloud meeting

  11. HL-LHC: Bunch doublets (Idea and RF simulations by S. Fartoukh) • 800 MHz RF cavity could split a bunch into two “bunchlets” separated by 1.25 ns Not interesting: - Additional LRBBI - Incompatibility with crab cavities - Incompatibility with detectors • 400 MHz + 440 MHz RF cavities could split a nominal bunch into two “bunchlets” separated by 2.5 ns • Only possible at top energy thanks to a lower phase space filling factor RF cycle used for the splitting and recombination processes. One second appears to be a time sufficient for carrying out the two processes adiabatically. 8 April 2013 - e-cloud meeting

  12. HL-LHC: Bunch doublets (Cortesy S. Fartoukh) 8 April 2013 - e-cloud meeting

  13. HL-LHC: Bunch doublets • One advantage of this kind of beams is the possibility of recombination • With 25 ns, the optimum run time is shorter than the recombination time • In terms of luminosity performance, it is identical to pure 12.5-ns bunch spacing (56% machine availability needed) 8 April 2013 - e-cloud meeting

  14. HL-LHC: Bunch doublets • Similar behaviour as observed with regular bunch spacings (backward tendence for 1.25 ns) • Longer bunch length explored (10 cm, as nowadays in the LHC)  Negligible reduction • These scenarios can be also discarded 8 April 2013 - e-cloud meeting

  15. HL-LHC: Bunch doublets And what if instead of 25 ns we use 50-ns bunch spacing? • Luminosity performance similar to pure 25 ns with Nb=1.75∙1011 ppb (~70% machine availability), although still trecomb. > topt • e-cloud activity much worse than pure 50 ns but considerably better than the base line 25 ns • In case of long fills (for whatever reason), recombination could still be used • It could be another backup option for operation 8 April 2013 - e-cloud meeting

  16. HL-LHC: Summary • From all the explored filling patterns, only two show a reduced e-cloud activity with respect to the base line 25-ns bunch spacing: • Microbatches (with 25 ns)  Minimizes LRBBI potential issues • 50 ns doublets (with 2.5 ns bunchlet spacing)  Reduces pile-up • Both present a worse performance in terms of integrated luminosity compared to the base line 25-ns bunch spacing (99% and 70% vs. 60%) • Both could be considered as fallback solutions in case 25 ns cannot be injected • Other 25 ns schemes with “holes” generated at the PS can be explored 8 April 2013 - e-cloud meeting

  17. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  18. (V)HE-LHC: Parameters 8 April 2013 - e-cloud meeting

  19. PY* choice ∝ E3 ∝ E (Considering Cu-sawtooth) We normally use at 7 TeV the value 0.049 from V.V. Anashin et al. / Vacuum 60 (2001) 255-260 • PY* (effective photoelectron yield) is a controversial parameter: • There are not many measurements in general and none in particular for Ec=575 and 5475 eV • Different labs measure different values ([1], [2], [3], [4]) • Ec divides the photon energy spectrum in two. Does it assures an average PY*? • So: • PY*∝ Ec ? • PY* ≈ constant ? • PY* ∝ (Ec)-0.4 ? • Can we really establish a certain dependence between PY* and Ec? 8 April 2013 - e-cloud meeting

  20. PY* choice • We also have to take into account that high energy photons could penetrate into the material reaching deep layers where no e- could be liberated • Hence a non monotonic behaviour with energy can be expected • I’ve considered an intermediate dependence (same as for h): PY*∝ (Ec)2/3 ∝ E2 • In any case the number of photons is big enough to need to think about suppression measures (saw tooth pipe, coatings, biased photon stops at warm temperature, etc.) F. Zimmermann, SLAC-PUB-7238 8 April 2013 - e-cloud meeting

  21. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  22. HE-LHC: Results First suggested bunch spacing: 50 ns • Very high heat load  Photon absorption techniques must be applied 8 April 2013 - e-cloud meeting

  23. HE-LHC: Results First suggested bunch spacing: 50 ns • If a high photon trapping efficiency can be achieve, e-cloud would not be a problem with a 50-ns bunch spacing • In addition, pile-up rates are also very high • (295 events/crossing!) We could think of shorter bunch spacing 8 April 2013 - e-cloud meeting

  24. HE-LHC: Results First suggested bunch spacing: 50 ns We could think of shorter bunch spacing 8 April 2013 - e-cloud meeting

  25. HE-LHC: Results • Shorter bunch spacings show a sharper behaviour at threshold (as for the HL-LHC) • The thresholds for 25 ns (1.55) and 5 ns (1.45) are higher than 25 ns at the LHC • Operation with 12.5 could be in principle discarded (highest e-cloud activity) • For shorter bunch spacing the photon trapping efficiency does not play a major role 8 April 2013 - e-cloud meeting

  26. HE-LHC: Results I also have considered different values for R 8 April 2013 - e-cloud meeting

  27. HE-LHC: Results • The higher the e-cloud activity, the less important are the values of R and efficiency • SEY thresholds: • The bunch spacing adopted for the European HEP Strategy has been 25 ns, although the pile-up is still high (147 events/crossing) • If compatible with experiments, 5 ns would be a good backup option for operation: • Acceptable heat load levels after some scrubbing • Its lower bunch charge constraints the e-cloud activity to a central region, what eases the placement of clearing electrodes 8 April 2013 - e-cloud meeting

  28. HE-LHC: Scrubbing? • The different bunch charges places the stripes in different regions of the chamber • The central stripes can be very easily scrubbed with any of these filling schemes • For operation with 25 ns, beam steering would be needed with 12.5 or 5-ns beams 8 April 2013 - e-cloud meeting

  29. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  30. VHE-LHC: Results All results and conclusions are very similar to the ones at the HE-LHC First suggested bunch spacing: 50 ns • Very high heat load  Photon absorption techniques must be applied 8 April 2013 - e-cloud meeting

  31. VHE-LHC: Results First suggested bunch spacing: 50 ns • If a high photon trapping efficiency can be achieve, e-cloud would not be a problem with a 50-ns bunch spacing • In addition, pile-up rates are also very high • (342 events/crossing!) We could think of shorter bunch spacing 8 April 2013 - e-cloud meeting

  32. VHE-LHC: Results First suggested bunch spacing: 50 ns We could think of shorter bunch spacing 8 April 2013 - e-cloud meeting

  33. VHE-LHC: Results • Shorter bunch spacings show a sharper behaviour at threshold (as for the HL-LHC) • The thresholds for 25 ns (1.55) and 5 ns (1.45) are higher than 25 ns at the LHC • Operation with 12.5 could be in principle discarded (highest e-cloud activity) • For shorter bunch spacing the photon trapping efficiency does not play a major role 8 April 2013 - e-cloud meeting

  34. VHE-LHC: Results I also have considered different values for R 8 April 2013 - e-cloud meeting

  35. VHE-LHC: Results • The higher the e-cloud activity, the less important are the values of R and efficiency • Since machine parameters are very similar to the HE-LHC, SEY threshold are almost identical • The bunch spacing adopted for the European HEP Strategy has been 25 ns, although the pile-up is still high (171 events/crossing) • If compatible with experiments, 5 ns would be a good backup option for operation: • Acceptable heat load levels after some scrubbing • Its lower bunch charge constraints the e-cloud activity to a central region, what eases the placement of clearing electrodes 8 April 2013 - e-cloud meeting

  36. VHE-LHC: Scrubbing? • The different bunch charges places the stripes in different regions of the chamber • The central stripes can be very easily scrubbed with any of these filling schemes • For operation with 25 ns, beam steering would be needed with 12.5 or 5-ns beams 8 April 2013 - e-cloud meeting

  37. Outline • HL-LHC • 1.1 Motivation • 1.2 Microbatches • 1.3 Reduced bunch spacing • 1.4 Bunch “doublets” • 1.5 Summary HL-LHC • (V)HE-LHC • 2.1 Machine and beam parameters • 2.2 Photoemission yield choice • 2.3 HE-LHC: Results for different bunch spacings • 2.4 HE-LHC: Scrubbing? • 2.5 VHE-LHC: Results for different bunch spacings • 2.6 VHE-LHC: Scrubbing? • 2.7 Summary (V)HE-LHC 8 April 2013 - e-cloud meeting

  38. (V)HE-LHC: Summary • Results for both accelerators are very similar due to similar machine parameters • Due to very strong SR, anti-photon measures must be considered for machine operation (warm biased photon absorbers, saw-tooth surface, coatings, etc.) • If high photon trapping efficiency is achieved, e-cloud build-up would not present problems for a bunch spacing of 50 ns (but very high pile-up) • 25 ns has been the bunch spacing adopted by the European HEP strategy for both accelerators (still high pile-up) • 12.5 ns can be discarded  highest e-cloud activity, very low SEY threshold • 5 ns could be a nice backup scheme (if compatible with detectors): • Reasonable e-cloud activity after some scrubbing • More suitable for clearing electrodes • Low pile-up (~LHC @ 7 TeV) • For new machines, suppressing techniques shall be preferred • If, after suppressing techniques, scrubbing is needed for 25 ns operation, beam steering with 12.5 or 5 ns would be needed • Other additional measures must be considered (higher operating temperature of the beam screen, solenoids where possible, NEG coating in warm straight sections, etc.) 8 April 2013 - e-cloud meeting

  39. Thank you for your attention! 8 April 2013 - e-cloud meeting

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