1. Collimation with retracted TCSGs R. Bruce, R. Kwee, S. Redaelli

2. Motivation N. Mounet, Evian 2014: for small emittances, beam stability might become an issue in Run 2 investigate possible ways of reducing impedance if needed Ways of reducing collimator impedance: Open collimators First results with IR3 more open shown by N. Mounet in CWG 04/11/2013. Some gain, but not dramatic Change optics Change materials This talk: Study the effect of opening secondary collimators in IR7 on cleaning and machine protection

3. Motivation for Simulation Completely retract single secondary collimators at IR7 Completely retract a selected set of secondary collimators at IR7 Select those collimators which still allow asynchronous beam dump protection (done by Roderik) Addressed: How does the cleaning inefficiency change for selected, retracted collimators (for B1 and B2, horizontal and vertical halo) compared to nominal settings? All my results were done in 2013 using the old SixTrack scattering routine Simulations at 7 TeV with nominal optics.

4. Lossmaps Simulate loss distributions for each retracted secondary collimator and selection of retracted collimators. IR7

5. “Benchmark clusters” benchmark cleaning inefficiency by 1.integrated cold losses at Q8-9 and Q11-12 2.maximum cold loss in either of the clusters (showing for better visibility only 10% of it)

6. Comparison for B1 Completely retract single TCSGs and selection of TCSGs selection 1: TCSG.A6L7.B1, TCSG.A6R7.B1, TCSG.A4L7.B1, TCSG.E5R7.B1 selection 2: ALL TCSGs selection 3: selection 1 +TCSG.D4L7.B1 selection 4: selection 3 + TCSG.D5R7.B1, TCSG.6R7.B1, TCSG.B5R7.B1 (only TCSG.B5L7.B1, TCSG.A5L7.B1, TCSG.B4L7.B1 left in) Q8-9 Q11-12

7. Similar Result for B2 Fully retract corresponding B2 TCSGs ● Conclusion: retracting a single TCSG hardly affects the cleaning. Retracting several TCSGs has a detrimental effect, and opening all (thus having the TCLAs as secondaries decreases performance by up to a factor 7. ● Selection 3 looks useful. Q8-9 Q11-12

8. Further studies for Run 2 ● Based on TCSG scan try to find option for possible OP configuration in Run 2, if beam stability turns out to be an issue ● Could imagine to use retracted TCSGs in squeeze only, and then insert them when beam is stabilized by collisions ● Selection 3 seems promising ● TCSGs selected based on phase advance from dump kicker, in order not to jeopardize machine protection ● With about half of the IR7 TCSGs open to 12 sigma, we should see a significant reduction of impedance... (see next talk) ● However, very significant increase in warm losses ● Solution: Don’t open TCSGs completely. Stay at 12 sigma

9. Configuration for further studies ● For better compatibility with 2015 operation: Look at 2 sigma retraction settings instead of nominal ● Using SixTrack to re-check cleaning and validate TCT/triplet protection during asynchronous dumps ● B1 (for now) ● Beta*=55cm, nominal optics ● Baseline settings: – TCP7 @ 5.5 sigma – TCS7 @ 7.5 sigma: TCSG.B5L7.B1, TCSG.A5L7.B1, TCSG.B4L7.B1, TCSG.B5R7.B1, TCSG.D5R7.B1, TCSG.6R7.B1 – Opening some TCS7 to 12 sigma: TCSG.A6L7.B1, TCSG.A4L7.B1, TCSG.A4R7.B1, TCSG.E5R7.B1, TCSG.D4L7.B1 – TCLA7 @ 9.5 sigma, TCS6 @ 8.3, TCDQ @ 8.8, TCT1/5 @ 10.3, IR3 @ nominal settings

10. Cleaning example: hor B1 2 sigma retraction settings 2 sigma retraction settings + Some TCSG7 @ 12 sigma … using SixTrack with updated scattering

11. Zoom in IR7 2 sigma retraction settings 2 sigma retraction settings + Some TCSG7 @ 12 sigma Can see differences in TCSG losses....

12. Summary of cleaning ● Looked at cases with hor, ver, skew halo, with standard 2 sigma retraction settings, and with in addition some selected TCSGs opened to 12 sigma. Increase of losses when opening some TCS to 12 sig: Hor. B1Ver. B1Skew B1 Highest cold loss60%50%70% (but high statistical error) Integrated losses cell 8-9, IR740%30%50% Integrated losses cell 11-12, IR740%20%50% Integrated warm losses, IR740%50%30% … quite significant increase in losses! Can we tolerate this?

13. Quench? Compare with FLUKA results in 2013 coll. Review (1) ● FLUKA result of IR7 power load with relaxed settings at 6.5 TeV shown in collimation review (E. Skordis https://indico.cern.ch/event/251588/session/1/contribution/16/material/slides/ 1.pdf) ● Radial average of head load of about 20mW/cm^3 for a 12 minute lifetime ● This should be on the safe side if quench limit is ~50mW/cm^3 (see talk P.P. Granieri in CWG 172 https://indico.cern.ch/event/304154/ ) ● With more conservative estimate of quench limit ~25mW/cm^3, it’s on the limit (A. Verweij in collimation review 2013 https://indico.cern.ch/event/251588/session/1/contribution/9/material/slides/0.pptx )

14. Quench? Compare with FLUKA results in 2013 coll. Review (2) ● Very rough assessment of quench: compare with losses in the underlying SixTrack simulation used for the review, using B2 relaxed settings. – Used back then OLD scattering – should introduce additional factor ~2 safety limit, since the FLUKA map was corrected with the higher SD cross section ● With some TCSGs at 12 sigma and NEW scattering: similar losses in IR7 DS. Both clusters and peak agree within 10% between a) relaxed with old scattering and b) 2 sigma retraction, but with some TCSGs at 12 sigma, and new scattering – With these settings, we should have a DS heat load significantly below what was shown in the review, possibly about a factor ~2. – Probably OK for quench, however, FLUKA simulation and/or experimental validation needed for more certainty

15. Loads on collimators ● Additional item to check: load on collimators ● Expect increased loads on the TCSGs left in if we rertract some to 12 sigma ● Also to be checked: TCLAs, TCTs (for background)

16. Loads on collimators Conclusion: TCPs: unchanged loads TCSGs: up to a factor ~2.5 higher load on the TCSGs left in. Increase only ~50% on most loaded (%-level) TCLAs: with vertical halo, one TCLA has increased load by factor 9. Acceptable? TCTs: with vertical halo, one TCLA has increased load by factor 9. => Higher background. Acceptable? A FLUKA sim. of real heat load during a lifetime drop could be useful, as well as dose to warm magnets B1H B1V B1skew

17. Asynchronous dumps ● Next step: verify machine protection ● Using modified SixTrack version to simulate asynch. dump (single-module pre-fire) ● Scanning TCT setting and recording the losses on TCTs. Example: TCT@ 6.9 sigmaTCT@ 8.1 sigmaTCT@ 10.3 sigma Losses on TCDQ and most loaded TCT vs bunch number Normalization: 1.5e11 p/bunch

18. Integrated losses over all bunches on TCT ● Integrated losses on most loaded TCT as function of setting (55 cm optics, 2σ retraction) (selected TCSGs at 12 sigma, the rest left at 7.5 sigma) (all TCSGs at 8.5 sigma) (all TCSGs at 7.5 sigma) With the selected TCSGs retracted, the TCT losses do not increase notably during asynch. dump (collimators chosen based on phase advance from MKDs) Visible effect if retracting ALL TCSGs by 1 sigma

19. Second most loaded TCT ● Too little statistics to draw conclusion on if it's worse or not. ● However, should not be a concern. We are here about factor 100 below damage limit.

20. Summary ● Retracting some TCSGs in IR7 could provide a significant gain in impedance and beam stability ● We can for example do this only in the squeeze, if stability is most critical there, to minimize the negative impact ● Studied configuration: Open about half of the IR7 TCSGs to 12 sigma, keeping the rest at 7.5 sigma ● cleaning is worse by some 20%-50% compared to same settings and no TCSGs retracted. Still better than relaxed settings and possibly still OK for quench at 12 minute lifetime ● Loads on other collimators increase. Most critical: One TCT increasing by factor 9 at V halo. Acceptable for background? ● No effect on TCT/triplet protection during asynch. dump ● Impact could possibly be acceptable, if it provides significant gain in stability, especially if used only in squeeze

21. Next steps ● Possible follow-up – Repeat cleaning + asynch dump study with 2 σ retraction, selection 3, for B2 – Maybe FLUKA study of dose to warm magnets and energy deposition in cold region with retracted TCSGs – Are increased loads on TCTs and TCLAs acceptable? – Estimates of gain in impedance and beam stability (first results in next talk) – If deemed promising and gain in stability is important, formulate MD proposal for 2015 – Obvious steps: Measure tune shifts / impedance, do loss maps, do asynch dump tests

22. Backup

23. Comparison for B2 selction 1: TCSG.A6R7.B2 TCSG.A4R7.B2 TCSG.A4L7.B2 TCSG.E5L7.B2 selction 2: all TCSGs selction 3: sel.1 + TCSG.D4R7.B2 selction 4: sel.3 + TCSG.D5L7.B2 TCSG.B5L7.B2 TCSG.6L7.B2 Fully retract corresponding B2 TCSGs Q8-9 Q11-12