1. LHC Scrubbing Runs Overview H. Bartosik, G. Iadarola, K. Li, L. Mether, A. Romano, G. Rumolo, M. Schenk, G. Arduini ABP information meeting 03/09/2015 Many thanks to all the people who participated!

2. Scrubbing to mitigate electron cloud ABP information meeting 03/09/20152 Electron clouds*  Build up at each passage of a bunch train due to secondary electron emission (above certain threshold yields)  Many unwanted effects: beam degradation, heat load, vacuum degradation… Scrubbing  Secondary electron yield (SEY) can be lowered through electron bombardment  Reduce SEY by exposing machine to maximum possible electron cloud dose LHC scrubbing runs  10 days scrubbing for 50 ns physics ramp-up (June 24 – July 3)  2 weeks scrubbing for 25 ns physics (July 25 – August 8) * See April 2015 meeting, presentation of G. Iadarola

3. Introduction ABP information meeting 03/09/20153 Electron clouds*  Secondary electron emission (above certain threshold yields) causes build-up of electron clouds  Many unwanted effects: beam degradation, heat load, vacuum degradation… Scrubbing  Secondary electron yield (SEY) can be lowered through electron bombardment  Reduce SEY by exposing machine to maximum possible electron cloud dose LHC scrubbing runs  10 days scrubbing for 50 ns physics ramp-up (June 24 – July 3)  2 weeks scrubbing for 25 ns physics (July 25 – August 8) * See April 2015 meeting, presentation of G. Iadarola 50 ns Scrubbing 25 ns Scrubbing Doublet Scrubbing Doublet beam ~900 doublets 0.7 x 10 11 p/b 25 ns beam ~2800 bunches 1.15 x 10 11 p/b 50 ns beam ~1400 bunches 1.7 x 10 11 p/b

4. 4 LHC Scrubbing Run I First phase with 50 ns beams  Quick improvement of beam quality  Total intensity limited by vacuum in injection kickers (MKI)  SEY in the arcs quickly approached multipacting threshold (as shown by heat load measurement) ABP information meeting 03/09/2015

5. 5 LHC Scrubbing Run I Second phase with 25 ns beam  Important boost to scrubbing efficiency  Beam often unstable, more challenging operation  Beam quality severely degraded, gradually improving with scrubbing  Vacuum in MKI8 was limiting the intensity which could be injected for Beam 2 ABP information meeting 03/09/2015

6. 6 LHC Scrubbing Run II: Week 1 Intensity ramp up with trains of 72 bunches  Vacuum in TDI8 limiting injections in Beam 2  Filling the beams difficult due to large number of injections (up to 39/beam) Injection of trains of 144 bunches  Gradually reducing the spacing between injected trains of 144b  First intense scrubbing period at the end of the week ABP information meeting 03/09/2015

7. 7 LHC Scrubbing Run II: Week 2 Continued scrubbing with trains of 144 bunches  Injection process in general slowed down by cryogenics  Injection of Beam 2 severely slowed down after ~1000b because of vacuum in TDI and/or in the MKI ABP information meeting 03/09/2015

8. 8 LHC Scrubbing Run II: Week 2 Finished with validation fills for physics, with 1176 bunches per beam  Trains of both 72 and 144 bunches  Identified settings to guarantee beam quality: Q’H = 15 (no margin), Q’V = 10 Octupole knob set to -1.5, damper gain 0.25  Lifetime at end of injection worse for Beam 1 (~10h) than for Beam 2 (~25h) ABP information meeting 03/09/2015

9. 9 Scrubbing Run II: Arc heat loads Hard to spot a decreasing trend in heat load, but beam quality steadily improving … ABP information meeting 03/09/2015

10. 10 Scrubbing Run II: Losses Packing trains closer together, and exploring settings Fixed filling pattern, still trying different settings Evolution of losses for fills with trains of 144 bunches  Diminished losses due to parameter optimisations, made possible by scrubbing ABP information meeting 03/09/2015

11. 11 End of Scrubbing Run II: Beam quality ABP information meeting 03/09/2015

12. 12 Conclusions Scrubbing for 50 ns physics  Quickly reached an SEY in the arcs close to multipacting threshold for 50 ns  Thanks to scrubbing with 25 ns, beam quality for 50 ns at end of run excellent Scrubbing for 25 ns physics  Injection speed in general limited by cryogenics  Number of injections limited for Beam 2 by vacuum in TDI and MKI  Clear improvement of beam quality thanks to scrubbing and adapted machine settings, but electron cloud still present  Beam quality after the injection process still good for 1176 b. (physics filling schemes), with validation fill settings ABP information meeting 03/09/2015

13. Thank you for your attention! ABP information meeting 03/09/201513

14. 14 LHC Scrubbing Run II: Losses Fill 4086 Fill 4167 Beam 1 ABP information meeting 03/09/2015

15. 15 LHC Scrubbing Run II: Losses Fill 4086 Fill 4167 Beam 2 ABP information meeting 03/09/2015

16. 16  Heat load still significant for the ‘physics’ fills. Two potential problems:  Transient during the ramp: The heat load increase during the ramp should be handled by the cryogenic system  Heat load at 6.5 TeV: Looking at the outliers (S12, S23), a fourfold increase of the heat load (like in 2012!) would be already beyond the cooling capacity of the cryogenic system. And Q6L8? ABP information meeting 03/09/2015

17. 17 The LHC Scrubbing Run 1 Validation with 50 ns beam: o No visible degradation from ecloud on bunch-by-bunch intensity and emittance o No ecloud pattern on bunch-by-bunch stable phase measurement Beam 2 Beam 1 Beginning of the scrubbing run50 ns validation fill ABP information meeting 03/09/2015

18. 18 SEY evolution as extrapolated from HL measurements 50 ns25 ns ABP information meeting 03/09/2015

19. 19 Heat load on arc beam screens: overview 50 ns25 ns Switching to 25 ns is the only key to efficient scrubbing, although it makes beam operation more challenging!!! ABP information meeting 03/09/2015