G. Rumolo, G . Iadarola , H. Bartosik , G. Arduini

1. Follow up on electron cloud studies in LHC:Possible strategy for scrubbing LHC for 25 ns operation G. Rumolo, G. Iadarola, H. Bartosik, G. Arduini

2. Outline • Recapitulation of the main facts (2011-2012 experience with 25 ns beams) • A possible path for LHC start up in 2015 • Use of the doublet beam • Motivation and experience • Options for a doublet beam in the LHC • Summary and back up scenario

3. Scrubbing in the arcs in 2011-2012 After 50 ns scrubbing (2011)

4. Scrubbing in the arcs in 2011-2012 Based on what we learnt from the triplets analysis With 25 ns scrubbing @450 GeV (2011 + 2012) • 450 GeV • Arc quads: still significant heat load (SEY ≈ 1.3) • Arc dipoles: e-cloud 10x – 15x lower (SEY ≈ 1.45, integrated effect about same as quads) • Beam still affected by e-cloud (emittance blow up, lifetime)

5. Scrubbing in the arcs in 2011-2012 • 4 TeV • SEY threshold in dipoles decreases (lower transverse beam sizes, photoelectrons) ? • SEY increases with magnetic field ? • Either way, the dipoles become again dominant and no further scrubbing occurs

6. Scrubbing in the arcs in 2011-2012 • Electron cloud not detrimental to beams in collision at 4 TeV (from emittance data), emittance blow up @450 • Arc heat load would exceed capacity of the cryogenic system (~ x2) extrapolating to full 25 ns beam @7 TeV • Do we have a means to achieve better scrubbing of the dipoles at 450 GeV ?

7. Proposed strategy for LHC start up in 2015 • Start up after LS1  Phase 1 (conditioning + 50 ns) • Unconditioned machine with vented and new surfaces • Need for decrease of SEY (scrubbing), but also decrease of desorption yield, h  “intense” conditioning phase • Further conditioning will benefit from enhanced synchrotron radiation at 6.5 – 7 TeV 6.5 – 7 TeV 450GeV δdip≥2.2 δdip≈2.2 δdip≤2.

8. Proposed strategy for LHC start up in 2015 • Phase 2 (25 ns) • Several improvements (thanks to TE/CRG, BE/RF, BE/OP) • Increased available cooling power of the SAMs  Faster injections • Heat load and stable phase shift measurements available online for scrubbing monitoring and steering • New monitoring equipment (e.g., 8 new thermometers in 3 half-cells of sector 45 to disentangle quad and dipole heating, new vacuum gauges) • Possibility to use doublet beams (see next slides) • Thermal cycle on the beam screen before scrubbing run to remove excess gas on wall ? (V. Baglin, in Chamonix Proc. 2003 and 2004) 6.5 – 7 TeV 450GeV δdip≤2. δdip≈1.45 δdip≤1.4

9. Motivation for the scrubbing beam dipoles with scrubbing beam Doublet beam, hopefully … (2015) 9

10. Scrubbing with 5 ns doublets • The 5 ns doublet beam exhibits a much lower multipacting threshold compared to the standard 25 ns beam LHC dipoles

11. Scrubbing with 5 ns doublets • The 5 ns doublet beam exhibits a much lower multipacting threshold compared to the standard 25 ns beam • Efficient scrubbing with the doublet beam expected from e- energy spectrum for a wide range of intensities • Population ≥ 0.8x1011 p/b preferable to cover similar horizontal region as the standard 25 ns beam with nominal intensity LHC dipoles LHC dipoles

12. 2012-13 experience with 5 ns doublet beam (SPS) • First machine tests in the SPS at the end of 2012-13 run in order to • validate the doublet production scheme at SPS injection • demonstrate the e-cloud enhancement • The production scheme based on splitting at injection has been successfully tested • Injection of two trains of 72 bunches with 1.7e11 p/doublet • Cycle included the start of acceleration to estimate capture losses (around 10%) 4 3 1st inj. 200 MHz RF Voltage [MV] 2 1 0 4 -2 0 8 2 6 Time [ms]

13. 2012-13 experience with 5 ns doublet beam (SPS) • Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the SPS arcs • Direct comparison of standard and doublet beam within the same supercycle 25ns std. (1.65e11p/bunch) the curves represent pressure gauges in the center of the SPS arcs (1.65e11p/doublet) 25ns “doublet”

14. 2012-13 experience with 5 ns doublet beam (SPS) • Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the SPS arcs • Direct comparison of standard and doublet beam within the same supercycle • Clear enhancement observed also in the dedicated e-cloud monitors (MBA and MBB), as predicted by PyECLOUD simulations Measurements Simulations

15. Schemes for doublet production • “Long” bunch splitting at SPS injection (5 ns doublets) • Extracting long bunches (~10ns) from the PS and capturing them in two neighboring LHC buckets  5 ns doublet spacing • Demonstrated in the SPS @26 GeV • Bunch splitting at high energy in the SPS (5 ns doublets) • By sudden phase jump by 180° and recapturing each bunch in 2 neighboring buckets • A controlled phase jump will be possible with new module presently under development for operation with ions (to be tested in 2014) • Preferably done at intermediate energy to clean up uncaptured beam before extraction and have shorter bunches at extraction • Not tested yet • “Long bunch splitting” at LHC injection (2.5 ns doublets) • Extracting long bunches (~5ns) from the SPS and capturing them in two neighboring LHC buckets  2.5 ns doublet spacing • Not tested yet

16. Schemes for doublet production • “Long” bunch splitting at SPS injection (5 ns doublets) • Extracting long bunches (~10ns) from the PS and capturing them in two neighboring LHC buckets  5 ns doublet spacing • Demonstrated in the SPS @26 GeV • Bunch splitting at high energy in the SPS (5 ns doublets) • By sudden phase jump by 180° and recapturing each bunch in 2 neighboring buckets • A controlled phase jump will be possible with new module presently under development for operation with ions (to be tested in 2014) • Preferably done at intermediate energy to clean up uncaptured beam before extraction and have shorter bunches at extraction • Not tested yet • “Long bunch splitting” at LHC injection (2.5 ns doublets) • Extracting long bunches (~5ns) from the SPS and capturing them in two neighboring LHC buckets  2.5 ns doublet spacing • Not tested yet

17. Possible issues investigated • In the SPS • Transverse beam stability due to enhanced e-cloud  losses and emittancegrowth  controllable with high chromaticity • Transverse damper  with new 200 MHz electronics will be able to damp the common oscillation mode • Acceleration: RF power limitation, longitudinal stability, LLRF  Slow cycle, maybe Q26 • Beam quality at extraction  need a different algorithm for BQM anyway • In the LHC (from LBOC + follow up) • BI  False readings of interlocked BPMs (up to 2-4 mm in “extreme” conditions of unbalance within a doublet), being followed up  MD foreseen at the SPS to check electronics of LHC BPMs with doublets • RF compatibility and ADT  OK • Beam induced heating  cos2 modulation of the beam spectrum, should not make more heating than 50ns with equivalent intensity/doublet

18. Proposed strategy for LHC start up in 2015: Summary 6.5 – 7 TeV 450GeV δdip≥2.2 δdip≈2.2 δdip≤2. • Intensity ramp up & physics with 25ns 6.5 – 7 TeV 450GeV δdip≤2. δdip≈1.45 δdip≤1.4

19. Back up strategy for LHC start up in 2015 6.5 – 7 TeV 450GeV δdip≥2.2 δdip≈2.2 δdip≤2. • Intensity ramp up & physics with 25ns 6.5 – 7 TeV 450GeV • With electron cloud (degraded beam, scrubbing) δdip≤2. δdip≈1.45 • With low e-cloud filling patterns δdip≤1.4