Giovanni Rumolo, G. Iadarola and O. Dominguez in LHC Beam Operation workshop - Evian 2011, 13 December 2011 For all LHC data shown (or referred to) in this presentation: V. Baglin, H. Bartosik, P. Baudrenghien, G. Bregliozzi, S. Claudet, J. Esteban- Müller, G. Lanza, G. Papotti, F. Roncarolo, E. Shaposhnikova, L. Tavian Electron cloud effects in the LHC

Outline 2 Focus of this talk  Analysis of the 2011 observations and measurements – How we observe electron cloud in the LHC – Resume of the effect of the scrubbing run with 50ns beams (1 – 11 April 2011) – Experience and progress with 25ns beams → Historical: MD sessions from the 29 June 2011 to the 24 October 2011 → Scrubbing of unbaked/uncoated field free regions → Scrubbing of the arcs → Evolution of some beam observables – Concluding remarks

Electron cloud observation in the LHC 3 → The electron flux to the chamber wall  e is revealed through 1) Pressure rise 2) Heat load Beam chamber

Electron cloud observation in the LHC 4 → The presence of electrons with density  e around the beam causes 1) Beam coherent instabilities, single or coupled-bunch type, for the last bunches of a bunch train 2) Incoherent emittance growth, degrading lifetime, slow losses Beam Obviously, both  e and  e depend on the beam structure and on the surface properties, e.g. R 0 and  max  From the evolution of the observables during scrubbing, we can infer the decrease of  max !

2011 scrubbing run in one slide! 5 ⇒ A 10-days scrubbing run took place at the beginning of April 2011, during which 50ns spaced beams with up to 1020 bunches per beam were injected into the LHC and kept at 450 GeV/c. ⇒ It resulted into a very efficient machine cleaning – The dynamic vacuum decreased by one order of magnitude – The heat load on the beam screen in the arcs →was significant at the beginning of the scrubbing run →disappeared at the end of the scrubbing run, even with higher number of bunches injected – The average stable phase over the beam decreased by one order of magnitude – Instabilities and emittance growth, clearly visible at the beginning of the scrubbing run, disappeared at the end even with low chromaticity settings ⇒ After the scrubbing run and first test ramps, the machine became ready to operate for physics with 50ns beams ⇒ The number of bunches per beam was ramped up to its maximum (1380) within two months

 max in the arcs after the 50ns scrubbing 6 ⇒ Heat load measured during the ramp in physics fill 1704 compared with the one predicted by numerical simulations ⇒ The measured heat load is compatible with values of  max = for R 0 = ⇒ The expected  max threshold is about 450 GeV and 3.5 TeV mW/m Measurement in fill 1704 Simulation scan Simulations by H. Maury-Cuna

 max in the uncoated and/or unbaked sections: estimation technique 7 The evaluation of  max is done in the field-free regions in proximity of the pressure gauges – Used Beam1 data from gauges (Cu): VGI.141.6L4.B and VGPB.2.5L3.B – A solution (R 0,  max ) is found comparing the pressure rises  P i measured at different injections with the electron fluxes  i from simulations  Baked but uncoated: SEY ~  Length 0.3 m  Pumping speed from NEG and maximum for CH 4 ≈ 10 L/s NEG Measured pressures Simulated electron fluxes

 max in the uncoated and/or unbaked sections: results 8 Pressure rise measurements with 50ns beam to estimate  max in the field-free regions in proximity of the pressure gauges (R 0 ≈0.2) – Measurements done at the beginning and at the end of the scrubbing run – Measurements done during the 50ns operation of LHC (19 May) – As expected, we are asymptotically approaching the  max threshold for 50ns beams 29 June 2011, date of the first injections of 25ns beams in LHC Calculated threshold for 50ns beam On the 29 June, a new story begins, with the 25ns beams in LHC …

25ns experience in 2011 DATESHORT DESCRIPTION 29 JuneInjections of 9 x 24b trains per beam with different spacings between them 28 AugustFirst attempt to inject a 48b train: fast instability dumps the beam within less than 1000 turns after injection 07 OctoberHigh chromaticity (Q’ x,y ≈15): Injection tests with trains of bunches from the SPS + ramp & 5h store with 60b ( ) per beam 14 OctoberHigh chromaticity: injection of up to 1020 bunches per beam in 72b trains (decreasing spacings between trains:  s) October Injection of up to 2100 bunches in Beam 1 and 1020 in Beam 2 (1  s train spacing) Scrubbing 29/0614/1024/10

29/0614/1024/10 Pressure rise measurements during most of the 25ns fills were found hard to be used for the  max estimation because of beam losses leading to rapidly changing regimes After considerable 25ns scrubbing, i.e. at the end of the 24/10 MD session, 8 x 72b batches with different spacings could be injected for Beam 1 into the LHC and remain stable to allow the pressure values to level 10  max in the uncoated and/or unbaked sections: results (II)

Scrubbing with 25ns beam (~40h) has lowered  max to 1.35 ! Again, we are not far from the threshold for 25ns beams, but further scrubbing is needed 11 Start of 25ns beams in LHC Calculated threshold for 50ns beam Calculated threshold for 25ns beam  max in the uncoated and/or unbaked sections: results (II)

12  max in the arcs: estimation technique 14/10 24/1029/06 Measured heat loads [W/hcell] averaged over sectors from cell by cell data Five snapshots in the 25ns MDs to reproduce the measured heat load by simulations!

13  max in the arcs: estimation technique 24/10 fastBCT + bblength (B1) fastBCT + bblength (B2)

14  max in the arcs: estimation technique Measured heat load Simulated heat loads fastBCT + bblength (B1) fastBCT + bblength (B2) Simulator PyECLOUD

15  max in the arcs: results  max has decreased from the initial 2.1 to 1.55 in the arcs ! 29/0614/1024/10 Calculated threshold for 25ns beam (450 GeV) Calculated threshold for 25ns beam (3.5 TeV) R 0 = 0.7

16  max in the arcs: results Simulations   max fixed to 1.55 for the last fill on the 25 October Measurements  the energy loss per bunch is obtained from the stable phase shift Beam 1 ZOOM

 max in the arcs: results ⇒ Excellent agreement between bunch by bunch synchronous phase shift and simulated energy loss at the saturation of the e-cloud ⇒ Build up phase of the electron cloud still not reproduced by simulations with the same accuracy and level of detail – Simulation underestimates the primary electron generation? – In reality, stronger memory effect between batches →Larger R 0 ?  unlikely, because 0.7 is already a high value →Uncaptured beam between batches? (SPS experience) – Dynamic range of the measurements? – Energy loss from impedance, dominant for the first bunches in each batch and for the last batches in the full train? – Further check with the bunch by bunch position data from BQM ⇒ Model confirms cross-calibration between stable phase shift measurements to measured heat load data

Beam observables: Transverse emittances 14 October  batches injected with 4  s spacing, Q’ x,y =15 24 October  batches injected with 1  s spacing, Q’ x =3, Q’ y =15 Both beams still unstable in the two planes, or anyway affected by emittance growth Some visible benefits from scrubbing: – The effect of the electron cloud manifests itself later along the trains, in spite of the closer spacing! – First 1 – 2 trains seem to be hardly affected now – In general, improvement in vertical Lowering horizontal chromaticity did not seem to degrade the beam horizontally, but rather it slightly improved it: effect of scrubbing?

HEADTAIL simulations by Kevin Li Electron Cloud Instability What do we expect ? Calculated coherent ECI threshold for central density in dipoles is around  e =10 12 m -3 for nominal intensity at 450 GeV (simulations were run assuming the whole LHC made of dipoles) It can be stabilized with chromaticities Q’ x,y >15, but emittance growth due to electron cloud + chromaticity remains! Right plot shows that this could be achieved only for  max ≤ 1.5

Beam observables: Losses and lifetimes 24 October  batches injected with 1  s spacing Beam 1 Steady improvement visible on 2 nd and 3 rd train Fast losses Degrading lifetime Lifetime degrades and then recovers

Concluding remarks (25ns) 21 ⇒ Further scrubbing is needed to suppress the electron cloud ⇒ Tricky, as the efficiency of scrubbing decreases with scrubbing itself… – The electron dose measured in lab to decrease the  max on Cu by an extra 0.1 from 1.55 to 1.45 is about the same needed to decrease it from 2.1 to – The flux of scrubbing electrons decreases with lowering  max ⇒ Instability threshold for 25ns beams very close to the build up threshold −Not much margin to be in the comfortable situation of scrubbing without significant beam degradation ⇒ Significant extra gain could be boosted by −Multi-train injections from the SPS −Find a comfortably stable filling pattern at 450 GeV and ramp to 3.5 TeV to benefit from photoelectrons and from the lower electron cloud build up threshold  max (estimated)  max GeV)  max TeV) StSt (straight section) Beam screen (arcs)

Concluding remarks (50ns) 22 ⇒ As could be expected, before the 25ns beams in the LHC, the  max values were just about the build up threshold for nominal 50ns beams ⇒ After the 25ns MDs, the LHC beam chambers have been cleaned to  max values well below the build up threshold for nominal 50ns beams ⇒ Simulation work on  max thresholds as a function of bunch intensity is ongoing, but first results show little dependence at least up to bunch populations of 1.8 x ppb ⇒ If we keep the present level of conditioning, ‘ecloud-less’ operation of LHC with 50ns beams up to high intensities should be guaranteed (bar specific situations in common beam chambers, which need to be checked)  max (estimated)  max GeV)  max TeV) StSt (straight section) Beam screen (arcs)

Very special thanks to G. Iadarola, H. Bartosik, O. Dominguez, J. Esteban- Müller, and F. Roncarolo for their careful off-line analysis of large amounts of MD data and the huge simulation effort that improved the general understanding of electron cloud and scrubbing! Many thanks to V. Baglin, P. Baudrenghien, G. Bregliozzi, S. Claudet, G. Lanza, G. Papotti, E. Shaposhnikova, L. Tavian for all the beautiful data they kindly provided us with and the numerous discussions Thanks to G. Arduini, B. Goddard, V. Kain, K. Li, H. Maury-Cuna, E. Métral, S. Redaelli, B. Salvant, F. Zimmermann, and all those who promoted and/or actively participated in the MDs Thank you for your attention

Some references 24 ⇒ Past observations, measurements and studies – 2010 experience reviewed in the note “50 and 75 ns operation in the LHC: Vacuum and Cryogenics observations”, G. Arduini et al., CERN-ATS-Note MD – 2010 experience scrubbing run reviewed in several LBOC/LMC talks and in IPAC 2011 paper and talk “Electron Cloud observation in the LHC”, G. Rumolo et al., CERN-ATS – “Observations of Electron Cloud Effects with the LHC Vacuum System”, V. Baglin et al. TUPS01 in IPAC 2011 – “Electron Cloud Parameterization Studies in the LHC”, O. Domínguez et al., CERN-ATS – “Simulation of Electron-cloud Build-Up for the Cold Arcs of the LHC and Comparison with Measured Data”, H. Maury Cuna et al., CERN-ATS – “Review of beam instabilities in the presence of electron clouds in the LHC”, K. Li and G. Rumolo, CERN-ATS – “Injection into LHC of bunches at 25 ns spacing” G. Arduini, B. Goddard, et al., CERN-ATS-Note MD – “Benchmarking Electron-Cloud Build-Up and Heat-Load Simulations against Large-Hadron-Collider Observations” H. Maury-Cuna et al., CERN-ATS

 max in the uncoated and/or unbaked sections: method of estimation 25 The evaluation of  max is done in the field-free regions in proximity of the pressure gauges – Gauges explored (StSt): VGI.141.6L4.B and VGPB.2.5L3.B, Beam 1 data – Beams are injected with a filling pattern with different spacings, to try to extrapolate memory effects  Baked but uncoated: SEY ~  Length 0.3 m  Pumping speed from NEG and maximum for CH 4 ≈ 10 L/s NEG 3  s 2  s 1  s 16  s

26 The evaluation of  max is done in the field-free regions in proximity of the pressure gauges – The pressure rise  P is read at the gauge when injecting a new batch into LHC and is assumed to be proportional to the electron flux  e – Numerical simulations can be used for finding the pairs (R 0,  max ) compatible with the measured pressure rise ratio – By using more measured points, the point/region where the curves intersect will define the pair (R 0,  max ), or narrow range, solution of of our problem Simulated surface Plane of measured ratio (161) Intersection of the two in the (R 0,  max ) plane Plausible solution R 0 ≈0.2,  max =1.35 – 1.4  max in the uncoated and/or unbaked sections: method of estimation

27  max in the arcs: results Simulations   max fixed to 1.55 for the last fill on the 25 October Measurements  the energy loss per bunch is obtained from the stable phase shift Beam 2 ZOOM

 max in the arcs: results

Improvement visible on 2 nd and 3 rd train Beam observables: Losses and lifetimes 24 October  batches injected with 1  s spacing Beam 2 Fast losses Degrading lifetime