HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study - PDF document

CERN-ACC-SLIDES-2014-0068 HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study Presentation Possible Strategy for Scrubbing LHC for 25 ns Operation Rumolo, G (CERN) et al 27 November 2013 The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. This work is part of HiLumi LHC Work Package 2: Accelerator Physics & Performance . The electronic version of this HiLumi LHC Publication is available via the HiLumi LHC web site <http://hilumilhc.web.cern.ch> or on the CERN Document Server at the following URL: <http://cds.cern.ch/search?p=CERN-ACC-SLIDES-2014-0068> CERN-ACC-SLIDES-2014-0068

Possible strategy for scrubbing LHC for 25 ns operation G. Rumolo, G. Iadarola, H. Bartosik, G. Arduini Electron cloud simulation team: H. Bartosik, O. Dominguez, G. Iadarola, K. Li, S. Rioja Fuentelsaz, G. Rumolo, F. Zimmermann Acknowledgments: S. Claudet, L. Tavian (heat load data and general support for cryo), J. Esteban-Müller and E. Shaposhnikova (stable phase shift data), C. Zannini (power loss calculations from impedance), J. Wenninger, W. Höfle, G. Kotzian, D. Valuch, P. Baudrenghien, T. Lefevre, B. Salvant, V. Baglin, R. Cimino, M. Taborelli, BE-ABP, BE-RF, BE-BI, BE-OP, TE-ABT, TE-CRG, TE-VSC LMC, 27 November 2013

Outline o Recapitulation of the main facts (2011-2012 experience with 25 ns beams) o A possible path for LHC start up in 2015 o Use of the doublet beam Motivation • Options (and issues) for a doublet beam in the LHC • − SPS experience and comparison with simulations − Production schemes (SPS or LHC) − Possible issues in the SPS and LHC o Conclusions LMC, 27 November 2013 1

Scrubbing in the arcs in 2011-2012 4 10 After 50 ns scrubbing (2011) Heat load [W/hc/beam] 2 10 0 10 -2 Dipole 50 ns 10 Dipole 25 ns Quadrupole 50 ns Quadrupole 25 ns -4 10 1 1.2 1.4 1.6 1.8 2 2.2 SEY LMC, 27 November 2013 2

Scrubbing in the arcs in 2011-2012 25 ns 4 10 Based on what we learnt from the triplets analysis Heat load [W/hc/beam] 2 10 With 25 ns scrubbing 0 @450 GeV 10 (2011 + 2012) 450 GeV • Arc quads : still significant heat -2 10 load (SEY ≈ 1.3) • Arc dipoles : e-cloud 10x – 15x Dipole lower (SEY ≈ 1.45, integrated Quadrupole effect about same as quads) -4 10 • Beam still affected by e-cloud 1 1.2 1.4 1.6 1.8 2 2.2 (emittance blow up, lifetime) SEY LMC, 27 November 2013 3

Scrubbing in the arcs in 2011-2012 25 ns 4 10 Heat load [W/hc/beam] 2 10 0 10 4 TeV • SEY threshold in dipoles decreases (lower transverse beam sizes, photoelectrons) ? -2 10 • SEY increases with magnetic field ? Dipole • Either way, the dipoles Quadrupole become again dominant and -4 10 no further scrubbing occurs 1 1.2 1.4 1.6 1.8 2 2.2 SEY LMC, 27 November 2013 4

Scrubbing in the arcs in 2011-2012 25 ns 4 10 Heat load [W/hc/beam] 2 10  Electron cloud not detrimental 0 10 to beams in collision at 4 TeV (from emittance data), emittance blow up @450  Arc heat load would exceed -2 10 capacity of the cryogenic system (~ x2) extrapolating to Dipole full 25 ns beam @7 TeV Quadrupole  Do we have a means to -4 10 achieve better scrubbing of 1 1.2 1.4 1.6 1.8 2 2.2 the dipoles at 450 GeV ? SEY LMC, 27 November 2013 5

FAQ: how do we know it’s all electron cloud? The enhanced heat load at 4 TeV (in the dipoles) seems NOT to o Degrade the beam further a. Lead to further scrubbing (at least, visibly on the time scale of a fill) b. Is it electron cloud ? ① These observations are compatible with electron cloud: The beam is more rigid and other effects are dominant over the e-cloud a. Scrubbing in a cold dipole has saturated or become inefficient, at least with the b. same nominal 25 ns beam used to get to that point and at 4 TeV LMC, 27 November 2013 6

FAQ: how do we know it’s all electron cloud? ② Comparison between 50 ns and 25 ns 50 ns heating of the arc beam screen is fully consistent with impedance model • 50 ns Heat load measurement Estimation (impedance + synchrotron rad.) from cryogenics LMC, 27 November 2013 7

FAQ: how do we know it’s all electron cloud? ② Comparison between 50 ns and 25 ns 50 ns heating of the arc beam screen is fully consistent with impedance model • 25 ns heating is 10x the value expected from impedance and synchrotron radiation • 25 ns Heat load measurement Estimation (impedance + synchrotron rad.) from cryogenics LMC, 27 November 2013 8

FAQ: how do we know it’s all electron cloud? ② Comparison between 50 ns and 25 ns 50 ns heating of the arc beam screen is fully consistent with impedance model • 25 ns heating is 10x the value expected from impedance and synchrotron radiation • Lines of the 25 ns spectrum twice less dense than those of the 50 ns spectrum • LMC, 27 November 2013 9

FAQ: how do we know it’s all electron cloud? ③ Bunch-by-bunch stable phase shift measurements Clearly show the build up shape along the bunch train at 450 GeV • Reveal the same structure, but with steeper increase and larger saturation value, at • 4 TeV Fill 3429: 11 trains of 72 bunches Beam 2 LMC, 27 November 2013 10

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, η  “intense” conditioning phase ⇒ Further conditioning will benefit from enhanced synchrotron radiation at 6.5 – 7 TeV, especially during the intensity ramp up 450GeV 6.5 – 7 TeV Commissioning 50ns Vacuum conditioning Scrubbing (low (50ns/short trains 25ns) with 25ns (intensity ramp up + intensity/luminos (5-7 days) (2 days) physics) ity) δ dip ≥2.2 δ dip ≈2.2 δ dip ≤2. LMC, 27 November 2013 11 11

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 5 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) 450GeV 6.5 – 7 TeV Scrubbing with 25ns test ramps + 25ns scrubbing doublet beams commissioning (5 days) (≈ 5 days) (≈ 5 days) δ dip ≤2. δ dip ≈1.45 δ dip ≤1.4 LMC, 27 November 2013 12 12

Motivation for the scrubbing beam 25 ns 4 10 dipoles with scrubbing beam Heat load [W/hc/beam] 2 10 0 10 Doublet beam, hopefully … (2015) -2 10 Dipole Quadrupole -4 10 1 1.2 1.4 1.6 1.8 2 2.2 SEY LMC, 27 November 2013 13 13

Scrubbing with 5 ns doublets o The 5 ns doublet beam exhibits a much lower multipacting threshold compared to the standard 25 ns beam 11 x 10 2 Beam prof. [p/m] 10 4 LHC dipoles 2 Scrubbing dose (50eV) [mA/m] 0 10 0 10 20 30 40 50 60 70 3.5 0.50e11ppb -2 10 0.60e11ppb 3 0.70e11ppb N e / N e (0) 0.80e11ppb 2.5 0.90e11ppb -4 10 2 1.00e11ppb 1.10e11ppb 1.5 Std. 25 ns -6 1 10 10 20 30 40 50 60 70 1.1 1.2 1.3 1.4 1.5 Time [ns] SEY LMC, 27 November 2013 14

Scrubbing with 5 ns doublets o The 5 ns doublet beam exhibits a much lower multipacting threshold compared to the standard 25 ns beam o Efficient scrubbing with the doublet beam expected from e - energy spectrum for a wide range of intensities o Population ≥ 0.8x10 11 p/b preferable to cover similar horizontal region as the standard 25 ns beam with nominal intensity sey = 1.50 sey = 1.50 0 2 10 10 LHC dipoles 0.70e11ppb 0.70e11ppb LHC dipoles 0.80e11ppb 0.80e11ppb 1 10 0.90e11ppb Scrubbing current (50eV) [A/m 2 ] 0.90e11ppb Normalized energy spectrum -1 1.00e11ppb 10 1.00e11ppb nom. 25 ns nom. 25 ns 0 10 -2 -1 10 10 -2 10 -3 10 -3 10 -4 -4 10 10 0 200 400 600 800 1000 -15 -10 -5 0 5 10 15 Energy [eV] Position [mm] LMC, 27 November 2013 15

Scrubbing with 2.5 ns doublets o The 2.5 ns doublet beam exhibits lower multipacting threshold compared to the standard 25 ns beam, but higher threshold compared to 5 ns doublets o Similar e - energy spectrum as with 5 ns doublets o E-cloud build-up is concentrated in central part of the chamber  less favorable compared to the 5 ns doublets !! sey = 1.50 1 2 10 10 0.70e11ppb LHC dipoles 0.80e11ppb 0 1 10 10 0.90e11ppb Scrubbing current (50eV) [A/m 2 ] Scrubbing dose (50eV) [mA/m] 1.00e11ppb nom. 25 ns -1 0 10 10 0.50e11ppb -2 -1 10 10 0.60e11ppb 0.70e11ppb -3 0.80e11ppb -2 10 10 0.90e11ppb 1.00e11ppb -4 -3 10 1.10e11ppb 10 Std. 25 ns -5 -4 10 10 1.1 1.2 1.3 1.4 1.5 -15 -10 -5 0 5 10 15 SEY Position [mm] LMC, 27 November 2013 16