Vacuum Cleaning / Scrubbing measurements in the LHC J.M. Jimenez on behalf of G. Arduini, V. Baglin, G. Bregliozzi, P. Chiggiato, G. Lanza, OP

J.M. Jimenez – LMC, 20 Oct’10 Main Topics Introduction Electron Cloud Build-up Vacuum Cleaning / Scrubbing Closing Remarks

J.M. Jimenez – LMC, 20 Oct’10 Introduction Electron Cloud induced Pressure Blow-up in the SPS ring Red = P >10 -4 Pa, Green = P < Pa Bending areasStraight areasBending areas Mid arc 1-2 Mid arc 2-3 Mid arc 3-4 Mid arc 4-5 Mid arc 5-6 Mid arc 6-1 Mid arc 1-2 Mid arc 2-3 Mid arc 3-4 Mid arc 4-5 Mid arc Before the Scrubbing Run -

J.M. Jimenez – LMC, 20 Oct’10 Introduction [cont.] Electron Cloud Build-up Driving parameters Bunch Intensity –Threshold phenomenon Ramp and RF can shrink the bunches resulting in an increase of the beam potential and thus the energy kick to the electrons Bunch train –Number of bunches required to start the electron avalanche (18/20 1.1x10 11 p/bunch) Filling Pattern –Spacing between bunch train –Surviving low-energy electrons, seed electrons 3.5 TeV) Secondary Electron Yield (SEY)  –Reflectivity of low-energy electrons Stimulated Desorption Yields  –Energy of the electrons, state of the surface, temperature has limited effect Other parameters affecting the electron cloud build-up –Size of the beam vacuum pipe –Magnetic field: field free, dipole and quadrupole fields, crossing angles and orbit modifications / displacements, stray fields (solenoid) –Temperature of the beam pipe walls: effect of condensed gasses

J.M. Jimenez – LMC, 20 Oct’10 Introduction [cont.] Electron Cloud limiting factors in LHC Vacuum pressure rise –Interlock to the vacuum sector valves triggering the beam dump Cryogenic cooling capacity –Cannot run the scrubbing run above the cooling capacity in the capilaries Beam stability –Bunch instabilities Beam-gas scattering induced radiation –Radiation dose rates to cables and single events on electronics –Quench limit of the cryomagnets Background to Detectors –Limit acceptable by the detectors

J.M. Jimenez – LMC, 20 Oct’10 Electron Cloud Build-up Evidence of surviving electrons between bunch trains 225 ns between bunch trains550 ns between bunch trains 5.25  s between bunch trains Measurements made with 25 ns bunch spacing Low-energy electrons can survive long gaps without beams thus inducing a crosstalk between bunch train’s build-ups

J.M. Jimenez – LMC, 20 Oct’10 Electron Cloud Build-up [cont.] Surviving low-energy electrons and seed electrons Measurement made with 2 trains of 24 bunches with a 50 ns bunch 450 GeV Crosstalk between bunch train’s build-ups start at 10  s, increasing very quickly below 3  s bunch train spacing Seed photo-electrons to be considered above 2 TeV Secondary Electron Yield (SEY) measurement of a scrubbed copper sample Low-energy electrons are reflected by the surface R. Cimino, I.R. Collins, App. Surf. Sci. 235, , (2004)

J.M. Jimenez – LMC, 20 Oct’10 Vacuum Cleaning / Scrubbing Guidelines from SPS Measurements Measurements of the Pressure decrease in the SPS as a function of the cumulated LHC-type beam time Dipole field conditions showed a decrease by 15 in 58 hours Measurements of the Pressure decrease in the SPS as a function of the cumulated LHC-type beam time Field free conditions showed a decrease by 50 in 58 hours

J.M. Jimenez – LMC, 20 Oct’10 Vacuum Cleaning / Scrubbing IR1 Arcs extremities, left/right sides: Beam 1 (Blue) Beam Intensity on Beam 1 Very stable during the Fill

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR1 Triplets, left side: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR1 SAMs, left side: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR4 SAMs, left/right sides: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR1 LSS RT, left/right sides: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR4 LSS RT, left/right sides: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing IR6 LSS RT, MKD zone: Beam 1 (Blue)

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing Expected decrease of pressures with beam time No correlation between P Beam and position along the ring Pressure increase reduced by factor 2/3 within 3 hours

J.M. Jimenez – LMC, 20 Oct’10 Normalized  P vs. Cumulated Time of Beam in the machine 48Bunches at 450GeV – Buckets 31/10 01/11 04/11 All data refer to the NEG-NEG Transition in LSS3 for the same beam configuration at 00:18 AM: Fill 1459 with bunch spacing of 50ns 108 bunches and 12 buckets injection at 3:40:00 AM: Fill 1466 with bunch spacing of 50ns 108 Bunches and 24 buckets injection. During that fill due to the ALICE solenoid off/on the pressure around the IP2 is very high and variable. Did not succeed to ramp this fill at 3.5TeV before the “scrubbing” period was over. Pressures reached after the Scrubbing Run compatible with design values

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing Expected decrease of pressures with beam time Linear scale Valid for a given bunch intensity and filling pattern Cleaning/Scrubbing ONLY if running with ECloud! Memory effect will stay (partly/totally) for other schemes

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing  P as a function of the number of injected trains

J.M. Jimenez – LMC, 20 Oct’10 Beam Cleaning / Scrubbing Expected decrease  and  (calculations)

J.M. Jimenez – LMC, 20 Oct’10 Mitigation Solutions [Preliminary] Scrubbing Runs –Use maximum bunch intensity –Increase the number of bunches per train –Keep the beam potential at the highest – avoid beam emittance blow-up to keep the energy lick of the electrons to the maximum –Adjust the pressure interlocks to higher value where feasible Solenoids –Only applicable on Cold/Warm transitions and bellows of the long straight sections housing the Experimental Areas (IR1/2/5/8) Re-cooling sequence of SAM in case of failure of the cryogenics –Beam Screen is kept at a higher temperature than Cold Bore during normal cool down – Standard procedure –In case of stoppage of a cryoplant, same procedure shall be applied during the re- cooling of the SAMs Takes longer BUT is absolutely required to avoid gas condensation on Beam Screens!

J.M. Jimenez – LMC, 20 Oct’10 Closing Remarks Vacuum Cleaning versus Beam Scrubbing G. Vorlaufer et al., CERN VTN, 2000  max V. Baglin et al., Chamonix, 2001 Log scale for  versus linear scale for  (scrubbing) 6 orders of magnitude on  while  goes down to 1.4  impacts the pressure rise as  affects the electron cloud density Electrons with energies between 5 and 50 eV decrease  BUT their efficiency on the reduction of  is significantly lower Same dose

J.M. Jimenez – LMC, 20 Oct’10 Closing Remarks Evolution of the SEY=f(E) with the conditioning Courtesy of B. Henrist and N. Hilleret Total number of electrons contributing to the build up Number of secondary electrons generated by a primary electron