LHC Scrubbing Runs

LHC Scrubbing Runs J.M. Jimenez On behalf of the Electron Cloud Study Team, a Collaboration between AT and AB Departments

Main Topics • Introduction • Review of the Limitations • Expectations from the Scrubbing Runs • Main Results in the SPS • Electron Cloud Build up • Vacuum Cleaning and Beam Conditioning • Role played by the Physisorbed Gasses • Diagnostics foreseen in the LHC • RT and Cryogenic Vacuum and Electron Cloud Pilot Sectors • Scrubbing runs scenarios

Introduction (1)Review of the Limitations

Introduction (2)Expectations from the Scrubbing Runs • Scrubbing runs shall improve: • The dynamic pressure by: • Reducing the Photons (synchrotron radiation) and Electrons Stimulated Desorption yields • The electron stimulated desorption by reducing the electrons (electron cloud) flux to the wall as a consequence of the reduction of the SEY • Vacuum Cleaning Effect • The electron cloud-induced heat loads, beam instabilities and emittance growths by: • Reducing the SEY i.e. the total number of electrons in the cloud • Beam Conditioning Effect

Courtesy of B. Henrist and N. Hilleret Introduction (3)Beam Conditioning Mechanism Evolution of the SEY=f(E) withthe conditioning Total number of electronscontributing to the build up Number of secondary electronsgenerated by a primary electron

Main Results in the SPS (1)Electron Cloud Build-up …is a threshold phenomenon: • 3 1010 p/bunch in Dipole Field regions • 5 1010 p/bunch in Field Free regions …Intensity varies linearly with the bunch intensity • Non-homogeneous spatial distributions in dipole and quadrupole fields …Intensity varies linearly with the filling pattern • No extinction during the 225 ns batch spacing  evidence of surviving electrons • In the RT parts, NEG coatings will decrease the electron cloud activity due to their intrinsic low SEY: 1.1 after activation and 1.3 if saturated by water

Main Results in the SPS (3)Vacuum Cleaning and Beam Conditioning • Vacuum Cleaning observed in the SPS during the 25 ns / 75 ns Periods  Factor 10 during the 75 ns period, 100 during the 25 ns period for both FF and DF • Beam Conditioning on the cold surfaces has been observed in the SPS, probably a different physical process than at RT • Beam Conditioning @ 75 ns • Dipole field at Cold  Factor 100 after 7 hours • Beam Conditioning @ 25 ns • Dipole field at Cold  Factor 10 after 1½ day • Quadrupole field at RT  Factor 2.5 after 2 days • Gasses physisorbed on the cryogenic surfaces will play a predominant role (see next slide) • Effect of a Cycling in Temperature As expected from Lab measurements, a water condensation will reset the beam conditioning. A temperature cycling did not helped to recover the initial value.  Electron cloud intensity back to the initial value before the conditioning

Courtesy of V. Baglin (CERN) Main Results in the SPS (5)Role played by the Physisorbed Gasses • Physisorbed water identified as a potential problem: • Conditioning has been observed in the SPS if the cold detector is protected against water back streaming from the unbaked parts • In the LHC, low water coverage is expected: • Pumping down to 10-6 torr of the cold parts prior to the cooling • Controlled cool down sequence where the cold bore is cooled while the beam screen is kept as warm as possible • A air leak in the arcs will have severe consequences on the operation

Reduction of the SEY by the physisorbed CO Starting point – baked sample Starting CO injection Courtesy of B. Henrist and N. Hilleret (CERN) 4-7 monolayers of CO Main Results in the SPS (6)Role played by the Physisorbed Gasses CO2 and H2O could have a detrimental effect as the others like H2 and CO will decrease the SEY

Diagnostics foreseen in the LHC (1) Electron Cloud and Vacuum Pilot Sectors • The SPS electron Cloud test bench will be kept operational for the dedicated electron cloud studies while running the LHC • The LHC diagnostics will be designed to provide indications on: • The beam conditioning levels of the non-NEG coated RT components and the cryogenic sections • The vacuum cleaning efficiency i.e. the dynamic pressure decrease on both the RT and cryogenics sections • The saturation level of the NEG coated sections  triggering of reactivations • These diagnostics will be dedicated to the operation follow-up and not to the electron cloud studies  If used during MDs, the subsequent additional conditioning of the detectors will result on an unknown “shift” between the detectors status as compared to the LHC machine.

Diagnostics foreseen in the LHC (2) Electron Cloud and Vacuum Pilot Sectors …aimed to provide indications on the: • Vacuum Cleaning • Pressure rises and partial pressure evolution • Quantity of gas released by the photon and electron bombardment and physisorbed within a given period of run  recalculate the evolution of the desorption yields (η)  requires a specific sectorisation • Beam Conditioning • In situ SEY measurement  only achievable in field free condition • Decrease of the electron cloud flux to the wall  Strip Detectors or Electron Collectors • Decrease of the electron cloud density  Swapping detector (R. Macek) • Decrease of the e- cloud-induced heat load  Calorimetric measurements: dedicated calorimeters or upgraded thermometry and flow rate measurements on the magnets Q5 and D2Q4 in IR4 or IR5  ECR proposal being prepared  IR5 is accessible with beam  important for delicate measurements • NEG saturation level • Bayard-Alpert Pressure gauges • Hydrogen transmission facility • Partial pressure evolution with time i.e. Hydrogen become “visible” or CO or CO2 crossing the CH4 level

Scrubbing Runs Scenarios (1)Maximizing the Efficiency… … the beam emittance, beam losses and beam instabilities has to remain under control • Not an issue in the SPS since a new beam is injected every 21 s • Bunch Spacing, Filling Pattern and Beam energy • Bunch Spacing • No limitation expected with the 75 ns bunch spacing up to nominal • No limitation expected with the 25 ns bunch spacing if < 3 1010 p/bunch • > 3 1010 p/bunch  will depend on the induced heat load provided that beam instabilities and emittance growths are kept under control ~8 1010 p/bunch is the expected limit prior to the beam conditioning • Filling pattern • A modification of the filling pattern by increasing the gaps (RHIC case) between the batches shall be preferred to the reduction of the bunch intensity: • Displacement of the lateral strips in a dipole field with the bunch intensity • Decreases the average energy of the electrons from the cloud  reduction of the conditioning efficiency

Scrubbing Runs Scenarios (2)Maximizing the Efficiency… • Scrubbing runs at injection energy • Will increase the cooling budget available for the electron cloud-induced heat load, • Will only work if not limited by other effects like beam instabilities or emittance growths, • Will require a short scrubbing run period @ top energy in case of a small orbit displacement during the ramp in energy • De-conditioning effect • Has been observed both in the Lab and in accelerators (EPA and SPS) when the surface is no longer bombarded • Subsequent conditioning is 10 times faster • Is expected as a consequences of a partial warming up of the cold parts during the shutdown  physisorbed gasses will go back to the gas phase and be recondensed during the following cooling down • Part of the molecules in the gas phase will be pumped by the mobile pumping stations

Bunch length shortenedduring the ramp Scrubbing Runs Scenarios (3)Energy Ramp induced an Orbit Displacement Ramp to 450 GeV Ramp to 55 GeV

Scrubbing Runs Scenarios (4)Maximizing the Efficiency… • The continuous bombardment of the surfaces by the electrons will release a non negligible amount of gasses which will: • be pumped by the NEG coatings in the RT parts and contribute to their saturation  re-activation required  several weeks or • be transferred to the cold bore in the cryogenic systems due to the high desorption yields of the electrons (e- cloud). Thick coverages will lead to a high equilibrium pressure in the arcs and therefore thermal cycling shall be foreseen to remove these gasses physisorbed mainly on the cold bore or on the less bombarded beam screens surfaces. • This operation requires a long stop and therefore shall take place during the shutdown  free of cost since the cooling is stopped

Scrubbing Runs Scenarios (5)Maximizing the Efficiency… • Vacuum cleaning and Beam conditioning expected • Scrubbing runs required Prior to the shutdown allow the temperature cycling and NEG reactivationAfter the upgrade of the diluters: • Short scrubbing run at start-up • Long Scrubbing run right before the shutdown…non-contractual dates and scheduling…

Conclusions • No limitation is expected with the 75 ns beams • The electron cloud build up always saturates and should not lead to over runs like the ion instability avalanche • If not dominated by the beam instabilities and emittance growths, the available cooling capacity should allow running with intensities up to 8 1010 p/bunch in presence of an electron cloud • Scrubbing runs will be required if running with intensities > 5 1010 p/bunch with a 25 ns bunch spacing: • After shutdowns or after venting of an arc sector • In case of long stops > 1 month (de-conditioning) • In case of several quenches or severe leaks (see V. Baglin’s talk) • Beam Conditioning and Vacuum Cleaning can only take place in presence of an intense electron cloud • A temperature cycling of the beam screen will help in removing the physisorbed molecules by sending them back to the gas phase could not help to remove condense water

On going studies in 2005… …SPS is stopped, no result shall be expected. …will continue the: • Laboratory measurement on the effect of the physisorbed gasses on the SEY and ESD and mainly when using gas mixtures. • RHIC collaboration: Installation of a set of detectors (strip detector, pick-ups and retarding field detectors) to study the effect of the filling pattern and the role played by the surviving electrons.

Acknowledgements • Many thanks to G. Arduini, V. Baglin, P. Collier,O. Gröbner, G. Ferioli, J. Hansen, B. Henrist, N. Hilleret, L. Jensen, D. Schulte, P. Strubin, A. Rossi, F. Ruggiero, J. Wenninger and F. Zimmermann for their help • To the operation crew of both PS and SPS • To the AT/VAC-SL Section collaborators for the installation of the detectors • To the TS Department collaborators for the design and manufacturing of the detectors

LHC Scrubbing Runs