1 Possible Vacuum Issue V. Baglin CERN TE-VSC, Geneva Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October LHC present collimator’s base line 2. Cold / Warm Collimator 3. Some though and observations 4. Conclusions

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Present LHC Collimator’s Base Line

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Vacuum Specification Ref : Vacuum requirements for the LHC collimators, EDMS and Invitation for tender FC 553EDMS Bake able to 250 deg during 24h. Minimum cycle of 60 over 20 years Cleaned, no traces of hydrocarbons, organic and inorganic residues (partial pressure < mbar) Leak tight : global helium leak rate < mbar.l/s Outgassing rate mbar.l/s.cm 2 i.e. furnace treatment at 1000 deg under vacuum (carbon surface, stainless steel, ferrites …) Total outgassing rate ~ mbar.l/s Transition to the standard aperture (ID80) in the collimator tank Permanent bakeout system and remotely controlled (reduce dose to personnel, ALARA) Avoid trapped volumes No in-situ welding (requires in-situ cleaning) Number and length of weld should be minimised (leaks due corrosion) No crossing welds Fully penetrating welds and brazing between vacuum and cooling circuit are not allowed Easy access to potentially leaking components (reduce dose to personnel) Bellows should be bake able and leak tight after 4000 cycles Quick release flange with a Conflat knife design

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Material List Each components were carefully selected and vacuum cleaned according to UHV CERN standards (EDMS ) Stainless steel vacuum chamber Stainless steel welded bellows Tungsten, copper and graphite jaws Glidcop Ferrite to damped HOM Cu/Be Ag coated and Cu/Ni RF contacts Ceramic temperature sensors Temperature sensors feedthrough Kapton cable ….

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October LSS layout At each end of a collimator is connected a pumping dome (VPIA) It allows an exchange of a collimator when required

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Cold/Warm Collimator

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Warm Collimator This solution offers : -A bake able collimator -A vacuum sectorisation to allow a “fast” exchange of a collimator -A port to allow RF ball test to check beam aperture before cool down (PIM !) -Access to vacuum system to boost the pumping speed if required (allows to upgrade the vacuum performances) Cryostat (“by- pass”) (QTC) Cryostat (“by- pass”) (QTC) Collimator Module (TCLD) A. Bertarelli et al. This solution needs to be shorten and revisited

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October What about a Collimator Operating at Cryogenic Temperature ? This solution : -Does not offer any of the previous aspect -But is smaller D. D. Ramos et al. This solution needs to be fully evaluated theoretically and experimentally by VSC

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Outgassing rate baked vs. unbaked Total outgassing rate: - 2 orders of magnitude lower with bake out J. Kamiya et al., EVC 11 Salamanca, 2010 Residual gas analysis: - H 2 O is the main component before bake out (65 %) - H 2 is the main component after bake out (85 %) Q (N 2 eq) [mbar l/s] Unbaked7.3x10 -6 After 1 st bake-out6.7x10 -8 After 2 nd bake-out5.2x10 -8 After 3 rd bake-out4.0x

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Unbaked/baked what does it mean ? Unbaked state : -The total outgassing rate of an unbaked collimator is equivalent to ~ 400 m of unbaked cold bore concentrated in 1 m length - In terms of gas load, it is equivalent ~ 300 x the synchrotron radiation gas load -During bake out, the total amount of removed H 2 O is equivalent to at least 100 monolayers desorbed from a 1 m long cold bore Baked sate: - This is the LHC base line - The total outgassing rate of a baked collimator is equivalent to ~ 10 km of stainless steel ID80 vacuum fired beam tube This quantity of gas will be available for future stimulated desorption Despite all precautions taken for LHC, there are still operation issues from time to time …

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Jaws : RF contact’s Movement Moving the collimator’s jaws induce gas desorption (due to friction) Hydrocarbons show up and gas load x 100 The outgassing rate is linear with the displacement length ~ mbar.l/s.mm A lot of hydrocarbons are degassed under displacement No differences with an unbaked collimator (except water) J. Kamiya et al. IPAC 2010, Kyoto, Japan. Almost no conditioning is observed after a few tens movements Observed in the LHC machine CO and CO 2 outgassing rate are ~ mbar.l/s for a 30 mm displacement LHC life time: cycle (1.2 km) i.e. large gas load J. Kamiya et al. EVC , Salamanca, Spain. The desorption due to friction must be reduced

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Some Though and Observations

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Operating temperature of Beam Screens LHC beam screens operate in the range 5 to 20 K (EDMS ) Holes are produced through the beam screens to allow pumping of H 2 vapour pressure onto the cold bore The H 2 saturated vapour pressure decrease from mbar at 4.2 K to mbar at 2 K C. Benvenuti et al. J. Vac.Sci.Technol., Vol. 13. No6, Nov/Dec 1976

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Why perforated Beam Screens ? SSC studies in 1994 V.V. Anashin et al. J. Vac.Sci.Technol. A. 12(5), Sep/Oct 194 No perforationsWith perforations

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Vacuum Transients Transients are due to an excess of physisorbed gas onto the beam screen : beam screen’s surface must be bared Their level varies with the gas species, the local pumping speed, the temperature, the driving mechanism (temperature excursion, electron cloud, synchrotron radiation, ion bombardment, particle loss …) Appropriate cooling scenario with decoupling between cold bore and beam screen with possibility of BS warming up to 80 K have been implemented in the LHC base line V. Baglin, Chamonix 2004 In a LHC-type mock –up (SR driven) In LHC (T driven) Fill 2177, 1 st October 2011

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Possible operating temperature of cold collimator To provide necessary cooling power (~ 150 W during 10 s and ~ 30 W in steady state) the only available cooling line is K / 20 bar The zone around 80 K must be avoid to allow flushing of CO 2 from outside the beam tube and avoid that of CO 2 is physisorbed in view of the beam We need to operate above ~ 90 K Based on measurements by V.V Anashin et al.

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Implication of a non uniform source of gas in the arc Without beam, static gas load are dominated by unbaked collimator scheme During operation, a baked collimator or an unbaked collimator will bring dynamic gas load However, it is expected that the stimulated gas load from baked will be much less than unbaked collimator Moreover, desorption yields of physisorbed gas are much larger than chemisorbed gas En vrac : - Photon stimulated molecular desorption (synchrotron radiation) - Electron stimulated molecular desorption (photoelectron, electron cloud) - Ion stimulated molecular desorption (gas ionisation by protons, electrons …) - Friction induced molecular desorption (RF bridges) - Particle loss stimulated molecular desorption (by essence of a collimator) - Vacuum stability - … All this must be studied in detailed

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Observation of proton loss in the LHC Stimulated outgassing is observed at the (baked) TDI due to proton losses at perpendicular incidence J. Kamiya et al. IPAC 2010, Kyoto, Japan. Extrapolation of p/m/s loss yields to ~ to mbar.l/s i.e times the SR gas load Similarly, proton losses at collimator produce stimulated gas desorption but at grazing incidence …. G. Bregliozzi, private communication

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Observation at Betatron Collimation in the LHC A pressure increase of ~ mbar is observed at …. 50 m from the TCPs It consists only of hydrocarbons not pumped by the NEG According to literature, gas load to other gas species is times larger (see e.g. E. Mahner et al., Phys. Rev. ST Accel. Beams. 8, (2005)) Fill 2181, 4 st October 2011

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Observation at RHIC I guess that RHIC advice to CERN would be “ bake your collimator” …. after rebucketing, the pressure increases exponential with a time constant of about 10 seconds until the vacuum interlock system aborts the beams. A vacuum instability in the Blue ring The Blue beam intensity was limited by pressure rises in the collimator region. The collimators were not baked due to scheduling conflicts during the last shut-down. The Yellow collimators were baked, and no vacuum instabilities were observed there. Remark : it reminds me my everyday work !

Conclusions 21 Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 LHC base line is to bake all components operating at room temperature Unbaked system i.e. system operating at cryogenic temperature have continuous pumping, that means a perforated beam screen A solution at room temperature exists and need to be consolidated/upgraded The proposal with a collimator operating at cryogenic temperature must be studied in detail (issue with resources & time) Our present observation with LHC shows that collimators areas (TDI, TCTVB-D1 ….) are delicate regions subjected to pressure excursions RHIC, which suffer of electron cloud like LHC. has a bad experience with unbaked collimator operating at room temperature !

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October Thank you for your attention !!!