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Vacuum Stability Issues in the New Inner Triplet

This document explores vacuum stability challenges in the LHC Inner Triplet, including issues like photon-stimulated desorption, electron multipacting, and ion-induced pressure instability. It discusses vacuum requirements, beam screens, and conclusions from studies on ion-induced pressure instability. The text covers operational parameters, beam screen functionalities, and proposals for short and long-term developments in managing vacuum stability.

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Vacuum Stability Issues in the New Inner Triplet

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  1. Vacuum Stability Issues in the New Inner Triplet V. Baglin CERN AT-VAC, Geneva 1. Vacuum physics in LHC 2. Vacuum requirements for IT phase 1 3. Beam screens 4. Conclusions V. Baglin LIUWG, 27/09/07

  2. 1. Vacuum physics in LHC • Photon stimulated desorption • photon stimulated desorption • Electron multipacting (electron cloud) • electron stimulated desorption • heat load onto the beam screen • emittance growth • Ion induced pressure instability • Ion stimulated desorption • pressure runaway • beam loss V. Baglin LIUWG, 27/09/07

  3. Ion induced pressure instability : ISR • Beam current stacking to 1 A • Pressure increases to 10-6 Torr • (x 50 in a minute) • Beam losses current pressure First documented pressure bump in the ISR E. Fischer/O. Gröbner/E. Jones 18/11/1970 V. Baglin LIUWG, 27/09/07

  4. Ion induced pressure instability : model O. Gröbner, CERN 99-05 A.G. Mathewson, CERN ISR-VA/76-5 Reduction of the net pumping speed • Increase pumping speed (large conductance) • Decrease desorption yield (cleanliness) V. Baglin LIUWG, 27/09/07

  5. 2. Vacuum requirement for IT phase 1 • Ensure vacuum stability • Provide an equivalent gas density less than 1013 H2/m3 i.e. 2.5 10-11 Torr at RT (A. Rossi, LHC PR 674). • IT characteristics • An operating temperature of 1.9 K • A gradient 120 T/m gradient • A length of ~ 10 m long for each quadrupole and a total of ~40 m for the IT • A cold bore diameter of ~ 15 cm • Due to the crossing angle, the beam are ~ 5 mm off-axis in the IT. This generates a synchrotron radiation of ~ 3 eV critical energy and 1/10 of the nominal LHC arc photon flux. • D1 generates also synchrotron radiation in the IT of ~ 6 eV critical energy and 1/4 of the nominal LHC arc photon flux (I.R. Collins, O.B. Malyshev, LHC PN 274). • Even in the case of the absence of multipacting, a gas load will be due to photon stimulated desorption • This gas will condense on the cold bore V. Baglin LIUWG, 27/09/07

  6. Parameters at cryogenic temperature • ’H2 ~ 1 000 at 1 keV and 1 monolayer • (N. Hilleret, R. Calder, IVC, 1977). • The critical current is given by : • It is driven by the geometry, the gas species, the sticking probability, the primary and the recycling desorption yields V. Baglin LIUWG, 27/09/07

  7. Option without beam screen • Critical current • Given the IT diameter (15 cm), a vacuum chamber longer than 5 m is considered to be infinite so that the pumping speed at the ends remain negligible. • We assume a sticking coefficient of unity (optimistic case) • Critical current : ’H2 ~ 2 000 at 1 monolayer ’co ~ 200 at 1 monolayer When the surface coverage approaches a few monolayers, the vacuum may become unstable V. Baglin LIUWG, 27/09/07

  8. Option without beam screen When is the pressure runaway reached ? H2 CO The pressure runaway could be reached within 100 days and … V. Baglin LIUWG, 27/09/07

  9. Option beam screen When is the gas density limit reached ?  = 1, photon flux = 1/10 of arc The gas density limit is reached in a few days !! V. Baglin LIUWG, 27/09/07

  10. So, we need to control the gas density… V. Baglin LIUWG, 27/09/07

  11. Option with beam screen No holes BS ~ 5 K Short vacuum chamber BS ~ 19 K CB ~ 4 K ~ 2% holes V. Baglin et al., LHC PR 435 A perforated beam screen allows to control the gas density V. Baglin LIUWG, 27/09/07

  12. 3. Beam screens • Functionalities • Provide an equilibrium density and an equilibrium surface coverage defined by the hole’s pumping speed, C • Beam screens have a critical current of • In the arcs : (ion I)crit ~ 30 A for CO2 to 103 A for H2 • Actively cooled beam screens avoid thermal transients and therefore vacuum transients • Beam screens heaters allow a warm up to remove the condensed gas • The transparency of the beam screen defines the level of the pressure and of the surface coverage V. Baglin LIUWG, 27/09/07

  13. Operation of beam screens • Beam screens operate in the range of 5 to 20 K (operation above 20 K must be avoided) • The cold bore at 1.9 K provides large pumping capacity for all gases except helium. • To avoid vacuum transients during operation, the beam screen temperature must be held above the cold bore temperature during cool down • After a quench, or in the case of oscillation in temperature, the beam screen must be warm up to flush the gas towards the cold bore • In the case of large heat load and / or pressure increase due to thick coverage of gases, the beam screen must be warm up to flush the gas towards the cold bore V. Baglin LIUWG, 27/09/07

  14. Beam screen short term developments for IT phase 1 • Define the required transparency i.e. estimate of the gas load • Define the geometry, the material • Built prototypes • Validate • Production V. Baglin LIUWG, 27/09/07

  15. Beam screen long term developments for LHC upgrade • Learn from LHC operation • Investigate and validate electron cloud mitigations (clearing electrodes, grooved chambers …) • Define beam screen parameters • Built prototypes • Scientific and engineering validation • Production V. Baglin LIUWG, 27/09/07

  16. 4. Conclusion • The vacuum group recommends the installation of beam screens in the IT • A “scaling” of the present design is proposed in a first phase • After acquisition of know-how, upgrading of the beam screen technology could be foreseen for the LHC upgrade V. Baglin LIUWG, 27/09/07

  17. Some references • Ion induced desorption coefficients for titanium alloy, pure aluminium and stainless steel. A.G. Mathewson. ISR-VA/76-5, March 1976. • Ion desorption of condensed gases. N. Hilleret, R. Calder. IVC 7, 1977. • Overview of the LHC vacuum system. O. Gröbner. Vacuum 60 (2001) 25-34. • Ion desorption stability ion the LHC. O.B. Malyshev, A. Rossi. VTN 99-20, December 1999. • First results from COLDEX applicable to the LHC cryogenic system. V. Baglin et al. LHC PR 435, September 2000. • Dynamic gas density in the LHC interaction regions 1&5 and 2&8 for optic version 6.3. I.R. Collins, O.B. Malyshev. LHC PN 274, December 2001. • Synchrotron radiation studies of the LHC dipole beam screen with COLDEX. V. Baglin et al. LHC PR 584, July 2002. • Running-in commissioning with beam. V. Baglin. LHC Performance Workshop – Chamonix XII, January 2003 • Residual gas density estimations in the LHC experimental interaction regions. A. Rossi, N. Hilleret. LHC PR 674, September 2003. • Vacuum transients during LHC operation. V. Baglin. LHC Project Workshop – Chamonix XIII, January 2004. • Performance of a cryogenic vacuum system (COLDEX) with an LHC type beam. V. Baglin et al. Vacuum 73 (2004) 201-206. • Gas condensates onto a LHC type cryogenic vacuum system subjected to electron cloud. V. Baglin, B. Jenninger. LHC PR 742, August 2004. • Results from the scrubbing run 2004. N. Hilleret. LHC MAC December 2004. V. Baglin LIUWG, 27/09/07

  18. SEY of gas condensate N. Hilleret. LHC MAC December 2004 V. Baglin LIUWG, 27/09/07

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