Chapter 12: Vacuum system

Abstract

The luminosity upgrade programme (HL-LHC) requires modifications of the present LHC’s vacuum system, in particular in the triplets, crab cavities, matching section and experimental areas. Such modifications must follow guidelines similar to those followed for the present machine. The increased stored current implies a higher thermal power in the beam screen from the image current moving along with the stored particles and stronger synchrotron radiation (SR) and electron cloud (EC) effects, which in turn translate into higher degassing rates.

One of the main tasks of the HL-LHC vacuum work package is to produce new beam screens in the new superconducting (SC) Inner Triplet (IT) and D1-D2 magnets together with the vacuum layout along the Insertion Region (IR). It is also necessary to assemble and insert with the beam screens high-density shielding material into IT magnets. This is mandatory for protecting the magnets from collision debris coming from the experiments’ interaction points (IPs). A balance between cold bore size and vacuum pumping system is defined based on experience gained with the present machine and recent advances with new materials. Indeed, a number of new ideas have emerged recently for the mitigation of the e-cloud effect in cryogenic beam pipes: amorphous carbon (a-C) coating for which validation is ongoing and laser structured surface currently under study.

The change of the aperture of the IT at IR1 and IR5 implies that the experimental vacuum chambers of CMS and ATLAS require a review of aperture, impedance, and vacuum (dynamic and static). The forward regions of CMS and ATLAS will need to be adapted to cope with the new beam geometry in IR1 and IR5. New vacuum systems at the tunnel/cavern interfaces are needed to mitigate the additional activation from the increased luminosity. New access procedures and tooling will be also needed to allow the minimization of the integrated dose to personnel. With the HL-LHC, less flexibility will be available for the optics of LHCb and ALICE; therefore, the vacuum chambers at IR2 and IR8 must be validated for operating conditions to ensure that these chambers do not impose a limitation. In-situ a-C coating of the beam screens of the IT magnets, located in LSS2 and 8, is under validation to reduce the electron cloud heat load onto the cryogenic system. Finally, positions of mechanical supports, pumps, and gauges must be analyzed to ensure that layouts are optimized for the new machine configuration. Bake-out equipment will be redefined depending on activation and specific needs. To deliver good vacuum conditions, all chambers held at room temperature must be treated with Non-Evaporative Getter (NEG), to provide low dynamic outgassing with large pumping speed and to minimize secondary electron yield (SEY), to reduce electron cloud effects.