WP12: Vacuum & Beam Screen

The luminosity upgrade program (HL-LHC) of the current LHC in relation to its vacuum system must follow strict guidelines; similar to those followed for the present machine, with particular emphasis on the new areas around the triplets and the experimental chambers. The increased stored current means more synchrotron radiation (SR), which in turn translates into higher SR power being dissipated along the beam screen and higher SR photon fluxes potentially generating more outgassing and/or electron cloud (EC).

One of the main tasks of WP12 is to define a strategy for the dimensions and size of the beam screen in the superconducting (SC) new triplet/D1 magnets, in particular how to accommodate the high-density shielding material which is mandatory in order to protect the SC magnets from outgoing collision debris. A balance between cold bore size and vacuum pumping is going to be defined based on experience gained with the present machine and, probably, thanks to recent R&D on new materials. A number of new ideas have surfaced recently, namely new coatings (amorphous-carbon, or exotic high-temperature superconductors) but these have not yet been validated.

The change to the aperture of the triplets at IR1 and IR5 means that the experimental vacuum chambers of CMS and ATLAS will require a number of analyses. These analyses will validate the aperture, impedance, and vacuum (dynamic and static) for the experimental layouts. From preliminary analyses, the forward regions of CMS and ATLAS will need to be adapted to cope with the new beam geometry coming from IR1 and IR5. New materials and new access methods will likely be needed to deal with with the additional activation from the increased luminosity.

With HL-LHC, less flexibility will be available for the optics of LHCb and ALICE; therefore the vacuum chambers at points 2 and 8 must be validated for ultimate running conditions to ensure that these chambers do not impose a limitation on the machine. Positions of mechanical supports, pumps, and gauges must be analysed to ensure that layouts are optimised for the new machine configuration. Bakeout equipment will be redefined depending on activation and specific needs. All experimental chambers must be NEG coated or equivalent to minimise secondary electron yield (SEY), thus reducing electron cloud effects.