Overview of the LHC vacuum system

The Large Hadron Collider (LHC) project, now in the advanced construction p hase at CERN, comprises two proton storage rings with colliding beams of 7 + 7 TeV energy. The machine is housed in the existing LEP tunnel with a cir cumference of 26.7 km and requires a bending magnetic held of 8.3 T with 14 m long superconducting magnets operating at 1.9 K. The beam vacuum system comprises the inner 'cold bore' walls of the magnets which provide a nearly perfect cryopump. In view of reducing the cryogenic power consumption, bot h the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipation by the beam image currents have to be absor bed on a 'beam screen', which operates between 5 and 20 K and is inserted i nside the cold bore. The design operating pressure must provide a beam life time of several days and this requirement comes from the power deposition i n the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and interrupt the machine operatio n. Cryopumping of gas on the cold surfaces provides the necessary low gas d ensities but it must be ensured that the vapour pressure of cryosorbed mole cules, of which H-2 and He are the most critical species, remains within ac ceptable limits. (C) 2001 Elsevier Science Ltd. All rights reserved.