Overview of the LHC vacuum system

The Large Hadron Collider (LHC) project, now in the advanced construction phase 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 circumference of 26.7 km and requires a bending magnetic field 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, both the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipation by the beam image currents have to be absorbed on a `beam screen', which operates between 5 and 20 K and is inserted inside the cold bore. The design operating pressure must provide a beam lifetime of several days and this requirement comes from the power deposition in the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and interrupt the machine operation. Cryopumping of gas on the cold surfaces provides the necessary low gas densities but it must be ensured that the vapour pressure of cryosorbed molecules, of which H/sub 2/ and He are the most critical species, remains within acceptable limits. (27 refs).