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-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 8.4 T with 14-m long superconducting magnets. The beam vacuum chambers comprise the inner 'cold bore' walls of the magnets. These magnets operate at 1.9 K, and thus serve as very good cryo-pumps. In order to reduce 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 vacuum chamber. The design of this beam screen represents a technological challenge in view of the numerous and often conflicting requirements and the very tight mechanical tolerances imposed. The synchrotron radiation produces strong outgassing from the walls. The design pressure necessary for operation must provide a beam lifetime of several days. An additional stringent 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. In the warm straight sections of the LHC the pumping speed requirement is determined by ion induced desorption and the resulting vacuum stability criterion. (27 refs).