The cold vacuum system of the Large Hadron Collider

In the CERN Large Hadron Collider (LHC) two 7 TeV proton beams will circulate in a storage ring 27 km in circumference in cold bore vacuum chambers at 1.9 K. The relativistic protons emit synchrotron radiation which deteriorates the vacuum situation by desorbing gas. An inner beam screen at some intermediate temperature (probably 520K) will be necessary to protect thermally the cold bore. Nuclear scattering on the residual gas results in decreased beam lifetimes and the deposition of the high energy protons in the superconding magnets which may then quench. The cold beam screen can only function to a limited extent as a cryopump. For example, only a few monolayers of condensed H2 already have a vapour pressure at 4.2 K of 7 10-7 Torr; in addition, beam induced ion and electron bombardment of the condensed layer can cause vacuum instabilities. Perforations in the beam screen can limit the pressure increases by providing steady cryopumping on the 1.9 K cold bore.