Vacuum technology for superconducting colliders

In high energy proton-proton colliders such as the CERN Large Hadron Collider (LHC) project with a centre of mass collision energy of over 14 TeV and the American Superconducting Super Collider (SSC) where the centre of mass collision energy reaches 40 TeV the relativistic protons lose energy in the form of synchrotron radiation. For adequate beam-residual gas lifetimes in these machines the pressure should typically be in the 10^-10 Torr range. However the synchrotron radiation impinging on the walls of the vacuum chamber desorbs gas and may result in large pressure increases detrimental to the operation of the collider. To achieve the required strong bending in these machines it is necessary, in the case of the LHC, to employ dipole fields up to 9.0 T which need superconducting magnets operating at 1.9 K. The vacuum chamber is therefore at cryogenic temperature and functions as a cryopump. At first sight this free cryopumping may appear beneficial but in practice introduces several liabilities-for example, only a few monolayers of cryopumped H₂ already has a vapour pressure at 5 K in excess of 10^-6 Torr. These and other effects and constraints on the design of the cold vacuum system will be described in detail