The Large Hadron Collider vacuum system

The two rings of the LHC beam vacuum system have a total length of about 54 km of which almost 48 km will be at 1.9 K, the temperature of the superconducting magnets. The total synchrotron radiation power emitted by the two beams is 0.41 Wm/sup -1/. A a so-called beam screen, maintained at a temperature between 5 K and 20 K by gaseous helium flow, is inserted in the magnet cold bore to intercept this power. We discuss the beam screen, magnetic permeability/vapour pressure aspects, beam screen vacuum behaviour, photon-induced gas desorption, intermagnet connection, pressure measurement, warm sections and the insulation vacuum.