A 12/18 kW at 4.5 K helium refrigerator for CERN's LEP superconducting acceleratingcavities

Abstract The superconducting resonant cavities of the LEP accelerator, which are operated at 4.5 K, need an important amount of cryogenic refrigeration. Two custom designed refrigerators of an equivalent power of 12 kW at 4.5 K each, have been built. They must be able, with some further modifications, to reach 18 kW and operate in a special LHC mode. For reasons of room, the cold box had to be split in two parts located one at ground level, the other in the tunnel. The thermodynamic cycle is introduced and discussed. Information is given about the cycle screw compressors and the refrigerator operations and performances.