Helium cryogenic systems for the LEP2 and LHC projects at CERN

CERN is presently operating a large distributed 4.5 K helium cryogenic system (48 kW@4.5 K equivalent) for cooling the superconducting acceleration cavities of the 26.7 km circumference LEP2 lepton collider. This also constitutes the first part of the 1.8 K cryogenic system (about 150 kW@4.5 K equivalent) for the future Large Hadron Collider (LHC), the high-field superconducting magnets of which will operate in superfluid helium. We briefly describe the main features of each system, and review the progress of their development, construction and operation.