Long-term operational experience and upgrade of the cryogenic system for the superconducting cavities in the CERN SPS accelerator

Abstract Cryogenics at liquid helium temperature has been requested in the SPS for testing and operating superconducting accelerating RF cavities: in 1988 and 1989 a single LEP prototype cavity made of bulk niobium, in 1990 two cavities made of niobium sputtered copper and finally, in 1991, a single niobium sputtered cavity. To cope with the various cooling needs, we used the same refrigerator (120 watt at 4.5 K) and the associated cryosystem has been optimized to reduce the thermal load. The original transfer line has been replaced by a flexible and thermally shielded one made of six coaxial bellows. The achieved thermal performance for the various combination of superconducting cavities, the long-term behaviour and operational experience of the cryosystem are given. While the first cryosystem is maintained in operation, a new one (using a refrigerator of 400 watt at 4.5 K) is now being tested to increase, in 1992, the SPS accelerating field by means of two additional superconducting cavities. A description and the preliminary results of the new cryogenic facility are presented.