Operation, Testing and Long Term Behaviour of the LHC Test String Cryogenic System

Since the end of 1994 we have been operating a prototype half-cell of the machine lattice, accumulating more than 10,000 hours at superfluid helium temperatures and recovering from 150, mainly provoke d, magnet resistive transitions. The system has confirmed the validity of the basic design choices of the LHC cryogenic system. Furthermore, extensive testing on the response of the system to current ramp and discharge, and to magnet resistive transition, has provided sufficient information to enable a simplification of the cryogenic scheme that fulfils the LHC requirements. We report on the cryog enic operation, testing and long-term behaviour of the LHC Test String during the last 4 years of operation.