The Model & Experimental Basis for the Design Parameters of the JET Divertor Cryopump Protection System including variations in Divertor Geometry & First Wall Materials

ABSTRACT A large In-Vessel Cryopump has been operational for several years as part of the JET Pumped Divertor. A model has been developed to analyse the behaviour of the Cryopump under all possible operating or fault conditions, and a suitable Protection System has been defined and implemented. It protects the Cryopump system during baking, “restart”, normal operation, glow discharge cleaning, loss of vacuum, loss of cryogens or water flow (in the associated water cooled components). It ensures that no excessive thermal stresses are generated and no freezing or boiling occurs in the neighbouring water cooled components. The model has been validated through a series of experiments and applied to other Cryopump devices at JET. A freeze-up incident that took place at the early operating stages of the LHCD Cryopump was well simulated. The In-Vessel Cryopump Protection System has been adapted to respond to changes in the First Wall Material (C, Be) and Divertor Geometry.