Performance of the JET pumped divertor cryopump system

An integrated chevron type cryopump system for the JET pumped divertor program consisting of two independent 180° elements has been operational at JET since mid 1994. The pumping speed of the pump in situ in the torus is ~200000 l/s being limited by the conductance of the water-cooled high heat flux elements in front of the cryopump. The LN shield provides a high pumping speed for impurities such as water and hydrocarbons. An argon spray facility enables the deposition of argon frost directly onto the cryopump and to pump He-gas by cryosorption. The pumping speed for He has been found to be nearly the same as for D₂ . The operation of the pump has been very stable and unaffected by all kind of plasma operation conditions, disruptions or deposition of large quantities of argon. The operation of the cryopumps was so beneficial for the vacuum and the series of experimental programs that cryopumps were kept practically in permanent operation during the full 94/95 experimental campaign