The use of cryopumps for impurity pumping during the TFTR D-T phase

Introducing tritium into the plasmas of the controlled fusion program will require stringent control and processing of effluents from the vacuum system. The reduction in the amount of gas required for discharge cleaning of the vacuum vessel during D-T operations is discussed. A steady-state glow discharge at pressures of 5×10^-3 torr (D₂) for periods of up to 24 h are currently performed as part of the procedure to prepare the TFTR (Tokamak Fusion Test Reactor) vacuum system for high-power pulsing. The present vacuum system, with a speed of 5000 L/s, would generate about 5 mol. of tritium-contaminated effluent per hour. Purification and real-time recycling of the discharge cleaning gas using a Stirling cycle cryopump (10 K to 20 K) in series with a turbopump would reduce the discharge cleaning gas requirements to a single gas filling. The usual charcoal coating on the low-temperature stage of the cryopump will be removed to prevent hydrogen isotope sorption pumping. Impurities such as CO, H₂O, CH₄, etc., will be cryopumped on the 80 K and 10-20 K cryo-surfaces. At the termination of glow discharge, the cleaning of a single charge (250 torr-L) of tritium contaminated gas will be transferred to a tritium cleanup system, possibly for recycling. The cryopump is then regenerated and the tritium-contaminated impurities are transferred to the tritium clean up system