The influence of water on the performance of ITER cryosorption vacuum pumps

A cryopump system is being developed for high vacuum operation of the ITER torus. It is based on combined sorption and condensation of gases at cooled surfaces at about 5 K, coated with activated charcoal. During the operation of ITER water-like species which show strong sorptive interaction with charcoal have to be coped with by the cryopumps. To achieve optimum performance they have to be regenerated from these species at regular, less frequent intervals. The different aspects to be considered when designing this step are reflected in the present paper using water as a typical representative. The poisoning effect is demonstrated to be acceptably moderate. The potentials of both temperature increase and pressure decrease to stimulate water desorption are critically assessed and their specific advantages and drawbacks are discussed.