Pumping characteristics of a cryopump with Ar sorbent in He and in a D2/He mixture

Abstract

The He generated as a result of D/T burn in a fusion reactor must be exhausted from the plasma to avoid fuel dilution effects. In view of this, transport and exhaust studies of He in fusion plasmas are getting increasing attention in recent years. In fusion plasmas, the exhaust gas will be a combination of D2 and He, with He forming only a small fraction (about 10%). The cryosorption pumping characteristics of pure He and a mixture of D2 and He (90% D2) have been investigated using a cryosorption pump with condensed layers of Ar as sorbent. A cryocondensation pump cooled by liquid He at 4.3 K, and located in the outboard divertor region of the DIII‐D tokamak, was used for the experiment. The investigation was conducted in a pressure regime that is relevant for particle exhaust from fusion plasmas. The experiment revealed that (1) the cryosorption pumping speed of pure He drops precipitously if the Ar/He ratio falls below about 20; (2) the pumping speed for He in a D2/He mixture decreases in an exponential manner with the amount of D2 pumped; (3) increasing the thickness of Ar in the range of 1–12 μm had little effect on the pumping speed for He in a D2/He mixture; and (4) for a pumping surface coated with a thick (≳2 μm) layer of Ar, surrounded by a radiation shield having a transparency factor of about 6, a He pumping speed in the range of 15–25 m3 s−1 m−2 in the mTorr pressure range for a pulse duration of about 5 s can be obtained after pumping about 80 Torr l of D2.