INVESTIGATION OF NITROGEN AND HELIUM CRYOSORPTION ON MOLECULAR-SIEVES

The helium-containing plasma exhaust gases of Tokamaks can be pumped by cryopumps using cryosorption panels coated with solid porous materials. The best pumping characteristics are achieved with activated charcoal and molecular sieves at temperatures below 10 K. In order to select the most suitable material with respect to cryosorption properties and reactivation behaviour, comparative tests on various molecular sieve types differing in cavity structure, pore size and polarity were carried out. Nitrogen and helium adsorption isotherms were determined at LN2 temperature and at 14.5 K, respectively, expecting that an extrapolation mechanism between the adsorption properties of both gases on molecular sieves could be obtained. For investigating the reactivation behaviour, molecular sieves were saturated with water prior to testing and outgassed at temperatures ranging from 100-degrees-C to 400-degrees-C. The results of the experiments demonstrate that even extremely unfavourable preconditioning has hardly any influence on the adsorption capabilities of the zeolites tested. This can be attributed to the reactivation pressure of 10(-2)-10(-3) mbar on the one hand and to the relatively low polarities of the tested zeolites on the other. As a further result of the experiments, it can be concluded that zeolites with narrow pores lend themselves particularly well to helium adsorption.