INVESTIGATION OF NITROGEN AND HELIUM CRYOSORPTION ON MOLECULAR-SIEVES

The helium-containing plasma exhaust gases of Tokamaks can be pumped b y cryopumps using cryosorption panels coated with solid porous materia ls. The best pumping characteristics are achieved with activated charc oal and molecular sieves at temperatures below 10 K. In order to selec t the most suitable material with respect to cryosorption properties a nd reactivation behaviour, comparative tests on various molecular siev e types differing in cavity structure, pore size and polarity were car ried out. Nitrogen and helium adsorption isotherms were determined at LN2 temperature and at 14.5 K, respectively, expecting that an extrapo lation mechanism between the adsorption properties of both gases on mo lecular sieves could be obtained. For investigating the reactivation b ehaviour, molecular sieves were saturated with water prior to testing and outgassed at temperatures ranging from 100-degrees-C to 400-degree s-C. The results of the experiments demonstrate that even extremely un favourable preconditioning has hardly any influence on the adsorption capabilities of the zeolites tested. This can be attributed to the rea ctivation pressure of 10(-2)-10(-3) mbar on the one hand and to the re latively low polarities of the tested zeolites on the other. As a furt her result of the experiments, it can be concluded that zeolites with narrow pores lend themselves particularly well to helium adsorption.