The use of active carbons as cryosorbent

At Forschungszentrum Karlsruhe, a special cryosorption vacuum pump system is being developed for use in the next generation tokamak fusion reactor ITER-FEAT. At the operation temperature of about 5 K, practically all gases can be condensed easily, except of helium and hydrogen. To bind these species as well, the panels are coated with active carbon as cryosorbent. This paper has to some extent overview character and summarises the qualification results of different carbon materials. An experimental programme has been performed to develop an optimum cryopanel set-up with respect to pumping efficiency. Within that framework, material properties were measured (pore-size, sorption isotherms) and the vacuum pumping performance was determined (influence of mixture composition). Judging from these results, the candidate carbon was chosen with which more in-depth studies were made. Strong selectivity effects for the simultaneous sorption of hydrogen and helium were revealed. He sorption, which is very sensitive to temperature fluctuation, was shown to have the predominant influence on the overall pumping speed. The poisoning influence due to the accumulation of air-like and water-like substances was also quantitatively assessed. The observed effects were moderate. To gain a better understanding of cryosorption, a cryocooler-based sorption device which is currently being built, was developed to perform parametric measurements of sorption characteristics in the 5–15 K temperature range.