Atom-driven permeation of deuterium through Nb

In order to investigate the difference of the hydrogen transport behavior o f IDP (ion-driven permeation, i.e. permeation under impingement of the ener getic hydrogen ion beam) and ADP (atom-driven permeation, i.e. permeation u nder exposure of thermal atom beam), an ADP experiment through Nb (niobium) was performed. The dependence of the ADP flux on the specimen temperature turned out to be different from the IDP where the maximum was observed in t he temperature range of 500-1000 K. Such a case has not been seen in the ID P or GDP (gas-driven permeation) case. The deuterium concentration at the downstream-side end bulk volume was esti mated employing the phenomenological recombination rate coefficient. Comput er calculation showed that the profile of the deuterium concentration becom es almost "flat" under the experimental conditions. Though the experiment w as performed under UHV conditions, the assumption of Sieverts' law-like rel ation between P-U* (virtual pressure of atomic gas, or pressure equivalent to "atomic flow" in the upstream-side vacuum in terms of the molecular flow theory), bulk concentration and PD (pressure of permeated molecules) expla ins well the strange temperature dependence of the permeation plot.