Atom-driven permeation of deuterium through Nb

In order to investigate the difference of the hydrogen transport behavior of IDP (ion-driven permeation, i.e. permeation under impingement of the energetic hydrogen ion beam) and ADP (atom-driven permeation, i.e. permeation under 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 the temperature range of 500-1000 K. Such a case has not been seen in the IDP or GDP (gas-driven permeation) case. The deuterium concentration at the downstream-side end bulk volume was estimated employing the phenomenological recombination rate coefficient. Computer calculation showed that the profile of the deuterium concentration becomes almost ''flat'' under the experimental conditions. Though the experiment was performed under UHV conditions, the assumption of Sieverts' law-like relation between PU* (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) explains well the strange temperature dependence of the permeation plot. (orig.)