Anomalous isotope effect in the permeation, retention, and reemission at interaction of energetic hydrogen with niobium

Abstract

A niobium membrane sample was placed in H or D plasma and electrically biased. Isotope effects for H vs. D in factors of 20, 40, and 40, respectively, were observed in plasma driven permeation, retention, and in the reemission, within a narrow range of bias voltages (40–80 V) at the lowest metal temperature investigated (910 K). The phenomenon occurred at the “superpermeation” of suprathermal hydrogen arising from an oxygen monolayer at the metal surface. The phenomenon is supposed to be caused by dynamics of the oxygen monolayer under the action of ion sputtering and surface segregation of dissolved O. Such and even much stronger isotope effects are also expected on other metals with a similar “real” surface. This isotope effect may be important for D/T-mixture recycling, retention and permeation at its interactions with plasma facing components of fusion reactors as well as for the applications of superpermeable membranes for pumping of hydrogen isotopes and their separation from He.