Membrane bias effects on plasma-driven permeation of hydrogen through niobium membrane

Niobium membrane modeling a pumping system in the divertor region of a fusion plasma machine was immersed in hydrogen plasma in order to study interaction of the membrane with hydrogen ions in the energy range from a few eV to 200 eV, controlled by changing the membrane bias voltage. Bombardment by ions of an energy higher than 50 eV resulted in a decrease of the plasma-driven permeation/retention and in an enhancement of the boundary processes of absorption/reemission of thermal molecules. At ion energies below 50 eV, the effect of ion bombardment on plasma driven permeation and on the kinetic coefficients of boundary processes was non-monotonic in ion energy having a maximum at 10 eV. Both dissolution of oxygen in the membrane and increase of membrane temperature lead to diminution of the bias effect due to rise of recovering rate of surface impurity layer.