Applications of superpermeable membranes in fusion: The flux density problem and experimental progress

Abstract Superpermeable membranes whose permeability to energetic hydrogen approaches the permeability of an opening of the same area can be employed to separate D T and He in fusion machine exhausts, to control the edge plasma and divertor conditions (by pumping and/or arranging of gas circulation through SOL or divertor) and to pump and recuperate D T in auxiliary systems e.g. in pellet or neutral beam injection. One of the key points is the operation at permeation flux densities of up to 10 16 –10 19 cm −2 s −1 . Theory predicts that the highest flux densities may be reached with superpermeable membranes based on the V group metals: the limit conditioned by a maximum permissible hydrogen concentration in bulk metal is expected to be as high as ∼ 10 19 cm −2 s −1 . The experimental membrane system comprised a cylindrical Nb membrane and an incandescent Ta Nb atomizer placed inside. The hydrogen pumping speed by this system amounts to ∼ 10 3 1/ s , with a specific pumping speed of ∼ 1 1/ s per cm 2 membrane area and ∼ 8 1/ s per cm 2 atomizer area. Superpermeability was observed at record parameters referring both to the flux density of 3 × 10 17 H / cm 2 / s (by one order of magnitude larger than ever before) and to the operational pressure of 3 × 10 −2 Torr . A long-term reliable operation of this system proved being possible even in a vacuum far inferior to UHV conditions.