APPLICATIONS OF SUPERPERMEABLE MEMBRANES IN FUSION - THE FLUX-DENSITYPROBLEM AND EXPERIMENTAL PROGRESS

Superpermeable membranes whose permeability to energetic hydrogen appr oaches 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 edg e plasma and divertor conditions (by pumping and/or arranging of gas c irculation 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 densiti es may be reached with superpermeable membranes based on the V group m etals: the limit conditioned by a maximum permissible hydrogen concent ration in bulk metal is expected to be as high as similar to 10(19) cm (-2) s(-1). The experimental membrane system comprised a cylindrical N b membrane and an incandescent Ta/Nb atomizer placed inside. The hydro gen pumping speed by this system amounts to similar to 10(3) 1/s, with a specific pumping speed of similar to 1 1/s per cm(2) membrane area and similar to 8 1/s per cm(2) atomizer area. Superpermeability was ob served at record parameters referring both to the flux density of 3 x 10(17) H/cm(2)/s (by one order of magnitude larger than ever before) a nd to the operational pressure of 3 x 10(-2) Torr. A long-term reliabl e operation of this system proved being possible even in a vacuum far inferior to UHV conditions.