Basic study on the development of "superpermeable" membrane for pumping and refinement of plasma exhaust

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Experimental devices installed with ion, atom as well as plasma sources are developed to simulate the plasma exhaust in fusion device, and the permeation behavior of "superpermeable" membrane was investigated. An atomic beam source was employed to study the atom-driven permeation behavior of deuterium through Nb. "In-situ" surface analyses due to AES (Auger Electron Spectroscopy) revealed that the upstream-side surface was covered with sulfur, which was found to be very stable even under the atomic exposure. The permeation behavior was considered to be under the influence of this surface state, where the permeation probability was found to be nearly constant in the temperature range between 600 and 900 K. Such a dependence reflects one feature of "superpermeation" and it is expected from the present study that "superpermeation" can be controlled through careful treatment of the surface conditions. An plasma source installed with a carbon target was purchased from Russia and was incorporated with a permeation test stand to investigate the plasma-driven permeation behavior of "superpermeable" membrane (Nb). The device is so constructed that ignition of plasma bombards the carbon target to deposit carbon impurity layer on the membrane surface. In the preliminary experiment, a steady-state permeation rate was achieved initially under exposure to atomic hydrogen, when the plasma discharge was ignited. It was clearly shown that the permeation rate decreased as a result of carbon deposition.