Deuterium Plasma-Driven Permeation in Heliotron E During Discharge Cleaning and in a Small Plasma Device

Two experiments on the plasma-driven permeation (PDP) of deuterium through nickel membranes were conducted. One was an observation of the permeation during the discharge cleaning of Heliotron E, one of the largest helical systems. Significant amounts of deuterium permeated. The deuterium permeation was also observed during a helium discharge, where deuterium recycled between the plasma and chamber walls. Because the permeation was strongly influenced by impurities such as oxygen on the plasma-facing surface of the membrane, the PDP would be useful for a diagnosis of the discharge cleaning. The second experiment was a study on transient behaviors of the permeation when nickel membranes were exposed to a deuterium plasma for short times. The small device with a radio-frequency plasma was used to simulate pulse operations of large plasma devices. The maximum permeation flux at the nonsteady state was found to be nearly proportional to the exposure time of the plasma. Numerical calculations reproduced very well the transient behaviors of the deuterium permeation in the pulse-exposure experiments. Based on the results of the two experiments, it is expected that the deuterium PDP from neutral beam injection-heated plasmas in Heliotron E will be observed.