POS density measurements on GIT-8

Summary form only given. A collaborative experiment was carried out on the GIT-8 generator at the High Current Electronics Institute, Tomsk, Russia, to measure the plasma density in a 3 MA 1 /spl mu/s conduction time plasma opening switch (POS). A He-Ne laser interferometer, similar to one used on the Hawk POS experiment at NRL, was adapted to GIT-8. The line-integrated electron density was measured using an axial line-of-sight through the plasma injection region. Sixty-four capillary guns inject plasma in the axial direction opposite to the direction of power flow. One gun was removed to allow access for the laser beam. A total of 26 GIT-8 shots were taken, first: to measure the line-integrated density at different radial locations between the inner (cathode) and outer (anode) conductors, and second: to vary the conduction time at one interferometer location. The radial density measurements show an increase in the plasma inventory during conduction; evidence of ionization. At radial locations near the cathode, the density decreases (sometimes dramatically) prior to opening. The density near the anode increases monotonically during conduction and opening. We conclude that the POS probably opens a small gap in the vicinity of the cathode, but we were not able to find a line-of-sight where the density decreased to zero, either because the gap is smaller than the laser beam (3 mm), the gap is curved or slanted, or because plasma moves axially into the tube used to separate the exit window from the coaxial POS section.