Optical diagnostics of 4 MV water switches in the Z-20 test facility

We are studying the behavior of self-breaking, high voltage water switches for the Z refurbishment project. In Z-20, 3 or 4 water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm and the enhancement of the positive and negative electrodes is varied to study current sharing and switch simultaneity. In addition to the standard electrical diagnostics (V, I) we are looking at one of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. We find that the velocity of the streamers is approximately a linear function of macroscopic electric field, with streamers from positive electrodes having velocities about twice as fast as streamers from negative electrodes. We find that optical closure of the gap correlates well with the beginning of the main current pulse. We demonstrate that only about 2% of the total light emitted by the switches is emitted during the accelerator power pulse.