Optical Diagnostics of the ZR Gas Switch

Summary form only given. The ZR gas switch is a multistage gas switch designed to hold off a voltage of ~6 MV and then transmit a 600-kA current pulse to the water section of ZR. The first stage of the switch, which holds off approximately 900 kV, is laser triggered by the focused beam from a frequency-quadrupled ND:YAG laser operating at 266 nm. The breakdown of the laser-triggered section causes a wave of breakdowns in the remaining 22 gaps in the switch, starting near the laser-triggered section and cascading to the other end of the switch. The gaps in this "cascade" section normally break down in multiple arc channels, which lowers the overall inductance of the switch. We are observing the details of normal and abnormal switch operation with fast framing and streak cameras, observing the time-history of the light emitted by the switch with fast photo-detectors and taking time-integrated photographs of the switch. The diagnostics indicate that the laser-triggered gap normally begins conducting current 10-to-15 ns after the laser arrives in the gap, with the entire switch breaking down 45-to 55-ns after the laser beam arrives in the switch. Details of the experiments and the operation of different switches are presented.