Performance improvements of the 6.1-MV laser triggered gas-switch on the refurbished Z

A 6.1-MV, 790-kA laser triggered gas switch (LTGS) is utilized to synchronize the 36-modules of the Z-machine at Sandia National Laboratories. The switch is ~81-cm in length, 45-cm in diameter, and is immersed in transformer oil. The switch is pulse-charged from a 780-kJ, 6-MV Marx generator in 1.4-?s. Closure of the switch allows energy stored in a 24-nF intermediate-store water capacitor to flow into subsequent pulse-forming stage. The entire system (36-modules) generates a ~70-TW at the present system operating level, but after near term improvements are complete, this level will increase to ~100-TW. The initial design of the LTGS exhibited exceptional performance at a 5.4-MV, 700-kA level (1-? jitter of ~5 ns, prefire and flashover rate less than 0.1%, lifetime in excess of 150 shots) on an engineering test-module utilized for component development. When initially implementing the same design on the 36-module Z-machine, the switch exhibited a prefire probability of ~3%, a flashover probability of ~7%, and a jitter increase of a factor of 3. The difference in performance is attributed to several factors such as higher Coulomb transfer, exposure to a more debris prone environment, and more stressful dynamic mechanical loading when the machine discharges, compared to that of the engineering test module. Under these conditions the replacement lifetime was less than 10 shots, which is unacceptably frequent. Since refurbishment of Z in October 2007, there have been three major LTGS design iterations to improve the performance at a 6.1-MV operating point. The most recent design iteration includes modifications to the switch geometry and electrode and insulating materials that reduces the random prefire rate to less than 0.1%, reduces the flashover rate to -0.2%, reduces single switch jitter to ~6-ns, while increasing the average switch life to greater than 50 shots. Modifications and performance improvements are detailed in this manuscript.