6.1-MV, 0.79-MA laser-triggered gas switch for multimodule, multiterawatt pulsed-power accelerators

Abstract

A 6.1-MV, 0.79-MA laser-triggered gas switch (LTGS) is used to synchronize the 36 modules of the Z machine at Sandia National Laboratories. Each module includes one switch, which serves as the last command-fired switch of the module, and hence is used to determine the time at which each module electrically closes relative to the other modules. The switch is $81-cm in length, $45-cm in diameter, and is immersed in mineral oil. The outer switch envelope consists of six corrugated monomer-cast acrylic insulators and five contoured stainless-steel rings. The trigger electrodes are fabricated from copperinfused tungsten. The switch is pressurized with several atmospheres of sulfur hexafluoride (SF 6 ), which is turbulently purged within 2 seconds after every shot. Each switch is powered from a 6-MV, 0.78-MJ Marx generator which pulse charges a 24-nF intermediate-store water capacitor in 1:4-s. Closure of the switch allows power to flow into pulse-forming transmission lines. The power pulse is subsequently compressed by water switches, which results in a total accelerator output power in excess of 70-TW. A previous version of the LTGS performed exceptionally at a 5.4-MV, 0.7-MA level on an engineering test module used for switch development. It exhibited a 1-jitter of $5 ns, a prefire and flashover rate less than 0.1%, and a lifetime in excess of 150 shots. When installed on the Z accelerator, however, the switch exhibited a prefire probability of $3%, a flashover probability of $7%, and a 15-ns jitter. The difference in performance is attributed to several factors such as higher total charge transfer, exposure to more debris, and more stressful dynamic mechanical loading upon machine discharge. Under these conditions, the replacement lifetime was less than ten shots. Since refurbishment of Z in October 2007, there have been three LTGS design iterations to improve the performance at 6.1-MV. The most recent design exhibits a prefire rate of less than 0.1%, a flashover rate of $0:2%, a single switch jitter of $6-ns, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.

High-reliability 50-kV switch with 532-nm laser triggering

“…Multi-Photon Ionization (MPI) is important but only as a source of seed electrons from ionization of impurities or aerosols. The threshold for Barrier Tunnel Ionization (BTI) φ BTI is above the intensities generated here; φ BTI = 8.6 × 10 13 Dry air is the preferred switch gas to save cost, so the runtime as a function of f and P are shown in Figure 9 and Figure 10, respectively. The dramatic decrease in t sw with f is universally characteristic of laser-triggered switches and the present data substantiates the advantages of operating with f > 0.8, i.e., with a higher electric field which decreases the duration between CI avalanche and streamer formation.…”

“…Prominent, broad lines at 486.13 (H β ) and 656.28 (H α ) nm were readily observed for a spark at V c = 20 kV and P = 1.98 bar (H 2 ); the estimated ion density is n i = 6 × 10 17 cm -3 . Measuring and understanding the emission characteristics of the spark are important for prevention of dielectric flashover [12], which may occur on the interior walls of switch insulators [13]. Based upon evidence of surface tracking and measurement of intense radiation as noted here, the interior profile of the insulator was altered to reduce the probability of flashover.…”

Measurement of trigger and cascade section runtimes in 6MV switches using fiber coupled photo detectors.

“…mounts and an elaborate beam imaging system. Proper alignment of the laser into the trigger section of the switch [2] (see Fig. 3) is ultimately verified by directly looking at a bright plasma spark in the center of the trigger gap.…”

Low jitter, high voltage, repetitive laser triggered gas switches

“…The extensive work on the ZR switches at Sandia National Laboratories [46][47][48] let to a redesign of the electrode configuration for the Electra laser triggered switches. It is believed that the switch gap should have a more uniform electric field along the switch axis.…”