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.

Laser triggered, megavolt, megampere gas switches are frequently utilized to synchronize multiple pulsed power driver modules for inertial-confinement fusion, isentropic compression, and radiation physics experiments. The device developed to synchronize the 36 modules of the refurbished Z accelerator at Sandia National Laboratories is a 5.4 MV, 700 kA, sulfur-hexafluoride (SF 6 ) filled, laser triggered gas switch. At this operating level, switch jitter is 5 ns, the prefire rate is less than 0.1%, the average optic lifetime is greater than 200 shots, and the flashover rate is less than 1%. Over 1000 shots on a single-module test facility were conducted while iterating several potential design improvements, including utilizing low-erosion electrode material, varying SF 6 pressure, and modifying internal switch geometry all while keeping the basic switch architecture and footprint constant. Results of this development effort are presented herein.

This paper reviews recent work on the focusing of high-power relativistic electron beams in diodes and discusses concepts for pulsed fusion based on this technology. The physics of high-current relativistic electron beam focusing using plasmas in high-current diodes is studied experimentally and with computer simulation. The physics of the beam interaction with dense targets and the requirements for break-even are briefly discussed.

The Z pulsed power driver at Sandia National Laboratories is used to develop high energy density z-pinch x-ray sources for inertial confinement fusion research and radiation effects testing, and drive megabar pressures in material samples for equation of state studies. The pulsed power system is in the process of being replaced, improving reliability and increasing energy delivered to the load.The upgraded pulsed power system will deliver more than nine megajoules of forward wave energy in the first one hundred nanoseconds of its pulse. The system is comprised of thirty-six nominally identical modules, each producing a 3.3-terawatt pulse in 6Q water-insulated transmission lines. The peak forward-going voltage is about 5 MV. The pulse rise time is -75 ns; the full width at half maximum is -190 ns. The thirty-six modules are combined in parallel and drive twenty to twenty-five MA into the single load. In such a system, reliable insulation and precise switching are primary concerns. We will show key components of the system and results from a test module. We will also show performance results from the energy storage, triggering, and pulse-forming systems. We will also show the differing constraints of power flow from the 175 kA from each Marx generator, to currents in excess of 24 MA in the final feed to the load.

No abstract