We have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator's water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The accelerator generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

We are investigating several alternate gas-switch designs for use in linear transformer drivers. To meet linear-transformer-driver (LTD) requirements, these air-insulated switches must be DC charged to 200 kV, be triggerable with a jitter of 5 ns or less, have very low prefire and no-fire rates ( $ 1 in 10 4 shots), and have a lifetime of at least several thousand shots. Since the switch inductance plays a significant role in limiting the rise time and peak current of the LTD circuit, the inductance needs to be as low as possible. The switches are required to conduct current pulses with $100-ns rise times and 20-80 kA peak currents, depending on the application. Our baseline switch, designed by the High Current Electronics Institute in Tomsk, Russia, is a six-stage switch with an inductance on the order of 115 nH that is insulated with 47-67 psia of air. We are also testing three smaller two-stage switches that have inductances on the order of 66-100 nH. The smaller switches are insulated with 92-252 psia of air.

We have developed two new gas switches that are designed to be used with linear transformer drivers. The switches, which can be DC charged to 200 kV and triggered with less than a 2-ns 1-jitter, have overall inductances ranging from 69 to 85 nH. When transferring 400 J of energy per shot, the switches have lifetimes in excess of 5000 shots. The two switches are insulated with 130-270 PSIA of air and are submerged in transformer oil during operation. These switches should allow development of linear transformer drivers that are more compact and have higher peak power.

We have developed a conceptual design of a next-generation pulsed-power accelerator that is optimized for megajoule-class dynamic-material-physics experiments. Sufficient electrical energy is delivered by the accelerator to a physics load to achieve-within centimeter-scale samples-material pressures as high as 1 TPa. The accelerator design is based on an architecture that is founded on three concepts: single-stage electrical-pulse compression, impedance matching, and transit-time-isolated drive circuits. The prime power source of the accelerator consists of 600 independent impedance-matched Marx generators. Each Marx comprises eight 5.8-GW bricks connected electrically in series, and generates a 100-ns 46-GWelectrical-power pulse. A 450-ns-long water-insulated coaxial-transmission-line impedance transformer transports the power generated by each Marx to a system of twelve 2.5-m-radius water-insulated conical transmission lines. The conical lines are connected electrically in parallel at a 66-cm radius by a water-insulated 45-post sextuple-posthole convolute. The convolute sums the electrical currents at the outputs of the conical lines, and delivers the combined current to a single solid-dielectric-insulated radial transmission line. The radial line in turn transmits the combined current to the load. Since much of the accelerator is water insulated, we refer to it as Neptune. Neptune is 40 m in diameter, stores 4.8 MJ of electrical energy in its Marx capacitors, and generates 28 TW of peak electrical power. Since the Marxes are transit-time isolated from each other for 900 ns, they can be triggered at different times to construct-over an interval as long as 1 μs-the specific load-current time history required for a given experiment. Neptune delivers 1 MJ and 20 MA in a 380-ns current pulse to an 18-mΩ load; hence Neptune is a megajoule-class 20-MA arbitrary waveform generator. Neptune will allow the international scientific community to conduct dynamic equation-of-state, phase-transition, mechanical-property, and other material-physics experiments with a wide variety of drive-pressure time histories.

Linear Transformer Drivers (LTDs) represent a new pulsed power architecture that could dramatically reduce the size and cost of large pulsed-power drivers. Large LTD systems, however, will require hundreds, to tens-of-thousands, of lowinductance gas switches that can be DC-charged to -200kV and then be triggered with low jitter and low prefire probability. We are studying several competing gas switch geometries in an attempt to design an optimum switch for these applications as well as to increase our knowledge of the physics of the switching process. In addition to standard electrical diagnostics (V, I), we are studying the switches with a variety of optical diagnostics including fast photodiodes, a framing camera and a time-resolved spectroscopy system. In our test system, 20-nF capacitors on top and bottom of the switch are charged to voltages up to +100 kV, then the switch is triggered and current flows through the switch and a load resistor in a geometry that resembles the LTD architecture.Initial results have been obtained with a 6-stage, air-insulated switch based on a design by B. Koval'chukl. When the trigger arrives at the switch, two of the 6 gaps break down promptly but there is a delay of up to 50 ns before the other 4 gaps break down. Despite this delay, the 1-sigma jitter is + 10 ns. The 10%-to-90% risetime of the current pulse is 45 ns. Results from several competing switches will be presented.