Publications

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Herein we present details of the design, simulation, and performance of a 100-GW linear transformer driver (LTD) cavity at Sandia National Laboratories. The cavity consists of 20 "bricks." Each brick is comprised of two 80 nF, 100 kV capacitors connected electrically in series with a custom, 200 kV, three-electrode, field-distortion gas switch. The brick capacitors are bipolar charged to ±100 kV for a total switch voltage of 200 kV. Typical brick circuit parameters are 40 nF capacitance (two 80 nF capacitors in series) and 160 nH inductance. The switch electrodes are fabricated from a WCu alloy and are operated with breathable air. Over the course of 6,556 shots the cavity generated a peak electrical current and power of 1.03 MA (±1.8%) and 106 GW (±3.1%). Experimental results are consistent (to within uncertainties) with circuit simulations for normal operation, and expected failure modes including prefire and late-fire events. New features of this development that are reported here in detail include: (1) 100 ns, 1 MA, 100-GW output from a 2.2 m diameter LTD into a 0.1 Ω load, (2) high-impedance solid charging resistors that are optimized for this application, and (3) evaluation of maintenance-free trigger circuits using capacitive coupling and inductive isolation.

The Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17-18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10-1000 MJ) should be possible with pulsed power machines producing larger drive currents (45-60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.

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Saturn is a short-pulse ( 40 ns FWHM) x-ray generator capable of delivering up 10 MA into a bremsstrahlung diode to yield up 5 × 10^12 rad/s (Si) per shot at an energy of 1 to 2 MeV. With the machine now over 30 years old it is necessary to rebuild and replace many components, upgrade controls and diagnostics, design for more reliability and reproducibility, and, as possible upgrade the accelerator to produce more current at a low voltage ( 1 MV or lower). Thus it has been necessary to reevaluate machine design parameters. The machine is modeled as a simple LR circuit driven with an equivalent a sine-squared drive waveform as peak voltage, drive impedance, and vacuum inductance are varied. Each variation has implications for vacuum insulator voltage, diode voltage, diode impedance, and radiation output. For purposes of this study, radiation is scaled as the diode current times the diode voltage raised to the 2.7 power. Results of parameter scans are presented and used to develop a design that optimizes radiation output. Results indicate that to maintain the existing short pulse length of the machine but to increase output it is most beneficial to operate at an even higher impedance than originally designed. Also discussed are critical improvements that need to be made.

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Absolute calibration of streaked visible spectroscopy systems has been performed at Z-machine at Sandia National Labs in order to determine temperatures of electrode surfaces during the current pulse. The ability to calibrate the full system, including all fiber optic runs and probes is crucial to understanding errors in the calibration process. The calibration procedure involves imaging a blackbody light source, with a known spectral radiance which is coupled to an integrating sphere. This source is streaked slowly over a few ns using Sydor streak cameras. The slow sweep is converted to a 100-500ns sweep by imaging a bright light source on both sweep rates, and obtaining wavelength and time dependent correction curves. Any broadband light source or several laser lines of differing wavelengths can be used for this correction. This technique has yielded temperature estimates of several eV in the Z convolute.

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The Z machine is a 36-module, multi-megavolt, low impedance magnetic pressure driver for high-energy-density physics experiments. In 2007, a major re-build doubled the stored energy and increased the peak current capability of Z. The upgraded system routinely drives 27 MA through low inductance dynamic loads with 110 nanosecond time to peak current. The Z pulsed power system is expected to be prepared for a full-energy experiment every day, with a small (<2%) chance of pulsed power system failure, and ±2 ns timing precision. To maintain that schedule with 20 MJ stored, it becomes essential to minimize failures that can damage hardware. We will show the results of several improvements made to the system that reduce spurious breakdowns and improve precision. In most cases, controlling electric fields is key, both to reliable insulation and to precision switching. The upgraded Z pulsed power system was originally intended to operate with 5 MV peak voltage in the pulse-forming section. Recent operation has been above 6 MV. Critical items in the pulsed power system are the DC-charged Marx generators, oil-water barriers, laser-triggered gas switches, and the vacuum insulator. We will show major improvements to the laser-triggered gas switches, and the water-insulated pulse forming lines, as well as delivered current reproducibility results from user experiments on the machine.

National Security Technologies (NSTec) is developing dense plasma focus (DPF) systems for applications requiring intense pulsed neutron sources. Sandia National Laboratories participated in a limited number of experiments with one of those systems. In collaboration with NSTec, Los Alamos National Laboratory, and Lawrence Livermore National Laboratory, we installed additional electrical and X-ray image measurements in parallel with normal operation of the system. Dense plasma focus machines have been studied for decades, but much of the experimental interest has been on neutron and X-ray yield. The primary goal for the present work was to develop and field high-fidelity and traceably-calibrated current and voltage measurements for comparison to digital simulations. The secondary goals were to utilize the current and voltage measurements to add general understanding of vacuum insulator behavior and current sheath dynamics. We also conducted initial scoping studies of soft X-ray diagnostics. We will show the electrical diagnostics and the techniques used to acquire high-fidelity signals in the difficult environment of the 2 MA, 6 μ plasma focus drive pulse. We will show how we measure accreted plasma mass non-invasively, and the sensitivity to background fill density. We will present initial qualitative results from filtered X-ray pinhole images and spectroscopic data from the pinch region.

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We have conceived a new class of prime-power sources for pulsed-power accelerators: impedance-matched Marx generators (IMGs). The fundamental building block of an IMG is a brick, which consists of two capacitors connected electrically in series with a single switch. An IMG comprises a single stage or several stages distributed axially and connected in series. Each stage is powered by a single brick or several bricks distributed azimuthally within the stage and connected in parallel. The stages of a multistage IMG drive an impedance-matched coaxial transmission line with a conical center conductor. When the stages are triggered sequentially to launch a coherent traveling wave along the coaxial line, the IMG achieves electromagnetic-power amplification by triggered emission of radiation. Hence a multistage IMG is a pulsed-power analogue of a laser. To illustrate the IMG approach to prime power, we have developed conceptual designs of two ten-stage IMGs with LC time constants on the order of 100 ns. One design includes 20 bricks per stage, and delivers a peak electrical power of 1.05 TW to a matched-impedance 1.22-Ω load. The design generates 113 kV per stage and has a maximum energy efficiency of 89%. The other design includes a single brick per stage, delivers 68 GW to a matched-impedance 19-Ω load, generates 113 kV per stage, and has a maximum energy efficiency of 90%. For a given electrical-power-output time history, an IMG is less expensive and slightly more efficient than a linear transformer driver, since an IMG does not use ferromagnetic cores.

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In this study, we have developed a conceptual design of a next-generation pulsed-power accelerator that is optmized for driving megajoule-class dynamic-material-physics experiments at pressures as high as 1 TPa. The design is based on an accelerator architecture that is founded on three concepts: single-stage electrical-pulse compression, impedance matching, and transit-time-isolated drive circuits. Since much of the accelerator is water insulated, we refer to this machine as Neptune. The prime power source of Neptune consists of 600 independent impedance-matched Marx generators. As much as 0.8 MJ and 20 MA can be delivered in a 300-ns pulse to a 16-mΩ physics 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 well-defined drive-pressure time histories. Because Neptune can deliver on the order of a megajoule to a load, such experiments can be conducted on centimeter-scale samples at terapascal pressures with time histories as long as 1 μs.