Scott C Hsu

Spherically imploding plasma liners 1 are a proposed low-cost, reactor-relevant magneto-inertial-fusion (MIF) driver for compressing magnetized plasma targets to fusion conditions. The Plasma Liner Experiment-ALPHA (PLX-α aims to demonstrate the formation of subscale plasma liners via dozens of merging supersonic plasma jets (with initial ion density ~ 10 16 cm -3 , velocity ≈50 km/s, mass ~ 1 mg, and using various gas species). In the ongoing, first set of PLX-α experiments, we plan to merge six and seven plasma jets to form a conical section of a spherically imploding plasma liner in order to assess the shock heating (and associated Mach-number degradation) and uniformity of the liner upon jet merging and during further convergence, before proceeding to fully spherical liner-formation experiments (if warranted by the conical-liner results). In this talk, we will summarize experimental findings to date on characterizing plasma jets formed by the newly designed PLX-α guns and conical-plasma-liner formation with up to seven guns. Gated fast-framing-camera images from initial shakedown experiments suggest that shock formation between adjacent merging jets is consistent with oblique-shock formation as observed in earlier two- and three-jet merging experiments. 2,3

Spherically imploding plasma liners 1 are a potential reactorrelevant approach to compressing magnetized plasma targets to fusion conditions, for example as in magneto-inertialfusion (MIF). We use the USim code, a multi-dimensional hydrodynamic code with treatments for MHD and fusion effects, to estimate the gain for various liner conditions. We performed 1D and 2D simulations, with and without density perturbations, and with different models for alpha particle energy deposition. Specifically, we show that gain greater than one is possible for liners of argon gas with density 85 kg/m 3 and velocity 60 km/s, even with 10% density perturbation. We also show gain \sim 20 for ideal conditions. We also compare these results with recent semi-analytic work 2 .

The Plasma Liner Experiment-ALPHA (PLX-α) is investigating the merging of supersonic plasma jets into a spherically imploding plasma liner as a driver for use in magneto-inertial fusion (MIF) architectures. 1 The present work is focused on characterizing the merging of six and/or seven plasma jets, converging in a cone of solid angle 0.4 \pi over a distance of 1.3 meters. Results from high-speed imaging, photodiode arrays, self-emission visible survey spectroscopy, and self-emission visible high-resolution spectroscopy will be presented, yielding measurements of plasma velocity, number density, electron/ ion temperatures, and mean ionization state pre- and post-merge. Anticipated plasma parameter regimes are \mathrm {n}\sim 10 ^{15}-10 ^{17} cm^{-3}, \mathrm {T}\sim 1-10 eV, and \mathrm {v}\sim 50 km/s, with gas species varied among argon, nitrogen, neon, krypton, and xenon. Images and spectra will be compared with synthetic data generated from 3D fronttracking and smooth-particle-hydrodynamic simulations coupled with atomic physics / opacity analysis codes. Results will inform questions of liner-Mach-number and lineruniformity evolution throughout the jet-merging and subsequent liner-convergence process.

The Plasma Liner Experimnent-ALPHA (PLX-α project is developing spherically imploding plasma liners as a standoff driver for a plasma-jet-driven magneto-inertial-fusion (PJMIF) concept, in which merging supersonic plasma jets form a plasma liner that compresses a magnetized plasma target to fusion conditions. One main aim of the simulation component of the PLX-α project is to perform highly resolved simulations of supersonic plasma jets at experimental conditions, their propagation and merger, and formation and implosion of liners. Detailed numerical studies of hydrodynamic processes in plasma jets and liners have been performed using the FronTier and SPH codes enhanced with radiation, physical diffusion, and plasma-EOS models. In this talk, we discuss predicted properties of plasma liners, in particular 4π-averaged liner density, ram pressure, and Mach number, the degree of nonuniformity, strength of primary and secondary shock waves, and the scaling of these quantities with the number of plasma jets, initial jet parameters, and other input data. In addition to direct analysis of liner states, simulations also provide synthetic data for direct comparison to experimental data from a multi-chord interferometer, survey and high-resolution spectrometers, and high-speed cameras. A detailed comparison of simulations with the first series of PLX-α experiments operating with 6 and 7 jets will be presented. Verified against experimental data, both codes will be used for predictive simulations of spherical plasma liners for upcoming PLX-α experiments and potential scaled-up future experiments.

Spherically imploding plasma liners 1 are a proposed reactorfriendly magneto-inertial-fusion (MIF) driver for compressing magnetized plasma targets to fusion conditions. The Plasma Liner Experiment-ALPHA (PLX-α) is aiming to demonstrate the formation of such liners via dozens of merging supersonic plasma jets (with ion density \sim 10 ^{16} cm^{-3}, velocity \approx 50 km/s, mass \sim 1 mg, and various species). In the ongoing, first set of PLX-α experiments, we are merging 6 and 7 plasma jets to form a conical section of a spherically imploding plasma liner in order to assess the shock heating (and associated Mach-number degradation) and uniformity of the liner upon jet merging and during further convergence. Diagnostics include single- and multi-frame fast-gated ICCD cameras, visible survey spectrometer, high-resolution spectrometer, 12-chord visible interferometer, and visible photodiodes. Presented here will be the first results of the density and electron temperature uniformity of 6- and 7-jet merging using several gas species (argon, nitrogen, neon, krypton, xenon). Several 12-chord laser interferometry configurations are fielded with both end-on and crosssectional laser chord paths to determine density distribution and evolution. Results presented here will be compared to synthetic data from 3D simulations run using both smoothedparticle-hydrodynamics and the FronTier codes.

Summary form only given. We present comparison of simulations of plasma-jet-driven magneto-inertial fusion (PJMIF) with analytic results and with results from established codes 1 . The liners are spherically symmetric xenon plasma with implosion velocities of 60-120 km/s (assumed to be formed by merging plasma jets). At the center of the liner is a DT target that is compressed by the liner and the resulting high density and temperature creates neutron yield. We investigate, in particular, the effect of equation of state, radiation, thermal conduction and alpha particle production on the neutron yield. The goal of this work is to guide design of high gain configurations of PJMIF for a fusion power reactor 2 . A prototype of such a device is the PLX-Alpha project, currently operating at Los Alamos National Laboratory 3 .

Summary form only given. We describe the ongoing effort to design, build, and test coaxial plasma guns [1] appropriate for a scaling study of spherically imploding plasma liners as a standoff magneto-inertial-fusion driver under ARPA-E's Accelerating Low-Cost Plasma Heating And Assembly (ALPHA) program. HyperV joins LANL, UAH, UNM, BNL, and Tech-X to develop, build, operate and analyze a 60-plasma-gun experiment using the existing PLX facility [2] at LANL. The guns are being designed to operate over a range of operating parameters: 0.5-5.0 mg of Ar, Ne, N 2 , Kr, and Xe; 20-60 km/s; 10 16 -10 17 cm -3 muzzle density; and up to 7.5 kJ stored energy per gun. Each coaxial gun incorporates a contoured gap designed to suppress the blow-by instability, fast dense gas injection and triggering, and innovative integral sparkgap switching. The switch and pfn configurations are designed to reduce inductance, cost, and complexity, and to increase efficiency and system reliability. Each gun is driven by a 600μF, 5kV capacitor bank mounted directly onto the back of the gun to reduce inductance. The pfn is sufficiently low weight to allow mounting of the gun/pfn module directly on the vacuum-tank port without any additional supports. This also eliminates the need for racks and thick-cable bundled transmission lines to 60 guns, resulting in vastly improved experimental access to the vacuum tank, guns, and diagnostics. We will provide a brief overview of the PLX-α project, describe the overall design approach for the guns and pulsed-power systems, the projected performance over the parameter ranges mentioned above, and experimental results from testing of the first gun, AlphaGun-1.

Capsules driven with polar drive [1, 2] on the National Ignition Facility [3] are being used [4] to study mix in convergent geometry. In preparation for experiments that will utilize deuterated plastic shells with a pure tritium fill, hydrogen-filled capsules with copper doped deuterated layers have been imploded on NIF to provide spectroscopic and nuclear measurements of capsule performance. Experiments have shown that the mix region, when composed of shell material doped with about 1% copper (by atom), reaches temperatures of about 2 keV, while undoped mixed regions reach about 3 keV. Based on the yield from these implosions, we estimate the thickness of CD that mixed into the gas as between about 0.25 and 0.43 mu\m of the inner capsule surface, corresponding to about 5 to 9 mu g of material. Using 5 atm of tritium as the fill gas should result in over 1013 DT neutrons being produced, which is sufficient for neutron imaging [5].

In ongoing experiments at the Plasma Liner Experiment (PLX) facility at Los Alamos National Laboratory, two high Mach number plasma jets, composed of gases such as H and Ar, will be collided. We describe numerical simulations using particle-in-cell (PIC) and hybrid-PIC methods using the code LSP. Using expected experimental plasma conditions (n ~ 10 14 -10 16 cm -3 ) large scale transport simulations demonstrate that the jets are essentially collisionless at the merge point. In smaller-scale 1D and 2D simulations we show that collisionless shocks are generated by the merging jets when immersed in applied magnetic fields (B ~ 0.1-1 T). Unmagnetized collisionless shocks are not found in simulations at the expected jet velocities (10-100 km/s). Considerably higher velocities are required to see this effect. The orientation of the magnetic fields and the axial and transverse gradients of the jets are shown to a have strong effect on the nature of the interaction.

Summary form only given. Imploding spherical plasma liners formed by an array of merging supersonic plasma jets are a potential standoff compression driver for magneto-inertial fusion. 1, 2 From 2009-2012, a multi-institutional collaboration led by LANL pursued an integrated theory/modeling and experimental effort aimed at fielding targetless, spherical-plasma-liner formation experiments via the merging of thirty argon plasma jets, to reach 0.1-1 Mbar of peak stagnation pressure with a total (thirty-jet) initial kinetic energy of ~375 kJ. Specific goals included: (i) developing scaling laws for peak achievable pressures as a function of initial plasma jet parameters; (ii) achieving the plasma gun technology to deliver argon plasma jets with the requisite parameters (n≈10 17 cm -3 , V≈50 km/s, mass≈8 mg); (iii) experimentally characterizing single-jet propagation and multiple-jet oblique merging; (iv) exploring 3D effects of discrete merging jets; and (v) fielding thirty-jet spherical implosion experiments. The new Plasma Liner Experiment (PLX) facility was constructed at LANL for this project, and substantial progress was made 3-7 on topics (i)-(iv) before the project was terminated. This presentation reviews, for the first time, the project's research accomplishments, and closes with remarks on the concept's readiness to proceed to thirty-jet spherical plasma liner formation experiments.