Samuel Brockington

Spherically Imploding Plasma Liners as a Standoff Driver for Magnetoinertial Fusion

Spherically imploding plasma liners formed by merging an array of high Mach number plasma jets are a proposed standoff driver for magnetoinertial fusion (MIF). This paper gives an updated concept-level overview of plasma liner MIF, including advanced notions such as standoff methods for forming and magnetizing the fuel target and liner shaping to optimize dwell time. Results from related 1-D radiation-hydrodynamic simulations of targetless plasma liner implosions are summarized along with new analysis on the efficiency of conversion of the initial liner kinetic energy to stagnation thermal energy. The plasma liner experiment (PLX), a multi-institutional collaboration led by the Los Alamos National Laboratory, plans to explore the feasibility of forming spherically imploding plasma liners via 30 merging plasma jets. In the near term, with modest pulsed power stored energy of ≲1.5 MJ, PLX is focusing on the generation of centimeter-, microsecond-, and megabar-scale plasmas for the fundamental study of high energy density laboratory plasmas. In the longer term, PLX can enable a research and development path to plasma liner MIF ultimately requiring compressing magnetized fusion fuel to ≳100 Mbar.

A contoured gap coaxial plasma gun with injected plasma armature

A new coaxial plasma gun is described. The long term objective is to accelerate 100-200 microg of plasma with density above 10(17) cm(-3) to greater than 200 km/s with a Mach number above 10. Such high velocity dense plasma jets have a number of potential fusion applications, including plasma refueling, magnetized target fusion, injection of angular momentum into centrifugally confined mirrors, high energy density plasmas, and others. The approach uses symmetric injection of high density plasma into a coaxial electromagnetic accelerator having an annular gap geometry tailored to prevent formation of the blow-by instability. The injected plasma is generated by numerous (currently 32) radially oriented capillary discharges arranged uniformly around the circumference of the angled annular injection region of the accelerator. Magnetohydrodynamic modeling identified electrode profiles that can achieve the desired plasma jet parameters. The experimental hardware is described along with initial experimental results in which approximately 200 microg has been accelerated to 100 km/s in a half-scale prototype gun. Initial observations of 64 merging injector jets in a planar cylindrical testing array are presented. Density and velocity are presently limited by available peak current and injection sources. Steps to increase both the drive current and the injected plasma mass are described for next generation experiments.

Plasma density gradient measurement using laser deflection

For a given chord through a plasma, changes in the line integrated index of refraction as a result of a transverse density gradient can be observed by measuring the angle of deflection of a laser beam. In contrast to laser interferometers, this method of density profile measurement places modest requirements on laser quality and alignment procedures, allowing measurements to be conducted with short coherence length commercial laser diodes and segmented photodiode detectors. A prototype implementation of this scheme has been constructed and tested on the compact toroid injection experiment (CTIX). At densities comparable to magnetic fusion plasmas, laser deflections in the nanoradian range were measured. By assuming a particular density profile, a sensitivity of ∼1012cm−3∕nrad was obtained. This produced estimates of CTIX peak density in reasonable agreement with conventional interferometry data. The final goal of this diagnostic is a simple, reliable, array deployable density profile diagnostic.

Experiment to Form and Characterize a Section of a Spherically Imploding Plasma Liner

We describe an experiment to form and characterize a section of a spherically imploding plasma liner by merging six supersonic plasma jets that are launched by newly designed contoured-gap coaxial plasma guns. This experiment is a prelude to forming a fully spherical imploding plasma liner using many dozens of plasma guns, as a standoff driver for plasma-jet-driven magnetoinertial fusion. The objectives of the six-jet experiments are to assess the evolution and scalings of liner Mach number and uniformity, which are important metrics for spherically imploding plasma liners to compress magnetized target plasmas to fusion conditions. This paper describes the design of the coaxial plasma guns, experimental characterization of the plasma jets, six-jet experimental setup and diagnostics, initial diagnostic data from three- and six-jet experiments, and the high-level objectives of associated numerical modeling.

Poloidal field amplification in a coaxial compact toroid accelerator

The Compact Toroid Injection Experiment (CTIX) produces spheromak-like compact toroids (SCTs) without external power switching, initiating a discharge by pulsed gas injection into a formation region containing a seed magnetic field generated by a solenoidal coil. After formation, the plasma is driven by an inductively delayed capacitor bank into an acceleration region, where surface axial and toroidal magnetic fields are measured at several axial positions. Due to strong eddy-current effects, formation-region magnetic field cannot be simply computed; instead, it is measured using the response of axial and radial test coils in the formation region to short solenoid test current pulses. A temporal and spatial reconstruction method is developed allowing formation-region field to be computed from the test-coil data for any CTIX discharge of identical solenoid geometry. By varying the peak value and timing of solenoidal current, curves of peak accelerator-region field as a function of initial formation-region field are developed. Curves of peak accelerator-region axial magnetic field are thereby found to be highly nonlinear functions of formation-region field, showing a threshold value for the formation-region field of approximately 5 G, above which acceleration-region field saturates at values between 2 and 12 kG. The direction of acceleration-region axial field reverses sign when the direction of solenoid current is reversed. Saturated accelerator-region axial field is a function of axial position and accelerator voltage, and is typically comparable to toroidal field at the same location. The ratio of accelerator-region to formation-region axial field commonly exceeds 1000 near the onset of saturation. This large amplification is of practical advantage for delayed plasma breakdown on CTIX, allowing a modest seed field to produce high poloidal fields, which are necessary for intense SCT acceleration. The results may also provide a useful benchmark for numerical simulation of the conversion of abundant toroidal field into poloidal field in a plasma with comparatively low dissipation.

Method of reconstructing a moving pulse

We present a method of analyzing a set of N time signals fi(t) that consist of local measurements of the same physical observable taken at N sequential locations Zi along the length of an experimental device. The result is an algorithm for reconstructing an approximation F(z,t) of the field f(z,t) in the inaccessible regions between the points of measurement. We also explore the conditions needed for this approximation to hold, and test the algorithm under a variety of conditions. We apply this method to analyze the magnetic field measurements taken on the Compact Toroid Injection eXperiment (CTIX) plasma accelerator; providing a direct means of visualizing experimental data, quantifying global properties, and benchmarking simulation.

Coaxial guns for the ARPA-E PLX-α project — Design and initial experimental results

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-Es 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, N2, Kr, and Xe; 20-60 km/s; 1016-1017 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.

Toward imploding spherical plasma liner formation via an array of merging supersonic plasma jets

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≈1017 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 made3-7 on topics (i)-(iv) before the project was terminated. This presentation reviews, for the first time, the projects research accomplishments, and closes with remarks on the concepts readiness to proceed to thirty-jet spherical plasma liner formation experiments.

Minirailgun accelerator for plasma liner driven HEDP and magneto-inertial fusion experiments

A spherical array of minirailgun plasma accelerators is a potential driver for forming imploding spherical plasma liners that can reach HEDP-relevant (∼0.1 Mbar) pressures upon stagnation. The liners would be formed via merging of 30 or more dense, high Mach number plasma jets (n ∼ 1016⁻¹⁷ cm^∓3, M ∼10−35, v ∼ 50–70 km/s, r<inf>jet</inf> ∼5 cm) in a spherically convergent geometry. The small (typically 1–2 cm square bore × 15–50 cm length) parallel-plate railguns with ceramic insulators would use pulsed injection of high-Z gas at the breech via fast opening valves to produce high density plasma jets with velocity in the 50–100 km/s range. Recent tests at HyperV using a single pulsed capillary discharge injecting into the minirailgun breech have achieved plasma densities in the bore approaching 10¹⁸ cm⁻³, with densities in the jet plume exceeding 10¹⁷ cm⁻³ at velocities above 50 km/s. Total plasma jet mass in these 1 cm square bore tests has not yet been determined, but similar tests of an earlier 6 mm square bore 13 cm long device, with a roughly 3 µs, 100 kA current pulse using an aluminized mylar fuse starting from rest, yielded 90 µ;g of plasma at 50 km/s, and about 40µ;g at 63 km/s. A modest scaleup of the railgun to a 2 cm square bore operating at longer pulse widths of 200–300 kA should be capable of accelerating a few thousand micrograms of high-Z gas (e.g. xenon) to above 50 km/s. This performance should be sufficient for reaching HEDP-relevant pressures¹.

Calibration of magnetic probes in the vicinity of a conducting well

Measuring magnetic fields near the edge of a plasma device can be complicated by the geometric effects of the ports through which such measurements are made. The primary effect is an attenuation of the magnetic field at the probe coil due to the field expanding into the finite sized conducting well of the port. In addition, it is possible to determine the correspondence between the location of a field line as it intersects the probe coil inside the well, with its location far from the perturbation of the well. Here we explore several methods of experimentally characterizing the magnetic fields in the vicinity of the magnetic probe ports of a vacuum vessel, with the aim of improving the interpretation of magnetic measurements needed for experiments in plasma physics.