Session JP18: Poster Session: ICF: Magneto-Inertial Fusion (2:00pm - 5:00pm)

Magnetized Plasma Gun Development for the PLX PJMIF Project

We present a description of the engineering and technical development, including a detailed overview of the design choices, of magnetized plasma guns for the PLX PJMIF Project. Intended to form a magnetized hydrogen plasma target, the magnetized plasma gun is an extension of HyperJet Fusion's latest coaxial plasma gun, called HJ1, which was developed for the 4$\pi$ scaling study of spherically imploding plasma liners as a standoff driver for plasma-jet-driven magneto-inertial fusion (PJMIF). Each magnetized gun incorporates a high power, pulsed magnet coil designed to inject sufficient helicity into the forming plasma within the gun, such that the resultant hydrogen plasma jet exits the gun with an embedded magnetic field. The desire is to achieve a magnetized hydrogen plasma jet with $\sim$3x10$^{14}$ cm$^{-3}$ muzzle density at 100 km/s with an embedded field of $\sim$1 kG. Each magnetized gun also includes a compact capacitor drive module with integral transmission line and sparkgap switching, an ultra-fast precision gas dispensing valve and a gas pre-ionization system utilizing a self-switching glow discharge. [1] Hsu et al., IEEE Trans. Plasma Sci. 40 (2012). [2] Y.C.F. Thio et al., Fus. Sci. Tech., Vol. 75, 581–598 (2019). [3] Yates et al., Phys. Plasmas 27, 062706 (2020).   

Magnetized Target Formation and Liner Uniformity for Plasma-Jet-Driven Magneto-Inertial-Fusion

The plasma-jet-driven magneto-inertial fusion (PJMIF) concept seeks to use a spherical plasma liner to compress a magnetized plasma target to thermonuclear conditions. The desired target plasma has $\beta>1$ and Hall magnetization parameter $\omega\tau>1$. Additionally, the parameters of such a target plasma are of interest as a laboratory platform to study fundamental physics related to astrophysical systems. Research on the Big Red Ball at the Wisconsin Plasma Physics Laboratory (WiPPL) has collided pre-magnetized plasmas to successfully form a plasma with $\beta>1$ and $\omega\tau>1$. At the Plasma Liner Experiment (PLX) at Los Alamos National Laboratory, we explore formation of a spherical plasma liner using up to 36 discrete plasma jets with high standoff distance from the region of plasma interaction and compression. Work is in progress to create target plasmas with $\beta>1$, $\omega\tau>1$, at higher densities than the plasmas formed at WiPPL, and then to compress this target plasma with the PLX liner. Utilizing the FLASH code, we study effects on target compression of the liner uniformity (from collision of discrete jets) and of the anisotropic thermal conductivity (from the target magnetic field). In this work, we continue to characterize and evaluate the PJMIF concept.   

Diagnostics for the PLX PJMIF magnetized target plasma jets

Our new SEED program is aimed at developing a suitable target plasma for Plasma Jet Magneto-Inertial Fusion (PJMIF) experiments at LANL [1,2]. The target parameters are density of $\sim$3 $\times$ 10$^{14}$~cm$^{-3}$, temperature above 5 eV, velocity of 100 km/s, and embedded magnetic field of 1 kG. In order to verify that the plasmoid meets these parameters we need to measure electron temperature (T$_e$), density (n), and magnetic field (B). The diagnostics used to verify that the plasmoid meets these criteria are interferometry (line integrated density), movable B-dot probe array (spatially resolved magnetic field), movable triple probe array (spatially resolved electron temperature, density), spatially resolved photodiodes (velocity) and time integrated spectroscopy (temperature, impurities). The fact that the plasma is supersonic poses certain challenges which are discussed along with the means used to address these issues. [1] Hsu et al., IEEE Trans. Plasma Sci.~{\bf 40}, 1287 (2012). [2] Yates et al., Phys. Plasmas \textbf{27}, 062706 (2020).   

Three-Dimensional Modeling of High Beta Magnetized Targets for Plasma-jet-driven Magneto-inertial-fusion (PJMIF)

Numerical simulations of compact toroid formation from supersonic plasma jets have been performed using Smooth Particle Fluid with MAXwell equation solver (SPFMax), a smooth particle hydrodynamics (SPH) code supporting the PLX-BETHE project. The physics includes radiation, Braginskii thermal conductivity and ion viscosity, separate ion and electron temperatures, tabular EOS (LTE and non-LTE), nonlocal fusion product deposition, and a novel electromagnetic field solver based on a combination of transmission line theory and Biot-Savart's law. Initial plasma jet conditions are derived from the experimental output of HyperJet-designed plasma guns. Variation in initial velocity, density, temperature, ion species, and formation coil geometry will be explored to assess the plasma beta and lifetime of the magnetic field. Preliminary simulations of 2 or more magnetized plasma jets will be performed to help guide experiments to follow.   

Modeling Neutron Transport for MagLIF experiments at the Z Facility using Attila and MCNP

For inertial confinement fusion experiments, the neutron yields is an important metric of the experiment. Accurately inferring neutron yields in experiments conducted at the Z facility is a difficult problem that requires understanding the neutron transport in a complex experimental assembly. In this, we demonstrate the utility of using Attila to develop a detailed geometrical model of MagLIF load hardware for neutron transport modeling in MCNP. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.   

Assessing the Origins of the Helical Instability in Axially Magnetized Liner Implosions

Magneto-inertial fusion concepts leverage magnetic fields to reduce thermal conduction losses and relax fuel areal density requirements. In MagLIF [Slutz, Phys. Plasmas 17, 056303 (2010)], an axial magnetic field is applied to a cm-scale metallic liner containing fusion fuel. Radiographs of magnetized implosions show helical instability structures despite the axial field being much lower than the azimuthal drive field [Awe, Phys. Rev. Lett. 111, 235005 (2013)]. Several hypotheses for the origin of these helical structures exist including magnetic flux compression [Ryutov, AIP Conf. Proc. 1639, 63 (2014), Seyler, Phys. Plasmas 25, 062711 (2018)], a seed from the electrothermal instability [Awe, Phys. Plasmas, 21, 056303 (2014)], and plasma bombardment of the liner surface [Sefkow, BAPS.2016.DPP.UI3.6]. Determining the physical mechanism responsible for the helical instability is a necessity to be able to predict how the instability will scale to higher currents. We have developed an experimental platform to begin discerning between these hypotheses on the Z machine. Specifically, the experiments address the hypothesis of magnetic flux compression through a change in the available flux while maintaining the same initial magnetic field. Simulations, experimental designs, and results will be presented. *Sandia National Laboratories is a multi-mission laboratory managed and operated by NTESS, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE's NNSA under contract DE-NA0003525.   

A Simulation Resource Team for Innovative Fusion Concepts in the BETHE Program

Computer simulations are indispensable tools in the development of all areas of science and engineering. For any innovative fusion scheme, simulations are essential to help interpret data and to extrapolate from the first experiments to a prototype design. Here we present a project that assembles a theory/modeling Capability Team at the University of Rochester to provide, under the auspices of the DOE ARPA-E BETHE Program, simulation support for Concept Teams and independent theoretical analysis of the physics underlying leading Concepts. We discuss the suite of simulation codes-fluid, hybrid, and kinetic-we will use in this effort, and how they will be applied to engage with Concept Teams that focus on plasma-jet-driven magneto-inertial fusion, field-reversal configurations, and the staged Z-pinch. The codes central to this project are FLASH, TriForce, and OSIRIS, chosen because they are flexible, high-performance computing codes, capable of one-, two-, and three-dimensional simulations, and can be used by Concept Teams to sustainably continue their modeling efforts. The Capability Team also leverages OSHUN, a Fokker-Planck code to develop models of magnetized transport.   

Understanding the relationship between current loss and load hardware geometry in Z machine experiments

Inertial confinement fusion, x-ray source development, and dynamic material physics studies on the Z Pulsed Power Facility at Sandia National Laboratories rely on dependable coupling of the $\sim$20 MA drive current to the experiment load. Multi-MA current losses have been observed in a variety of experimental configurations and historically have been attributed to the double post-hole convolute, where current is joined from four magnetically insulated transmission lines (MITL). More recently, there have been indications that current is also lost in the final transmission line that connects the convolute to the load. We compare tens of experiments with varied transmission line and load configurations, that produce varied loss, in order to comprehend the nature of the current loss mechanism in this critical region. A key goal of this work is to understand the underlying physics of current loss and use that to identify the characteristics that make a transmission line optimized for load coupling. Generator and load currents are determined via B-dot monitors and velocimetry, respectively. Trends in current loss with various load hardware parameters will be presented.   

Fusion from liner driven implosions of Field Reversed Configurations

MagLIF experiments$^{\mathrm{\thinspace }}$have demonstrated the basic principles of Magneto-Inertial Fusion. We present another approach using AutoMag liners to form and implode Field Reversed Configurations. External coils provide an initial bias magnetic field and the reverse field is then supplied by an AutoMag liner, which has helical conducting paths imbedded in an insulating substance. Experiments have demonstrated that AutoMag can generate magnetic fields greater than 30 Tesla within the liner. The fuel needs to be heated to 1-2 eV so that the initial bias field is partially frozen in. This can be done without a laser using radio frequency heating. We have performed 2D Radiation MHD simulations of the formation and implosion of an FRC on the Z machine, which indicate that DT yields greater than 30 kJ and plasma temperatures greater than 5 keV should be possible. We present an analytic model, which predicts the gain should scale with the implosion kinetic energy as E$_{\mathrm{k}}^{\mathrm{1/3}}$ and thus experiments on accelerators delivering only 1 MA should be interesting.   

Experimental and simulation investigations on a solid-state switched spiral generator for triggering of large scale pulsed power accelerators

This paper presents experimental and simulation work on a solid-state switched spiral generator, designed to trigger high current switches in the next generation of pulsed power devices. The spiral generator utilized new ultra-fast Thyristor as an input switch and a polarity dependent gap to sharpen the output pulse. It can produce 50 kV from a 3.6 kV charging voltage, with a risetime of \textless 50 ns and a jitter of 1.3 ns - directly comparable, if not better than a generator employing triggered spark gap as the input switch. The entire spiral generator, along with control and charging electronics fitted into a case 210\texttimes 145\texttimes 33 mm. The behavior of the spiral generator was modelled through a combination of the telegraph equations to account for the voltage waveforms as they travel along the spiral and an equivalent circuit exchanging charge between the spiral with the input switch and load. The influence of the spiral geometry, input switch and load were investigated. The model is compared to experimental measurements and shows remarkable agreement -- with the predicted output voltage waveform being within 10{\%} of the experimental values. The model will enable spiral generators to be readily optimized for different uses.