Session CP16: Poster Session: ICF: Dense Plasma Focus (2:00pm - 5:00pm)

Dense Plasma Focus Simulations at LLNL

Dense plasma focus (DPF) Z-pinches are compact pulse power driven devices consisting of two coaxial electrodes, separated by an insulator and filled with a low-density gas. The discharge of DPF consists of three distinct phases: first generation of a plasma sheath, plasma rail gun phase where the sheath is accelerated down the electrodes and finally an implosion phase where the plasma stagnates into a z-pinch geometry. A DPF is similar in nature to a traditional gas puff z-pinch, with the rail gun phase serving as an opening switch for a fast-current rise into an imploding load. MHD/XMHD, Hybrid Kinetic and Fully Kinetic techniques are employed in simulating dense plasma focus (DPF) loads at LLNL for optimizing neutron generation. Simulations of two of LLNL's DPFs will be presented: a 300 J, 80 kA small scale DPF and the newly commissioned MJOLNIR DPF which operates at 1 MJ and 2.25 MA. Simulations and results from the 2019-20 commissioning campaign of MJOLNIR will be presented.   

Insulator Surface Conditioning Effects on X-Ray Yield of a 10 kJ-Scale Dense Plasma Focus.

The insulator sleeve plays an important role in Dense Plasma Focus (DPF) current sheath dynamics, which can affect production of X-rays and neutrons. We have conducted experiments with 250 kA current and 2.5 $\mu $s rise time to investigate the effect of insulator sleeve conditions on X-ray production. A variety of borosilicate insulator sleeves with smooth and rough surfaces have been used. Insulators with the smoothest surfaces resulted in the highest soft X-ray yield to date when filled with Ne between 0.1 and 1.0 torr. Introducing surface roughness has a detrimental effect on X-ray yield and can substantially delay the X-ray burst.   

DPF Experiments at LLNL

A dense plasma focus is a relatively compact coaxial plasma gun which completes its discharge as a Z-pinch. These devices have been designed to operate at a variety of scales in order to produce short (\textless 100 ns) pulses of ions, X-rays, or neutrons. LLNL has recently constructed and brought into operation a new device, the MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF which is designed for radiography and high yield operations. This device has been commissioned over the last year and has achieved neutron yields up to 3E11 neutrons/pulse at 2.2 MA pinch current while operating at up to 1 MJ of stored energy. MJOLNIR is equipped with a wide range of diagnostics, including activation foils, neutron time of flight detectors, a fast framing camera, optical light gates, and a time-gated neutron and x-ray imager. LLNL also runs unique particle-in-cell (PIC) simulations of DPF discharges, and has been able to gain significant insight into the various physical factors that influence neutron yield. To that end, MJOLNIR is one of the first DPFs whose design and continual upgrades are heavily influenced by model predictions. We will present device operation, recent results, and first x-ray and neutron images..   

Seeking Experimental Evidence for Radiative Collapse in Hybrid X-pinches

We present results of initial studies of the X-ray bursts produced by Hybrid X-pinches (HXPs) in the soft X-ray spectral range using \textasciitilde 10 ps time resolution X-ray streak cameras. The first goal is to collect X-ray spectra that can illuminate the role of radiative collapse in the formation of the micropinches that produce the X-ray bursts. The second goal is to obtain time-resolved source size measurements in the 2.5-5 keV X-ray energy range. To do that and capture the spectrum or source image on the streak camera on its 1ns full screen streak, we must have a strong X-ray burst from the HXP at a time that is reproducible within \textpm 1 ns. As a first step, we tested the results produced by HXPs made of Al, Ag, Mo, and Ti by varying the gap distance between the two conical electrodes and the wire sizes, keeping the mass per unit length constant across all the different materials, using the 250-300 kA, 50 ns rise time current pulse on the XP pulsed power generator. Using 40 \textmu m Ti wires, more than 50{\%} of 44 pulses produced X-ray bursts between 33-35 ns after the start of the current pulse. Initial time-resolved source size measurements and X-ray spectra from Ti and Mo with \textasciitilde 10 ps time resolution have been obtained. Further studies with higher magnification are being carried to accurately determine the source size.   

Development of a Gas-Puff Z-Pinch Neutron Source for the 1-MA, 100-ns MAIZE LTD

The Z-machine at Sandia National Laboratories is instrumental in plasma physics research across a range of applications. University-scale z-pinch experiments, such as gas-puff z-pinches, can inform the high-value experiments conducted on the Z facility. A gas-puff z-pinch requires gas to be puffed into the anode-cathode gap, which is then pulsed with a high voltage. The gas is ionized, accelerated, and compressed as the current flows across the electrodes, allowing for study of pinch phenomena including fusion reactions. The initial ionization condition of the gas-puff prior to compression is poorly understood. Additionally, how this affects fusion, which is largely the result of micro-pinch instabilities, is also poorly understood. We report on the progress made in developing this experimental capability for the 1-MA, 100-ns MAIZE Linear Transformer Driver at the University of Michigan. Specifically, we discuss the construction and integration of the fast-valve and nozzle assembly, valve drivers, gas manifold, MAIZE transmission lines, and pre-ionization generator.   

Calculational Study of the Z-Pinch Dynamics of Resistively Thick Aluminum Rods

The fundamental limits of high-current conduction are of interest to magnetically driven ICF and other applications. Nonlinear Ohmic heating and conductor motion lead to instabilities such as the Electrothermal Instability (ETI) and Magneto-Rayleigh Taylor (MRT) that disrupt current flow. Here, the LANL, resistive MHD code, FLAG, is used to model, well-diagnosed, uncoated Al rod loads (R$_{\mathrm{0}}$ \textasciitilde 400 $\mu $m \textgreater skin-depth) in a Z-pinch configuration fielded on the Sandia Mykonos pulse generator (t$_{\mathrm{rise}}$ \textasciitilde 0.1 $\mu $s, I$_{\mathrm{peak}}$ \textasciitilde 1 MA). Results are compared to PDV measurements. Initial rod compression due to Lorentz forces in the solid-state agree well with experiments. After melt, during expansion, results with a tabular EOS that utilizes Maxwell constructs in the bi-phase region show better agreement to data than ones with Van der Waals loops. As predicted, the state of the outer layer of the rod follows the liquid/vapor coexistence curve. Finally, calculational sensitivity to EOS and conductivity are studied to better understand the expansion dynamics.   

Numerical Investigations of $m=0$ sausage instabilities in Axisymmetric Z-pinch in 5-moment Two-Fluid Model

The growth rates for a $m=0$ sausage instability in axisymmetric Z-pinch are investigated in ideal MHD and 5-moment two-fluid model via Washington Approximate Riemann Plasma (WARPXM) code. The WARPXM code developed at the University of Washington implements numerical simulations by using Discontinuous Galerkin (DG) methods. The growth rates obtained by using 5-moment two-fluid models for the shear-free Z-pinch are larger compared with the ideal MHD simulations but comparable to the Hall MHD simulations presented by V.I. Sotnikov et al. at the small-$k$ modes. As increasing the wave number $k$, the growth rates show the peak at ${kr}_{p}\approx r_{p}/r_{Li}$, where $r_{p}$ is the pinch radius and $r_{Li}$ is ion Lamor radius, and the further stabilizing effects at the large-$k$ modes that are consistent to the former numerical studies done by J. Scheffel et al. using the Vlasov-fluid model and K. Tummel et al. using the fully kinetic model. Applying the radial sheared flow, $\partial_{r}v_{z}\ne 0$, is our future work and expected to further stabilize the sausage instabilities as observed in Fusion Z-pinch Experiment (FuZE) and the numerical studies done by V.I. Sotnikov et al. using ideal and Hall MHD models.   

Experimental Measurements of X-Ray Driven Plasma Ablation From Solid Density Silicon Targets

In this poster we present preliminary observations of fast plasma outflows which are generated when prompt X-Ray bursts impinge upon silicon targets. The X-Ray bursts are produced by the implosion of wire array Z-Pinches on the MAGPIE pulsed power facility ($1.4 \; \mathrm{MA}$ peak-current, $240 \; \mathrm{ns}$ rise-time). The X-Rays emitted by the arrays have spectra which are dominated by continua (color-temperature $\sim 150 \; \mathrm{eV}$), and persist for long timescales ($\sim 30 \; \mathrm{ns}$). The plasma outflows are diagnosed with a state of the art suite of spatially and temporally resolved diagnostics including interferometry, optical Thomson scattering, and fast frame optical self-emission imaging. They are observed to have a uniform structure, and a characteristic velocity . The plasmas expand into strong magnetic fields ($B \sim 10 \; \mathrm{T}$), generated by the pulsed-power drive. The well-defined spatial structure of the plasma outflows mean that the setup represents a promising testbed for radiation-hydrodynamics problems. The experiments could also be tuned to facilitate the study of extended MHD phenomena, particularly the Nernst effect.   

Characterization of the Imploding Plasma Sheath in Triple Nozzle Gas-Puff Z-pinches at 1 MA

Triple nozzle gas-puff implosions on the 1 MA, 220 ns COBRA generator at Cornell University provide an efficient source of intense x-ray radiation and are of interest for magneto-inertial fusion studies with an applied magnetic field. These implosions are susceptible to the magneto-Rayleigh-Taylor instability (MRTI); however, observations indicate that they are more stable than predicted by simple MRTI theory. Furthermore, the instability growth rate, characterized by an effective Atwood number, is observed to depend on gas species and initial fill density. Detailed measurements of the plasma parameters in the imploding plasma sheath can help to provide an explanation for these observations and can be used to validate simulation codes. To this end, we have used collective Thompson Scattering, Zeeman polarization spectroscopy, and laser shearing interferometry to characterize the imploding plasma sheath at a radius of 1 -- 1.5 cm with 0.25 mm spatial resolution. The preliminary results of this study are presented here.   

Magnetic Mirror for Exploding Wire Plasma Confinement

An auto-magnetizing liner producing a mirror magnetic configuration has the potential to increase fuel density in MAGLIF experiments by reducing end losses [1]. To demonstrate the feasibility of forming a mirror trap on a pulsed power platform, we have fabricated and fired wire arrays that produce magnetic mirror fields on COBRA, a pulsed power machine with 1 MA peak current and 100 ns rise time. In initial experiments, we exploded 127 \textmu m diameter Al wires that are connected in parallel with twisted copper wire arrays in either mirror or cusp configuration. Generation of a mirror configuration by the copper wire array was observed with micro bdot probes placed at multiple axial positions; a mirror ratio of 3.8 was measured at 0.5 MA load current. Up to twelve visible light images were obtained with a high-speed camera at 40 ns intervals and plasma was observed to follow magnetic field line in the cusp configuration. We are planning a series of experiments to measure the flow of particles that have escaped from the mirror using a retarding potential analyzer. [1] Shipley et al., Physics of Plasmas 25(5) (2018).