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We examine the effects of varying the magnitude of an applied, uniform, axial magnetic field ( $B_{z}$ ) on the formation of laboratory plasma jets produced by a 1-MA 100-ns rise time pulsed power generator in a radial foil configuration. A Helmholtz coil applies the external magnetic field (0 to 2 T). With a small-enough applied field, the foil surface ablation is relatively azimuthally uniform, and $J\times B$ forces drive the ablated plasma inward and upward to form a well-defined azimuthally symmetric jet. Using applied field strengths larger than a critical $B_{z} = 1.1\pm 0.1$ T with aluminum foils, the plasma ablation is not azimuthally uniform, discrete bursts of plasma initiate from the foil surface, and the formation of a well-defined jet is disrupted. The critical $B_{z}$ for this plasma jet formation disruption correlates with the foil material’s electrical resistivity and equation of state (EOS). To better understand the material-dependent phenomenon, we compare the experimental results with 3-D extended magnetohydrodynamics simulations of ablation of a 2 mm $\times 2$ mm by 25- $\mu \text{m}$ slab that represents a section of the radial foil. The simulations initialize the slab in the solid state and include the material resistivity and EOS from the solid to plasma phases. As is observed in the experimental disruption, these simulations show enhanced nonuniform plasma ablation with an applied $B_{z}$ , which can inhibit the azimuthal uniformity necessary to produce a well-defined plasma jet. Furthermore, the simulations also show a material and resistivity dependence on the ablation process similar to the trend shown by the experiments. These results demonstrate the necessity of accurate, detailed modeling of material properties during the transition from solid to plasma phases.

We present experimental data providing evidence for the formation of transient ( ${\sim }20\ \mathrm {\mu }\textrm {s}$ ) plasmas that are simultaneously weakly magnetized (i.e. Hall magnetization parameter $\omega \tau > 1$ ) and dominated by thermal pressure (i.e. ratio of thermal-to-magnetic pressure $\beta > 1$ ). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $\lambda _{\textrm {gun}}=\mu _0 I_{\textrm {gun}}/\psi _{\textrm {gun}}$ of gun current $I_{\textrm {gun}}$ to applied magnetic flux $\psi _{\textrm {gun}}$ is an experimental knob for exploring the parameter space of $\beta$ and $\omega \tau$ . These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high- $\beta$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.

We numerically model the ablation process of a 25- $\mu \text{m}$ -thick aluminum foil driven by a pulsed-power machine that provides a 1-MA peak current in a 100-ns zero-to-peak rise time. The extended magnetohydrodynamics simulation is a discontinuous Galerkin code with Cartesian coordinates in 3-D and with 25- $\mu \text{m}$ spatial resolution. We investigate the influence of an external magnetic field normal to the foil surface, $B_{z}$ . During the foil ablation, $B_{z}=1$ T causes more nonuniform distributions of density and current compared to $B_{z}=0$ T. $B_{z}=4$ T delays the generation of surface plasma relative to the 0- and 1-T cases. The understanding of a material’s ablation as it undergoes transition from the solid to plasma phases requires detailed knowledge of a material’s equation of state and conductivity. This paper of warm dense matter and how instabilities propagate from a solid material to plasma motivates improvements to both numerical simulations and experimental diagnostics.

Ar gas puff z-pinch experiments were conducted on the 0.9-MA, 200-ns COBRA generator at Cornell University. In the experiments, a triple-nozzle gas valve was used to produce the z-pinch load with concentric outer and inner gas puffs and a central gas jet. We used a planer laser-induced fluorescence to measure the initial gas profile of the load, two four-frame 4-ns gated extreme ultraviolet cameras to image the imploding plasma, and filtered photoconducting diamond detectors to measure the X-ray output. We have observed the shock heating and/or photoionization of the dense center jet by the imploding plasma. With an outer-to-inner plenum pressure ratio of 1:3, deacceleration of the imploding plasma occurred when the outer plasma shell imploded on the inner shell resulting in a stable implosion. By varying the plenum pressure for the center jet, the X-ray emission was optimized. An Ar K-shell X-ray (3–4 keV) yield of 77 ± 17 J was produced at a current level of 0.87 ± 0.2 MA and an implosion time of 200 ns.

Thomson scattering measurements were performed on plasma jets created from a 15- $\mu \text{m}$ -thick radial Al foil load on a 1-MA pulsed power machine. The laser used for these measurements has a maximum energy of 10 J at 526.5 nm. Using the full energy, however, significantly heats the $5\times 10^{18}$ cm−3 jet by inverse bremsstrahlung, creating a density bubble in the jet. To measure the evolving plasma parameters of this laser-heated jet, a streak camera was used to record the scattered spectrum, resulting in the sub-ns time-resolved Thomson scattering. Analysis of the streak camera image showed that the electron temperature of the jet was about 25 eV prior to the laser pulse. The laser then heated the plasma to 80–100 eV within about 2 ns. The electron temperature then stabilized for about 0.5 ns prior to falling at the end of the laser pulse. Jets made from a radial Ti foil showed more heating by the laser than the Al jets, going from 50 to over 150 eV, and heating was detected even when only 1 J of laser energy was used. Also, the ion-acoustic peaks in the scattered spectrum from the Ti jets were significantly narrower than those from Al jets, a result of several possible differences in the plasma created from these two materials.

Recent work reported on the current density in pulsed-power-driven liners where a vacuum gap was introduced in the power feed connecting the liner to the generator. The resultant gap flashover generates azimuthally localized current-carrying plasma channels, which can create an azimuthal nonuniformity in the current density. The current density evolves during the current pulse, but nonuniformity is observed through the experiment timescale. Magnetohydrodynamic simulation work in 3-D demonstrates the difficulty in reproducing the experimental data within a limited computational domain, and those boundary conditions may dominate this paper. The development of current-driven instabilities in the plasma channels can explain the liner current density evolution, and specifically the ion acoustic instability can account for the main features observed in the experiments.

In this paper, collimated plasma jets form from Joule heating and ablation of a radial foil (Al 15- $\mu \text{m}$ -thin disk) using a pulsed power generator (Cornell Beam Research Accelerator) with 1-MA peak current and 100-ns rise time. Plasma dynamics of the jet are diagnosed with and without an applied uniform external magnetic field ( $\sim 1$ -T axial $B_{z}$ produced from a pulsed Helmholtz coil) and under a change of current polarity, which corresponds to current moving either radially inward or outward from the foil’s central axis. Experimental results are compared with predictions made by numerical simulations (Plasma as an Extended-MHD Relaxation System using an Efficient Upwind Scheme). The influence of the Hall effect on the jet development is observed under opposite current polarities.

In this paper, we introduce a novel experimental platform for the study of the Richtmyer–Meshkov instability in a cylindrically converging geometry using a magnetically driven cylindrical piston. Magnetically driven solid liner implosions are used to launch a shock into a liquid deuterium working fluid and, ultimately, into an on-axis rod with a pre-imposed perturbation. The shock front trajectory is tracked through the working fluid and up to the point of impacting the rod through the use of on axis photonic Doppler velocimetry. This configuration allows for precise characterization of the shock state as it impacts the perturbed rod interface. Monochromatic x-ray radiography is used to measure the post-shock interface evolution and rod density profile. The ALEGRA MHD model is used to simulate the dynamics of the experiment in one dimension. We show that late in time the perturbation growth becomes non-linear as evidenced by the observation of high-order harmonics, up to n = 5. Two dimensional simulations performed using a combination of the GORGON MHD code and the xRAGE radiation hydrodynamics code suggest that the late time non-linear growth is modified by convergence effects as the bubbles and spikes experience differences in the pressure of the background flow.

We observe shock-front separation and species-dependent shock widths in multi-ion-species collisional plasma shocks, which are produced by obliquely merging plasma jets of a He/Ar mixture (97% He and 3% Ar by initial number density) on the Plasma Liner Experiment [S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018)]. Visible plasma emission near the He-I 587.6 nm and Ar-II 476.5-514.5 nm lines are simultaneously recorded by splitting a single visible image of the shock into two different fast-framing cameras with different narrow bandpass filters (589 +/- 5 nm for observing the He-I line and 500 +/- 25 nm for the Ar-II lines). For conditions in these experiments (pre-shock ion and electron densities ~5*10^14 cm^-3, ion and electron temperatures of ~2.2 eV, and relative plasma-merging speed of 22 km/s), the observationally inferred magnitude of He/Ar shock-front separation and the shock widths themselves are < 1 cm, which correspond to ~50 post-shock thermal ion-ion mean free paths. These experimental lengths scales are in reasonable qualitative and quantitative agreement with results from 1D multi-fluid simulations using the Chicago code. However, there are differences between the experimentally-inferred and simulation-predicted ionization states and line emission intensities, particularly in the post-shock region. Overall, the experimental and simulation results are consistent with theoretical predictions that the lighter He ions diffuse farther ahead within the overall shock front than the heavier Ar ions.

We present a method to seamlessly self-assemble, disassemble, transport, and reconstruct ordered 2-D structures of fluid-borne microspheres on a surface using an array of magnetic zigzag wire traps. Competition between and control over: 1) the trapping forces of underlying magnetic patterns; 2) magnetic dipole repulsion; and 3) Brownian motion gives rise to reproducible cluster structures. Weak external magnetic fields $({ tune the spacing between particles and enable the assembled structures to be remotely manipulated and reassembled on the platform. This method could be used in biological and photonic applications by utilizing the microspheres, for example, as carriers of host biomolecules or as linkers to fluorescent emitters.

The dynamics of Magnetized Liner Inertial Fusion (MagLIF)1, a new and promising approach to pulsed power fusion, are presently under detailed study at Sandia National Laboratories. Alongside this, a comprehensive analysis of the influence of the specific liner design geometry in the MagLIF system on liner initiation is underway in the academic community.

A single channel sub-nanosecond time-resolved Thomson scattering system used for pulsed power-driven high energy density plasma measurements has been upgraded to give electron temperatures at two different times and from two different angles simultaneously. This system was used to study plasma jets created from a 15 μm thick radial Al foil load on a 1 MA pulsed power machine. Two laser pulses were generated by splitting the initial 2.3 ns duration, 10 J, 526.5 nm laser beam into two pulses, each with 2.5 J, and delaying one relative to the other by between 3 and 14 ns. Time resolution within each pulse was obtained using a streak camera to record the scattered spectra from the two beams from two scattering angles. Analysis of the scattering profile showed that the electron temperature of the Al jet increased from 20 eV up to as much as 45 eV within about 2 ns by inverse bremsstrahlung for both laser pulses. The Thomson scattering results from jets formed with opposite current polarities showed different laser heating of the electrons, as well as possibly different ion temperatures. The two-angle scattering determined that the electron density of the plasma jet was at least 2 × 1018 cm-3.

We experimentally measure the effects of an applied axial magnetic field (Bz) on laboratory plasma jets and compare the experimental results with numerical simulations using an extended magnetohydrodynamics code. A 1 MA peak current, 100 ns rise time pulse power machine is used to generate the plasma jet. On application of the axial field, we observe on-axis density hollowing and a conical formation of the jet using interferometry, compression of the applied Bz using magnetic B-dot probes, and azimuthal rotation of the jet using Thomson scattering. Experimentally, we find densities ≲5 × 1017 cm−3 on-axis relative to jet densities of ≳3 × 1018 cm−3. For aluminum jets, 6.5 ± 0.5 mm above the foil, we find on-axis compression of the applied 1.0 ± 0.1 T Bz to a total 2.4 ± 0.3 T, while simulations predict a peak compression to a total 3.4 T at the same location. On the aluminum jet boundary, we find ion azimuthal rotation velocities of 15–20 km/s, while simulations predict 14 km/s at the density peak. We discuss possible sources of discrepancy between the experiments and simulations, including surface plasma on B-dot probes, optical fiber spatial resolution, simulation density floors, and 2D vs. 3D simulation effects. This quantitative comparison between experiments and numerical simulations helps elucidate the underlying physics that determines the plasma dynamics of magnetized plasma jets.

We present a study of the time varying current density distribution in solid metallic liner experiments at the 1 MA level. Measurements are taken using an array of magnetic field probes which provide 2D triangulation of the average centroid of the drive current in the load at 3 discrete axial positions. These data are correlated with gated optical self-emission imaging which directly images the breakdown and plasma formation region. Results show that the current density is azimuthally non-uniform and changes significantly throughout the 100 ns experimental timescale. Magnetic field probes show clearly motion of the current density around the liner azimuth over 10 ns timescales. If breakdown is initiated at one azimuthal location, the current density remains non-uniform even over large spatial extents throughout the current drive. The evolution timescales are suggestive of a resistive diffusion process or uneven current distributions among simultaneously formed but discrete plasma conduction paths.

Awe et al. found on the 20 MA Z machine [Acta Phys. Pol. A 115, 956 (2009)] that applying an externally generated axial magnetic field to an imploding liner leads to a helical pattern in the liner when viewed with soft x-ray radiography ([Phys. Rev. Lett. 111, 235005 (2013)] and [Phys. Plasmas 21, 056303 (2014)]). Here, we show that this phenomenon is also observed in extreme ultraviolet self-emission images of 10 mm long cylindrical metal liners having varying diameters and varying wall thicknesses on a 1 MA, 100–200 ns pulsed power generator. The magnetic field in these experiments is created using either twisted return current wires positioned close to the liner, generating a time-varying Bz, or a Helmholtz coil, generating a steady-state Bz.