Point projection radiography of electromagnetically accelerated flyer plates with an external X-pinch driver.

A platform for flyer plate benchmarking experiments has been developed, with an external X-pinch driver for point projection radiography. The experiments were performed using CEPAGE, a low inductance pulsed power machine at First Light Fusion (2 MA, 1.4 µs), with a new vacuum transmission line and flyer load hardware designed specifically to give a line of sight for radiography. A broadband 10-20 keV x-ray source was produced by a portable X-pinch driver (140 kA, 350 ns) [Strucka et al., Matter Radiat. Extremes 7, 016901 (2021)] and was used to image the flyer. Radiography compliments the pre-existing diagnostic suite, which consists of current probes, velocimetry, and side-on optical probing of the impact shock transmitted into a transparent sample. The platform allows for significant insights into the 2D and 3D nature of the flyer launch, such as deformation and instability formation. It was used to diagnose a 10 × 9 × 1 mm3 aluminum flyer, which reached a peak velocity of 4.2 km s-1 before impact with a poly(methylmethacrylate) sample. The experimental configuration, on-shot source characterization, and the results from two flyer plate experiments on CEPAGE are discussed.

Perpendicular Subcritical Shock Structure in a Collisional Plasma Experiment.

We present a study of perpendicular subcritical shocks in a collisional laboratory plasma. Shocks are produced by placing obstacles into the supermagnetosonic outflow from an inverse wire array z pinch. We demonstrate the existence of subcritical shocks in this regime and find that secondary shocks form in the downstream. Detailed measurements of the subcritical shock structure confirm the absence of a hydrodynamic jump. We calculate the classical (Spitzer) resistive diffusion length and show that it is approximately equal to the shock width. We measure little heating across the shock (<10% of the ion kinetic energy) which is consistent with an absence of viscous dissipation.

An imaging refractometer for density fluctuation measurements in high energy density plasmas.

We report on a recently developed laser-probing diagnostic, which allows direct measurements of ray-deflection angles in one axis while retaining imaging capabilities in the other axis. This allows us to measure the spectrum of angular deflections from a laser beam, which passes through a turbulent high-energy-density plasma. This spectrum contains information about the density fluctuations within the plasma, which deflect the probing laser over a range of angles. We create synthetic diagnostics using ray-tracing to compare this new diagnostic with standard shadowgraphy and schlieren imaging approaches, which demonstrates the enhanced sensitivity of this new diagnostic over standard techniques. We present experimental data from turbulence behind a reverse shock in a plasma and demonstrate that this technique can measure angular deflections between 0.06 and 34 mrad, corresponding to a dynamic range of over 500.

Counterpropagating Radiative Shock Experiments on the Orion Laser.

We present new experiments to study the formation of radiative shocks and the interaction between two counterpropagating radiative shocks. The experiments are performed at the Orion laser facility, which is used to drive shocks in xenon inside large aspect ratio gas cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently three-dimensional shocks, provides a novel platform particularly suited for the benchmarking of numerical codes. The dynamics of the shocks before and after the collision are investigated using point-projection x-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modeling of the experiments using the 2D radiation hydrodynamic codes nym and petra shows very good agreement with the experimental results.

Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment.

We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B=3 T), advected by supersonic, sub-Alfvénic carbon plasma flows (V_{in}=50 km/s), are brought together and mutually annihilate inside a thin current layer (δ=0.6 mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (T_{e}=100 eV, T_{i}=600 eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.

Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows.

We present experiments characterizing the detailed structure of a current layer, generated by the collision of two counterstreaming, supersonic and magnetized aluminum plasma flows. The antiparallel magnetic fields advected by the flows are found to be mutually annihilated inside the layer, giving rise to a bifurcated current structure-two narrow current sheets running along the outside surfaces of the layer. Measurements with Thomson scattering show a fast outflow of plasma along the layer and a high ion temperature (T_{i}∼Z[over ¯]T_{e}, with average ionization Z[over ¯]=7). Analysis of the spatially resolved plasma parameters indicates that the advection and subsequent annihilation of the inflowing magnetic flux determines the structure of the layer, while the ion heating could be due to the development of kinetic, current-driven instabilities.

Monochromatic radiography of high energy density physics experiments on the MAGPIE generator.

A monochromatic X-ray backlighter based on Bragg reflection from a spherically bent quartz crystal has been developed for the MAGPIE pulsed power generator at Imperial College (1.4 MA, 240 ns) [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (2005)]. This instrument has been used to diagnose high energy density physics experiments with 1.865 keV radiation (Silicon He-α) from a laser plasma source driven by a ∼7 J, 1 ns pulse from the Cerberus laser. The design of the diagnostic, its characterisation and performance, and initial results in which the instrument was used to radiograph a shock physics experiment on MAGPIE are discussed.

Diagnosing collisions of magnetized, high energy density plasma flows using a combination of collective Thomson scattering, Faraday rotation, and interferometry (invited).

A suite of laser based diagnostics is used to study interactions of magnetised, supersonic, radiatively cooled plasma flows produced using the Magpie pulse power generator (1.4 MA, 240 ns rise time). Collective optical Thomson scattering measures the time-resolved local flow velocity and temperature across 7-14 spatial positions. The scattering spectrum is recorded from multiple directions, allowing more accurate reconstruction of the flow velocity vectors. The areal electron density is measured using 2D interferometry; optimisation and analysis are discussed. The Faraday rotation diagnostic, operating at 1053 nm, measures the magnetic field distribution in the plasma. Measurements obtained simultaneously by these diagnostics are used to constrain analysis, increasing the accuracy of interpretation.

Interpenetration, deflection, and stagnation of cylindrically convergent magnetized supersonic tungsten plasma flows.

The interpenetration and interaction of supersonic, magnetized tungsten plasma flows has been directly observed via spatially and temporally resolved measurements of the Thomson scattering ion feature. A novel scattering geometry allows independent measurements of the axial and radial velocity components of the ions. The plasma flows are produced via the pulsed power driven ablation of fine tungsten wires in a cylindrical wire array z pinch. Fits of the data reveal the variations in radial velocity, axial velocity, and temperature of the ion streams as they interpenetrate and interact. A previously unobserved increase in axial velocity is measured near the array axis. This may be the result of v[over →]×B[over →] bending of the ion streams by a toroidal magnetic field, advected to and accumulated about the axis by the streams.

Optical Thomson scattering measurements of plasma parameters in the ablation stage of wire array Z pinches.

A Thomson scattering diagnostic has been used to measure the parameters of cylindrical wire array Z pinch plasmas during the ablation phase. The scattering operates in the collective regime (α>1) allowing spatially localized measurements of the ion or electron plasma temperatures and of the plasma bulk velocity. The ablation flow is found to accelerate towards the axis reaching peak velocities of 1.2-1.3×10(7) cm/s in aluminium and ∼1×10(7) cm/s in tungsten arrays. Precursor ion temperature measurements made shortly after formation are found to correspond to the kinetic energy of the converging ablation flow.

Suppression of the ablation phase in wire array Z pinches using a tailored current prepulse.

A new wire array configuration has been used to create thin shell-like implosions in a cylindrical array. The setup introduces a ~5 kA, ~25 ns current prepulse followed by a ~140 ns current-free interval before the application of the main (~1 MA) current pulse. The prepulse volumetrically heats the wires which expand to ~1 mm diameter leaving no dense wire core and without development of instabilities. The main current pulse then ionizes all the array mass resulting in suppression of the ablation phase, an accelerating implosion, and no trailing mass. Rayleigh-Taylor instability growth in the imploding plasma is inferred to be seeded by µm-scale perturbations on the surface of the wires. The absence of wire cores is found to be the critical factor in altering the implosion dynamics.