L. Suttle

Oblique shock structures formed during the ablation phase of aluminium wire array z-pinches

A series of experiments has been conducted in order to investigate the azimuthal structures formed by the interactions of cylindrically converging plasma flows during the ablation phase of aluminium wire array Z pinch implosions. These experiments were carried out using the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. The main diagnostic used in this study was a two-colour, end-on, Mach-Zehnder imaging interferometer, sensitive to the axially integrated electron density of the plasma. The data collected in these experiments reveal the strongly collisional dynamics of the aluminium ablation streams. The structure of the flows is dominated by a dense network of oblique shock fronts, formed by supersonic collisions between adjacent ablation streams. An estimate for the range of the flow Mach number (M = 6.2-9.2) has been made based on an analysis of the observed shock geometry. Combining this measurement with previously published Thomson Scattering measurements of the plasma flow velocity by H...

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.

Optical Thomson scattering measurements of cylindrical wire array parametersa)

A Thomson scattering diagnostic has been used to measure the parameters of cylindrical wire array Z pinch plasmas. The scattering operates in the collective regime (α>1) allowing spatially localised 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 × 107 cm/s in aluminium and ∼1 × 107 cm/s in tungsten arrays. Measurements of the precursor ion temperature shortly after formation are found to correspond to the kinetic energy of the converging ablation flow. Measurements during the implosion phase of tungsten arrays show the main imploding mass reaches velocities of ∼1.4–1.7 × 107 cm/s and is non-zero even at large radii close to the start of the x-ray pulse indicating current flow in the trailing mass.

The formation of reverse shocks in magnetized high energy density supersonic plasma flows

A new experimental platform was developed, based on the use of supersonic plasma flow from the ablation stage of an inverse wire array z-pinch, for studies of shocks in magnetized high energy density physics plasmas in a well-defined and diagnosable 1-D interaction geometry. The mechanism of flow generation ensures that the plasma flow (ReM ∼ 50, MS ∼ 5, MA ∼ 8, Vflow ≈ 100 km/s) has a frozen-in magnetic field at a level sufficient to affect shocks formed by its interaction with obstacles. It is found that in addition to the expected accumulation of stagnated plasma in a thin layer at the surface of a planar obstacle, the presence of the magnetic field leads to the formation of an additional detached density jump in the upstream plasma, at a distance of ∼c/ωpi from the obstacle. Analysis of the data obtained with Thomson scattering, interferometry, and local magnetic probes suggests that the sub-shock develops due to the pile-up of the magnetic flux advected by the plasma flow.

Shock-less interactions of ablation streams in tungsten wire array z-pinches

Shock-less dynamics were observed during the ablation phase in tungsten wire array experiments carried out on the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. This behaviour contrasts with the shock structures which were seen to dominate in previous experiments on aluminium arrays [Swadling et al., Phys. Plasmas 20, 022705 (2013)]. In this paper, we present experimental results and make comparisons both with calculations of the expected mean free paths for collisions between the ablation streams and with previously published Thomson scattering measurements of the plasma parameters in these arrays [Harvey-Thompson et al., Phys. Plasmas 19, 056303 (2012)].

BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS

The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counter-streaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame and the experiments are driven over many times the characteristic cooling time-scale. The initially smooth bow shock rapidly develops small-scale non-uniformities over temporal and spatial scales that are consistent with a thermal instability triggered by strong radiative cooling in the shock. The growth of these perturbations eventually results in a global fragmentation of the bow shock front. The formation of a thermal instability is supported by analysis of the plasma cooling function calculated for the experimental conditions with the radiative packages ABAKO/RAPCAL.

Commissioning of a Rotated Wire Array Configuration for Improved Diagnostic Access

A new rotated wire array

Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments

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Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

-pinch configuration has been developed for use in experiments on the Magpie generator at Imperial College London. The wire array is rotated onto its side such that the array axis lies perpendicular to the axis of the pulsed power electrodes. This arrangement provides greatly improved end-on diagnostic access to the array and has a number of potential experimental applications; the design has recently been used to make novel Thomson scattering measurements of ablation flow interactions in tungsten wire arrays. Turning the wire array on its side leads to an uneven distribution of current through the wires due to the variation in the inductance of the current path through each wire. The forces acting on each wire will therefore be imbalanced, leading to uneven ablation of the wire cores. An experimental campaign was carried out to inductively retune the current distribution in the wire array. The results of these experiments are presented along with discussion of potential future experimental applications.

Structure of a magnetic flux annihilation layer formed by the collision of supersonic, magnetized plasma flows

We describe magnetic reconnection experiments using a new, pulsed-power driven experimental platform in which the inflows are super-sonic but sub-Alfvenic. The intrinsically magnetised plasma flows are long lasting, producing a well-defined reconnection layer that persists over many hydrodynamic time scales. The layer is diagnosed using a suite of high resolution laser based diagnostics, which provide measurements of the electron density, reconnecting magnetic field, inflow and outflow velocities, and the electron and ion temperatures. Using these measurements, we observe a balance between the power flow into and out of the layer, and we find that the heating rates for the electrons and ions are significantly in excess of the classical predictions. The formation of plasmoids is observed in laser interferometry and optical self-emission, and the magnetic O-point structure of these plasmoids is confirmed using magnetic probes.