M. Bennett

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.

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

Interpenetration and deflection phenomena in collisions between supersonic, magnetized, tungsten plasma flows diagnosed using high resolution optical Thomson scattering

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Experimental investigations of ablation stream interaction dynamics in tungsten wire arrays: Interpenetration, magnetic field advection, and ion deflection

-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.

Thomson scattering measurements of supersonic tungsten plasma flow interpenetration in wire array z-pinches

An optical Thomson scattering diagnostic has been used to investigate collisions between supersonic, magnetized plasma flows, in particular the transition from collisionless to collisional interaction dynamics. These flows were produced using tungsten wire array z-pinches, driven by the 1.4 MA 240 ns Magpie generator at Imperial College London. Measurements of the collective-mode Thomson scattering ion-feature clearly indicate that the ablation flows are interpenetrating at 100 ns (after current start), and this interpenetration continues until at least 140 ns. The Thomson spectrum at 150 ns shows a clear change in the dynamics of the stream interactions, transitioning towards a collisional, shock-like interaction of the streams near the axis. The Thomson scattering data also provide indirect evidence of the presence of a significant toroidal magnetic field embedded in the “precursor” plasma near the axis of the array over the period 100–140 ns; these observations are in agreement with previous measuremen...

Photo-ionisation of gas by x-rays from a wire array z-pinch

Experiments have been carried out to investigate the collisional dynamics of ablation streams produced by cylindrical wire array z-pinches. A combination of laser interferometric imaging, Thomson scattering, and Faraday rotationimaging has been used to make a range of measurements of the temporal evolution of various plasma and flow parameters. This paper presents a summary of previously published data, drawing together a range of different measurements in order to give an overview of the key results. The paper focuses mainly on the results of experiments with tungsten wire arrays. Early interferometric imagingmeasurements are reviewed, then more recent Thomson scattering measurements are discussed; these measurements provided the first direct evidence of ablation stream interpenetration in a wire array experiment. Combining the data from these experiments gives a view of the temporal evolution of the tungsten stream collisional dynamics. In the final part of the paper, we present new experimental measurements made using an imagingFaraday rotationdiagnostic. These experiments investigated the structure of magnetic fields near the array axis directly; the presence of a magnetic field has previously been inferred based on Thomson scattering measurements of ion deflection near the array axis. Although the Thomson and Faradaymeasurements are not in full quantitative agreement, the Faraday data do qualitatively supports the conjecture that the observed deflections are induced by a static toroidal magnetic field, which has been advected to the array axis by the ablation streams. It is likely that detailed modeling will be needed in order to fully understand the dynamics observed in the experiment.

Rotating plasma disks in dense Z-pinch experiments

We present direct Thomson Scattering measurements of flow interpenetration and heating in the axial region of tungsten wire array z-pinches. These experiments were carried on the MAGPIE (1.4MA, 240ns) Pulsed Power generator at Imperial College London. Measurements were taken at early times in the evolution of the arrays (~120ns), before the formation of the dense precursor column. At this time the array is producing a set of eight cylindrically converging supersonic plasma streams (v~1.5×107cms-1, ni~1017cm-3). On axis, the collision of these streams results in the formation of an azimuthally isotropic interaction region. No shock structures are observed to form at the points where the flows collide; instead the density profile (as measured by end-on interferometry) varies smoothly between the ablation streams and the inter-wire regions. The Thomson scattering geometry used in these experiments allowed independent measurements of the radial and axial velocity distribution of the plasma through the axis, based on the analysis of the form-factor of the ion feature of the TS spectra. These measurements were both temporally (4ns) and spatially resolved across the array diameter. The spectra appear to be formed of two “peaks”, corresponding to counter-streaming interpenetrating flows. The flows were observed to decelerate over an extended distance (~1.5mm), gradually heating one another to Ti ~30keV as they stagnate. The independent measurements of the axial flow velocities show, for the first time, the appearance of a significant axial flow of the ions directed from cathode to anode, against the applied electric field. A magnetic field of ~20T is required to produce the observed deflection of the W ions.

PPPS-2013: Interactions of plasma ablation flows in wire array Z-pinches

Summary form only given. An experiment is presented in which the x-ray emission from a stagnated wire array z-pinch is used to photo-ionise gas inside a gas cell. Photo-ionisation studies with x-rays from wire array implosions on the Z facility at Sandia National Labs used a gas cell positioned side-on and at large distances from the pinch1. Due to the much smaller radiation power from a 1 MA z-pinch, in our experiments the gas cell is mounted in an end-on position, close to the pinch.The experiment is conducted on MAGPIE (1.4 MA, 250 ns current pulse) imploding AI or W wire arrays. Radiation enters the gas cell through a window situated lO rum above the top of the pinch. The window is transparent to photons above 300 eV. Gas composition and pressure are varied to observe different photo-ionisation regimes, in which the mean free path is on the order of, or larger than, the size of the system. We study the dynamics of the photo-ionisation process in gas, and the dynamics of the plasma which is formed. Two-colour multi-time laser-interferometry, Schlieren imaging and optical spectroscopy will diagnose the plasma density and temperature. We present the results of preliminary experiments with this system.

Experiments with colliding supersonic plasma jets on the magpie pulsed power facility

We present data from the first z-pinch experiments aiming to simulate aspects of accretion disk physics in the laboratory. Using off axis ablation flows from a wire array z-pinch we demonstrate the formation of a hollow disk structure that rotates at 60 kms−1 for 150 ns. By analysing the Thomson scattered spectrum we make estimates for the ion and electron temperatures as Ti ∼ 60 eV and ZTe ∼ 150 to 200 eV.