K. Gunasekera

Experimental Analysis of the Acceleration Region in Tungsten Wire Arrays

We present the first analysis of the ablated plasma flow acceleration region in tungsten cylindrical wire arrays within 1 mm of the wire core. We apply a recently developed modification to the Lebedev rocket model to infer the 2-D distribution of effective velocities which redistribute the array mass as a function of time. From these data, it is possible to directly observe the acceleration region in a wire array. Analysis of radiography data from the 1-MA Cornell Beam Research Accelerator machine suggests a region of rapid acceleration extending up to 300 μm from the wire core in 16 wire tungsten arrays.

Examination of Bow-Shock Formation in Supersonic Radiatively Cooled Plasma Flows

Radiative high-Z plasma bow shocks driven by a 200-kA current are investigated by using high-resolution dark-field laser Schlieren imaging and a Mach-Zehnder interferometer. Results demonstrate stationary high-density compressible bow shocks and provide data on the plasma Mach number and electron density.

A collinear self-emission and laser-backlighting imaging diagnostic

In this work we demonstrate a design for obtaining laser backlighting (e.g., interferometry) and time-resolved extreme ultraviolet self-emission images along the same line-of-sight. This is achieved by modifying a single optical component in the laser collection optics with apertures and pinhole arrangements suitable for single or multiple frame imaging onto a gated detector, such as a microchannel plate. Test results for exploding wire experiments show that machining of the optic does not affect the overall quality of the recovered laser images, and that, even with a multiple frame system, the area sacrificed to achieve collinear imaging is relatively small. The diagnostics can therefore allow direct correlation of laser and self-emission images and their derived quantities, such as electron density in the case of interferometry. Simple methods of image correlation are also demonstrated.

Analytical analysis of the ablation phase of low number wire arrays

Summary form only given. Whilst the dynamical evolution of wire arrays is well understood, and multi-dimensional Magneto-Hydrodynamic (MHD) modeling has demonstrated significant progress, a comprehensive predictive capability has not been realized to date. Recent experimental investigations have highlighted the need to more closely examine the ablation structure and its dependence on the initial parameters of the array. In particular, the range over which the ablated plasma is accelerated, and hence extent to which magnetic flux is convected into the array, is often a disputed point in the comparison simulation and analytical work. Recent work at UC San Diego [ 1 ] uses a modification of the Lebedev rocket model of wire ablation to fit the range of ablation velocities which are observed in experiments. This analysis can be extended to infer the 2D distribution of effective velocities which redistribute the mass as a function of time. From this data it may be possible to direct observe the acceleration region in a wire array. An analysis of the effect of array geometry on the determined acceleration region will be attempted using interferographic and radiographic data from experiments at 200 kA to 1 MA. Conclusions and possible extensions to this work will be presented and discussed.

Proton probing of magnetic fields in exploding wire experiments

Determination of B-field structures in pulsed power driven exploding wire experiments is vital to recover detailed information about the evolution, driving mechanisms of ablation, and subsequent instability development, but is complicated by the presence of large volumes of hot, dense plasma. Optical and electrical probe diagnostics typically fail early in the experiment. We present progress on a new project, which examines the use of proton deflectometry to measure magnetic fields in pulsed power plasmas.

Study of plasma diffusion across magnetic fields using double planar wire arrays

Accurately determining magnetic diffusion parameters in plasmas is important for benchmarking 3D MHD codes used for the design and interpretation of Z pinch and other plasma experiments. In this work, two planar arrays consisting of four wires each are at a fixed inter-plane spacing. The inter-wire spacing is then varied to alter the ratio of local to global magnetic field. This in turn determines the location of local precursor column structures, the current carrying regions, and the rate at which plasma may travel across the magnetic field toward the axis.

Investigation Of bow shock formation in pulsed-power-driven super-sonic plasma flows

Summary form only given. The development of shocks in plasma flows occurs in a wide range of environments, including fusion schemes and astrophysical objects. In wire array experiments, the plasma accelerated from the wire via the Lorentz force rapidly exceeds both the local sound and Alfven speeds, providing an interesting source for shock studies. Recently, the plasma flow in a 1 MA wire array z-pinch demonstrated both the formation of bow shocks around an obstacle in the plasma, and the feasibility of testing the effect of magnetic fields and radiation cooling on the shock formation in these systems.In this work we present examination of bow shock formation in wire array plasma flows driven by the 250 kA GenASIS Linear Transformer Driver (LTD). The plasma densities produced are lower than on the 1 MA device used in Ref 1, and this allows continuous 2-dimensional quantitative measurements of the electron density, which was not previously possible. A closer examination of the shock structure is therefore possible for comparison for both analytical theory and simulation work. Shock formation is examined as a function of obstacle geometry and size, and preliminary results from both standard and inverse wire arrays will be presented.

Quantitative analysis of the ablation of x-pinches at 80 kA

The ablation phase of exploding multi-wire experiments driven by fast-rising currents in which both global and local magnetic fields are dynamically significant is poorly understood at present. In particular, a quasi-periodic modulation in the plasma flow accelerated from the wire cores following initiation appears at all current levels, and is not fully explained. The lack of a complete description of the physical process which drives this ablation structure leads to uncertainties in the scaling of the plasma parameters with drive current. Performance at very high current levels cannot be predicted with a high degree of confidence. Such scaling is particularly important for wire array z-pinches which show promise as a driver for high yield Inertial Confinement Fusion (ICF), as well as the generation of novel High Energy Density Physics (FTEDP) states which may be achievable using exploding wire systems.