D. Mariscal

Laser generated neutron source for neutron resonance spectroscopy

A neutron source for neutron resonance spectroscopy has been developed using high-intensity, short-pulse lasers. This technique will allow robust measurement of interior ion temperature of laser-shocked materials and provide insight into material equation of state. The neutron generation technique uses laser-accelerated protons to create neutrons in LiF through (p,n) reactions. The incident proton beam has been diagnosed using radiochromic film. This distribution is used as the input for a (p,n) neutron prediction code which is validated with experimentally measured neutron yields. The calculation infers a total fluence of 1.8×109 neutrons, which are expected to be sufficient for neutron resonance spectroscopy temperature measurements.

Study of instability formation and EUV emission in thin liners driven with a compact 250 kA, 150 ns linear transformer driver

A compact linear transformer driver, capable of producing 250 kA in 150 ns, was used to study instability formation on the surface of thin liners. In the experiments, two different materials, Cu and Ni, were used to study the effect of the liners resistivity on formation and evolution of the instabilities. The dimensions of the liners used were 7 mm height, 1 mm radius, and 3 μm thickness. Laser probing and time resolved extreme ultraviolet (EUV) imaging were implemented to diagnose instability formation and growth. Time-integrated EUV spectroscopy was used to study plasma temperature and density. A constant expansion rate for the liners was observed, with similar values for both materials. Noticeable differences were found between the Cu and Ni instability growth rates. The most significant perturbation in Cu rapidly grows and saturates reaching a limiting wavelength of the order of the liner radius, while the most significant wavelength in Ni increases slowly before saturating, also at a wavelength clo...

Measurement of pulsed-power-driven magnetic fields via proton deflectometry

Measuring magnetic field and current distribution in Z-pinch plasma systems is crucial to the validation of Z-pinch theory. In this letter, the demonstration of proton deflectometry to pulsed-power-driven loads at the mega-amp scale is presented, which is capable of making more detailed field maps in high-density regions of plasmas. In this method, a laser-driven, broad-spectrum, MeV-energy proton beam is directed through a pulsed-power-driven plasma system, and the resulting deflections are measured to examine configuration of magnetic fields and to infer the currents that support them. The technique was first demonstrated on simple short-circuit loads, and the results are in excellent agreement with numerical simulations providing reliable estimates of the field and current configurations. It was then applied to a more complex—radial foil—plasma load. The measurements show unexpected proton deflections that exhibit the complexity of the plasma load and that with further analysis will reveal details about the current and magnetic field topology in this complex configuration.

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.

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.

Measurement of temperature and density using non-collective X-ray Thomson scattering in pulsed power produced warm dense plasmas

We present the first experimental measurement of temperature and density of a warm dense plasma produced by a pulsed power driver at the Nevada Terawatt Facility (NTF). In the early phases of discharge, most of the mass remains in the core, and it has been challenging to diagnose with traditional methods, e.g. optical probing, because of the high density and low temperature. Accurate knowledge of the transport coefficients as well as the thermodynamic state of the plasma is important to precisely test or develop theoretical models. Here, we have used spectrally resolved non-collective X-ray Thomson scattering to characterize the dense core region. We used a graphite load driven by the Zebra current generator (0.6 MA in 200 ns rise time) and the Ti He-α line produced by irradiating a Ti target with the Leopard laser (30 J, 0.8 ns) as an X-ray probing source. Using this configuration, we obtained a signal-to-noise ratio ~2.5 for the scattered signal. By fitting the experimental data with predicted spectra, we measured T = 2±1.9 eV, ρ = 0.6±0.5 gr/cc, 70 ns into the current pulse. The complexity of the dense core is revealed by the electrons in the dense core that are found to be degenerate and weakly coupled, while the ions remain highly coupled.

Investigation of Current Transport in

Here, we present an experimental and computational study using planar wire arrays to examine the transport of magnetic fields in wire array plasmas. Two groups of wires are placed adjacent to each other at a fixed distance, while the local spacing between the wires is varied. Experimental results employing the electrical B-dot probes are compared with the simulations performed with the numerical results from the resistive MHD code, Gorgon, and show that the magnetic field generated by each pair of wires determines the amount of current advected with the plasma flow to the central axis.

2\times 2

Summary form only given. Results of instability formation in metallic liners driven by GenASIS, a Linear Transformer Driver (LTD) capable of producing 250 kA current in 150 ns are presented. Different material liners (Aluminum, Copper and Nickel) with thicknesses ranging from 1 to 5 μm and diameter of 2 mm have been tested. We used laser probing techniques such as schlieren and interferometry to diagnose instabilities on the outside surface of the liners using four frames separated by 15 ns per frame. In addition, we used Extreme Ultra-Violet (XUV) radiation to study the evolution of the liners. We observed that the instabilities are produced within 100 ns of the current start and these grow with time. Experiments have been modeled using the hybrid code LSP. The relationship between electro-thermal and Magneto-Rayleigh Taylor instabilities has also been studied. We show that it is possible to study interesting physics relevant for MagLIF [1] concept using small-scale university device.

Wire Array Plasmas

Summary form only given. Presenting initial results from an experimental investigation into the interaction of supersonic, radiatively cooled plasma jets with solid targets. The jet is produced with a converging conical plasma flow from a cylindrically symmetric array of inclined wires (a conical wire array) driven by 1.4MA, 250ns current pulse on the MAGPIE Z-pinch. The produced jet has scalable characteristics (Mach number of 20) that allow exploration of astrophysically relevant shock structures arising from the jets interaction with the target and pre ionized material from the target. Different target materials both metallic and foam are explored as well as using both Al and W in the conical array.

Study of instability formation in liners driven with a compact linear transformer driver

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.