Browsing by Subject "Pulsed power"
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Item A radiative model for determining plasma dissociation using vacuum ultraviolet self-absorption spectroscopy(2013-05) Laity, George; Neuber, Andreas A.; Krompholz, Hermann G.; Hatfield, Lynn H.; Frank, KlausThis manuscript documents the first five years of collective knowledge gained from the Texas Tech University research program to study the emission and subsequent re-absorption of vacuum ultraviolet radiation present during the initiation of nano-second plasma discharges generated at atmospheric pressure. The initial experimental study resulted in direct observation of vacuum ultraviolet radiation produced by an open transient plasma at atmospheric pressure for the first time in the available literature. Upgrades to the spectral instrumentation enabled more efficient measurement of vacuum ultraviolet radiation for the wavelength range 115 - 135 nm, allowing for enhanced resolution in the recording of emission line profiles. A direct consequence of this effort was the conception of a passive optical diagnostic for measuring the absolute number density of atoms in the discharge plasma: the vacuum ultraviolet self-absorption spectroscopy (VUV-SAS) technique. An integral piece of this technique is a radiative transfer simulation for calculating the radiation trapping physics inside the plasma channel volume, executed in the MATLAB® environment and accelerated via GPU resources using the NVIDIA® CUDA architecture. The combination of experimental and modeling approaches resulted in successful demonstration of the VUV-SAS diagnostic for N2/H2 plasmas at atmospheric pressure, where spatially resolved N and H atom densities on the order 10^17 cm^-3 were observed without an invasive laser absorption diagnostic. Through approximation of the quasi-contiguous Stark broadening of H atoms in the discharge plasma, spatially resolved electron densities on the order 10^18 cm^-3 were observed along the plasma channel. All measured quantities are validated through detailed discussion of the relevant physics concerning the fast ionization shockwave characteristic to the inception of spark discharge plasmas. Finally, the extended VUV-SAS technique was successful in demonstrating the measurement of N and O atom densities present in air discharge plasmas, thereby finding practical application for a variety of future pulsed power laboratory experiments.Item Creating and measuring white dwarf photospheres in a terrestrial laboratory(2014-08) Falcon, Ross Edward; Winget, Donald Earl, 1955-As the ultimate fate of nearly all stars, including our Sun, white dwarfs (WDs) hold rich and informative histories in their observable light. To determine a fundamental parameter of WDs, mass, we perform the first measurement of the average gravitational redshift of an ensemble of WDs. We find a larger mean mass than that determined from the primary and expansive technique known as the spectroscopic method. The potential inaccuracy of this method has broad astrophysical implications, including for our understanding of Type 1a supernova progenitors and for constraining the age of the Universe. This motivates us to investigate the WD atmosphere models used with the spectroscopic method, particularly the input theoretical line profiles, by developing a new experimental platform to create plasmas at WD photospheric conditions (T_e ~ 1 eV, n_e ~ 10^17 cm^-3). Instead of observing WD spectra to infer the plasma conditions at the surface of the star, we set the conditions and measure the emergent spectra in the laboratory. X-rays from a z-pinch dynamic hohlraum generated at the Z Pulsed Power Facility at Sandia National Laboratories irradiate a gas cell to initiate formation of a large (120x20x10 mm or 24 cm^3) plasma. We observe multiple Balmer lines from our plasma in emission and in absorption simultaneously along relatively long (~120 mm) lines of sight perpendicular to the heating radiation. Using a large, radiation-driven plasma aides us to achieve homogeneity along our observed lines of sight. With time-resolved spectroscopy we measure lines at a range of electron densities that spans an order of magnitude, and we do this within one pulsed power shot experiment. Observing our plasma in absorption not only provides the signal-to-noise to measure relative line shapes, it allows us to measure relative line strengths because the lines share the same lower level population. This constrains the theoretical reduction factors used to describe ionization potential depression or the occupation probabilities associated with these Balmer lines. We compare our measured line shapes with the theoretical ones used in WD atmosphere models as part of the first fruits of this rich experimental platform.Item MegaGauss : a portable 40T magnetic field generator(2011-05) Wisher, Matthew Louis; Hallock, G. A.; Bengtson, Roger D.Fusion neutrons from high energy density plasmas generated by pulsed laser irradiation of nanoscale atomic clusters have been explored in recent experiments at the University of Texas at Austin. A sufficiently strong (~200 T) magnetic field is expected to produce a magnetized, high temperature (10 keV) plasma with beta [approximately equal to] 1. Such a field along the laser axis may confine the plasma’s radial expansion, thus increasing fusion yield. As part of a multi-stage project to implement this experiment, a scaled (~40 T, ~500 KA) version of the final 200 T, 2.2 MA pulsed power device has been designed and built by Sandia National Laboratories and is now at UT-Austin. This apparatus, named MegaGauss, is meant to serve as a preparation tool for the 200 T system; as such, its current pulse was recorded for analysis, and is compared to a theoretical model to verify its response parameters (e.g. peak current, time to peak). Techniques and results of this comparison are discussed, followed by explanations of basic construction of the 40 T device and current sensing instrumentation. Discussion of MegaGauss is completed with a survey of notable failure modes, and a description of the often severe effects the miniature field-generating Helmholtz coil experiences due to the current pulse and magnetic field. Finally, a novel data archive scheme, structured around the familiar MDSplus archive system, is implemented in Labview and integrated into the main pulsed power control program. Specifically, methods for linking MDSplus’s robust functionality with Labview’s intuitive development environment are realized by means of a specialized software bridge between the two. These methods are used in software that allows MDSplus archives to be written and read exclusively through Labview.Item Pulsed evaluation of silicon carbide majority carrier devices(2011-08) Lawson, Kevin J.; Bayne, Stephen B.; Giesselmann, Michael G.In the power electronics industry power converters are reaching their physical limits for efficiency, power density, and output power. This is due to the fact that the heart of these power converters relies on silicon switches. High power silicon semiconductors are being pushed to the extreme boundaries of operation in order to maximize these converters. If further advancements are to take place, a new semiconductor material must be investigated, which can overcome limitations of previous generations of silicon based power electronics. As a result, silicon carbide (SiC) has been identified as the material capable of replacing silicon in state-of-the-art high power semiconductor switches used in industrial and military grade power electronics. Experiments to test new SiC JFETs and MOSFETs were designed to study the effects of high stress operation on these devices. These devices were tested under various combinations of high current discharges, high temperature operation (up to 150 °C), and high rate of voltage change (dV/dt). These conditions stress these new devices to physical extremes not capable in a silicon device. The results from these tests are analyzed and correlated back to semiconductor physics to better understand the effects of the stress induced on these devices. Overall these new SiC devices prove to be very resilient to the tests conducted and new tests are proposed to further investigate the safe operating area boundaries.Item Pulsed magnetic field generation for experiments in high energy density plasmas(2014-08) Wisher, Matthew Louis; Hallock, G. A.; Bengtson, Roger D.Experiments in high energy density (HED) plasma physics have become more accessible with the increasing availability of high-intensity pulsed lasers. Extending the experiment parameters to include magnetized HED plasmas requires a field source that can generate fields of order 100 tesla. This dissertation discusses the design and implementation of a pulsed field driver with a designed maximum of 2.2 MA from a 160 kJ capacitor bank. Faraday rotation measurement of 63 T for a 1.0 MA discharge supported Biot-Savart estimates for a single-turn coil with a 1 cm bore. After modification, the field driver generated up to 15 T to magnetize supernova-like spherical blast waves driven by the Texas Petawatt Laser. The presence of the high field suppressed blast wave expansion, and had the additional effect of revealing a cylindrical plasma along the laser axis.Item Study of picosecond-scale electron dynamics in laser-produced plasmas with and without an external magnetic field(2013-12) McCormick, Matthew Warren; Ditmire, ToddThe interaction of ultra-short laser pulses and cluster targets can be used to explore a number of interesting phenomena, ranging from nuclear fusion to astrophysical blast waves. In our experiments, we focused on exploring very fast plasma dynamics of a plasma created by ionizing clusters and monomer gas. By using a 115 fs laser pulse, we can even study sub-picosecond plasma dynamics. In addition, we also wanted to impose an external magnetic field on these plasmas to study how the plasma evolution would change. The results of this work produced two significant results. First, a new, extremely fast ionization mechanism, with velocities as high as 0.5 c, was discovered which allows for significant plasma expansion on a picosecond time-scale. Experimental studies measured the velocity of the ionization wave, while particle-in-cell simulations helped explain the source and longevity of the wave. It was also observed that this ionization wave was not affected by the external magnetic field. Second, the external field was shown to inhibit plasma expansion on a time-scale of tens of picoseconds, which seems to be one of the first demonstrations of magnetic confinement on such a fast time-scale. Simple 1D simulations tell us that the field appears to slow electron heat transport in the plasma as well as inhibiting collisional ionization of electrons expanding into the surrounding gas. The inhibition of plasma expansion by the field on this time-scale may provide some evidence that magnetic confinement of a fusion plasma created by exploding clusters could improve the fusion yield by slowing heat loss as well as possibly electrostatically confining the hot ions.