Publications

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Spectroscopic investigations in the visible and near UV are underway to study plasmas present in X-ray radiography diodes during the time of the electron beam propagation. These studies are being performed on the RITS-3 accelerator (5.25 MV and 120 kA) at Sandia National Laboratories using several diode configurations. The proper characterization of the plasmas occurring during the time of the X-ray pulse can lead to a greater understanding of diode behavior and X-ray spot size evolution. By studying these plasmas along with the use of selective dopants, insights into such phenomena as impedance collapse, thermal and non-thermal species behavior, charge and current neutralization, anode and cathode plasma formation and propagation, and beam/foil interactions, can be obtained. Information from line and continuum emission and absorption can give key plasma parameters such as temperatures, densities, charge states, and expansion velocities. This information is important for proper modeling and future predictive capabilities for the design and improvement of flash X-ray radiography diodes. Diagnostics include a gated, intensified multichannel plate camera combined with a 1 meter Czerny-Turner monochromator with a multi-fiber spectral input, allowing for both temporal and spatial resolution. Recent results are presented.

Single layer transition metal sulfides (SLTMS) such as MoS{sub 2}, WS{sub 2}, and ReS{sub 2}, play an important role in catalytic processes such as the hydrofining of petroleum streams, and are involved in at least two of the slurry-catalyst hydroconversion processes that have been proposed for upgrading heavy petroleum feed and other sources of hydrocarbon fuels such as coal and shale oils. Additional promising catalytic applications of the SLTMS are on the horizon. The physical, chemical, and catalytic properties of these materials are reviewed in this report. Also discussed are areas for future research that promise to lead to advanced applications of the SLTMS.

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The magnetically immersed (B{sub z}) diode is being investigated as a source for pulsed-power driven flash radiography. Experiments fielding this diode have revealed a limit on its achievable current density on target. Either a small spot produces a low dose, or a high dose is achieved with a large spot. It has been proposed that this limit is due to non-protonic ions liberated from the anode surface and subsequently ionizing to higher states. The three-dimensional particle-in-cell code LSP is used to investigate this proposal. Data from the recent immersed diode experiments conducted on the RITS-3 accelerator are compared to LSP models of the experimental configuration, including the B{sub z} field map. We report on how the non-protonic and protonic ion models compare to data, and proposals for future investigation.

Controlled seeding of perturbations is employed to study the evolution of wire array z-pinch implosion instabilities which strongly impact x-ray production when the 3D plasma stagnates on axis. Wires modulated in radius exhibit locally enhanced magnetic field and imploding bubble formation at discontinuities in wire radius due to the perturbed current path. Wires coated with localized spectroscopic dopants are used to track turbulent material flow. Experiments and MHD modeling offer insight into the behavior of z-pinch instabilities.

Experiments to study the implosion dynamics and radiation characteristics of copper z-pinches have been fielded at the 1 MA Zebra facility and the 20 MA Z facility. The impact of initial load mass, initial load diameter, and nesting of wire arrays on the precursor and the stagnated plasma has been evaluated through spectroscopy, shadowgraphy, and fluence measurements. Plasma parameters extracted from modeling of the time-integrated L-shell spectra indicate the presence of more than one plasma source contributing to the radiation, likely due to non-uniform hot spot x-ray emission or temporal gradients.

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Over the last several years, rapid progress has been made evaluating the double-z-pinch indirect-drive, inertial confinement fusion (ICF) high-yield target concept (Hammer et al 1999 Phys. Plasmas 6 2129). We have demonstrated efficient coupling of radiation from two wire-array-driven primary hohlraums to a secondary hohlraum that is large enough to drive a high yield ICF capsule. The secondary hohlraum is irradiated from two sides by z-pinches to produce low odd-mode radiation asymmetry. This double-pinch source is driven from a single electrical power feed (Cuneo et al 2002 Phys. Rev. Lett. 88 215004) on the 20 MA Z accelerator. The double z-pinch has imploded ICF capsules with even-mode radiation symmetry of 3.1 {+-} 1.4% and to high capsule radial convergence ratios of 14-21 (Bennett et al 2002 Phys. Rev. Lett. 89 245002; Bennett et al 2003 Phys. Plasmas 10 3717; Vesey et al 2003 Phys. Plasmas 10 1854). Advances in wire-array physics at 20 MA are improving our understanding of z-pinch power scaling with increasing drive current. Techniques for shaping the z-pinch radiation pulse necessary for low adiabat capsule compression have also been demonstrated.

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The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations, we find a transition in the nature of the shear as a characteristic height H* is exceeded. Below H* there is a central stationary core; above H* we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height, while the axial width remains narrow and fixed.

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