Publications

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Planar wire arrays are studied at 3-6 MA on the Saturn pulsed power generator as potential drivers of compact hohlraums for inertial confinement fusion studies . Comparison with zero-dimensional modeling suggests that there is significant trailing mass. The modeled energy coupled from the generator cannot generally explain the energy in the main x-ray pulse. Preliminary comparison at 1-6 MA indicates sub-quadratic scaling of x-ray power in a manner similar to compact cylindrical wire arrays. Time-resolved pinhole images are used to study the implosion dynamics. © 2009 American Institute of Physics.

Experiments with different stainless steel (SS) wire loads were performed on the 1 MA Zebra Z-pinch generator at University of Nevada, Reno. The wire array loads consisted of 7.6 (μm SS wires and had 10 wires for the planar wire array with an interwire gap of 1 mm and 8 wires for the cylindrical wire array of a 16 mm diameter. In addition, a single-wire experiment with a 25 (μm SS wire was carried out. The different wire loads were used to provide a broader spectrum of plasma conditions. Time-integrated and time-gated x-ray images, as well as time-integrated, spatially-resolved and spatially-integrated x-ray spectra, were collected and analyzed. Both K-shell and L-shell radiation were recorded using LiF and KAP crystal spectrometers, respectively. Non-LTE kinetic models of Fe and Ni are employed to derive plasma parameters. For axially resolved L-shell spectra, the resulting electron temperatures are between 230 and 300 eV (higher near the cathode) and electron densities vary from 10 19 to 10 20 cm -3 dependent on the load. The advantage of using Z-pinch plasmas for astrophysical applications is highlighted. © 2009 American Institute of Physics.

A 1D Lagrangian magnetohydrodynamic z-pinch simulation code is extended to include wire ablation. The plasma transport coefficients are calibrated to reproduce the K-shell yields measured on the Z generator for three stainless steel arrays of diameter 55 mm and masses ranging from 1.8 to 2.7 mg. The resulting 1D scaling model is applied to a larger SS array (65 mm and 2.5 mg) on the refurbished Z machine. Simulation results predict a maximum K-shell yield of 77 kJ for an 82 kV charging voltage. This maximum drops to 42 kJ at 75 kV charging. Neglecting the ablation precursor leads to a ∼10% change in the calculated yield. © 2009 American Institute of Physics.

Many physical effects can produce unstable plasma behavior that affect K-shell emission from arrays. Such effects include: asymmetry in the initial density profile, asymmetry in power flow, thermal conduction at the boundaries, and non-uniform wire ablation. Here we consider how asymmetry in the radiation field also contributes to the generation of multidimensional plasma behavior that affects K-shell power and yield. To model this radiation asymmetry, we have incorporated into the MACH2 r-z MHD code a self-consistent calculation of the non-LTE population kinetics based on radiation transport using multi-dimensional ray tracing. Such methodology is necessary for modeling the enhanced radiative cooling that occurs at the anode and cathode ends of the pinch during the run-in phase of the implosion. This enhanced radiative cooling is due to reduced optical depth at these locations producing an asymmetric flow of radiative energy that leads to substantial disruption of large initial diameter (>5 cm) pinches and drives ID into 2D fluid (i.e., Rayleigh-Taylor like) flows. The impact of this 2D behavior on K-shell power and yield is investigated by comparing ID and 2D model results with data obtained from a series of single wire array stainless steel experiments performed on the Z generator. © 2009 American Institute of Physics.

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Tracer aluminum alloyed wires (Al5056) are used to provide additional information for x-ray diagnostics of implosions of Cu planar wire arrays (PWAs). Specifically, the analysis of combined PWA experiments using the extensive set of x-ray diagnostics is presented. In these experiments, which were conducted at the 1MA pulsed power generator at University of Nevada, Reno, the Z-pinch load consisted of several (eight) Cu alloyed (main material) and one to two Al alloyed (tracer) wires mounted in a single plane row or double parallel plane rows, single planar wire array (SPWA) or double planar wire array (DPWA), respectively. The analysis of x-ray spatially resolved spectra from the main material indicates the increase in the electron temperature Te near the cathode. In general, the axial gradients in Te are more pronounced for SPWA than for DPWA due to the more "columnlike" plasma formation for SPWA compared to "hot-spot-like" plasma formation for DPWA. In addition, x-ray spectra from tracer wires are studied, and estimated plasma parameters are compared with those from the main material. It is observed that the x-ray K -shell Al spectra manifest more opacity features for the case of SPWA with about 18% of Al mass (to the total load mass) compared to the case of DPWA with about 11% of Al mass. The analysis of time-gated spectra shows that the relative intensity of the most intense K -shell Al line, small before the x-ray burst, increases with time and peaks close to the maximum of the sub-keV signal. © 2008 American Institute of Physics.

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A series of ten shots were performed on the Saturn generator in short pulse mode in order to study planar and small-diameter cylindrical tungsten wire arrays at {approx}5 MA current levels and 50-60 ns implosion times as candidates for compact z-pinch radiation sources. A new vacuum hohlraum configuration has been proposed in which multiple z pinches are driven in parallel by a pulsed power generator. Each pinch resides in a separate return current cage, serving also as a primary hohlraum. A collection of such radiation sources surround a compact secondary hohlraum, which may potentially provide an attractive Planckian radiation source or house an inertial confinement fusion fuel capsule. Prior to studying this concept experimentally or numerically, advanced compact wire array loads must be developed and their scaling behavior understood. The 2008 Saturn planar array experiments extend the data set presented in Ref. [1], which studied planar arrays at {approx}3 MA, 100 ns in Saturn long pulse mode. Planar wire array power and yield scaling studies now include current levels directly applicable to multi-pinch experiments that could be performed on the 25 MA Z machine. A maximum total x-ray power of 15 TW (250 kJ in the main pulse, 330 kJ total yield) was observed with a 12-mm-wide planar array at 5.3 MA, 52 ns. The full data set indicates power scaling that is sub-quadratic with load current, while total and main pulse yields are closer to quadratic; these trends are similar to observations of compact cylindrical tungsten arrays on Z. We continue the investigation of energy coupling in these short pulse Saturn experiments using zero-dimensional-type implosion modeling and pinhole imaging, indicating 16 cm/?s implosion velocity in a 12-mm-wide array. The same phenomena of significant trailing mass and evidence for resistive heating are observed at 5 MA as at 3 MA. 17 kJ of Al K-shell radiation was obtained in one Al planar array fielded at 5.5 MA, 57 ns and we compare this to cylindrical array results in the context of a K-shell yield scaling model. We have also performed an initial study of compact 3 mm diameter cylindrical wire arrays, which are alternate candidates for a multi-pinch vacuum hohlraum concept. These massive 3.4 and 6 mg/cm loads may have been impacted by opacity, producing a maximum x-ray power of 7 TW at 4.5 MA, 45 ns. Future research directions in compact x-ray sources are discussed.

Planar wire arrays are studied at 3-6 MA on the Saturn pulsed power generator as potential drivers of compact hohlraums for inertial confinement fusion studies. Comparison with zero-dimensional modeling suggests that there is significant trailing mass. The modeled energy coupled from the generator cannot generally explain the energy in the main x-ray pulse. Preliminary comparison at 1-6 MA indicates sub-quadratic scaling of x-ray power in a manner similar to compact cylindrical wire arrays. Time-resolved pinhole images are used to study the implosion dynamics.

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