Publications

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Z-pinch plasmas are susceptible to the magnetic Rayleigh-Taylor (MRT) instability. The Z-pinch dynamic hohlraum (ZPDH), as implemented on the Z machine at Sandia National Laboratories, is composed of an annular tungsten plasma that implodes onto a coaxial foam convertor. The collision between tungsten Z pinch and convertor launches a strong shock in the foam. Shock heating generates radiation that is trapped by the tungsten Z pinch. The radiation can be used to implode a fuel-filled, inertial confinement fusion capsule. Hence, it is important to understand the influence that the MRT instability has on shock generation. This paper presents results of an investigation to determine the affect that the MRT instability has on characteristics of the radiating shock in a ZPDH. Experiments on Z were conducted in which a 1.5 cm tall, nested array (two arrays with initial diameters of 2.0 and 4.0 cm), tungsten wire plasma implodes onto a 5 mg/cc, CH{sub 2} foam convertor to create a {approx}135 eV dynamic hohlraum. X-ray pinhole cameras viewing along the ZPDH axis recorded time and space resolved images of emission produced by the radiating shock. These measurements showed that the shock remained circular to within +/-30-60 {micro}m as it propagated towards the axis, and that it was highly uniform along its height. The measured emission intensities are compared with synthetic x-ray images obtained by postprocessing two-dimensional, radiation magnetohydrodynamic simulations in which the amplitude of MRT perturbations is varied. These simulations accurately reproduce the measured shock trajectory and spatial profiles of the dynamic hohlraum interior emission as a function of time, even for large MRT amplitudes. Furthermore, the radiating shock remains relatively uniform in the axial direction regardless of the MRT amplitude because nonuniformities are tamped by the interaction of the tungsten Z-pinch plasma with the foam. These results suggest that inertial confinement fusion implosions driven by a ZPDH should be relatively free from random radiation symmetry variations produced by Z-pinch instabilities.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

Trends in radiation production from dynamic-hohlraums driven by single and nested wire arrays were studied. The axial radiation developed from the interior of an imploding dynamic hohlraum target was compared with that generated using a standard nested array on Z. Measurements over a range of single-array masses showed a decrease in radiation power for masses above 3.5 mg.

The intense magnetic field generated in the 20 MA Z-machine is used to accelerate metallic flyer plates to high velocity (peak velocity {approx}20-30 km/s) for the purpose of generating strong shocks (peak pressure {approx}5-10 Mb) in equation of state experiments. We have used the Sandia developed, 2D magneto-hydrodynamic (MHD) simulation code ALEGRA to investigate the physics of accelerating flyer plates using multi-megabar magnetic drive pressures. Through detailed analysis of experimental data using ALEGRA, we developed a 2D, predictive MHD model for simulating material science experiments on Z. The ALEGRA MHD model accurately produces measured time dependent flyer velocities. Details of the ALEGRA model are presented. Simulation and experimental results are compared and contrasted for shots using standard and shaped current pulses whose peak drive pressure is {approx}2 Mb. Isentropic compression of Al to 1.7 Mb is achieved by shaping the current pulse.

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Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km s{sup -1}. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD{sub 2}) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 22--100 GPa. The results of these experiments disagree with the previously reported Hugoniot measurements of LD2 in the pressure range above {approx}40 GPa, but are in good agreement with first principles, ab initio models for hydrogen and its isotopes.

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This work is an attempt to elucidate effects that may limit efficiency in magnetrons operated at relativistic voltages (V {approximately} 500 kV). Three-dimensional particle-in-cell simulation is used to investigate the behavior of 14 and 22 cavity, cylindrical, rising-sun magnetrons. Power is extracted radially through a single iris located at the end of every other cavity. Numerical results show that in general output power and efficiency increase approximately linearly with increasing iris width (decreasing vacuum Q) until the total Q becomes too low for stable oscillation in the n-mode to be maintained. Beyond this point mode competition and/or switching occur and efficiency decreases. Results reveal that the minimum value of Q (maximum efficiency) that can be achieved prior to the onset of mode competition is significantly affected by the magnitude of the 0-space-harmonic of the {pi}-mode, a unique characteristic of rising-suns, and by the magnitude of the electron current density (space-charge effects). By minimizing these effects, up to 3.7 GW output power has been produced at an efficiency of 40%.

We have conducted an extensive investigation of the split cavity oscillator (SCO) using particle-in-cell simulation. The goal of this work is to test and optimize an inverse diode rf convertor for use with a cylindrical SCO, while simultaneously determining factors that control rf extraction efficiency. We present results from simulations of several configurations including the SCO with inverse diode extractor, the SCO in conjunction with post-acceleration and inverse diode extraction, and the SCO, using electron beams with a variety of currents, voltages, and radii. 7 refs., 8 figs.

The split cavity oscillator is a resonant cavity device which can be used to highly modulate the current of a relativistic electron beam for the purpose of generating high power microwaves. It consists of a cylindrical cavity resonator which has been divided into two identical cavities by a conducting foil whose radius is less than the inner radius of the cylinder. The gap between the foil endpoint and cylinder walls forms an annular slot which couples the cavities electromagnetically. Using a particle-in-cell code, we have performed a numerical investigation of SCO structures driven by high current, magnetized, annular, relativistic electron beams. Results are presented which illustrate the dependence of current modulation efficiency and oscillation frequency on injected beam current density. In addition, simulation results are presented for a new, double-foil SCO which indicate significantly enhanced performance in comparison to the single-foil device. 10 refs., 10 figs., 2 tabs.

The utility of the magnetically insulated transmission line oscillator (MILO) as a high power microwave source depends on how efficiently power can be extracted from it. We have designed a slow-wave stepped transformer for the purpose of axially extracting microwave power from a 3.6 GHz coaxial MILO. The slow-wave transformer design was optimized using particle-in-cell simulation, and tested in experiments performed on the HPM Simulation Division's GEMINI and GYPSY water Blumlein pulse power sources. In this paper we summarize the slow-wave stepped transformer design, and describe MILO axial power extraction experiments which yielded up to 300 MW of radiated power. 10 refs., 4 figs., 2 tabs.