Publications

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This paper presents an analytic theory on the linear and nonlinear evolution of the most unstable azimuthal clumping mode, known as the pi-mode, in a discrete wire array. In the pi-mode, neighboring wires of the array pair-up as a result of the mutual attraction of the wires which carry current in the same direction. The analytic solution displays two regimes, where the collective interactions of all wires dominate, versus where the interaction of the neighboring, single wire dominates. This solution was corroborated by two vastly different numerical codes which were used to simulate arrays with both high wire numbers (up to 600) and low wire number (8). All solutions show that azimuthal clumping of discrete wires occurs before appreciable radial motion of the wires. Thus, absence of azimuthal clumping of wires in comparison with the wires' radial motion may imply substantial lack of wire currents. While the present theory and simulations have ignored the plasma corona and axial variations, it is argued that their effects, and the complete account of the three-dimensional feature of the pi-mode, together with a scaling study of the wire number, may be expediently simulated by using only one single wire in an annular wedge with a reflection condition imposed on the wedge's boundary. © 2007 American Institute of Physics.

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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.

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Complex simulations (in particular, those involving multiple coupled physics) cannot be understood solely using geometry-based visualizations. Such visualizations are necessary in interpreting results and gaining insights into kinematics, however they are insufficient when striving to understand why or how something happened, or when investigating a simulation's dynamic evolution. For multiphysics simulations (e.g. those including solid dynamics with thermal conduction, magnetohydrodynamics, and radiation hydrodynamics) complex interactions between physics and material properties take place within the code which must be investigated in other ways. Drawing on the extensive previous work in view coordination, brushing and linking techniques, and powerful visualization libraries, we have developed Prism, an application targeted for a specific analytic need at Sandia National Laboratories. This multiview scientific visualization tool tightly integrates geometric and phase space views of simulation data and material models. Working closely with analysts, we have developed this production tool to promote understanding of complex, multiphysics simulations. We discuss the current implementation of Prism, along with specific examples of results obtained by using the tool.

ALEGRA is an arbitrary Lagrangian-Eulerian finite element code that emphasizes large distortion and shock propagation in inviscid fluids and solids. This document describes user options for modeling resistive magnetohydrodynamics, thermal conduction, and radiation transport effects, and two material temperature physics.

ALEGRA is an arbitrary Lagrangian-Eulerian finite element code that emphasizes large distortion and shock propagation in inviscid fluids and solids. This document describes user options for modeling resistive magnetohydrodynamic, thermal conduction, and radiation emission effects.

ALEGRA is an arbitrary Lagrangian-Eulerian multi-material finite element code used for modeling solid dynamics problems involving large distortion and shock propagation. This document describes the basic user input language and instructions for using the software.

The impact of 3D structure on wire array z-pinch dynamics is a topic of current interest, and has been studied by the controlled seeding of wire perturbations. First, Al wires were etched at Sandia, creating 20% radial perturbations with variable axial wavelength. Observations of magnetic bubble formation in the etched regions during experiments on the MAGPIE accelerator are discussed and compared to 3D MHD modeling. Second, thin NaF coatings of 1 mm axial extent were deposited on Al wires and fielded on the Zebra accelerator. Little or no axial transport of the NaF spectroscopic dopant was observed in spatially resolved K-shell spectra, which places constraints on particle diffusivity in dense z-pinch plasmas. Finally, technology development for seeding perturbations is discussed.

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ALEGRA is an arbitrary Lagrangian-Eulerian finite element code that emphasizes large distortion and shock propagation in inviscid fluids and solids. This document describes user options for modeling magnetohydrodynamic, thermal conduction, and radiation emission effects.

An understanding of the dynamics of z-pinch wire array explosion and collapse is of critical interest to the development and future of pulsed power inertial confinement fusion experiments. Experimental results clearly show the extreme three-dimensional nature of the wire explosion and collapse process. The physics of this process can be approximated by the resistive magnetohydrodynamic (MHD) equations augmented by thermal and radiative transport modeling. Z-pinch MHD physics is dominated by material regions whose conductivity properties vary drastically as material passes from solid through melt into plasma regimes. At the same time void regions between the wires are modeled as regions of very low conductivity. This challenging physical situation requires a sophisticated three-dimensional modeling approach matched by sufficient computational resources to make progress in predictive modeling and improved physical understanding.

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