Heath L. Hanshaw

ALEGRA : an arbitrary Lagrangian-Eulerian multimaterial, multiphysics code.

ALEGRA is an arbitrary Lagrangian-Eulerian (multiphysics) computer code developed at Sandia National Laboratories since 1990. The code contains a variety of physics options including magnetics, radiation, and multimaterial flow. The code has been developed for nearly two decades, but recent work has dramatically improved the code’s accuracy and robustness. These improvements include techniques applied to the basic Lagrangian differencing, artificial viscosity and the remap step of the method including an important improvement in the basic conservation of energy in the scheme. We will discuss the various algorithmic improvements and their impact on the results for important applications. Included in these applications are magnetic implosions, ceramic fracture modeling, and electromagnetic launch.

Three-dimensional effects in trailing mass in the wire-array Z pinch

The implosion phase of a wire-array Z pinch is investigated using three-dimensional (3D) simulations, which model the mass ablation phase and its associated axial instability using a mass injection boundary condition. The physical mechanisms driving the trailing mass network are explored, and it is found that in 3D the current paths though the trailing mass can reduce bubble growth on the imploding plasma sheath, relative to the 2D (r,z) equivalent. Comparison between the simulations and a high quality set of experimental radiographs is presented.

Impact of Time-Varying Loads on the Programmable Pulsed Power Driver Called Genesis

The success of dynamic materials properties research at Sandia National Laboratories has led to research into ultralow impedance, compact pulsed power systems capable of multi-MA shaped current pulses with rise times ranging from 220 to 500 ns. The Genesis design consists of two hundred and forty 200 kV, 80 kA modules connected in parallel to a solid dielectric disk transmission line and is capable of producing 280 kbar of magnetic pressure (>; 500 kbar pressure in high Z materials) in a 1.75 nH, 20-mm wide stripline load. Stripline loads operating under these conditions expand during the experiment resulting in a time-varying load that can impact the performance and lifetime of the system. This paper provides analysis of time-varying stripline loads and the impact of these loads on system performance. Further, an approach to reduce dielectric stress levels through active damping is presented as a means to increase system reliability and lifetime.

On the validation of a 3D inflow model for simulating wire array z-pinches, z-pinch energetics, and scaling of radiated power with current

Summary form only given. Development of a predictive computational model for wire array z-pinches has been inhibited by the 3D nature of the dynamics. We have developed a 3D computational model for simulating cylindrical wire array z-pinches. In lieu of trying to simulate individual wires in the array from the beginning of the initiation phase, which is computationally impractical, we have incorporated a steady state model of wire ablation physics into our 3D, radiation MHD code ALEGRA. Results are presented from a validation study using radiation pulses, currents, and density profiles from a variety of experiments with single wire arrays on the Z accelerator. The wire array is modeled using full length, periodic wedges with angles of 1 and 60 degrees, and 1 degree azimuthal cells. Radiation power pulses produced by arrays with different masses are matched by tuning the ablation rate. Results indicate that the mass ablation rate scales with wire diameter to the -0.49 power, slower than the -0.66 power deduced from experiments. While both wedge models produce power pulses in reasonable agreement with measurements, only the 60 degree wedge with uncorrelated perturbations in azimuth produces both the measured density profile and radiated power. The resulting web-like, azimuthal structure that develops in the z-pinch plasma has a significant influence on the current distribution, density profile and power pulse. Simulations show that shock heating and pdV work account for most of the energy radiated by the z-pinch at stagnation, through peak power. Results indicate that the sub quadratic scaling of peak power (P) with peak current (I) observed in experiments on Z4 (P~I1.24plusmn0.18) is correlated with significant precursor mass stagnating on axis before the main pinch. For 2.5 and 6.0 mg arrays, the simulated power scales as P~I1.35 with a stagnating precursor plasma, and P~I1.8 when stagnation of the precursor is precluded by decreasing the implosion time, which is consistent with P~I2 based on energy conservation.

Z machine circuit model development

The transmission line circuit model of the Z machine is used extensively to aid in the design and analysis of experiments conducted on Z. The circuit model consists of both 1-D and 2-D networks of transmission lines modeling Zs 36 pulselines, vacuum insulator stack, MITLs, vacuum convolute, and load [1].

Double shock experiments on the sandia z machine

The double shock layered high-velocity flyer plate is one new capability being developed on Sandias Z machine. With this technique, dynamic material data at high energy densities can be obtained at points in phase space which lie neither on principal Hugoniots nor on quasi-isentropic ramp curves. We discuss the double shock capability development experiments being performed on Z.