Magnetic diffusion in conductors at ultra-high current density

Recent experiments on the Z-Accelerator at Sandia National Laboratories were performed to characterize magnetic diffusion in aluminum, copper, and stainless steel at currents of 3–6 MA/cm and pulse widths of 200 ns. Low, medium, and high pressure shots were taken in aluminum and copper to establish a magnetic diffusion versus current scaling curve and to characterize relative propagation times between the pressure wave front and the onset of magnetic field diffusion at varying depths in the material. A medium pressure shot was taken in stainless steel to benchmark against aluminum and copper.; Principal diagnostics include unique B-dot probes and VISAR laser interferometry. The diagnostics were fielded on square short circuit loads designed to produce a symmetric current on each of the diagnostic surfaces. Linear current values were scaled by varying the anode surface area and charge voltage of the Z-Accelerator. Target pressure values of 500 kbar, 1.0 Mbar, and 2.0 Mbar were achieved in this manner.; These experiments are motivated by the need to understand several different physical properties in materials at the extreme conditions produced in the Z-Accelerator where temperatures range from ambient to several keV and material densities range from compressed-solid to low density plasma. Several separate physical properties are of particular interest. As the current ramps up in the load a pressure wave is launched into the conductor. Magnetic field diffusion and thermal diffusion are also initiated in the conductor. The pressure wave front compresses the material to densities greater than ambient solid resulting in dissipative forces which equate to mechanical losses in the material, and magnetic field diffusion produces ohmic heating in the conductor, also producing dissipative losses in the material. These are highly nonlinear effects and not well known at these current densities.; Magnetic diffusion rates were measured in all three materials with average speeds ranging from 2.1–3.5 km/s in aluminum, 1.1–2.7 km/s in copper, and 8.3 km/s in the stainless steel shot. The diffusion rate was faster in aluminum than in copper and the diffusion rate in stainless was faster than in aluminum or copper by nearly a factor of four at equal pressures, as would be expected.; Numerical modeling of these experiments was also performed using the MHD code Mach2. Recently updated resistivity tables and pertinent thermal conductivities were used in these simulations. It was determined that the updated Lee-Moore-Desjarlais (LMD) resistivity tables provide the best fit of the models to the VISAR data. By scaling the 1-D current we achieved very good agreement with experimental VISAR data, and good agreement with B-dot data.