Kevin Colligan Yates

Electrothermal instability plays an important role in applications of current-driven metal, creating striations (which seed the magneto-Rayleigh-Taylor instability) and filaments (which provide a more rapid path to plasma formation). However, the initial formation of both structures is not well understood. Simulations show for the first time how a commonly occurring isolated defect transforms into the larger striation and filament, through a feedback loop connecting current and electrical conductivity. Simulations have been experimentally validated using defect-driven self-emission patterns.

Electrothermal instability is responsible for degrading numerous applications of pulsed-power technology, yet the initial conditions from which it grows are not well understood. For the first time, metal surfaces have been tracked from characterization to self-emission. This reveals no clear correlation between non-uniform thermal emissions and surface metallurgical defects or crystallographic grains, while correlations are observed with surface topography for 5N metal but not 6061 metal. Finally, for 5N metal, surfaces with average roughness as small as 5nm still admit thermal perturbations with δT/T >0.1.

Complex plasma-material interactions are found in many fusion and high energy density experiments. These can dramatically alter the performance of a fusion device, as contaminated fusion fuel does not achieve as high of a temperature and thus fusion yield, due to radiative losses. Magneto-inertial fusion schemes, such as Sandia's MagLIF concept, have metal components that carry increasingly high current density in the vicinity of the plasma. Electrothermal instability of the metal can mix the metal into the plasma, both directly and by seeding magnetohydrodynamic (MHD) instabilities. Accurate numerical modeling of electrically driven conductors is currently challenging due to uncertainties in the equation of state (EOS) and electrical conductivity, especially during the metal-insulator transition. To supply data for comparison to MHD modeling, photonic Doppler velocimetry (PDV) was used to measure the surface motion of mm-diameter (6061 and 5N Al, 5N Cu, 4N Ni, and Ti) rods driven to 0.8 MA in 100 ns by the Sandia Mykonos generator. The high quality of the data permitted the initial magnetic compression of rods to be measured for the first time. Uncoated pure (5N) Al rods compressed 40 nm radially before expanding. Subsequently, the reflective surface experiences several changes in acceleration during the current rise. Last, taking advantage of PDV's sensitivity to multiple simultaneous velocities, the time dependence of the distribution of velocities in the reflective material is being investigated to compare with computer simulations of electrothermal and MHD instabilities. The experimental measurements are being used to benchmark MHD calculations, and thereby inform the choice of EOS and conductivity tables for modeling.