Photonic Doppler Velocimetry Measurements of Compression and of the Duration of the Solid-Liquid Phase Transition on the Surface of Electrically Thick Aluminum Rods Driven by Intense Current

The evolution of metal carrying intense current is of fundamental importance to high-energy-density physics and applications. However, accurate numerical modeling of electrically pulsed conductors is challenging due to large uncertainties in the equation of state (EOS) and electrical conductivity when metal transforms from liquid to vapor. To benchmark MHD calculations and inform the choice of EOS and conductivity tables, extremely sensitive measurements have been made of the motion of the surface of electrically driven metal rods, using Photonic Doppler velocimetry (PDV). The rods were diamond-turned from 5N aluminum (99.999% pure AI) to diameters of
$800-801\ \mu\mathrm{m}$
, with surface roughness (Ra) 17–245 nm. The Sandia Mykonos linear transformer driver pulsed them to 860 kA with 71 ns risetime (10-90%), in vacuum. PDV data were compared with MHD simulations to diagnose the density-temperature phase-space trajectory of the reflective surface, including magnetic compression of the solid, the duration of the solid-liquid phase transition on the surface, and the subsequent motion until plasma formation. Using PDV, radially inward motion of the solid aluminum current-carrying surface was resolved experimentally for the first time. PDV subsequently observed rapid outward acceleration of the surface, in correspondence with MHD simulations showing surface melting. Then PDV observed a decrease in the radial acceleration of the surface, in correspondence with computations showing the surface had finished melting. The difference in time between these latter two events provides the first experimental measurement of the duration of surface melting on a current-driven rod. An important application of the new measurements and improved MHD simulations is more accurate knowledge of the conditions in which high current instabilities, such as electrothermal and magnetically driven instabilities, are observed to develop.