Compression and Melt of Electrically Thick Metal Driven by a Mega-Ampere Current Pulse

The electrothermal instability (ETI) is found in many fusion and HEDP experiments, dramatically affecting performance by seeding of MHD instabilities. Accurate modeling of the electrically driven conductors used in these experiments is challenging due to uncertainties in the equation of state and electrical conductivity, especially during the metal-insulator transition. Photonic Doppler velocimetry (PDV) was used to measure the surface motion of mm-diameter pure aluminum rods driven to 860 kA with 70 ns risetime (10-90%) by the Sandia Mykonos generator. For the first time, aluminum rods were measured to compress ~40 nm radially before expanding. The reflective surface experiences several changes in acceleration during the current pulse. PDV data was compared with MHD simulations to diagnose the phase-space trajectory of the metal surface, including magnetic compression, the duration of the solid-liquid phase transition, and the subsequent early-time motion until plasma formation, constraining the conditions from which ETI arises.