Magnetic compression and melt of electrically thick metal driven by lineal current densities characteristic of pulsed-power-driven fusion devices*

Approaches to magnetic fusion utilizing advances in electrical pulsed-power technology offer an attractive path toward commercialization due to their high energy conversion efficiency. In Z-pinch and liner-based fusion schemes such as MagLIF, the stability of the imploding conductor is critical to the fusion performance. The electrothermal instability (ETI) is found in many high-energy-density and fusion experiments, dramatically affecting performance by seeding MHD instabilities. Computational modeling of electrically driven conductors is challenging due to uncertainties in the equation-of-state (EOS) and electrical conductivity during the metal-insulator transition. Photonic Doppler velocimetry (PDV) was used to measure the surface motion of extremely smooth 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 with sufficient PDV resolution to observe radial compression before expansion. The reflective surface experiences several changes in acceleration during the Mykonos current rise. 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. The experimental measurements are being used to benchmark MHD calculations, and thereby inform the choice of EOS and conductivity tables for modeling.