Simulations of imploding solid liner melting and vaporization vs liner thickness, and evidence for "melt waves"

The Air Force Research Laboratory Directed Energy Directorate (AFRL/DE) has, over the last several years, conducted experiments on the magnetic pressure driven implosion of various metal shells (solid liners). More recently, AFRL/DE has reported on experiments that successfully imploded cylindrical aluminum liners suitable for compressing field reversed configurations (FRC's) to magnetized target fusion (MTF) conditions (1). We have recently done Mach2 (2) MHD simulations of the resistive heating of such imploding liners as a function of their thickness. This was to gain insight on diffusion time effects that conceivably could lead to melt waves for thicker liners, driven with higher currents. For example, scaling the thickness of a liner for successful experiment parameters with the implosion discharge energy might be expected to preserve the timing of liner liquification (or loss of material strength). However, diffusion time effects can complicate this. Our simulations indicate such effects, sometimes referred to as melt waves, for increasing the discharge energy and liner thickness a factor of 4, with the same 10 microsecond current risetime, relative to experimentally successful implosion parameters.