The Quest for the Wholly Stable Liner

Abstract

For several decades many have studied or conducted experiments to drive magnetic fields into metallic conducting materials. Examples include designs for electrically exploded fuses, exploding wires to generate high energy plasmas, and of course heavy metal liners as kinetic drivers for hydrodynamic experiments. When the material melts the surface can develop highly unstable dynamics. One of the most common results is the onset and growth of spatial perturbations taking on the form of spike and bubble like structures. This is usually identified as Magneto-Raleigh-Taylor (MRT) instability. A clear example is when excessive current is applied to accelerate a near normal density thick metal liner to velocities approaching 1.0 cm/musec or greater. Yet we have observed several experiments where melting of the liner was present but the outside liner surface was observed to remained stable (B-0.5 to 1.3 MG). Analysis of this and other cases compared to MHD simulations enabled us to examine this phenomenon under a variety of conditions. While the majority of the cases still are fundamentally acceleration driven instability of a fluid interface, other phenomenon have been observed to play a significant role such as the effect of liquid/vapor phase change at the surface. Additionally, this suggests there may be drive conditions that can maintain the aluminum at conditions well away from the saturated liquid line until the conditions are well above the triple point in aluminum. There are some indications that this may reduce or delay the MRT like instabilities. However excessive drive that pressurizes the melted layer too much produces unfavorable gradients in the material that grossly aggravate the traditional MRT instabilities. In this talk we will examine in detail the effects of EOS structure, conductivity dependence on state properties (e.g. density and temperature), and the magnitude and time dependence of the driving magnetic field on the evolution of surface conditions. Based on these observations we propose that controlling the surface stability may depend on careful adjustment of time scales associated with the driving waveform and kinetics of the liner in order to control the path in phase space (EOS) the material follows.