Initiation, ablation, precursor formation, and instability analysis of thin foil copper liners

Summary form only given. Solid liners are an attractive load design for use with magnetized liner inertial fusion (MagLIF) and for possible replacement of wire-array Z-pinches on large pulsed-power drivers. Experiments have been conducted on COBRA, our 1 MA, 100 ns rise time pulsed power generator, to investigate three aspects that can affect liner performance: initiation; ablation and precursor formation; and instability development. In initiation experiments, it was found that by decreasing the liner thickness, and thereby increasing the rate of rise of current density (dJ/dt), thin liners initiated quicker and more uniformly than thicker liners. At the time of breakdown, if dJ/dt exceeded 3.5x1016 Acm-2s-1, the liners initiated nearly instantaneously and uniformly. Ablation and precursor formation on the inside surface of the liners was investigated using axial x-ray radiography to measure average ion density in the r-θ plane. In general, thin liners ablate much less material than equivalently massed wire-arrays, but they are sensitive to initial conditions. If crinkles are present on the liner surface, ablation can be enhanced in those regions leading to stronger precursor formation. However, if the liner is relatively smooth, ablation is more uniform and the precursor mass is reduced. With liners much thinner than the skin depth, a mostly uniform prefill plasma was produced with a density of about 1x1018 cm-3. Finally, instabilities have been observed forming on the outside of the liners prior to implosion. These instabilities are formed in low density plasma that is observed to be accelerating away from the liner against the density gradient and therefore is not likely to be magneto-Rayleigh-Taylor in nature. This conclusion is supported by comparing simulations that have been performed using the extended magnetohydrodynamic (XMHD) code, PERSEUS2, with standard MHD simulations.