The Electrothermal Instability on Pulsed Power Ablations of Thin Foils.

Abstract

The electrothermal instability (ETI) is an exponentially growing temperature perturbation that arises due to nonuniformities in Ohmic heating of a current-carrying material with a temperature-dependent resistivity. When resistivity increases with temperature, as in most solid and liquid metals, ETI forms striations of hot and cold material perpendicular to the flow of current. On a pulsed-power driven ablation of an initially solid metal, these striations can cause local vaporization before the bulk material vaporizes, leading to a mass perturbation that can seed plasma instabilities, such as the magneto Rayleigh-Taylor (MRT) instability. These instabilities have been identified as the primary impediment to producing energy gain in a pulsed power-driven nuclear fusion concept called magnetized liner implosion fusion (MagLIF). Understanding of ETI may provide better means to mitigate plasma instabilities and achieve fusion gain on MagLIF experiments. A diagnostic has been developed to measure spatially resolved temperature using an ultrafast framing camera from self-emission of planar foil ablations conducted in atmospheric conditions. These temperature measurements provide the first time-resolved experimental observations of ETI as a growing temperature perturbation on ablations of initially solid metal targets. Growth rates of experimentally observed perturbations show good agreement with theoretical predictions of ETI and demonstrate expected quadratic dependence on current density. Additional experiments were conducted on the MAIZE linear transformer driver (LTD), a 1-MA pulsed power facility at the University of Michigan, to study the coupling of ETI to later-time plasma instabilities. Liners of aluminum, titanium, and tantalum were ablated to compare material-dependent effects, and ablations of aluminum with and without dielectric coatings (which had previously been shown to reduce the impact of ETI) were performed to compare instability growth on the same material with varying ETI seeding. It was observed that tantalum liners, which have lower predicted ETI growth, exhibit dramatically less plasma instability growth than aluminum or titanium. Additionally, ablations of aluminum liners with external dielectric coatings exhibited less azimuthal symmetry than bare aluminum liners, which was anticipated because ETI tends to azimuthally self-correlate. These results support the theory that ETI provides the surface perturbation that is responsible for seeding plasma instabilities on liner ablations.