The Electrothermal Instability on Pulsed Power Ablations of Thin Foils

Presented are results from the optical imaging of atmospheric ablations of thin aluminum foils. These experiments were performed to evaluate the growth of temperature perturbations attributed to the electrothermal instability (ETI). ETI has been shown to seed magnetohydrodynamic instabilities on pulsed power-driven ablations of initially solid metallic targets, a topic of interest to various programs in pulsed power-driven plasma physics that depend on stable liner implosions. Experimental observations presented herein demonstrate exponentially growing temperature perturbations perpendicular to the direction of current with growth rates consistent with the linear ETI theory. High-temperature regions were observed to enter the vapor phase before sufficient energy had been deposited in the bulk foil to overcome the latent heat of vaporization, indicating a significant spatial heterogeneity in energy deposition rates. The growth rates of these perturbations scale as the square of current density, the predicted behavior for long-wavelength ETI structures. The development of these structures was unchanged by physical deformation of the foil surface, but dramatically influenced by incorporating areas of local high resistance in the foil loads. Extending the observation window in time showed a transition from perpendicular to parallel filaments, which is significant because ETI is predicted to switch orientations when the bulk foil material transitions into the plasma state. Collectively, these results provide an experimental validation of many theoretical predictions regarding ETI.