Non-Uniform Joule Heating and Plasma Formation Driven by Machined 2D and 3D Surface Perturbations on Dielectric Coated and Bare Aluminum Rods
Maren Hatch, University of New Mexico
The electrothermal instability (ETI) is a Joule heating-driven instability that instigates runaway heating on conductors driven to high current density, altering the 3D evolution of the expansion and phase state. Most metals include complex distributions of imperfections (voids, resistive inclusions) which seed ETI. To simplify comparison with modeling and theory, experiments examined growth of ETI from various alloys of stainless steel as well as relatively void/inclusion free, 99.999% pure, diamond-turned, one mm-diameter aluminum rods. Aluminum surfaces included a variety of deliberately machined and well-characterized perturbations, including 10-micron-scale quasi-hemispherical voids, or “engineered” defects (ED), and sinusoidal patterns of varying wavelength and amplitude. Such perturbations were studied in isolation and colocation to evaluate which defect type drove more rapid heating. Larger diameter ED pairs exhibited qualitatively different behavior from smaller diameters, sourcing localized heating and plasma filaments from the center and azimuthal sides of the ED. Epoxy coatings were added to select loads to evaluate the effect of hydrodynamic tamping. Reduced expansion of the metal delays surface plasma formation, resulting in altered ETI evolution, sourcing from the ED center. Twenty-four-micron diameter ED were machined into sinusoidally perturbed surfaces of varying amplitudes and wavelengths. Epoxy-coated rods exhibit elongated azimuthal heating that follows sinusoidal contours. Data from high-resolution-gated-imagers of visible surface emission confirm theoretical predictions and present novel results suggesting collaborative heating between 2D and 3D surface perturbations. We propose to extend this platform by embedding prescribed sub-surface inclusions of varying depths to study their impact on ETI evolution, advancing experiments that provide critical data on extreme conditions relevant to the nuclear stockpile.