Electrical and Computer Engineering ETDs

Publication Date

Summer 7-9-2024

Abstract

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, 1 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. 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. Larger diameter ED pairs exhibited qualitatively different behavior from smaller diameters, defying theoretical predictions that these ED types would heat and evolve similarly. Emission peaks of sinusoidal perturbations heat faster and emission valleys heat more slowly than background metal at low current, and those perturbations with higher A/λ ratios tend to heat faster. Reduced expansion of the metal delays surface plasma formation, resulting in altered ETI evolution. 24-micron diameter ED have been machined into sinusoidally perturbed surfaces of varying amplitudes (0.191 – 3.056um) and wavelengths (24-48um). Maximum theoretical current density amplification (jmax/j0 = 1 + (2πA/λ)) for the sinusoids ranges from 1.025 to 1.4, while the maximum ED-driven j/j0 is ~1.25. Epoxy-coated rods exhibit elongated azimuthal heating that follows sinusoidal contours. These first-of-their-kind experiments seek to inform material choices in ICF experiments, validate critical MHD simulations, and contribute to the fundamental understanding of relevant instability phenomena.

Keywords

electrothermal instability, plasma physics, magnetohydrodynamics, ETI

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Mark Gilmore

Second Committee Member

Edl Schamiloglu

Third Committee Member

Bhuvana Srinivasan

Fourth Committee Member

Thomas Awe

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