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Experimental studies of the electrothermal instability using electrically thick, coated metal
AdvisorBauer, Bruno S.
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The electrothermal instability (ETI) plays an important role in the thermal and hydrodynamic uniformity of metallic systems carrying electrical current densities greater than approximately 1 MA/mm^2. Most importantly, the stratified form of the ETI generates and amplifies temperature and subsequently density perturbations perpendicular to the current. Later, the alternate manifestation of the instability organizes any plasma that forms into filamentary streamers parallel to the current. For a typical electrically thick bare metal with lineal current density increasing at 3×10^15 A/m/s, the time window between when the stratified instability is observable and the conditions for filamentary growth are met is only a few nanoseconds, and not typically sufficient for detailed growth studies of the stratified form of the instability.To understand the stratified ETI, this time window was enlarged by coating the metal with a thin dielectric layer, typically 35 − 70 μm. This insulating dielectric inertially tamps the hot, exploding metal, and keeps it at a larger density relative to uncoated metal at the same temperature. This prevents the metal from entering the liquid-vapor bi-phase region, which suppresses plasma formation, and enables prolonged observation of the stratified form of the ETI. Benefiting from this longevity, the surface emissions from the exploding metal are imaged through the transparent dielectric. These images may be converted to temperature maps of the emitting metal, and Fourier analysis of a sequence of such images shows discrete azimuthally-correlated stratified thermal perturbations perpendicular to the current whose wavenumbers, 1, grew exponentially with rate γ(k) = 0.06/ns − 0.4/ns∙μm^2/rad^2 k^2 in ~1 g/cm^3, ~7,000 K aluminum. Following the experimental confirmation that the ETI does grow in electrically thick environments, it is important to understand from what initial conditions. To answer this, surfaces with 5 − 100 nm average roughness were fabricated, and surface features were characterized and tracked through experimentation. This tracking revealed no clear correlation between non-uniform thermal emissions and surface metallurgical defects or crystallographic grains, while correlations are observed with surface topography. Thermal perturbations emerging from defect-laden aluminum alloy 6061 are approximately a factor of two larger in amplitude than those from pure aluminum, and surfaces with average roughness as small as 5 nm still admit appreciable thermal perturbations.