Non-uniform Joule heating and plasma formation driven by machined 2D and 3D surface perturbations on dielectric coated and bare aluminum rods*

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 target. Density perturbations formed through ETI can seed the magneto-Rayleigh-Taylor instability, which is acknowledged as one of the key factors limiting the conditions achievable in magnetically driven high energy density physics (HEDP) experiments. Understanding the formation and evolution of ETI is critical in developing predictive computational tools for HEDP experiments as well as strategies for mitigating its impact.

Most metals include complex distributions of imperfections (voids, resistive inclusions) which seed ETI. To simplify comparison with modeling and theory, our experiments examined growth of ETI from relatively void/inclusion free, 99.999% pure, diamond-turned aluminum rods. We then introduced a variety of deliberately machined and well-characterized perturbations into the target surface, including micron-scale quasi-hemispherical voids, or “engineered” defects (ED), and sinusoidal patterns of varying wavelength and amplitude. Larger diameter ED exhibited qualitatively different behavior from smaller diameters, an effect that we can accurately model. In this study, we evaluated the use of dielectric coatings to hydrodynamically tamp the target surface, effectively delaying surface plasma formation. ED were also machined into sinusoidally perturbed surfaces to understand the relative importance of surface roughness compared with voids, both of which can initiate non-uniform surface heating and plasma formation. We observed a transition from heating dominated by surface roughness to that by ED, consistent with theoretical predictions. These experiments constrain our computational tools, which will enable advances in magnetically driven HEDP target design.