Nano-Surface Finishing for Electrothermal Instability Evolution Studies

Implosion asymmetries generated by the Magneto-Rayleigh-Taylor (MRT) instability commonly limit the performance of the magnetically accelerated targets used to compress high-energy-density plasmas for radiation effects sciences and inertial confinement fusion studies, yet the seed perturbation from which MRT grows is not yet fully understood. Early in the current pulse an electrothermal instability (ETI), which is driven by non-uniform runaway Ohmic heating, may provide a mass perturbation on the target’s surface which exceeds the machining roughness; ETI may then provide the dominant seed from which MRT grows. First, surface finishing data and results from experiments on the Sandia National Laboratories Z Machine (20 MA in 100 ns) are presented which measure the implosion stability of magnetically accelerated liners that were diamond machined and DC electropolished. Experiments examined Al 1100 and Al 2099 liners. High surface precipitates of aluminum, iron, and silicon where found in Al 1100 samples whereas Al 2099 samples show surface precipitates rich in copper after DC electropolishing. While the targets used for Z experiments are typically diamond turned to 10-30 nm rms surface roughness, the observed MRT amplitude is unexpectedly large. Second, surface finishing data and results from experiments on the U. of Nevada, Reno Zebra Facility (1 MA in 100 ns) are presented. These experiments studied the non-uniform heating of high-purity Al 5N, and Al 6061. When intensely Ohmically heated, Al 6061 alloy, which contains distributed non-metallic inclusions, forms 2-3 times as many surface hot spots as 99.999% pure 5N Al rods (with no inclusions). Various surface finishes were studied, including “as-received” machined, diamond turned, and electropolished post-finished. All post-finishes required a two-step “onion-skin” approach to remove nano layers of material at a uniform rate across a large scale. This approach involved pulse anodizing the targets to form an oxide favoring high ridges created by the cutting tools. Next, the oxide was selectively removed to the aluminum substrate. After removing the oxide, the targets were pulse nano-electropolished using chemistries precisely tailored to the alloy. After the nano-electropolishing step, surface profiles were optically characterized using 3D interferometry. The “post-finish” process was repeated until machining defects were minimized. This work was funded in part by Sandia’s LDRD program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.