Experimental Examination of Plasma Formation and Current Loss in Post-Hole Convolutes.

In High Energy Density Physics, high current (10^7 A) pulses are applied to create states of matter that would be otherwise difficult, if not impossible, to achieve. Pulsed power drivers, which produce the high current, typically utilize vacuum transmission lines to deliver the electrical pulse to the load. In order to achieve the desired peak current and risetime, several transmission lines are often used in parallel; currents from the transmission lines are summed at the load using a post-hole convolute. Post-hole convolute designs have not changed significantly over the last 30 years despite the considerable increase in the power passing through them. After recent upgrades to Sandia’s Z-Machine (the world’s largest pulsed power driver), the efficiency of the convolute decreased to an unacceptable level. Particle-In-Cell (PIC) simulations of the system appear to accurately describe the losses observed, but are not yet benchmarked to be predictive. In this dissertation, experimental investigations of plasma formation within a post-hole convolute have been conducted both on a university scale accelerator (~ 1 MA, 100 GW, Linear Transformer Driver or LTD) and on the Z-Machine (~ 20 MA, 100 TW). The small-scale experiments conducted on the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) showed good agreement with PIC simulations and were useful in developing diagnostic techniques. Experiments conducted on the Z-Machine characterized the plasma timing, composition and density for comparison to simulations. A localized (LiF) dopant was used to confirm the location of plasma optical emission in these experiments. Time-resolved spectroscopic measurements of the plasma emission measured the plasma density as a function of time for several different experimental loads. Convolute plasma density increases more quickly and reaches a higher peak value when the electric field in the convolute increases. Electron densities of 10^18 cm^-3 were inferred from broadening of the hydrogen Balmer series spectral lines observed in absorption. It is expected that these experimental data will be utilized to benchmark PIC codes so new convolute hardware designs can be validated in simulations before being fielded experimentally.PhDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/84505/1/mrgomez_1.pd