A Parametric Study Of Polarity, Contact And Current Risetime Effects On Wire Array Z-Pinches
Data presented are the culmination of experiments studying the combined effects of polarity, contact and current risetime on wire array z-pinches operated on the 1 MA, 100-200 ns Cornell University COBRA pulsed power generator. Data were collected on 16-wire, 12.5 mu-m phi aluminum z-pinch arrays in both negative and positive polarity, with soldered and non-soldered contacts, and with slow and fast risetimes (100ns and 200ns). A minimum of five shots were collected for each of the eight cases to obtain statistical significance. The initiation, ablation, implosion and stagnation phases were compared for the various cases. Polarity was modified via a 12 nH bolt-on convolute, rotatable to provide positive or negative polarity with similar inductance in each case, allowing sideby-side comparisons. Contact was modified by applying Pb60Sn40 solder to the cathode wire-electrode contact point or leaving the array unsoldered. The current risetime was varied between a 100 ns "short" pulse and 200 ns "long" pulse by utilizing current-pulse shaping capabilities of the COBRA pulsed power generator. These experiments aimed to build upon previous research by Duselis et al. (2004), which showed that polarity, contact and current risetime have significant impact on energy deposited and uniformity of expansion of single wires under low-current ( less than 10 kA) conditions. These low current experiments were designed to mimic the conditions on a single wire of a wire-array z-pinch during initiation, also known as the resistive voltage phase. Experiments presented here aim to build upon this research by establishing a better understanding of the effects of polarity, contact and current risetime on the initiation, ablation, implosion and stagnation phases of wire array z-pinches, and also to understand the utility of low current, single wire results as they pertain to higher current, wire array z-pinches. The data were collected in two separate experimental runs: a preliminary dataset with only contact and current risetime as the variables (four distinct cases), and a second, full dataset examining polarity, contact and current risetime (eight distinct cases). The first dataset showed a significant improvement to long pulse shots when the wires were soldered to the cathode, but no improvements to short pulse shots. Long pulse, soldered shots from this dataset were also observed to exhibit the "Christmas tree" effect, or time-accelerated pinch dynamics at the cathode and axial flow towards the anode. The second dataset showed that, with short pulses only, operating arrays in positive polarity resulted in a 25% improvement in deposited energy, 30% smaller achieved current radius, 20% improvement in peak x-ray power and a 50% improvement in x-ray yield when compared to negative polarity arrays. Soldering contacts showed a 10% improvement in initiation energy deposited in positive polarity only, but no subsequent correlated improvement in x-ray yield. Short pulse shots showed a ten-fold increase in peak x-ray powers and yield over long pulse shots. The two datasets were compared, and recommendations for future work were developed.
dissertation or thesis