Azimuthal wire motion and ablation dynamics in z -pinches
This dissertation describes theoretical, simulation, and experimental work to study ablation dynamics and azimuthal 'clumping' of wires in multi-wire z-pinch arrays.
First, a resistive-inductive model of discrete filamentary conductors is developed to model wires in multi-wire z-pinches. These equations have been shown to decrease computation times over similar models in literature by as much as 5 orders of magnitude. From these new equations, a simulation code called REIN (REsistive-INductive) was developed to simulate discrete wire arrays.
REIN simulations revealed that wires 'clumped' azimuthally when an initial random azimuthal perturbation was placed on the wire positions. Theory was developed to describe the observed clumping. The fastest growing clumping mode was found to be the pairing of neighboring wires (π-mode).
Experiments were conducted by the author on the COBRA accelerator (1 MA, 100 ns risetime) at Cornell University to look for azimuthal wire clumping in closely spaced wires. Radiography of wire cores did not demonstrate azimuthal wire clumping. However, based on the lack of motion, the major conclusion of the experiment is that less than 7% of the total current was flowing in the wire cores. Another calculation indicated that for wire cores to remain unvaporized by resistive heating, less than 1% of the current could have flowed in the cores. The other 93--99% of the current must therefore have flowed in the coronal plasma. Results also indicated presence of axial non-uniform ablation.
Experiments performed at the University of Michigan on the MIZ-3 (9-10 kA, 400 ns risetime) and MIZ-4 (18 kA, 150 ns risetime) facilities are also described. On MIZ-4, plasma electron temperatures of ablated 30 μm Al wires were measured by emission spectroscopy to be 1.5-2 eV. Comparing results to previous experiments, it is observed that electron temperature scales very weakly with current (more than 10 times increase in current amplitude resulted in less than 2 times increase in electron temperature). On MIZ-3, more intense optical emission from wires preheated with DC currents indicated greater energy deposition than in slightly preheated wires.
0759: Fluid dynamics