Effects of Pre-Ionization on the Gas-Puff Z-Pinch

Abstract

This dissertation presents: (1) the development of a gas-puff z-pinch system for the MAIZE pulsed power facility; (2) experiments carried out to characterize the system’s performance on MAIZE; and (3) experiments to test the effects of pre-ionization, if any, on the implosion dynamics, energetics, and current distribution.

Z-pinches rely on the axisymmetric compression of a plasma by driving a current through its axis, a process which can result in efficient x-ray and neutron production. Understanding z-pinches is crucial to studies in nuclear fusion, materials properties, radiation science, and laboratory astrophysics. Gas-puff z-pinches, in particular, rely on the compression of a puff of gas delivered by a nozzle and fast-valve assembly. This dissertation describes the modeling, designing, and construction of such an experiment. The hardware creates a z-pinch target with two concentric hollow shells: an outer-shell with inner radius of 2.0 cm and outer radius of 2.2 cm, and an inner-shell with inner radius of 0.9 cm and outer radius of 1.0 cm. A central solid-filled jet can be added in the future. This experiment is driven by the MAIZE linear transformer driver, which is a 1-MA, 200-ns class current pulse generator. Diagnostics tools fielded include an interferometer, a Rogowski current probe, B-dot current probes, a 12-frame visible light imager, a 4-frame extreme ultraviolet light imager, polycrystalline-diamond x-ray detectors, a bolometer, bubble detectors, and a beryllium probe activation detector.

This dissertation then describes a series of experiments carried out to benchmark the performance of the newly built and integrated experimental platform. In particular, argon and deuterium shots were carried out at different densities in order to establish optimal pinch performance. It was found that when using argon gas for inner-shell-only experiments, total x-ray yields of up to 720 J were observed with strong K-shell signals. When using deuterium gas for inner-shell-only experiments, total neutron yields of up to 4.9 × 10^8 neutrons were observed. Outer-shell-only experiments were less prolific, in terms of both x-ray and neutron output. Total x-ray yields of only up to 420 J with weak K-shell signals were observed with argon gas. Total neutron yields of only up to 1.8 × 10^7 neutrons were observed with deuterium gas. It was also observed that the inner shell was useful in stabilizing the outer shell against instability growth when both were pinched simultaneously in a given experiment (i.e., a shell-on-shell stabilized implosion).

With an understanding of the performance of the gas-puff z-pinch, tests were carried out to study the effects of applying a separate pre-ionization step. Pre-ionization was seeded by emissions from a thin, velvet-tipped copper wire when pulsed with a high voltage. For both inner-shell-only and outer-shell-only experiments with argon gas, it was observed that pre-ionization did not impact current delivered to the pinches early in time, but it did increase current flowing through the pinch at late times. For the outer-shell-only experiment with deuterium, it was observed that pre-ionization resulted in improved current delivery to the pinches and increased neutron yields. By contrast, for inner-shell-only experiments with deuterium gas, it was observed that pre-ionization resulted in increased current losses and decreased neutron yields.