Pre-Initiation Features Investigation and Helical Structure Formation in Solid Metal Liner Z-Pinches
The work in this dissertation has focused on two phenomena that occur in a solid metal liner Z-pinch. The first involves features that develop before the liner’s surface ionizes and turns into plasma. The second concerns the formation of a helical structure in the liner when an external axial magnetic field is applied throughout the liner. We experimentally investigated micrometer scale features that develop in thin metallic liners driven by the 1 MA COBRA pulsed power machine. These features are associated in the literature with possible non-ionized matter electrothermal and electrochoric instabilities [Phys. Plasmas 22, 102701 (2015)]. To study the development of the micrometer structure, we first needed to optimize the X-pinch X-ray diagnostic for such high-resolution measurements in cylindrical liner experiments. We show images from two types of detectors, the SR image plate and the DR-50 film, demonstrating that using film is critical for the detection of the micrometer structure in thin Ti, Ni and Cu liners. Using that X-pinch diagnostic, we first examined the 17 - 25 µm features that develop in 16 µm thick Al liners without dielectric coatings. Results of areal density variation and the dominant wavelength of these features are compared with computer simulations using an extended MHD computational model with recently implemented Al Equation-of-State and resistivity models. Experimental results obtained with Al, Ni, Cu and Ti show the average feature size of the perturbation decreases in these materials in the order given. When applying a dielectric coating at the surface of the material, we demonstrate that expansion inhibition correlates with significantly reduced areal density variation and size of perturbations. We additionally show evidence that the insulator is predominantly heated by conduction, as opposed to radiation. Finally, we show that ingrained structure in our liner material a) influences the small scale features’ structure, changing the azimuthal correlation of the features from highly to weakly correlated depending on the orientation of current flow with respect to the material’s ingrained pattern and b) changes the density perturbation amplitude quantitatively. Beyond micrometer scale features, we also investigated the formation of a 600-750 µm wavelength helical pattern in the liner when viewed with extreme ultraviolet self-emission imaging on COBRA. The magnetic field in these experiments was created using either twisted return current wires positioned close to the liner, generating a time-varying Bz, or a Helmholtz coil, generating a steadystate Bz. We show that an upward external axial magnetic field generates a lefthanded twist of the helical pattern and a downward field a right-handed twist. We further show that the helix angle does not correspond to Bz-applied/Bθ(t) , where Bθ(t) is the expected Bθ at the time of measurement, and briefly present a few proposed explanations for the behavior. We conclude that section by proposing an evidence-based explanation as to why the self-emission diagnostics detect any pattern at all, whether helical or axial.
COBRA; Electrothermal; Helical; Liner; Z-pinch; Plasma physics
Hammer, David A.
Lovelace, Richard V. E.; Kusse, Bruce Raymond; Seyler, Charles Eugene
Electrical and Computer Engineering
Ph. D., Electrical and Computer Engineering
Doctor of Philosophy
dissertation or thesis