Investigation of the Compression of Magnetized Plasma and Magnetic Flux

• Main Author: Mikitchuk, Dimitry
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2017
• Subjects: Electromagnetics; Materials science; Nuclear physics; Plasma physics
• ISBN: 9798597008806
• DOI: 10.34933/wis.000238

In this research I investigated fundamental phenomena occurring as magnetic-field flux and magnetized-plasma are compressed by applied azimuthal magnetic fields. This subject is relevant to numerous studies in laboratory and space plasmas. Recently, it has gained particular interest due to the advances in producing plasmas of high temperature and density in experiments based on the approach of magnetized plasma compression [1]. Many in the plasma physics community consider this approach to be the most promising for achieving controlled nuclear fusion. To advance this approach, it is essential to study experimentally the governing physical mechanisms that take place during the compression. Performing the required systematic experiments is impractical in large scale facilities designed for fusion demonstration.In our experiment, we employ a cylindrical (Z-pinch) configuration, in which a current (300 kA, rise time 1.6 µs) driven through a cylindrical plasma causes implosion of the plasma under the self-generated azimuthal magnetic fields (Bθ). However, our cylindrical plasma is initially embedded in an axial magnetic field Bz. The field is quasi-statically applied prior to the high-current discharge, with a value of 0.4 T.Here, for the first time in these research, Zeeman-splitting observations are used to measure the evolution and spatial distribution of Bz and Bθ during the implosion and stagnation stages. The two fields are measured simultaneously, which is rather important due to the irreproducibility that characterizes such experiments of high-current pulses. The difficulties in these measurements are due to: 1) the high electron densities in the plasma giving rise to large Stark broadening that smears out the Zeeman pattern, 2) the difficulty in distinguishing between Bz and Bθ, and 3) the absence of light emission from the center of the plasma column. Indeed, in previous studies, under similar conditions, the B-fields were only indirectly estimated from the plasma radius. These challenges were achieved by employing a novel spectroscopic technique based on the polarization properties of Zeeman split emission, combined with a laser generated doping technique that provided mm-scale spatial resolution.Systematic measurements were performed for different initial conditions of Bz and gas loads. The measurements showed that estimates of the B-fields based on the plasma radius are subjected to large errors and thus unreliable. Indeed, the simultaneously measured Bz and Bθ, together with the plasma radius and the discharge current, showed that the application of an initial Bz has a dramatic effect on the current distribution in the plasma. While without Bz the entire current is found, as expected, to flow through the imploding plasma, when an initial Bz is applied the measured Bθ (through ∇ × B = µ0J) showed that only a small part of the current flows in the imploding plasma. Specifically, when Bz,t=0 = 0.4 T, the value of Bθ in the imploding plasma shell remains nearly constant (between 1.5 and 2 T) during the implosion, even though the current rises and the plasma radius drops. A theoretical model is suggested to explain this unexpected phenomenon. To rigorously test this model self-consistent 3D MHD simulations are required.In addition to these results, the measurements provide much information useful for the understanding of the Bz-embedded plasma implosion. We measure at stagnation a ∼ 15× compression of the initial axial B-field. This compression factor, together with the observed plasma radius allows for obtaining the Bz confinement efficiency, which is found to be ∼ 50%. This information is useful for testing MHD codes. Another phenomenon observed is an axial gradient of Bz in which its magnitude increases by a factor of 2 from the anode (low Bz) to the middle of the plasma column (z ∼ 5 mm, high Bz). This measurement demonstrates the existence of a transition region from the uncompressed Bz = Bz(t = 0) inside the electrodes to the compressed Bz farther away from the electrode surface.The spectroscopic measurements were complemented by 2D images of the plasma self-emission and by interferometric images. These measurements were important both for obtaining the B-field evolution and for the study of the dependence of instabilities on the different initial conditions. The measurements clearly showed the mitigation effect of Bz on the magneto-Rayleigh-Taylor instabilities.The 2D images have also shown the existence of axially directed, filament-like regions that have significantly higher emission than the surrounding plasma. These filaments were found to be plasma regions with higher electron density (by 10−20%), and slightly lower electron temperature (by a few percent) than of the surrounding plasma.