Magnetic Compression of Compact Tori, Experiment and Simulation
Date
2019-09-16
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
The magnetic compression experiment at General Fusion was a repetitive non-destructive test to study plasma physics applicable to magnetic target fusion compression. A compact torus (CT) is formed with a co-axial gun into a containment region with an hour-glass shaped inner flux conserver, and an insulating outer wall. External coil currents keep the CT off the outer wall (radial levitation) and then rapidly compress it inwards. The optimal external coil configuration greatly improved both the levitated CT lifetime and the recurrence rate of shots with good compressional flux conservation. As confirmed by spectrometer data, the improved levitation field profile reduced plasma impurity levels by suppressing the interaction between plasma and the insulating outer wall during the formation process. Significant increases in magnetic field, electron density, and ion temperature were routinely observed at magnetic compression in the final external coil configuration tested, despite the prevalence of an instability, thought be an external kink mode, at compression. Matching the decay rate of the levitation currents to that of the CT currents resulted in a reduced level of MHD activity associated with unintentional compression by the levitation field, and a higher probability of long-lived CTs. The DELiTE (Differential Equations on Linear Triangular Elements) framework was developed for spatial discretisation of partial differential equations on an unstructured triangular grid in axisymmetric geometry. The framework is based on discrete differential operators in matrix form, which are derived using linear finite elements and mimic some of the properties of their continuous counterparts. A single-fluid two-temperature MHD model is implemented in this framework. The inherent properties of the operators are used in the code to ensure global conservation of energy, particle count, toroidal flux, and angular momentum. The development of the discrete forms of the equations solved is presented. The code was applied to study the magnetic compression experiment. The numerical models developed to simulate CT formation, levitation, and magnetic compression are reported. A model for anisotropic thermal diffusion has been formulated and implemented to the code. A method for determining the q
profile of the CT was established - simulated CT q
profiles indicate that the magnetic compression experiment could be improved by modifying the q
profile to regimes with increased stability against kink modes. Comparisons between simulated and experimental diagnostics are presented. A model of plasma/neutral fluid interaction was developed and included in the framework. The source rates of species momentum and energy due to ionization and recombination were derived using a simple method that enables determination of the volumetric rate of thermal energy transfer from electrons to photons and neutral particles due to radiative recombination, which has been neglected in other studies. The implementation of the model has enabled clarification of the mechanisms behind the increases in CT electron density that are routinely observed on the SPECTOR plasma injector well after CT formation. This understanding helps account for the exceptionally significant increase in electron temperature, and markedly reduced electron density, observed during the electrode edge biasing experiment conducted on SPECTOR. It is thought that edge fueling impediment, a consequence of a biasing-induced transport barrier, is largely responsible for the observed modifications to temperature and density.
Description
Keywords
Magnetic Compression,
Compact Tori,
Helicity Injection,
Tokamak,
MHD Simulation,
Finite element method,
Conservative numerical schemes,
Plasma/neutral interaction,
H-mode induced by edge biasing
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics