Analyses of the multi-step sawtooth crash behavior in the highly-elongated tokamak plasmas with the soft x-ray tomographic diagnostics

Abstract

Development of the thermonuclear fusion, a quasi-unlimited energy source suitable for large scale electricity production, has been one of the main goals of plasma physics research for decades. Among the many fusion devices explored to achieve the goal, the use of toroidal device called tokamak to create and confine the hot plasmas for a sufficiently long period using strong magnetic field is particularly promising. In tokamak plasmas, the magnetohydrodynamic instabilities are the important causes degrading the fusion performance, and the development and optimization of diagnostics for observing these plasma instabilities are parts of this thesis work. This thesis work covers the implementation and exploitation of a multi-channel soft x-ray array diagnostic system with high spatial resolution together with the development of the tomographic inversion methods for data analysis. The soft x-ray array system will be installed onto Korea Superconducting Tokamak Advanced Research (KSTAR). The detector position was determined by the condition for optimizing the tomographic performance, and the optimized detector system is used to accurately study the magnetohydrodynamic phenomena of tokamak plasmas. To analyze the line-integrated soft x-ray data, the modified fast maximum entropy method was developed. It was utilized for Tokamak \\`{a} Configuration Variable (TCV) soft x-ray array data for reconstructing the two-dimensional structure of the sawtooth crash. In general, plasmas with elongated cross-sections offer the higher energy confinement as well as the higher plasma current and pressure limits. Since the ohmically heated plasma analyzed in this thesis has a high elongation and a broad emission structure and current density profile, the radius of the $q=1$ surface is large, and the magnetic shear is weak in the core region. Due to this, there exists several steps during a sawtooth crash phenomenon, and the crash time becomes long. The sawtooth crash process reve...