Density Limit Disruptions in the Compact Toroidal Hybrid Experiment

Abstract

Density limit disruptions in toroidal plasma experiments have been an active area of research for decades. Density limit disruptions are plasma terminations that occur as the plasma density is increased. Plasma disruptions in magnetically-confined, current-carrying toroidal devices result in a sudden loss of confinement leading to a rapid drop of the plasma temperature immediately followed by drop of the toriodal plasma current. Heat and particles are expelled to the edge of the plasma at a rate higher than in a normal discharge which can damage the plasma facing components on large experiments and future fusion reactors. Tokamak-like disruptions do not occur in current-free stellarators, where the required rotational transform is produced by currents in external coils, but exists as a radiative collapse. This thesis presents the observations of density limit-induced disruptions in a currentcarrying stellarator, the Compact Toroidal Hybrid (CTH) experiment at Auburn University. In addition, the development of an in situ wavelength calibration system for the x-ray imaging crystal spectrometers (XICS) on the Wendelstein 7-X (W7-X) experiment is described. To further the study of CTH density limit disruptions, new bolometer arrays were installed on CTH. A synthetically trained De-Convolution Neural Network (DeCNN) based inversion method has been developed for the new bolometer arrays to capitalize on the available spatial information. The inverted bolometer fluctuations show spatial and temporal correlations with the poloidal magnetic field fluctuations indicating that the source of the radiation is magnetohydrodynamic (MHD) instabilities. The physics underlying the tokamak-like density limit in CTH plasmas may be ascribed to a radiative instability localized to a specific rational surface. The best predictor of the tokamaklike density limit characteristics across all ranges of vacuum rotational transform was found to be Hugill density limit, not the Greenwald limit. Of important significance, at increasing levels of vacuum rotational transform, the performance of the plasma (i.e. the slow collapse) is not dictated by either Greenwald or Hugill limits but seems to be caused by increased plasma resistivity possibly due to toroidally trapped particles.