Edge plasma phenomena in the Alcator C-Mod tokamak measured by high resolution X-ray imaging diagnostics

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.Includes bibliographical references (p. 203-206).In this thesis high resolution soft x-ray measurements from the Alcator C-Mod plasma edge are presented for a variety of different plasma conditions. These measurements provide radial profiles of the soft x-ray emissivity with 1.5 mm resolution or better, and temporal resolution down to 12 [mu]s. These profiles show a distinct and very narrow pedestal shape in H-mode, indicative of the H-mode transport barrier. The soft x-ray emissivity pedestal at the outboard edge is typically 10 mm inside the last closed flux surface, near the top of the electron density and temperature pedestals. Modelling shows that the inward shift of the x-ray pedestal implies an inward shift of the impurity density pedestal. This inward shift is explained by an inward impurity pinch located in the region of strong electron density gradient, as predicted by neoclassical impurity transport theory. Calculations using the impurity transport code MIST support the existence of a neoclassical-like inward pinch. Changes in the soft x-ray pedestal width can be interpreted as changes in the edge impurity diffusion coefficient. We find several scaling laws of the edge diffusion coefficient with various plasma parameters in EDA H-mode. A second array views the top of the plasma. The x-ray emissivity measured with this array also shows a distinct and narrow pedestal in H-mode. However, it is located significantly closer to the separatrix and is often narrower. Both of these differences increase with the safety factor at the edge, q95 . Thus, there is a significant poloidal asymmetry in the impurity density in the H-mode edge region, which increases with q95 . Therefore, the impurity transport in the H-mode edge is highly two-dimensional. The strong poloidal asymmetries measured show some quantitative agreement with theories developed to explain poloidal impurity asymmetries. However, none of the theories are strictly applicable to the Alcator C-Mod edge, and they all significantly underestimate the actual asymmetries that we observe.by Thomas Sunn Pedersen.Ph.D