Turbulence and transport phenomena in edge and scrape-off-layer plasmas

Abstract

This thesis reports recent experimental studies of edge turbulence structure using gas-puff-imaging at high time resolution in a radially broad zone at the low-field-side of Alcator C-Mod[1], and highlights the connections between its characteristics and particle transport. Within the framework of this thesis a new detector system has been designed and built for viewing the steep gradient outboard midplane region at minor radii 0.9 < r/rma, < 1.1, where turbulence is postulated to be driven [2, 3]. The design is optimized for the study of edge and scrape-off-layer turbulence, based on previous studies of these regions. Analysis of the data from this diagnostic in Ohmic L-modes from the electron diamagnetic drift propagating turbulence in the closed field line region of the plasma edge shows a strong scaling of the fluctuation power 5/n with the density normalized to the tokamak density limit ne/ng; in addition, a critical physical scale is found at ... ~ 0.1, where the critical wavenumber is normalized to ps, the ion Larmor radius at the sound speed, also known as the drift scale. The characteristic size scale shows very little variation with operating parameters. The analysis of the physical scale includes spectral analysis, yielding robust spectral indices beta= ~4.5 for larger wavenumbers and a sensitive spectral shape for lower wavenumbers, and the first spectral transfer measurement of turbulence on C-Mod, showing that the critical scale feeds power into the turbulence. A connection is found to the quasi-coherent mode (QCM), a modelike edge fluctuation characteristic of the Enhanced D-Alpha H-mode, which also appears at ... ~ 0.1 and has decreased inverse transfer but greatly enhanced forward transfer compared to the L-mode levels.