Investigation of triggering mechanism of internal transport barriers on the Alcator C-Mod tokamak using Thomson scattering diagnostic

Abstract

Internal transport barriers (ITBs) in tokamak plasmas are characterized by the reduction of transport in one or more of the particle, momentum, or energy channels in the core plasma region. On Alcator C-Mod, significant contributions to ITB studies were made possible with the core Thomson scattering (TS) diagnostic, which measures profiles of electron temperature (0.03 </= Te[keV] </= 10) and density (0.05 </= ne[1020m-3] </= 5) with ~1 cm resolution in the ITB region. For the transport analysis purposes, TS profiles are complemented by measurements from other diagnostics and fitted with smooth curves. This thesis research examines the plasma conditions necessary to trigger ITBs on C-Mod. ITBs can be routinely produced in C-Mod steady enhanced D[alpha] (EDA) high-confinement (H-mode) plasmas by applying off-axis ion cyclotron range of frequencies (ICRF) heating (Ir/al </= 0.5). They are observed primarily in the electron particle channel and are marked by the steepening of the density and pressure profiles. ITB formation in C-Mod appears to be the result of competition between an inward particle pinch and the outward diffusion caused by various fine scale plasma instabilities. Several experiments were performed on C-Mod to verify whether ITB formation can be explained within a paradigm of marginal stability. Analyses of the temperature profiles reveal that the Ti profile widens and the Te profile exhibits flattening in the ITB region when the ICRF resonance is moved off-axis. Transport and gyrokinetic stability analyses demonstrate that reduction of the temperature gradient suppresses the temperature gradient driven instabilities. Nonlinear gyrokinetic simulations of plasma microturbulence show that this results in a significant reduction of the outward diffusion, and allows the inward particle pinch to dominate, peaking the density profile.