Alcator C-Mod

Rachael McDermott

In 1982 the H-mode (high mode) was discovered on the ASDEX tokamak and has since become a very active and important area in fusion research. The H-mode is a plasma operation regime characterized by the suppression of edge turbulence, the formation of an edge transport barrier just inside the separatrix, and the resultant approximate factor-of-two improvement in energy confinement time. The H-mode has attracted a lot of attention from both the experimental and theoretical fusion communities and several different theories have been proposed to explain its formation. However, our understanding of the physics of the H-mode is far from complete and requires detailed experimental measurements to place constraints on the proposed theories. Future fusion devices will require H-mode confinement in order to reach their operational goals thus driving the community's need to understand and control the H-mode transition.

The goal of this thesis project it to measure poloidal and toroidal ion velocity and temperature profiles near the edge of Alcator C-Mod via active charge exchange with a diagnostic neutral beam using the Boron+4 7-6 line. Charge exchange is a process by which an electron is transferred from a neutral atom to an ion. The ion is left in an excited state and subsequently de-excites to a lower energy level and emits a photon.  The spectrum obtained yields information about the population of emitting ions. Specifically, the Doppler width and shift of the spectral line give temperature and velocity information and the absolute intensity of the line provides information on the density of the emitting ions. This information will be used to study the transition between L-mode (low mode) and H-mode and also enable us to study the physics and formation of the edge pedestal, which has a direct impact on the core H-mode confinement.