Edge radial electric field studies via charge exchange recombination spectroscopy on the Alcator C-Mod Tokamak

• Main Author: McDermott, Rachael Marie
• Other Authors: Bruce Lipschultz and Ian H. Hutchinson.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2010
• Subjects: Nuclear Science and Engineering.
• Online Access: http://hdl.handle.net/1721.1/54462

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. === Cataloged from PDF version of thesis. === Includes bibliographical references (p. 189-197). === It is commonly accepted that ExB velocity shear is responsible for the suppression of edge turbulence, which reduces the losses of both energy and particles across magnetic field lines and results in the formation of edge transport barriers and high-confinement mode (H-mode) in tokamak plasmas. However, the self consistent evolution of the radial electric field profile (Er), pedestal shape and improvement in plasma confinement is not well understood. A better understanding of pedestal physics and the interplay between Er, turbulence suppression and pedestal formation should enable better control of edge transport and improve core confinement. A new, high-resolution, charge exchange recombination spectroscopy (CXRS) diagnostic has been installed on Alcator C-Mod to provide measurements of the B5+ population in the pedestal region. This diagnostic is capable of measuring the boron temperature, density, and poloidal and toroidal velocity with 3mm radial resolution and 5ms temporal resolution. These profiles, coupled with knowledge of the toroidal and poloidal magnetic fields, enable the determination of the edge radial electric field through the radial force balance equation. The new CXRS diagnostic has provided the first spatially resolved calculations of the radial electric field in the C-Mod edge and has made possible significant contributions to the study of pedestal physics. Detailed measurements of the boron population have been made in a variety of plasma regimes. The measured rotation profiles connect the SOL and core measurements and are consistent with both. The CXRS boron temperature profiles are observed to agree well with the Thomson Scattering electron temperature profiles in bothl shape and magnitude over a wide range of collisionalities. In H-mode plasmas both the boron temperature and density profiles form clear pedestals, similar to what is observed in the electron channel. The edge toroidal rotation increases in the concurrent direction at the onset of H-mode confinement and the poloidal rotation in the pedestal region increases in the electron diamagnetic direction forming a narrow === (cont.) peak (3-4mm) just inside of the LCFS. In Ohmic L-mode plasmas Er is positive near the last closed flux surface (LCFS) and becomes more negative with distance into the plasma. In H-mode plasmas E, is positive in the core, but forms a deep negative well, relative to its L-mode values, just inside of the LCFS. These results are qualitatively consistent with the observations made on other machines. However, the C-Mod H-mode Er wells are unprecedeited in depth (up to 300kV/m) and the narrow E, well widths (5mm), as compareJ to results from other tokamaks, suggest a scaling with machine size. The measured Er well widths have been compared to theoretical scalings for the edge pedestal and no significant correlation was observed with any of the predictions. In fact, very little variation of the E, well width is observed in general. Howc:ver, the depth of the E, well, or alternatively the magnitude of the E, shear (constant width), shows a strong correlation with improved plasma energy confinement. It also correlates well with the edge electron temperature and pressure pedestal heights (and gradients). It is not, however, very sensitive to variation in the edge electron density pedestal height. These results are an indication that the energy and particle transport have different relationships to Er, with energy transport more directly linked. The radial electric field results from ELM-free H-mode and I-mode plasmas support this interpretation. === by Rachael Marie McDermott. === Ph.D.