Studies on Mixed Slab-Toroidal Electron Temperature Gradient Mode Instabilities in the Columbia Linear Machine

Abstract

Studies on Mixed Slab-Toroidal Electron Temperature Gradient Mode Instabilities in the Columbia Linear Machine Abed Balbaky This thesis investigates the behavior of electron temperature gradient (ETG) driven instabilities in the Columbia Linear Machine (CLM). Building on prior work in CLM, the primary goal of this research is to produce, identify, and illuminate the basic physics of these instabilities, and explore the behavior of these instabilities under the presence of trapping and curved magnetic field lines. The first part of this thesis is focused on studying the saturated ETG mode, and the general behavior of the mode under varying levels of magnetic curvature. Measuring ETG modes can be problematic since they have large real frequencies, fast growth rates (~MHz) and small spatial scales, but carefully designed probe diagnostics can overcome these limits. In order to produce curved magnetic field lines, we modified CLM to operate with an internal movable mirror coil. We determined the temperature and density profiles under varying curvature, and measured changes in the mode structure and frequency. We found small changes in the azimuthal/poloidal structure and frequency, characterized by an increase in the m-number (mslab~10-13 and ∆m~1), along with small changes in the axial/toroidal structure (k∥,curvature < k∥,slab) and frequency (ωcurvatue < ωslab). We also present one of the first experimental scaling of ETG mode amplitude as a function of curvature. Our key finding was a that overall levels of saturated ETG mode amplitude had a modest increase (~1.5x) which is slightly larger than existing theory and simulations would predict, and that the power density and amplitude of individual mode peaks can increase more dramatically (~2-3x amplitude). The second part of this thesis studies the radial transport for saturated ETG modes in CLM. ETG modes are believed to be a significant source of anomalous electron energy transport in plasmas, and a better understanding of these modes and the transport they drive across magnetic field lines is of particular interest for advanced tokamaks and future fusion reactors, where these is a continued push for energy efficiency. A specially designed triple probe has been developed, which can measure fluctuations in temperature and potential simultaneously, with a high frequency and special resolution suitable for ETG studies. We present an experimental scaling of radial transport as a function of magnetic field curvature, again one of the first of its kind. Our findings indicate a modest increase in radial transport (~2x) with increased curvature, but unlike saturated mode amplitudes, we find that radial transport saturates for higher levels of curvature in CLM.