Particle Transport and Fluctuation Characteristics around the Neoclassically Optimized Configuration in LHD

Abstract

Density profiles in LHD were measured and particle transport coefficients were estimated from density modulation experiments in LHD. The dataset used in this article included a wide range of discharge conditions, e.g., for different heating powers, magnetic axes, and toroidal magnetic fields scanned to cover wide regions for neoclassical transport. The minimized neoclassical transport configuration in the dataset (R_ax = 3.5 m, B_t = 2.83 T) showed peaked density profiles, and its peaking factors increased gradually with decreasing collisionality. These results are similar to those observed in tokamaks. At some other configurations, peaking factors were reduced with decreasing collisionality and a larger contribution of neoclassical transport produced hollow density profiles. Comparison between neoclassically and experimentally estimated particle diffusivities showed different minimum conditions. This suggests that the condition for neoclassical optimization is not the same as that for anomalous optimization. A clear difference in spatial profiles of turbulence was observed between hollow and peaked density profiles. A major part of the fluctuations existed in the unstable region of the linear growth rate of the ion temperature gradient mode and trapped electron mode.