Improvement of Plasma Core Confinement Via Electron-Root Realization by Strongly Focused ECRH in LHD: Core Electron-Root Confinement

Core electron-root confinement (CERC), observed in the Large Helical Device as well as in other helical devices, is an improved electron energy confinement mode. It is characterized by a highly peaked electron temperature profile in the core region and appears when the centrally focused electron cyclotron resonance heating power exceeds a certain threshold value. This threshold value has been clarified to associate with the transition of the radial electric field (E_r) from the ion root (small negative value) to the electron root (large positive value greater than a few kV/m), based on the bifurcation nature of E_r due to the ambipolarity condition of neoclassical transport fluxes that is specific in nonaxisymmetric configurations. It has been experimentally recognized that a steeper T_e gradient is realized with a clear transition (power threshold nature) in target plasmas with counter neutral beam injection (NBI) than ones with codirectional NBI. It has been interpreted, based on the heat pulse propagation experiment, to be related to the rational surface or the island induced by the NBI-driven current. Transport analyses have shown that the incremental thermal diffusivity of electron heat transport becomes lower, and the standard thermal diffusivity decreases with the increase of heating power in CERC plasmas.