Analysis of Radial Electric Field Bifurcation in LHD Based on Neoclassical Transport Theory

Abstract

Radial electric field (E_r) properties in LHD have been investigated based on the neoclassical transport theory and have also been applied to LHD experimental results. The effects of the helicity of the magnetic configuration on the condition required to realize the electron root are examined. The larger helicity makes the threshold temperature lower for the same electron density. A higher threshold temperature is anticipated to be required in the plasma core region based on this fact and also due to the larger density there. This high electron temperature (T_e) has been successfully obtained with a centerfocused ECH. There is a threshold for the ECH power to achieve a steep gradient of T_e, and it seems to be qualitatively consistent with the transition of E_r, at least in the sense that the abrupt increase of T_e occurs after entering the anticipated electron root regime. These experimental results, consistent with those of analysis of the neoclassical ambipolar E_r, indicate that the transition phenomena of E_r in LHD are predominantly governed by neoclassical features.