Hot Cathode Biasing Experiments in Helical Systems

Electrode biasing experiments under electrode current control were carried out in the Tohoku University Heliac and the Compact Helical System to examine the role of an ion viscosity on a transition to a high-confinement regime and to investigate the dependence of the ion viscosity on magnetic structure. Observations included (a) an increase of electron density, (b) an increase of electron stored energy, (c) a formation of the steep gradient of electron density, and (d) a formation of a negative electric field in both devices during electrode biasing negatively. The dependence of the ion viscosity normalized by the ion pressure on the poloidal Mach number qualitatively agreed with the neoclassical theory based on the Shaing model. This result supported the transition mechanism of the neoclassical theory based on ion viscosity, which advocates that the transition to a high-confinement mode is the bifurcation phenomenon resulting from the existence of local maximum in ion viscosity.