Effects of rational surfaces and magnetic islands on radial electric fields and ion viscosity in Tohoku university heliac

The influence of the low-order rational surface (n, m) = (5, 3) on ion viscosity and radial electric field formation were investigated by electrode current sweep biasing with a hot cathode at the Tohoku University Heliac. In the improved mode, the position of the maximum electric field remained on the n/m = 5/3 rational surface. After the H-L transition, the local maxima shifted outward to the plasma periphery of ρ ∼ 0.8. The low-order magnetic islands were formed resonating the magnetic Fourier components of (n, m) = (5, 3) by external perturbation coils. As the widths of the magnetic islands were increased, the biasing electrode current required for the improved mode transition increased. It was suggested that the ion viscosity increased according to the increase in magnetic island width. The increase in the biasing electrode current is equivalent to the increase in the driving force for the poloidal rotation. This suggests the possibility of an active viscosity control assisted by externally controlled island width and magnetic island rotation.