Observation of the low to high confinement transition in the large helical device

The low to high confinement transition has been observed on the large helical device [A. Iiyoshi, A. Komori, A. Ejiri et al. , Nucl. Fusion 39, 1245 (1999)], exhibiting rapid increase in edge electron density with sharp depression of H α emission. The transition occurs in low toroidal field ( B t = 0.5 – 0.75 T ) discharges and are heated by high power neutral beam injection. The plasma thus has a relatively high value ( ∼ 1.5 % ) of the volume averaged β value. The electron temperature and density profiles have steep gradients at the edge region which has high magnetic shear but is at a magnetic hill. Formation of the edge transport barrier leads to enhanced activities of the interchange type of modes with m = 2 ∕ n = 3 ( m , n are the poloidal and toroidal mode numbers) in the edge region. At present, these magnetohydrodynamic activities limit the rise of the stored energy; the resultant increment of the stored energy remains modest.