Characteristic Features of Edge Transport Barrier Formed in Helical Divertor Configuration of the Large Helical Device

In a helical divertor configuration of the Large Helical Device (LHD), transport barrier was formed through low to high confinement (L-H) transition in the plasma edge region including ergodic field layer of which region is in the magnetic hill. The plasma stored energy or the averaged bulk plasma beta (derived from diamagnetic measurement) starts to increase just after the transition. In the case that both and line-averaged electron density at the transition are relatively high as ≥1.5% and ≥2x10m, the increase is hampered by rapid growth of edge MHD modes and/or small ELM like activities just after the transition. On the other hand, the transition at lower (≤ 1.5x10 m) and (< 2%) leads to a continuous increase in the stored energy with a time scale longer than the global energy confinement time, without suffering from these MHD activities near the edge. The ETB typically formed in electron density profile extends into ergodic field layer defined in the vacuum field. The width of ETB is almost independent of the toroidal field strength from 0.5T to 1.5T and is much larger than the poloidal ion gyro-radius. When resonant helical field perturbations are applied to expand a magnetic island size at the rational surface of the rotational transform ι/2π=1 near the edge, the L-H transition is triggered at lower electron density compared with the case without the field perturbations. The application of large helical field perturbations also suppresses edge MHD modes and ELM like activities.