Formation of edge transport barrier in the ergodic field layer of helical divertor configuration on the Large Helical Device

On the Large Helical Device (LHD), low to high confinement (L–H) transition and edge transport barrier (ETB) formation were observed in the low beta regime (⟨βdia⟩ < 1%, ⟨βdia⟩: volume-averaged beta derived from diamagnetic measurement) as well as in relatively high beta regime (>1.5%). In most of ETB plasmas electron density preferentially increases in the edge region without a substantial rise of the edge electron temperature. The ETB zone develops inside the ergodic field layer calculated in the vacuum field. The ETB formation strongly destabilizes edge coherent modes such as m/n = 2/3 or 1/2 (m, n: poloidal and toroidal mode numbers), because the plasma edge region is in the magnetic hill. The ETB is partially destroyed by the combination of these edge MHD modes and ELM-like activities. For a particular experimental condition, the forced generation of a sizable m/n = 1/1 magnetic island near the edge by application of external field perturbations facilitates the L–H transition at a lower electron density and suppresses edge MHD modes and ELM-like activities to lower levels.