Toroidally non-uniform formation of the edge-transport-barrier by low-to-high confinement transition on the Compact Helical System

In the Compact Helical System, the radial structure of the edge-transport-barrier (ETB) and the characteristics of electrostatic fluctuations were measured for the first time at three toroidally separated locations (the upper side of the vertically elongated section and the outboard and inboard sides in the horizontally elongated section) by using triple Langmuir probes. Electron density (ne) near the plasma edge increased noticeably across the low-to-high confinement (L–H) transition without a clear electron temperature rise. A rapid vertical expansion and outward shift of the formed particle ETB were observed just after the transition. Radial profiles of ne and radial electric field at the inboard of the torus evolved to a peculiar shape across the transition, suggesting the presence of a sizable magnetic island at the rational surface of ι/2π = 1 (ι/2π: rotational transform). Turbulent particle flux derived experimentally was clearly reduced at the upper and outboard locations just after transition, but enhanced at the inboard location. Total turbulent flux passing through a whole LCFS is inferred to be reduced.