Study of Edge Plasma Characteristics at H-mode Transition in Heliotron J

Characteristics of spontaneous transition to a high confinement mode (H-mode) have been studied in a helical-axis heliotron device, Heliotron J. The radiation profile measured with a silicon photodiode array shows that a interesting change in the profile occurs at the plasma edge region in the transition phase, forming a strong gradient near the last closed flux surface (LCFS). Relating to the transition, characteristics of bursty density and particle-flux fluctuations current in the scrape off layer (SOL) have been investigated. The probability distribution function for the fluctuations has a tail structure in the L-mode phase, while the tail is suppressed and its direction is reversed after the transition, which may be related to the suppression of edge-plasma turbulence.