Characteristic Formation of Edge Transport Barrier in the Compact Helical System

Abstract

The structure of the Edge Transport Barrier (ETB) was measured by means of a triple Langmuir probe in the Compact Helical System, this diagnostic method achieving high time and spatial resolutions. The radial profiles of electron temperature and electron density show a steep gradient inside the normalized minor radius ρ ˜ 0.96, having a plateau of T_e at 0.96 < ρ < 1 and a dip of n_e at 0.95 < ρ < 0.98. The radial electric field clearly changed in the H-phase in the region at ρ < 0.96 and 0.99 < ρ < 1, and its shear increased around ρ ˜ 0.97. The characteristic profile evolutions suggest an interaction between ETB formation and a magnetic island related to ι/(2π) = 1. Based on these data, the precise position of the ETB is not determined denitely; that is, it is not established whether it exists inside the magnetic island or closely inside the last closed flux surface (ρ = 1).