Ergodic edge region of large helical device

Ergodicity in various vacuum magnetic configurations in large helical device was quantitatively estimated, using the Kolmogorov length as a measure. It is found that the edge electron temperature profile changes its gradient at the position where the ergodicity begins to increase several cm outside the last closed flux surface (LCFS). In the profiles no distinguished change of plasma parameters was seen at the LCFS, which suggests the existence of a region just outside the LCFS where the confinement performance is relatively as good as that in the closed region. The radial electron heat conductivity was also estimated using a simple energy balance equation and it was confirmed that the conductivity is strongly affected by the ergodicity. Using the relationship between ergodicity and electron heat conductivity, a new practical definition of the LCFS based on the experiments is proposed. (C) 2003 Elsevier Science B.V. All rights reserved.