Low concentration of iron as first wall material in LHD plasmas with edge ergodic layer

Impurity behavior of iron, which is a main element of the first wall made of stainless steel, is studied in Large Helical Device (LHD). The total density of iron in core plasmas is found to be fairly low, at least four or five orders of magnitude smaller than the electron density, even if the impurity accumulation occurs. The density of edge Fe15+ ions analyzed from two-dimensional measurement of impurity line emissions in EUV range increases with density. It indicates that the screening effect developed in edge ergodic layer intrinsically existing in LHD works well for the iron. The simulation with EMC3-EIRENE also predicts the impurity screening is more effective for heavier impurities than light impurities. The present result strongly suggests a tolerant use of high-Z materials to the first wall of helical devices. (author)