Spectroscopic study of impurity behavior in the edge stochastic magnetic field layer of large helical device

Carbon impurity transport in a thick stochastic magnetic field layer called "ergodic layer" located at the edge plasma of LHD was studied for both deuterium (D) and hydrogen (H) discharges by EUV and VUV spectroscopy, and EMC3-EIRENE simulation. By comparing emission intensities from carbon impurity ions in high- and low- charge states, impurity screening is found to be more effective in D plasmas rather than H plasmas. Enhancement of the bulk-impurity collision frequency in D plasmas might be one of the reasons of the effective impurity screening. In high density discharges, the carbon impurity flow in the ergodic layer is directed toward the same direction of the friction force, indicating the presence of the impurity screening. The carbon flow velocity measured by the VUV spectroscopy is smaller in the deuterium plasmas. The difference of the flow velocity was reproduced by the simulation qualitatively. (author)