Effect of neutral hydrogen on edge impurity behavior in stochastic magnetic field layer of Large Helical Device

Abstract

Two-dimensional (2-D) distribution of impurity line emissions has been measured with 2-D extreme ultraviolet (EUV) spectroscopy in Large Helical Device (LHD) for studying the edge impurity transport in stochastic magnetic field layer with three-dimensional (3-D) structure. The impurity behavior in the vicinity of two X-points at inboard and outboard sides of the toroidal plasma can be separately examined with the 2-D measurement. As a result, it is found that the carbon location changes from inboard to outboard X-points when the plasma axis is shifted from Rax=3.6m to 3.75m. A 3-D simulation with EMC3-EIRENE code agrees with the result at Rax=3.75m but disagreed with the result at Rax=3.60m. The discrepancy between the measurement and simulation at Rax=3.60m is considerably reduced when an effect of neutral hydrogen localized in the inboard side is taken into account, which can modify the density gradient and friction force along the magnetic field.