Identification of forbidden emission lines from highly ionized tungsten ions in VUV wavelength range in LHD for ITER edge plasma diagnostics

Abstract

Abstract Magnetic-dipole (M1) lines of W29+~W39+ were successfully observed in the vacuum-ultraviolet (VUV) wavelength range of 500–900 A in the Large Helical Device (LHD). Tungsten ions are distributed in the NBI-heated LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. It is found that W37+ line at 646.7 A and W38+ lines at 532.9 A and 559.1 A are emitted when the central electron temperature ranges from 2 to 3 keV with relatively high intensity and isolated from other intrinsic impurity lines. The result is the first observation of the M1 forbidden lines of tungsten ions in the VUV wavelength range in fusion plasma experiments. Observation of the W37+ and W38+ lines could improve the tungsten diagnostics in ITER edge plasmas because they are expected to be distributed in the edge plasmas around the last closed flux surface including the scrape-off layer in ITER.