Observation of line emissions from Ni-like W46 + ions in wavelength range of 7–8 Å in the Large Helical Device

Abstract

Tungsten W⁴⁶⁺ lines were successfully observed in the extreme ultraviolet (EUV) wavelength range of 7~8 Å in the Large Helical Device (LHD). Tungsten ions are distributed in the neutral beam injection (NBI) heated LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. While the electron temperature has a sudden drop due to the pellet injection, it can be recovered by applying electron cyclotron heating (ECH) together with continuous NBI heating. It is found that a W⁴⁶⁺ line at 7.93 Å is emitted when the central electron temperature ranges around 3.4 keV with relatively high intensity and is isolated from other intrinsic impurity lines. The 7.93 Å line consists of two lines of forbidden transitions which are blended with each other; an electric quadrupole (E2) transition at 7.928 Å and a magnetic octupole (M3) transition at 7.938 Å. Observation of W⁴⁶⁺ lines in the stellarator experiments is reported for the first time in this paper while the lines have been already observed in several tokamak experiments. The electron temperature dependence of the emission intensity of the 7.93 Å line agreed well with that of the fractional abundance of W⁴⁶⁺ ions calculated using the ionization and recombination rate coefficients available in the ADAS database under the assumption of the collisional ionization equilibrium. The 7.93 Å line observed in this study will be used as tools for further spectroscopic researches, such as the measurements of spatial profile of W⁴⁶⁺ ions using a space-resolved EUV spectrometer developed in LHD.