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 the present study 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.
Highlights
Tungsten (W) is regarded as a possible candidate material for the plasma facing component (PFC) in divertor region of ITER and future fusion reactors because of its high melting point, low sputtering yield, and low tritium retention [1,2,3]
These unresolved transition array (UTA) spectra are clear signals indicating that the W ions are successfully distributed in the plasmas by the pellet injection, which have been observed in many fusion plasma experiments related to W impurity studies [13,14]
The 7.92 Å line observed in this study is considered to consist of two lines of forbidden transitions which are blended with each other; an E2 transition at 7.928 Å and an M3 transition at 7.938 Å
Summary
Tungsten (W) is regarded as a possible candidate material for the plasma facing component (PFC) in divertor region of ITER and future fusion reactors because of its high melting point, low sputtering yield, and low tritium retention [1,2,3]. Fractional abundance of W ions for each charge state calculated using the ionization and recombination rate coefficients registered in the ADAS database [4]. More integration of the spectroscopic data in the fusion plasma experiments is required to proceed with a comparison with the results of the electron beam ion trap (EBIT) experiments [10] and the atomic modelling calculations [11] in order to investigate characteristics of the W46+ line emissions more precisely. The EUV wavelength spectra and the temporal evolution and the electron temperature dependence of the intensities of the W46+ line will be investigated together with emission from tungsten ions in neighboring charge states such as W41+~W45+ observed simultaneously in a single discharge to ensure the line identification
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