Collisional-Radiative modeling of unresolved transition array spectra near 200 Å from W17+-W25+ emissions for diagnostics of ITER edge plasma

• Tungsten pellet injection experiment was conducted in the Large Helical Device. • Unresolved transition array including charge states around W 20+ was observed. • Collisional-Radiative modeling of W 17+ -W 25+ has been conducted. • Transitions from 5 s-5p and 5 s 2 -5s5p of W 17+ -W 25+ around 200 Å were identified. Tungsten (W) is one of the major impurities in ITER and future DEMO reactors. However, diagnosing the ion density, temperature, and spatial distribution of tungsten ions in intermediate charge states, such as W 8+ -W 27+ , is difficult because of the lack of spectral line data. In this study, we observed a tungsten Unresolved Transition Array (UTA) spectrum, which has a pseudo-continuum structure, around W 20+ in the Extreme Ultraviolet wavelength region in the Large Helical Device (LHD). We conducted Collisional-Radiative (CR) modeling for W 17+ -W 25+ . Two pseudo-continuum peaks corresponding to 5 s-5p transition and transition between doubly excited states of 5 s 2 -5s5p, are strongly emitted around 200 Å for each ion. The synthesized spectrum of W 17+ -W 25+ ions reproduced the observed LHD spectrum near 200 Å. From the electron temperature dependence of the UTA spectral shape, the UTA shifted toward longer wavelength region with several pseudo-continuum peaks appearing for ions in lower charge states, with decreasing electron temperature from 0.4 keV to 0.2 keV. This result qualitatively explained the observed time evolution of the spectrum.