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

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 W8+-W27+, 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 W20+ in the Extreme Ultraviolet wavelength region in the Large Helical Device (LHD). We conducted Collisional-Radiative (CR) modeling for W17+-W25+. Two pseudo-continuum peaks corresponding to 5 s-5p transition and transition between doubly excited states of 5 s2-5s5p, are strongly emitted around 200 Å for each ion. The synthesized spectrum of W17+-W25+ 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.