Interpretation of spectral emission in the 20 nm region from tungsten ions observed in fusion device plasmas

We have measured extreme ultraviolet spectra from tungsten ions in the 20 nm region in plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The spectra following injection of a tungsten pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer. A quasicontinuum spectral feature arising from an unresolved transition array (UTA) was observed around 20 nm in plasmas with temperatures below 1.0 keV. This structure is reasonably considered to be the same as observed in another tokamak device or laser-produced plasmas under low-temperature conditions. Atomic structure calculations have been performed for tungsten ions with open 5p, 5s and 4f subshells (W super(7+)-W super(27+)) to interpret the observed feature around 20 nm. Wavelengths and strengths for these transitions were calculated, and mean wavelengths and extent of the UTAs were compared with the observations, which suggests that the emission largely arises from n = 5-5 transitions in stages lower than W super(27+).