Interpretation of EUV spectra in the 20 nm region from tungsten ions observed in the LHD

We have measured extreme ultraviolet (EUV) spectra in the 20 nm region from tungsten ions in plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science. The spectra are recorded by a grazing incidence spectrometer following an injection of a tungsten pellet. A quasicontinuum feature has been observed in the 20 nm range under low temperature conditions below 1.0 keV, where large contributions of charged states lower than W27+ are expected. In order to give an insight into this spectral feature, we have carried out atomic structure calculations for W7+-W27+ using Cowan's code. The calculated distributions of wavelengths and gA values suggest that the emission largely arises from n = 5-5 transitions in stages lower than W27+.