Analysis of EUV spectra of Sn XIX–XXII observed in low-density plasmas in the Large Helical Device

Abstract

We have measured extreme ultraviolet (EUV) spectra from highly charged tin ions in low-density and high-temperature plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The EUV spectra emitted after injection of a tin pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer whose wavelength resolution is about 0.01 nm. Two different types of spectral feature were measured in the 13–17 nm region depending on whether the discharge was stably sustained or underwent radiation collapse. The measured EUV spectra were analyzed by considering the difference in dominant charge states observed in the two types of spectra. Apart from the complex quasi-continuum structure around 13.5 nm, several strong lines occurring at wavelengths longer than 14 nm were found to originate from the transitions between excited states of Sn XXI and Sn XXII by comparison with the other experimental data including the charge exchange collisions experiments. Most of the strong spectral lines which were not identified as Sn XXI or Sn XXII were assigned to the resonance transitions of Sn XIX and Sn XX from comparison with the results of theoretical calculations.