EUV spectra from highly charged tin ions observed in low density plasmas in LHD

In order to meet the increasing needs for experimental databases of tin (Sn) extreme ultraviolet (EUV) spectra relevant to the next generation EUV lithography, we have measured EUV spectra from highly charged tin ions in low density plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science. A small amount of tin was introduced into a background high temperature and low density hydrogen plasma by injecting a pellet. The EUV spectra were monitored by a grazing incidence spectrometer whose wavelength resolution is about 0.01 nm. Two different types of spectral features were found in the LHD depending on the discharge condition. The well known dense spectral structure around 13.5 nm is measured when the plasma is rapidly cooled and approaching radiative collapse after the pellet injection, while the sparse spectrum with several unidentified discrete lines from 13.8-14.6 nm is observed if the plasma is cooled more slowly and higher temperature is maintained for a while. According to comparison with other charge-separated experimental data, the dominant charge states in the former case are Sn¹¹⁺-Sn¹⁴⁺. The latter case may be explained by considering the spectral lines from charge states higher than Sn¹⁹⁺.