Direct Observation of the M1 Transition between the Ground Term Fine Structure Levels of W VIII

We present a direct observation of the M1 transition between the fine structure splitting in the 4f135s25p6 2F ground term of W VIII. The spectroscopic data of few-times ionized tungsten ions are important for the future ITER diagnostics, but there is a serious lack of data. The present study is part of an ongoing effort to solve this problem. Emission from the tungsten ions produced and trapped in a compact electron beam ion trap is observed with a Czerny–Turner visible spectrometer. Spectra in the EUV range are also observed at the same time to help identify the previously-unreported visible lines. The observed wavelength 574.47±0.03 nm (air), which corresponds to the fine structure splitting of 17,402.5 ± 0.9 cm −1 , shows reasonable agreement with the previously reported value 17,410 ± 5 cm −1 obtained indirectly through the analysis of EUV spectra [Ryabtsev et al., Atoms 3 (2015) 273]