Direct observation of the M1 transition between the ground state fine structure splitting of W VIII

TL;DR

This study directly observes the M1 transition in W VIII, providing crucial spectroscopic data for tungsten ions relevant to ITER diagnostics, and confirms previous indirect measurements with high precision.

Contribution

First direct measurement of the M1 transition between the fine structure levels of W VIII's ground state, filling a critical data gap for plasma diagnostics.

Findings

Observed wavelength: 574.47 nm with 0.03 nm uncertainty

Fine structure splitting measured as 17402.5 cm^-1

Results agree with previous indirect estimates

Abstract

We present direct observation of the M1 transition between the fine structure splitting in the 4f13 5s2 5p6 2F ground state 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 lack. 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 the identification of the previously-unreported visible lines. The observed wavelength 574.47 pm 0.03 nm (air), which corresponds to the fine structure splitting of 17402.5 pm 0.9 cm-1, shows reasonable agreement with the previously reported value 17410 pm 5 cm-1 obtained indirectly through the analysis of EUV spectra [Ryabtsev et al., Atoms 3 (2015) 273].