Visible emission spectroscopy of highly charged tungsten ions in LHD: I. Survey of new visible emission lines

TL;DR

This study identified 12 new visible emission lines from highly charged tungsten ions in the core plasma of the LHD, using a custom spectrometer, advancing plasma impurity diagnostics.

Contribution

First survey of new visible emission lines from highly charged tungsten ions in LHD plasma, with detailed spectroscopic analysis and identification.

Findings

12 unknown emission lines discovered

Lines observed only after tungsten pellet injection

Lines attributed to tungsten ions with charge states 25-28

Abstract

We found 12 unknown visible emission lines from the core plasma of large helical device with highly charged tungsten ions accumulated. The observation was made with our home-built echelle spectrometer, which covers the wavelength range of 450-715 nm with a wavelength resolution of<0.05 nm for two lines of sight; one line passes both the core and edge plasmas and the other passes only the edge plasma. These emission lines are attributed to highly charged tungsten ions because (1) they were observed only after a tungsten pellet was injected into the plasma, (2) they were observed only from the core plasma where the electron temperature is 1 keV, (3) they show line broadenings that are close to the Doppler widths of tungsten ions with 1 keV temperature and (4) the wavelengths of some of these emission lines are close to the calculation results for tungsten ions in the charge state of 25-28.