Component investigation of ionization stages on tungsten unresolved transition array spectra for plasma diagnostics based on space-resolved extreme-ultra violet spectroscopy in large helical device

Abstract

A series of experiments on tungsten spectroscopy has been carried out in a Large Helical Device with tungsten pellet injection to study the tungsten spectra. In particular, understanding of an unresolved transition array (UTA) is still insufficient, whereas the UTA is very important for the diagnostics and transport study of tungsten ions in edge plasmas of International Thermonuclear Experimental Reactor. For the purpose, extreme-ultra violet (EUV) spectra of UTA observed in the wavelength range of 15 Å–70 Å are analyzed at two different wavelength intervals of 15–45 Å and 45–70 Å, which mainly consist of Δn = 1, 2 and Δn = 0 transitions for n = 4 partially ionized tungsten ions, respectively. At first, the UTA line intensity is analyzed against central electron temperature at the temperature recovery phase after the pellet injection to examine the presence of blended lines. Next, vertical profiles measured with two space-resolved EUV spectrometers are analyzed against electron temperature profiles for further precise investigation of the UTA spectra. For the analysis, local emissivity profiles are obtained from the measured vertical intensity profiles with the Abel inversion method based on magnetic surface structures calculated by the VMEC code. It is then possible to investigate the ionization stage of tungsten ions comprising the UTA. As a result, it is found that the wavelength intervals of 49.24 Å ≤ λ ≤ 49.46 Å, 48.81 Å ≤ λ ≤ 49.03 Å, and 47.94 Å ≤ λ ≤ 48.15 Å, which are identified as W27+, W26+, and W24+, respectively, are applicable to the tungsten diagnostics. The result of the line component analysis on the tungsten UTA is summarized in tables.