Temperature Dependent Shape of Quasi-Continuum Spectra from Highly Charged Heavy Ions Observed in the Large Helical Device

The temperature dependent shape of quasi-continuum unresolved transition array (UTA) spectra from highly charged heavy ions has been examined based on the experimental spectra recorded in the Large Helical Device plasmas. The observed spectral shape of the n = 4-4 UTA emission strongly depends on the electron temperature especially for the lanthanide elements with the atomic numbers of 63- 66. As the temperature decreases, the UTA position moves to shorter wavelength and the UTA bandwidth becomes narrower. Eventually, characteristic narrowed spectra with the lines of palladium-like and silver-like ions are observed at the lowest peak temperature of a few hundred eV. The temperature dependence of the UTA shape can be explained by the change in ion abundance and the wavelength distributions of the weighted transition probabilities calculated with the Flexible Atomic Code (FAC). A collisional-radiative modeling of the narrowed spectrum for terbium ions is tried based on the FLYCHK code and the FAC. As a result of slight intentional shifts of the calculated line positions, the measured spectrum matches qualitatively with the simulation for the electron temperature of 230 eV. (c) 2025 The Japan Society of Plasma Science and Nuclear Fusion Research