Nuclear Materials and Energy (Mar 2021)
Progress of tungsten spectral modeling for ITER edge plasma diagnostics based on tungsten spectroscopy in LHD
Abstract
Strong radiative cooling by tungsten ions in the core plasma is one of issues in fusion reactors. Quantitative analysis of tungsten density in divertor plasma, scrape-off layer, and main plasma relies on accuracy of spectroscopic model of tungsten ions. We have developed a collisional-radiative (CR) model for tungsten ions to estimate line intensities of tungsten ions and also have measured tungsten spectra in wide wavelength region from extreme ultraviolet (EUV) to visible wavelengths simultaneously in the Large Helical Device (LHD) with tungsten pellet injection to validate the CR model for tungsten ions. So-called unresolved transition array (UTA) measured in EUV spectra at 4.5–7 nm are commonly seen in fusion plasmas with electron temperature of around 1 keV, which corresponds to edge temperature in ITER. The UTA is strong and largely contributes to the radiation power at this temperature, so the CR model calculation is required to reproduce the UTA to estimate the tungsten radiation power. We have developed the CR model to include recombination processes, which were not considered before for tungsten ions, as a hybrid model with fine-structure levels and configuration-averaged levels for W25+–W39+ ions. The 6 nm-peak of UTA is not still reproduced well, but the recombination processes and cascade from higher levels can enhance spectral intensities. Contribution of dielectronic recombination from the excited states of W28+ ions is examined but it is small for W27+ ions. The spectral profiles of n = 4–5 transition peaks at 2–4 nm do not change much with recombination processes. Analysis of measured EUV spectra in LHD plasma shows that tungsten is accumulated first into core plasma to produce hollow temperature distribution due to large radiation power of tungsten ions, and continuous NBI heating recovers central electron temperature and the UTA appears again in EUV spectra with weaker intensity than one at the earlier phase. Reduced intensity of UTA and electron temperature estimated from EUV spectra indicate that tungsten is transported outward and the amount of tungsten emitting EUV spectra is reduced to about 1/4.
