Modelling of measured tungsten spectra from ASDEX Upgrade and predictions for ITER

Tungsten (W) has moved into the focus of fusion research being a main candidate for the plasma facing components (PFCs) of ITER and a future fusion reactor. A main ingredient for understanding the influence of W as a plasma impurity and its impact on the plasma is the spatially resolved spectroscopic diagnosis of W. The focus of the experimental investigations at ASDEX Upgrade is on the most intense emissions of W ions (about I-like W21+ to Mn-like W49+) in the VUV to the soft x-ray region covering the electron temperature range from about 0.5–5.0 keV. The relative shape of the fractional abundances of the ionization stages Se-like W40+ to Ni-like W46+ and of the bundle of ionization stages between Sn-like W24+ and Y-like W35+ was determined. Calculated fractional abundances using published ionization and recombination rates do not accurately describe the experimental temperature dependence. Adjustments to the recombination rates were calculated to reconcile with the measurements. The spectral features of W at 0.4–0.8 nm, around 5 nm, between 12 and 14 nm and between 10 and 30 nm have been recorded and compared with modelling results. The quality of agreement is best for highly charged ionization stages and short wavelengths and decreases for lower charged ionization stages and longer wavelengths. However, in the latter case the predictions manage to reproduce the total emissivity in each considered spectral range and also the rough distribution of emissions versus wavelengths within these spectral ranges. The modelling of the SXR range at 0.4–0.8 nm looks very similar to the measurement. Further observations of weaker spectral features between 0.6 and 0.7 nm, between 1.8 and 3.5 nm and at 8 nm could be attributed to certain ionization stages. The modelling of W spectra for ITER predicts emissions of Cr-like W50+ to about C-like W68+ at 0.1–0.15 nm, 1.8–4.0 nm and around 8 nm.