Spectral lines from highly charged tungsten ions in the soft-X-ray region for quantitative diagnostics of fusion plasmas

X-ray spectra in the range from 5.5 to 19 Å from highly charged tungsten (Z=74) ions were recorded in the tokamak ASDEX Upgrade from plasma discharges with electron temperatures up to 5 keV. Lines emitted from W³⁹⁺ to W⁵⁰⁺ were identified by comparison with ab initio calculations using the fully relativistic parametric potential code RELAC. In the short wavelength region the measured spectra could be simulated to a high degree of coincidence by using ion density distributions from impurity transport calculations and line intensities calculated by a collisional-radiative model, whereas in the long wavelength region not all of the stronger predicted lines are found. The measured absolute intensities from several spectral lines are used to calculate the tungsten density present in the tokamak plasma. The extracted tungsten concentrations are in the range of 10^-5-10^-4 indicating that a sensitive W impurity diagnostic can be based on the presented spectral lines.