Observations of x-ray spectra from highly charged tungsten ions in tokamak plasmas

X-ray spectra ion the range 7-10 A from highly charged tungsten (Z = 74) ions were observed in the tokamak ASDEX Upgrade. Lines emitted from W37+ to W47+ were identified by comparison with ab initio calculations and with the expected ionization equilibrium charge-state distribution. Most of the features in the observed spectra coincide with predicted lines. A spectra line of As 1-like W41+ could be identified as the predominant one for plasmas with 2.5-3 keV central electron temperature. The calculations predict clearly separated spectral lines for Kr 1-like W at about 18.4 A and strong lines between 5.6 and 6.0 A for all charge states under consideration that can be used for monitoring W densities in the soft x-ray region. Tungsten concentrations in the range of 10-4 were extracted from the emissivities of spectral lines from W40+ and W41+. (author)