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

X-ray spectra in the range 7-10 Angstrom 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 spectral line of As I-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 I-like W at about 18.4 Angstrom and strong lines between 5.6 and 6.0 Angstrom 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+.