EUV spectroscopy of highly charged tungsten ions relevant to hot plasmas

The radiation from tungsten ions in the extreme ultraviolet spectral region was investigated using a 2-m grazing-incidence spectrometer in conjunction with the Berlin electron beam ion trap. Operating EBIT at beam energies between 900 eV and 1.7 keV In-like W²⁵⁺ to Sr-like W³⁶⁺ ions could selectively be excited, and a bright emission band of about 2 Å width was measured which shifts from 50 to 54 Å when raising the charge state. The band of partially unresolved lines originates from 4l-4l′ transitions of ions having an open 4d subshell. Atomic structure calculations with the HULLAC code package show that the narrowing and shift of the line band emission can be interpreted in the framework of the unresolved transition array using mixed configurations. The theoretical spectrum analysis applies a collisional-radiative model to account for the low electron density of EBIT and reproduces the variations of the observed emission pattern.