Line and band emission from tungsten ions with charge $21+$ to $45+$ in the $45–70-\mathrm{\AA }$ range

We have measured the radiation from medium-charge-state ions of tungsten in the spectral range from 45 to 70 Å. The ions were produced in an electron beam ion trap (EBIT), and the radiation was observed using a 2-m grazing-incidence spectrometer. Operating EBIT with beam energies between 500 eV and 4 keV allowed us to sample charge states ranging from I-like ${\mathrm{W}}^{21+}$ to Cu-like ${\mathrm{W}}^{45+}.$ Lines of Sr-like through Cu-like tungsten were observed; for the Sr-like to As-like ions new lines were identified. Analysis of the spectra is based on ab initio calculations using the relativistic HULLAC code. For charge states lower than Sr-like $W^{36+},$ the spectrum lines fuse into a bright emission band situated around $50\hspace{0.5em}\mathrm{\AA }.$ The band extends over an interval of approximately $2\hspace{0.5em}\mathrm{\AA }\mathrm{}$ and moves smoothly towards shorter wavelengths with decreasing ion-charge state. The shift and narrowing of the band emission cannot be interpreted in the framework of the standard unresolved transition array formalism assuming statistical population of the excited levels. Instead, the observations are explained by detailed calculations based on a collisional-radiative model. It is shown that the variation of the experimental spectrum with charge state is the signature of the low electron density in EBIT.