Line and band emission from tungsten ions with charge 21+to 45+in the 45-70-angstrom range - art. no. 012720

We have measured the radiation from medium-charge-state ions of tungsten in the spectral range from 45 to 70 Angstrom. The ions were produced in an el ectron beam ion trap (EBIT), and the radiation was observed using a 2-m raz ing-incidence spectrometer. Operating EBIT with beam energies between 500 e V and 4 keV allowed us to sample charge states ranging from I-like W21+ to Cu-like W45+. 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 spec tra is based on ab initio calculations using the relativistic HULLAC code. For charge states lower than Sr-like W36+, the spectrum lines fuse into a b right emission band situated around 50 Angstrom. The band extends over an i nterval of approximately 2 Angstrom and moves smoothly towards shorter wave lengths with decreasing ion-charge state. The shift and narrowing of the ba nd emission cannot be interpreted in the framework of the standard unresolv ed transition array formalism assuming statistical population of the excite d levels. Instead, the observations are explained by detailed calculations based on a collisional-radiative model. It is shown that the variation of t he experimental spectrum with charge state is the signature of the low elec tron density in EBIT.