A vacuum double-crystal spectrometer for reference-free X-ray spectroscopy of highly charged ions

•First use of a double crystal spectrometer to directly observe X-ray transitions in highly charged ions.•Accuracies of a few parts per million could be reached that provides test of quantum electrodynamics.•We have developed an ab initio simulation code that allows us to obtain accurate line profiles.•It can reproduce experimental spectra with unprecedented accuracy.•The details of the newly developed ab initio simulation code is presented.

We have built a vacuum double crystal spectrometer, which coupled to an electron-cyclotron resonance ion source, allows absolute measurements of low-energy X-ray transitions in highly charged ions with accuracies of the order of a few parts per million. We describe in detail the instrument and its performance. Furthermore, we present a few spectra of n=2→n=1n=2→n=1 transitions in Ar14+, Ar15+ and Ar16+. We have developed an ab initio simulation code that allows us to obtain accurate line profiles. It can reproduce experimental spectra with unprecedented accuracy. The quality of the profiles allows the direct determination of line width.