Grazing-incidence measurements of L-shell line emission from highly charged Fe in the soft x-ray region

Line emission from highly charged ions in the soft x-ray region represents an important diagnostic for high-temperature plasmas. The wavelength region of 10–30 Å is occupied by lines from the K-shell transitions in N through Ne and L-shell transitions in Ti through Ni. Crystal spectrometers have previously been used to investigate Fe line radiation under well controlled conditions using the LLNL electron beam ion trap facility in this wavelength region. To complement this work we have instituted a grazing-incidence spectrometer with a variable line-spaced, concave grating and a flat field of focus which is suited for a multichannel detector. Simultaneous observations of the Ne-like Fe16+ n=3→2 transitions were made with this instrument and a flat-crystal spectrometer in order to establish the linear polarization of the Fe XVII lines. We present a description of the grating spectrometer and discuss its performance in comparison with Rowland circle based grazing incidence instruments.