Extreme-ultraviolet spectroscopy of Fe VI-Fe XV and its diagnostic application for electron beam ion trap plasmas

Extreme-ultraviolet spectra of intermediately ionized iron ions (Fe VI-Fe XIV) in the wavelength range of 125.0-265.0 Å have been measured at the Heidelberg electron beam ion trap. Emission spectra were recorded sequentially while varying the electron energy over the range of 75-544 eV in steps of 5 eV. The observed spectra clearly show the evolution of each ionic stage as a function of the electron energy, allowing to distinguish the emission lines from neighboring ion charge species and helping to disentangle possible line blends. The collisional-radiative modeling satisfactorily reproduces the measurement. A comparison with previous astrophysical observations (Sun) reveals that some weak emissions may originate from Fe VI and Fe VII, resulting in incorrect assignment of transition lines. The calculated polarization effects due to nonthermal (monoenergetic) electrons are found to be negligible for most of the emission lines at low-energy electron impact, except for a few lines whose polarization can be over 20%. By line ratio technique, the effective electron density in the trap was estimated to be 7.1+2.4 -3.0 × 109-3.4+0.5 -0.5 × 1010 cm-3, slightly depending on the ion charge state.