Laboratory measurements of iron L-shell emission:: 3→2 transitions of Fe XXI-XXIV between 10.5 and 12.5 Å

Using the electron beam ion trap EBIT-II facility at Lawrence Livermore National Laboratory, we have measured the iron L-shell spectrum between 10.5 and 12.5 Angstrom for Fe XXI-XXIV with a spectral resolution of similar to 30 m Angstrom. The relative line intensities of strong 3 --> 2 transitions for each charge state are measured as functions of electron energy and compared to distorted wave (DW) calculations. The contributions of resonant processes, namely, resonant excitation (RE) and spectroscopically unresolved dielectronic recombination (DR) satellites, are investigated. The RE contributions are shown to be less than the experimental and theoretical uncertainties for plasma in collisional ionization equilibrium. The unresolved DR satellites, however, enhance the emissivities of almost all lines, some by as large as 15%, consistent with our earlier measurements for Fe XXIV. The DW results agree with our measurements to better than 20% under the condition of collisional ionization equilibrium. The line emissivities in the widely used spectral synthesis model, MEKAL, are also compared to our measurements and are found to be discrepant at a greater than 20% level for some lines.