Extreme ultraviolet transitions of Fe XXI in solar, stellar and laboratoryspectra

Recent R-matrix calculations of electron impact excitation rates for transi tions among the 2s(2)2p(2), 2s2p(3) and 2p(4) levels of Fe XXI are used to derive theoretical electron density (N-e) sensitive emission-line ratios in volving 2S2(2)p(2)-2s2p(3) transitions in the similar to 98-146 Angstrom wa velength range. A comparison of these with observations from the PLT tokama k plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and i n some instances removes discrepancies found previously. The observed Fe XX I ratios for a solar flare, obtained with the OSO-5 satellite, imply electr on densities which are consistent, with discrepancies that do not exceed 0. 2 dex. In addition, the derived values of N-e are similar to those estimate d for the high-temperature regions of other solar flares. The good agreemen t between theory and observation, in particular for the tokamak spectra, pr ovides experimental support for the accuracy of the present line-ratio calc ulations, and hence for the atomic data on which they are based.