The effects of radiative cascades on the x-ray diagnostic lines of Fe¹⁶⁺

Abstract

We present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 Å–17 Å range. These lines have been the subject of much controversy in the astrophysical and laboratory plasma community. Radiative transition rates are generated from fully relativistic atomic structure calculations. Electron-impact excitation cross sections are determined using a fully relativistic R-matrix method employing 139 coupled atomic levels through n = 5. We find that, in all cases, using a simple ratio of the collisional rate coefficient times a radiative branching factor is not sufficient to model the widely used diagnostic line ratios. One has to include the effects of collisional-radiative cascades in a population model to achieve accurate line ratios. Our line ratio results agree well with several previous calculations and reasonably well with tokamak experimental measurements, assuming a Maxwellian electron-energy distribution. Our modelling results for four EBIT line ratios, assuming a narrow Gaussian electron-energy distribution, are in generally poor agreement with all four NIST measurements but are in better agreement with the two LLNL measurements. These results suggest the need for an investigation of the theoretical polarization calculations that are required to interpret the EBIT line ratio measurements.

Benchmarking atomic data for astrophysics: Fe XVII EUV lines

“…Feldman et al (1985) used the morphology of the observed lines as an analysis tool, and used estimated intensities from the calculations as an aid in the identification process. However, these line intensities (listed within Table 4) are very different from those obtained on the basis of the calculations by Loch et al (2006), by factors of 2-3 for the strongest lines, up to a factor of 10 for the weaker ones.…”

“…It is now timely for us to review the Fe xvii EUV spectrum for the following reasons. First, the R-matrix scattering calculations of Loch et al (2006) predict Fe xvii line intensities that largely differ (factors of 2-3) from previous estimates. Hence, previous identifications need to be reassessed in light of these new data.…”

“…We studied the Fe xvii 2s 2p 6 3s, p, d levels closely and propose that all previous identifications, in particular those of Finkenthal et al (1985), are Loch et al (2006). The second column provides the transition description.…”

“…Line intensities were calculated with the rates and transition probabilities provided by Loch et al (2006), at the temperature of peak ion abundance for Fe xvii in ionisation equilibrium (log T [K] = 6.6). Table 4 shows the relative line intensities calculated at an electron density of 10 11 cm −3 , which is typical of solar flares but still in the low-density regime for this ion.…”

“…are our experimental wavelengths. λ CC are the wavelengths obtained from the scattering target energies (Loch et al 2006), while λ NIST are those from NIST v.3 and λ MR−MP are those obtained from the MR-MP calculations.…”