3 ADAS line emission calculations

The atomic data set for Fe⁷⁺, which is now available at the ORNL internet site, contains angular momentum quantum numbers and energies for each of the 77 LSJ levels, followed by radiative rates and effective collision strengths for all 2926 possible transitions among pairs of levels. The energies were adjusted to the experimental values, where known, and the radiative rates for those transitions shown in Table 2 were those determined from our extended CI calculation. Although specific for ADAS, the general format should allow easy interface with most plasma modeling codes.

By way of example, we used the ADAS collisional-radiative modeling codes and the Fe⁷⁺ atomic data set to calculate equilibrium populations for all 77 LSJ levels over a range of electron temperatures and densities. Specific spectral intensities were then used to construct an emission line ratio involving a blend of the 3p⁶4f ²F $\rightarrow$ 3p⁶3d ²D transitions at around 131 A and another blend of the 3p⁵3d²(³P) ²P $\rightarrow$ 3p⁶3d ²D and 3p⁵3d²(³F) ²D $\rightarrow$ 3p⁶3d ²D transitions at around 168 A. The first blend contains the transitions 3p⁶4f ²F $_{5\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{3\over 2}$ at 130.9 A, 3p⁶4f ²F $_{5\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{5\over 2}$at 131.3 A, and 3p⁶4f ²F $_{7\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{5\over 2}$at 131.2 A. The second blend contains the transitions 3p⁵3d²(³P)²P $_{1\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{3\over 2}$at 168.9 A, 3p⁵3d²(³P)²P $_{3\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{3\over 2}$at 168.0 A, 3p⁵3d²(³F)²D $_{5\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{3\over 2}$at 167.7 A, 3p⁵3d²(³F)²D $_{3\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{3\over 2}$at 167.5 A, 3p⁵3d²(³P)²P $_{3\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{5\over 2}$at 168.5 A, 3p⁵3d²(³F)²D $_{5\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{5\over 2}$at 168.2 A, and 3p⁵3d²(³F)²D $_{3\over 2}$ $\rightarrow$ 3p⁶3d ²D $_{5\over 2}$at 168.0 A.

The emission line ratio as a function of electron temperature at an electron density of 1.0 10⁸ cm^-3 is shown in Fig. 5. This line ratio provides a very useful electron temperature diagnostic; however, since it involves only strong dipole-allowed transitions, it is found to be very insensitive to changes in the electron density. With the Fe⁷⁺ atomic data set and the observed spectrum in hand, other choices for line emission ratios may be quickly chosen and processed to provide further plasma temperature and density information. Furthermore, with the low-energy grating on the Chandra X-ray satellite observatory, it should be possible to resolve many of the components that constitute these two blended lines and, in combination with this data set, could provide a much more complete set of diagnostics.

Figure 5: Emission line ratio for a blend of the 3p64f 2F $\rightarrow$ 3p63d 2D transitions at a wavelength of about 131 A divided by a blend of the 3p53d2(3P)2P $\rightarrow$ 3p63d 2D and 3p53d2(3F)2D $\rightarrow$ 3p63d 2D transitions at a wavelength of about 168 A