Diagnostic forbidden lines of highly ionized elements for tokamak plasmas

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Abstract
The possibility of observing forbidden lines of highly ionized elements in tokamak plasmas is discussed. The forbidden lines are magnetic‐dipole transitions between levels of the 2s22pk configuration of ions of the F I through B I isoelectronic sequences. Some of the forbidden lines of iron have already been found in spectra of solar flares obtained from Skylab. The 2s22p5 2P3/2‐2s22p5 2P1/2 forbidden line of the F I sequence, and the 2s22p 2P1/2‐2s22p 2P3/2 forbidden line of the B I sequence, are extrapolated from zinc through molybdenum. These lines fall in the wavelength region between 105 and 509 Å. The two allowed lines of the F I sequence, i.e., 2s22p5 2P3/2‐2s2p6 2S1/2 and 2s22p5 2P1/2‐2s2p6 2S1/2, are also extrapolated. The wavelengths of these lines fall between 38 and 87 Å. It is shown that the forbidden line of a given ion should be comparable in intensity to the allowed lines of the same ion due to transitions of the type 2s22pk‐2s2pk+1 for electron densities characteristic of tokamak plasmas. The intensity ratio between the forbidden line and the allowed lines is sensitive to electron density between ≈1012 and ≈1016 cm−3. Calculations of the ratios of the forbidden to one of the allowed lines are given for Cr XVI, Fe XVIII, Fe XXII, and Ni XX. The effect of electron temperature on the ratios is also calculated. The widths of the forbidden lines allow the ion temperature of the plasma or nonthermal motions to be determined.