Measurement and comparison with theory of the temperature dependence of satellite-to-resonance line ratios of heliumlike nickel from the JET tokamak

The line ratios of the intercombination line x (1s2p ${}_{}{}^3{}_{}{}^{}\mathrm{P}_2^{}{}_{}{}^{}$→1${\mathrm{s}}^2$ ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$), the dielectronic and inner-shell satellite line t [1s2p${(}^3$P)2s ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{1/2}^{}{}_{}{}^{}$→1${\mathrm{s}}^2$2${\mathrm{s}}^2$ ${\mathrm{S}}_{1/2}$], and the n≥3 dielectronic satellite lines (1s2pnl→1$s^2$nl) to the resonance line w (1s2p ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$→1${\mathrm{s}}^2$ ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$) in the spectrum of heliumlike nickel are measured with a high-resolution x-ray crystal spectrometer on the JET tokamak. The central electron temperatures for which the spectra are taken range from about one-third to twice the threshold energy for the line excitation. The measurements are compared with predictions from a model taking into account the spatial emission profiles of the different ionization stages of nickel calculated from the radial profiles of JET plasma parameters. The rate coefficients for the formation of excited states are taken from a single, self-consistent source of calculations of dielectronic and radiative recombination and electron-impact excitation. The rate coefficients for the calculation of the ionization balance are critically reviewed. Good agreement is found for the ratios t/w and (n≥3)/w, but for x/w significant disagreement is found. Possible causes for this disagreement are discussed. In particular, charge-exchange processes, which are omitted from the model, are found not to be plausible contributors. It is concluded that the present coronal equilibrium model correctly describes the ionization balance, the radiative and dielectronic recombination, and the electron-impact excitation of electric dipole transitions, but that there is a problem with the excitation of nondipole transitions.