Electron density dependence of dielectronic recombination in highly ionized atoms

A model is introduced for investigating the effect of electron-ion inelastic collisions on dielectronic recombination (DR) and predicting its electron-density dependence. A general method for obtaining effective DR rate coefficients for optically thin dense plasmas is described. Level-by-level relativistic calculations are performed for DR of Ne-like iron. The results clearly show that, as the electron density increases, the first density effect is an enhancement of the DR rates through collisional transitions among the doubly excited autoionizing levels. At higher densities, collisional stabilization and collisional ionization play an important role. The former process enhances the DR rate, whereas the latter reduces it. It is found that the total collisional effect, which is a combination of the collisional mixing, collisional stabilization, and collisional ionization effects, is an enhancement of the DR rate coefficients of Ne-like iron by a factor of 2 at an electron density of 10²⁴ cm^-3 and at an electron temperature of kT_e=200 eV. At lower temperatures (kT_e=100 eV), the total enhancement can be as much as one order of magnitude.