Plasma effects on atomic reaction rates in hydrogen plasmas

The perturbative effects of plasma electrons and ions on the effective capture and ionization rates in hydrogen plasma are analyzed. A simple model of hydrogen plasma is constructed to extract the salient features of the system, which is assumed to be in local thermal equilibrium. The interplay between the collisional and radiative processes as functions of the electron temperature and density is explicitly demonstrated. A large increase in the collisional radiative recombination rates at high density and low temperature is caused by (i) the shift in the dominant capture mechanism from the radiative to collisional cascade, and (ii) the presence of upper states in Saha equilibrium which act as a reservoir of population flux. Different sets of effective rates which reflect the approximate structure of the rate equations are evaluated, stressing the relationship between the rate coefficients and the rate equations. When the angular-momentum sublevels are averaged over, because of rapid l-changing collisions, the ion field distortion effect is shown to be negligible, in the lowest order approximation.