Recombination lasers utilizing vapors of chemical elements. I. Principles of achieving stimulated emission under recombination conditions

A general analysis is made of the characteristics of laser action resulting from plasma recombination in the afterglow of a gas discharge. It is shown that efficient pumping of the active levels by impact-radiative recombination can be achieved by establishing a fairly high plasma density, cooling as fast and as much as possible the electron gas after a current pulse, and utilizing the recombination of doubly charged ions with a high initial concentration. It is shown that the most convenient approach is the utilization of the recombination in the afterglow of a discharge in a vapor of an easily ionizable element mixed with helium. The requirements to be satisfied by the distribution of the active levels are formulated in the approximation of level groups bearing in mind that the transitions due to electron deexcitation are very important during the plasma relaxation period. A generalized criterion is obtained for the existence of a population inversion. The cases of radiative and colh'sional regimes are discussed separately for the case of widely spaced levels. The paper concludes with a formulation of the general requirements to be satisfied by the distribution of levels so that an inversion is obtained under recombination-collisional kinetics conditions.