Formation of a population inversion of transitions in Ne-like ions in steady-state and transient plasmas

The example of multiply charged Ne-like ions and a detailed model of radiative collisions are used to consider three approximations employed in determination of the level populations and of the gain G in the short-wavelength part of the far ultraviolet range. If the plasma heating process is fast, which corresponds to the transient approximation, the gain can reach G ~ 10² cm ^− 1. A population inversion appears then not because of strong radiative depletion of the lower active level, but because of transient radiative collisional processes. There is therefore no need to limit the transverse dimensions of a plasma in establishing optical transparency conditions and there are several other consequences of practical importance.