In a low-Z plasma subjected to photoionizing radiation, stationary inversions on hydrogenic level populations occur. A collisional-radiative model is used to calculate gains for the case where the electron temperature is determined by radiative cooling alone, and for the case where additional electron cooling is provided. In the first case, measurable gain on the Balmer-a line is found. In the second case, the Balmer-a gain is dramatically enhanced and, moreover, sizeable gain on the Lyman-a line is predicted.
Ernst E. Fill,
David G. Goodwin,
"Soft X-Ray Gain In A Plasma Pumped By Photoionization", Proc. SPIE 0831, X-Rays from Laser Plasmas, (4 February 1988); doi: 10.1117/12.965051; https://doi.org/10.1117/12.965051