Inversion and gain in hydrogenic ion levels induced by photoionization pumping

If a low-Z plasma is subjected to ionizing radiation, stationary inversions on hydrogenic ion level populations occur. The plasma conditions and pumping requirements to achieve gain on the Lyman-alpha and Balmer-alpha lines are investigated by means of a collisional-radiative model. The calculations are carried out for two cases: first, for the case in which the electron temperature is determined solely by radiative cooling, and second, for the case in which additional electron cooling is provided. In the first case, measurable gain on the Balmer-alpha line is found, although the Lyman-alpha transition remains uninverted. In the second case, the Balmer-alpha gain is dramatically enhanced, and sizable Lyman-alpha gain is predicted.