Studies of population inversions in the soft x-ray spectral region in CO2 laser produced plasmas

The recombination approach for development of a short wavelength laser is one of two schemes used in recent demonstrations of high gain in the soft x-ray region. In this work, a theoretical and experimental study of recombination pumped soft x-ray population inversions in Li-like carbon (CIV) and H-like carbon (CVI) in CO2 laser produced plasmas is presented. A collisional radiative model is used to examine the behavior of the gain coefficients for various transitions in the C5+ ion with plasma parameters, and the optimum conditions for gain defined. A general formalism that elucidates the separate roles of radiative power loss and electron cooling in dense transient plasmas is described: it is used to examine the feasibility of quickly radiatively cooling a dense plasma to bring about optimum conditions for gain in a recombination system. Experimental studies of the effectiveness of radiation cooling in generating recombination pumped soft x-ray population inversions in CIV are carried out in freely expanding CO2 laser produced plasmas. Measurements of gain length products as high as 6.5 on the 182 A transition in CVI in magnetically confined CO2 laser produced plasmas are reviewed and observations of stimulated emission at 182 A obtained with an XUV multilayer mirror in use are presented