Use of Multiple Photoexcitation in an Optically Thick Silicon-Aluminum Plasma to Obtain Lasing at 44 A.

It is a well established phenomenon of radiative transfer in stellarlike plasmas that as the optical depth of a line increases, the mean radiative energy flux in the line correspondingly increases to approach the Planck blackbody value. Recently, the possibility of achieving stimulated emission in the x-ray or soft x-ray region by utilizing the interaction of strong lines from different elements or neighboring ionization stages has been considered in the context of resonance line pumping in high density, albeit optically thin, plasmas. Proper utilization of photon trapping effects in the pumped and pumping plasmas can yield comparable gains at soft x-ray frequencies to those proposed, however, at plasma densities that are significantly below and over spatial extents that are significantly larger than those that would be required were one to be restricted to optically thin plasmas in which an exponentially decaying inversion density is generated. The demonstration is made by treating the physical situation of a non-collisionally dominated aluminum plasma with the aid of a new global rate equation formalism developed to include photoexcitation processes, on the average, into ionization equilibrium calculations involving optically thick laboratory plasmas.