Amplification of spontaneous emission in aluminum and magnesium plasmas

Experimental evidence is given along with the theoretical interpretation of population inversions occurring between the levels of main quantum number 3 and 5 of lithium-like ions in recombining plasmas. A new experimental method is described that permits direct measurements of amplification of spontaneous emission as well as contiguous background absorption. Consideration is being given to extending this method to various studies of radiation transfer in upcoming experiments. The population ratio between the f5 and the 3d levels of lithium-like aluminum has been found to be about 3. The gain coefficient for the discrete transition alone is of the order of 1 per cm, but in most of the experiments described the background photoabsorption had a similar magnitude. Thus, the main issue for the prospect of X-ray laser feasibility is to achieve a positive balance of gain to losses in the plasma itself.