Use of multiple photoexcitation in an optically thick silicon-aluminium plasma to obtain lasing at 44 AA

By means of a new analytic formalism, which is developed to describe the average effect of photoexcitation processes in optically thick media it is shown that gain can be achieved in the Al XII 1s2p 1 P-1s3d 1 D, 1s2s 1 S-1s3p 1 P and 1s2p 1 P-1s3s 1 S transitions at 44 AA at densities well below, and over spatial extents well beyond, those proposed previously. Photoexcitation of the n=3 level in Al XII by the 1s 2 1 S-1s2p 1 P transition in Si XIII is used to generate the population inversion between the n=2 and n=3 states in Al XII. The gain is enhanced in magnitude and spatial extent by photoexcitation of the 1s2p 1 P and 1s3p 1 P states of the Si and Al plasmas respectively. An estimate of the inherent experimental limitations of this inversion technique is also made by comparing the calculated gains with upper limits obtained by pumping with a strict black-body photon flux in the pumping line.