Influence of line overlapping and dynamical Stark broadening on calculated gains in soft-x-ray lasers

International audienceStark lineshapes of lasing radiation in the X-UV range in aluminium and sulphur plasmas are calculated with the help of a quick code. Intensities are then obtained from the radiative-transfer equation in the nonsaturated regime, and, in the same way as in the experiments, a gain coefficient is deduced from the variation with length of the intensity integrated over wavelength. In the cases investigated, many lasing transitions contribute to the x-ray beam, and the spectral gain as well as the emissivity have to be suitably defined. For 10.57 and 15.47 nm radiation in Al10+ and 20.65 nn radiation in S13+, the gain coefficients thus calculated are much smaller than those obtained with symmetrical Doppler profile functions. When the amplification length is increased each lineshape becomes more and more homogeneous and tends towards the shape of the most intense transition. In other words, as the beam is amplified the individual transitions, except the most intense one, contribute less and less to the output intensity. The effect of the x-ray beam on the populations of the lasing levels is investigated and a rate is calculated which is found to be much smaller than the collisional and spontaneous-emission rates, but increases rapidly with the gain-length product