Experimental and theoretical investigation of neonlike selenium x-ray laser spectral linewidths and their variation with amplification

This paper describes our recent x-ray laser spectral linewidth measurements and discusses the consequences of these measurements. The experiments observed the 206.38- and 182.45-\AA{} laser transitions in neonlike selenium using an extremely high-resolution soft-x-ray spectrometer, and measured the spectral linewidths and relative spectral intensities of the laser lines from amplifiers of varying lengths. The data allow the intrinsic (unamplified) linewidths of the 206- and 182-\AA{} lasers to be extrapolated as 50 and 35 m\AA{}, respectively. The intrinsic spectral width of the 206-\AA{} laser is somewhat greater than the predicted 43-m\AA{} Voigt-profile width based on calculated transition rates and on the expected ion temperature in the plasma, and while the number of data points obtained with the 182-\AA{} laser is small and a conclusive comparison with calculations is not possible, the extrapolated intrinsic linewidth of this laser is clearly consistent with the predicted 37-m\AA{} Voigt-profile width and is not significantly broader or narrower than expected. The data also show significant gain narrowing of both lines with increasing amplifier length and show no significant rebroadening of the 206-\AA{} laser in long, saturated amplifiers. We discuss line broadening and line transfer calculations we have performed which show that the saturated 206-\AA{} laser line transfer behavior can be treated homogeneously due to non-negligible homogeneous lifetime broadening contributions, which significantly reduce inhomogeneous Doppler saturation rebroadening, and due to collisional redistribution rates which are large enough in any case to homogenize the otherwise inhomogeneous Doppler contributions. We also discuss the implications of these results with regard to earlier suggestions that Dicke narrowing effects could play a significant role in determining the intrinsic line profiles of these laser transitions, and we conclude that Dicke narrowing effects are not likely to be significant for either laser.