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Calculation of the X-ray gain in a recombining sodium plasma

The authors have numerically simulated previously-reported experiments first demonstrating soft-X-ray gain in the 3 to 2 transition in hydrogenic sodium ions at 54.2 AA and find a good agreement with the measured time evolution of this gain. Additionally, they derive from their theoretical model the possibility of obtaining a small gain in the 4 to 2 transition at 40.2 AA. They discuss the results, especially the duration of the gain under the aspect of different lengths of the plasma-producing laser pulses.