Amplification of spontaneous emissions at 19.6, 23.2 and 23.6 nm have been observed by a "Multiple-Target Series Coupling" design in Ge plasma. The combined length for four targets is up to 56 mm. The gain length product (GL) of small signal is up to 18 for both lines at 23.2 and 23.6 nm, and the effective GL is 16.4 and 15.7 for these two lines respectively. This two lines are obviously tending to saturation. The divergence of x-ray laser beam is about 4 mrad.
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Gu et al. (1991) studied this question.