Experimental study of soft X-ray laser for the sodiumlike recombination scheme, carried out at the LF12 Laser Facility of Shanghai Institute of Optics and Fine Mechanics, is reported. Soft X-ray amplification was observed in sodiumlike copper ions by line-focused laser irradiation of slab targets. Based on the spatially-resolved measurements, the spatial distribution of the laser gain for the 6g-4f (72.22 A) transition of sodiumlike copper ions was obtained. The maxima of gain coefficients and gain-length products at 550 microns from target surface are 2.0/cm and 3.6, respectively. The evolution of spectrally discriminated spatial uniformity of line-shaped plasmas as lasing gain media produced by uniform laser illumination in line-focus is also investigated. It is found that, under this experimental condition, the line-shaped plasma is very uniform after optical laser's heating.
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