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Experimental measurements of the optimum pulse delay and pulse irradiance ratio for lasing at 7.3 nm in Ni-like samarium are shown to be explained by a simple model. Experimental x-ray laser outputs are also shown to be well fitted with a simple one-dimensional ASE variation of the laser output with increasing target length through saturation. The ASE model is modified to take account of the two lasing lines present in Ni-like lasing and compared to experimental laser output measurements for Ni-like Dy lasing at 5.86 nm and 6.37 nm. The good agreement of experimental data with the ASE model indicates that refraction, non-uniformity of the laser medium and other problems traditionally associated with x-ray lasing are not significant with double pulse pumping.

We report here the experiments of the saturated Ni-like x-ray lasers at 14 -6 nm from a refraction compensating double target driven at 20 TW.cm(-2) by a pair of 75 ps Nd-glass laser pulses separated by 2.2 ns. The output source size as well as the time history, divergence, energy and spatial profile of the output beams of the Ni-like x-ray lasers have been fully characterised. The narrow divergence (similar to 1 mrad), short pulse duration (similar to 40 ps), high efficiency (similar to 10(-6)) and high brightness (similar to 10(25) photons.s(-1).mm(-2).mrad(-2) of these Ni-like x-ray lasers make them ideal candidates for x-ray laser applications.