Cavity experiment in a tabletop x-ray laser pumped by a pulse-train YAG laser

X-ray laser cavity experiments were performed using a recombination scheme pumped by a YAG laser. Two types of cavities, an unstable cavity and a cavity with a soft x-ray beam-splitter, were tried in the experiments. The unstable cavity consists of two flat Mo/Si multilayer mirrors, one of which has an output coupler made of a rectangular orifice. Clear cavity output signals were observed for the first time for Al XI 3p-4d (15.06 nm) and Al XI 3d-4f (15.47 nm) transition lines. The cavity output continued for more than 1 ns and was more than three times as intense as the signal of the double-pass amplification. Experiments on a cavity with a soft x-ray beam-splitter showed that clear evidences of enhancement in the two Li-like Al lines.