Demonstration of Soft X-Ray Lasing to Ground State in Li III

Soft x-ray lasing with a gain length $\mathrm{GL}\approx 5.5$ was demonstrated in hydrogenlike Li III at 13.5 nm (2-1 transition) in a 5 mm long LiF microcapillary using a 50 mJ, 250 fs UV laser beam at a 2 Hz repetition rate. The initial plasma was created in the 0.3 mm diameter microcapillary by a low power Nd/YAG laser. The strongly nonlinear increase of the 13.5 nm line intensity with increasing microcapillary length was compared with a linear increase of the Li III 11.4 nm (3-1 transition) line intensity.