Saturated and near-diffraction-limited operation of an XUV laser at 23.6 nm

Amplification of spontaneous emission (ASE) at 23.6 nm has been studied in a Ge plasma heated by a 1 TW infrared laser pulse. The exponent of the axial gain reached 21 in a geometry with Fresnel number ≤1. Two plasma columns of combined length up to 36 mm were used with an extreme ultraviolet mirror giving double-pass amplification. Saturation of the ASE output was observed. The beam divergence was about 8× diffraction limited with a brightness estimated at ${10}^{14}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$. The feedback from the mirror was significantly reduced probably by radiation damage from the plasma.