Focusing a multimillijoule soft x-ray laser at 21 nm

The authors focused the beam of currently the most powerful soft x-ray laser at 21nm using a spherical Mo:Si multilayer mirror. Focal spots were directly observed by imaging the fluorescence induced by the soft x-ray beam on a Tb doped phosphor screen. The energy density within the 40×60μm2 focal spot was 48J∕cm2, which corresponds to radiation peak intensity of 5×1011W∕cm2. The first observation of material ablation with a laser at 21nm is reported.