Final Activity Report Summary - FOXLA (Focusing of the optimised X-ray laser at PALS: the first step towards laboratory astrophysics) The main goal of this project was to explore the feasibility of focusing the multi-millijoule, deeply saturated soft X-ray laser at 21.2 nm. Precise optomechanic focusing system consisting of a spherical mirror at nearly normal incidence has been designed and fabricated. The system represents good compromise between simplicity, throughput, and acceptable optical aberrations, providing a reduced image of the X-ray laser source. The first focusing experiment with this optical tool was conducted at the Prague Asterix Laser System (PALS) using currently the most powerful Ne-like zinc soft X-ray laser. Focal spots were directly observed by imaging the fluorescence induced by the X-ray laser on a Tb:doped phosphor screen. We have successfully focused the X-ray laser beam with the size of 5.6 mm x 8.4 mm down to 40 microns x 60 microns elliptical spot. The actual spot size was determined predominantly by the geometrical demagnification of the emitting X-ray source and astigmatism. The available peak intensity and fluence represent the highest value reported to date at 21 nm, suitable for new applications in nanostructuring of solids, radiobiology and laboratory astrophysics. Material ablation at such a short wavelength was observed for the first time. Using the same focusing system we performed a joint experiment at LULI laser facility (France) with a low-energy transient X-ray laser at 13.9 nm. The smallest focal spot had an average diameter of 20 microns and a homogeneous profile which makes it suitable e.g. for experiments on irradiation of biological samples where smooth, large zone and moderate fluence are both required. To measure emission spectra of plasmas created by the interaction of an X-ray laser with a thin foil target, we have designed and fabricated a high-resolution keV crystal spectrometer.
