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New target geometries for collisional excitation x-ray laser experiments (in nickel) were proposed, analyzed and experimentally studied on the GDL laser. Experiments using a short line focus lens with new target geometries showed general agreement with predictions. The new geometries are designed to yield a higher gain and reduced refraction due to: (i) a higher plasma density, (ii) a wider lateral density profile, and (iii) a concave lateral density profile. These new geometries were: (i) two parallel exploding (thin) foils, irradiated from one side only; (ii) two ablating (thick) foils, one of which is irradiated on its inner face, and (iii) an exploding foil in front of an ablating foil, irradiated by a single laser beam incident on the thin foil. New experiments with a long line focus are in progress. The intensity ratio of Ne-like and F-like Ni lines is used to deduce the temperature, and these results together with the absolute intensity yields the density profile. The results show improvement achieved with the new target geometries: the density is higher (leading to a higher gain), and the concave density profile results in collimation rather than divergence of the x-ray laser beam. Theoretical developments included: (i) prediction of high gain on new type transitions in neon like ions, involving the excitation of an inner (2s) electron, and (ii) development of a ray tracing code for an amplifying medium of varying (e.g., collimating) lateral density profile. Results using this code are shown for convex as well as concave lateral density profiles.
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