Quality of an x-ray laser beam generated by irradiation of a thick target

A numerical and theoretical investigation was made of amplified spontaneous emission (ASE) from an x-ray laser in which a gain zone appears in a plasma on irradiation of a thick target and is characterised by an asymmetric concave profile of the electron density. An analysis is made of the power, angular divergence, and spatial coherence of the ASE from flat and bent targets. In the case of a flat target a confirmation is obtained of the experimental observation that the ASE coherence length has a strongly nonuniform distribution across the beam in the far-field zone. The optimal bending of the target makes it possible not only to reduce the divergence of the ASE and to increase considerably its power, but also to enhance coherence and ensure a uniform distribution of the coherence length across a beam. In the case of a bent target, the coherence can be improved additionally by creating a convex electron density profile in the gain zone.