We report on two different methods of producing a diffraction-limited focal spot for the study of the interaction of a high-intensity laser with a long underdense plasma: either a closed-loop adaptive optics or by selecting a reduced beam aperture. The influence of the intensity level on the propagation, filamentation and self-focusing of the resulting single hot spot has been studied. We have also studied the influence of a tranverse plasma flow, compared to a longitudinal, on the propagation.
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