Strong self-focusing in quasi-stationary laser plasmas

Collective Thomson scattering imaging has been used to study the propagation and self-focusing processes taking place during the interaction of a nanosecond laser beam with a preionized gas-jet plasma. The experiments have been carried out with a laser beam power PL exceeding greatly the critical power for ponderomotive self-focusingPc. It has been found that the position of the ion acoustic waves excited by stimulated Brillouin scattering depends only weakly on the initial focal position of the interactionlaser beam. These results, together with theoretical and numerical modeling, demonstrate that in such a regime (PL/Pc≫1)self-focusing is the dominant mechanism governing the localization of the interaction processes.