Interaction of an ultra-intense laser pulse with a nonuniform preformed plasma

The propagation of an ultra-intense laser pulse in a preformed plasma channel was investigated experimentally. Different regimes of propagation were observed when the pulse duration was varied. For a long pulse and powers lower than the critical power for self-focusing, P_L/P_C<1 (I₀ = 2×10¹⁷ W/cm²), the laser pulse was guided by the preformed plasma channel over three Rayleigh lengths (4 mm) and a longitudinal plasma wave was generated by envelope self-modulation of the pulse. For a short pulse and P_L/P_C≫1, the interaction was dominated by self-focusing and Raman instabilities. Numerical simulations were run for the latter case, giving results comparable to the experiment. The simulations were also used to investigate the dynamics of the instabilities at high power. They showed that strong Raman side scattering first occurs at the beginning of the interaction and is then followed by self-focusing and envelope self-modulation.