Plasma density fluctuations driven by a randomized laser beam and application to self-smoothing

Laser beam smoothing consists in introducing a random phase delay on the laser electric field in a transverse plane. In this work we focus our attention on the density fluctuations driven by the random laser beam itself. We study ponderomotively driven fluctuations and derive their average level and spectrum. Then we introduce corrections to account for the collisional effects and discuss their importance in the fluctuation enhancement. Our theoretical calculations are compared to the interaction code PARAX. The multiple scattering of random laser beams on self-induced density fluctuations is responsible for the modification of the spatial and temporal coherence properties of the transmitted light. We investigate within a statistical approach the excitation of density fluctuations by randomized laser beams. Their spectrum is derived and their level is estimated as a few percents. The temporal coherence loss of the beam during the propagation induces an increase of the fluctuation level as √T. The thermal effects on density fluctuations are important even in the weakly collisional limit and therefore have to be accounted for in the context of laser Megajoule (LMJ) and national ignition facility (NIF)