This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 mum of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

International audienceA two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations

The propagation of an ionization wave through a subcritical foam is studied under inertial confinement fusion conditions. Independent measurements of the ionization wave velocity are compared with hydrodynamic simulations and analytical models. It is shown that simulations of a homogeneous material at equivalent density strongly overestimate the front velocity. The internal foam structure can be accounted for with a simple model of foam homogenization that allows improving agreement between experiment and calculations.

The nonlinear paraxial propagation of a laser beam inside a hot plasma has been simulated by three-dimensional simulations, where the laser beam is smoothed by spectral dispersion and the plasma response modeled by ion acoustic waves. The typical coherence times and transverse correlation sizes of the beam pattern are provided as a function of the propagation distance, the incident irradiance, the initial laser bandwidth, and the plasma density damping.