This thesis is divided in two parts : i) the laser channeling in hundreds of microns long under-dense plasmas (0.1nc<n<nc, nc being the critical density) of a laser pulse of intensity 10^18 to 10^20 W/cm² and duration 1-10 ps; ii) the saturation mechanisms of stimulated Raman back-scattering of a laser pulse of intensity 10^14 to 10^16 W/cm² and duration of about 1 ps. A parametric study was performed to study the channeling of a very intense laser pulse, using a 2D PIC (Particle In Cell) code. Various kinds of channels were obtained depending on the laser and plasma parameters, thereby reproducing and enlarging previous studies. Moreover, the channeling velocity was measured and scaling laws were established for homogeneous plasmas. They are then applied to inhomogeneous plasmas, similar to those encountered in inertial confinement fusion (ICF). It is then possible to estimate the energy necessary to channel to the critical density, an important step for the fast ignition scheme of ICF. Raman saturation was studied using numerical simulations, in order to determine if it is due to dephasing or to the growth of sidebands, using different approaches. The first is to study Raman simulations (electromagnetic) performed with kinetic PIC and Vlasov codes. The second, is to study the evolution of a plasma initialized with a distribution function after the adiabatic theory, using a Vlasov code (electrostatic). In this case, we observe the growth of a sideband, with dominant wavenumber and growth rate in good agreement with kinetic simulations. The saturation of the plasma wave can be caused by both saturation mechanisms.
