We investigate the production of electron beams from the interaction of relativistically-intense laser pulses with a solid-density SiO(2) target in a regime where the laser pulse energy is approximately mJ and the repetition rate approximately kHz. The electron beam spatial distribution and spectrum were investigated as a function of the plasma scale length, which was varied by deliberately introducing a moderate-intensity prepulse. At the optimum scale length of lambda/2, the electrons are emitted in a collimated beam having a quasimonoenergetic distribution that peaked at approximately 0.8 MeV. A highly reproducible structure in the spatial distribution exhibits an evacuation of electrons along the laser specular direction and suggests that the electron beam duration is comparable to that of the laser pulse. Particle-in-cell simulations which are in good agreement with the experimental results offer insights on the acceleration mechanism by the laser field.

In the following work, we analyze one-dimensional (1D) and two-dimensional (2D) full particle-in-cell simulations of stimulated Raman scattering (SRS) and study the evolution of Langmuir waves (LWs) in the kinetic regime. It is found that SRS reflectivity becomes random due to a nonlinear frequency shift and that the transverse modulations of LWs are induced by (i) the Weibel instability due to the current of trapped particles and (ii) the trapped particle modulational instability (TPMI) [H. Rose, Phys. Plasmas 12, 12318 (2005)]. Comparisons between 1D and 2D cases indicate that the nonlinear frequency shift is responsible for the first saturation of SRS. After this transient interval of first saturation, 2D effects become important: a strong side-scattering of the light, caused by these transverse modulations of the LW and the presence of a nonlinear frequency shift, is observed together with a strong transverse diffusion. This leads to an increase of the Landau damping rate of the LW, contributing to the limiting of Raman backscattering. A model is developed that reproduces the transverse evolution of the magnetic field due to trapped particles. Based on a simple 1D hydrodynamic model, the growth rate for the Weibel instability of the transverse electrostatic mode and magnetic field is estimated and found to be close to the TPMI growth rate [H. Rose et al., Phys. Plasmas 15, 042311 (2008)].

An efficient method to describe the nonlinear evolution of Stimulated Brillouin Scattering in long scale-length plasmas is presented. The method is based on a decomposition of the hydrodynamics variables in long-and short-wavelength components. It makes it possible to describe the selfconsistent coupling between the plasma hydrodynamics, Stimulated Brillouin Scattering, and the generation of harmonics of the excited ion acoustic wave (IAW). This description is benchmarked numerically and proves to be reliable even in the case of an undamped ion acoustic wave. The momentum transferred from the electromagnetic waves to the plasma ions is found to induce a plasma flow which modifies the resonant three wave coupling between the IAW and the light waves.A novel picture of SBS arises, in which both IAW harmonics and flow modification reduce the coherence of SBS by inducing local defects in the density and velocity profiles. The spatial domains of Stimulated Brillouin activity are separated by these defects and are consequently uncorrelated, resulting in a broad and structured spectrum of the scattered light and in a temporally chaotic reflectivity.PACS numbers: 52.38. Bv, 52.35.Mw, 42.65.Es * Permanent address : Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester NY 14623, USA 1The description of parametric instabilities in laser-produced plasmas using simple coupled mode equations for three wave interaction is no longer sufficient whenever the longitudinal plasma waves are driven to large amplitudes. Then the nonlinearities of the longitudinal wave can induce detuning with respect to the three wave resonance. This is one of the reasons usually invoked to explain why these simplified models overestimate the scattering levels of Stimulated Brillouin Scattering (SBS). In this article we concentrate on SBS, which is the process by which the incident laser wave couples to an ion acoustic wave (IAW) togive rise to a scattered transverse wave. The generation of the harmonics due to the IAW fluid-type nonlinearity [1, 2, 3, 4, 5] is already known to be able to reduce significantly the SBS reflectivity when compared with the results involving simply a linearized IAW.However, the previous fluid-type models for SBS in Refs. [1, 2, 3, 4], aimed at taking into account the IAW nonlinearity, were incomplete because they did not properly describe the flow modification [6, 7] caused by the incident transverse wave momentum deposition. All the mentioned models [1, 2, 3, 4, 5] also ignored multi-dimensional effects. On the other hand, kinetic effects associated with particle trapping [8] give also rise to a nonlinear IAW frequency shift and therefore modify the SBS nonlinear behavior.In the present Letter, we reconsider the effect of the IAW nonlinearities on SBS by accounting properly for the flow modification caused by SBS. We first derive approximate equations describing simultaneously the plasma hydrodynamics (i.e. the long wavelength density and flow profiles), SBS, and the harmonic g...