Coherent forward stimulated Brillouin scattering of a spatially incoherent laser beam in a plasma and its effect on beam spray

A statistical model for forward stimulated Brillouin scattering (FSBS) is developed for a spatially incoherent, monochromatic, laser beam propagating in a plasma. A threshold for the average power in a speckle is found, well below the self-focusing one, above which the laser beam spatial incoherence can not prevent the coherent growth of FSBS. Three-dimensional simulations confirm its existence and reveal the onset of beam spray above it. From these results, we propose a new figure of merit for the control of the propagation through a plasma of a spatially incoherent laser beam.Comment: submitted to PR

Electron kinetic effects in the nonlinear evolution of a driven ion-acoustic wave

The electron kinetic effects are shown to play an important role in the nonlinear evolution of a driven ion-acoustic wave. The numerical simulation results obtained (i) with a hybrid code, in which the electrons behave as a fluid and the ions are described along the particle-in-cell (PIC) method, are compared with those obtained (ii) with a full-PIC code, in which the kinetic effects on both species are retained. The electron kinetic effects interplay with the usual fluid-type nonlinearity to give rise to a broadband spectrum of ion-acoustic waves saturated at a low level, even in the case of a strong excitation. This low asymptotic level might solve the long-standing problem of the small stimulated Brillouin scattering reflectivity observed in laser-plasma interaction experiments

Electron and ion kinetic effects in the saturation of a driven ion acoustic wave

The role of ion and electron kinetic effects is investigated in the context of the nonlinear saturation of a driven ion acoustic wave(IAW) and its parametric decay into subharmonics. The simulations are carried out with a full–particle-in-cell (PIC) code, in which both ions and electrons are treated kinetically. The full-PIC results are compared with those obtained from a hybrid-PIC code (kinetic ions and Boltzmann electrons). It is found that the largest differences between the two kinds of simulations take place when the IAW is driven above the ion wave-breaking limit. In such a case of a strong drive, the hybrid-PIC simulations lead to a Berstein-Greene-Kruskal-like nonlinear IAW of a large amplitude, while in the full-PIC the IAW amplitude decays to a small level after a transient stage. The electron velocity distribution function is significantly flattened in the domain of small electron velocities. As a result the nonlinear frequency shift due to the electron kinetic effects compensates partly the nonlinear frequency shift due to the ion kinetic effects, allowing then for the parametric decay of the driven IAW into subharmonics. These observations lead to the conclusion that electron kinetic effects become important whenever the nonlinear effects come into play

Turbulence and self-consistent fields in plasmas

This paper is concerned with the role of self-consistency of the electric field in 1-D plasma turbulence. We first show that in the non-self consistent electric field problem excellent agreement is found between numerical experiments and quasilinear theory whenever the imposed electric field Fourier components have random phase. A discrepancy is exhibited between quasilinear prediction and numerical simulations in the self-consistent electric field case. This discrepancy is explained by the creation of a long correlation time of the electric field resulting from a strong wave-particle interaction. A comparison is made between quasilinear and renormalized propagator theories, and the Dupree Clump theory. These three theories are found to be self-contradictory in the regime of strong wave-particle interaction because they make an a priori quasigaussian assumption for the electric field

Harmonic decomposition to describe the nonlinear evolution of stimulated Brillouin scattering

An efficient method to describe the nonlinear evolution of stimulated Brillouin scattering(SBS) in long scale-length plasmas is presented in the limit of a fluid description. The method is based on the decomposition of the various functions characterizing the plasma into their long- and short-wavelength components. It makes it possible to describe self-consistently the interplay between the plasmahydrodynamics,stimulated Brillouin scattering, and the generation of harmonics of the excited ion acoustic wave(IAW). This description is benchmarked numerically in one and two spatial dimensions [one dimensional (1D), two dimensional (2D)], by comparing the numerical results obtained along this method with those provided by a numerical code in which the decomposition into separate spatial scales is not made. The decomposition method proves to be very efficient in terms of computing time, especially in 2D, and very reliable, even in the extreme case of undamped ion acoustic waves. A novel picture of the SBS nonlinear behavior arises, in which the IAWharmonics generation gives rise to local defects appearing in the density and velocity hydrodynamics profiles. Consequently, SBS develops in various spatial domains which seem to be decorrelated one from each other, so that the backscattered Brillouin light is the sum of various backscatteredwaves generated in several independent spatial domains. It follows that the SBSreflectivity is chaotic in time and the resulting time-averaged value is significantly reduced as compared to the case when the IAWharmonics generation and flow modification are ignored. From the results of extensive numerical simulations carried out in 1D and 2D, we are able to infer the SBSreflectivity scaling law as a function of the plasma parameters and laser intensity, in the limit where the kinetic effects are negligible. It appears that this scaling law can be derived in the limit where the IAWharmonics generation is modeled simply by a nonlinear frequency shift

Kinetic effects in stimulated Brillouin scattering

The role of ion and electron kinetic effects in the nonlinear evolution of stimulated Brillouin scattering (SBS) is investigated by means of particle-in-cell numerical simulations. The simulations were carried out in one and two spatial dimensions (1D and 2D), with a full PIC code, in which both ions and electrons are kinetic. The full PIC simulations are compared with those obtained from a hybrid PIC code (kinetic ions and Boltzmann electrons), making it possible to determine in which limit the electron kinetic effects are important. The simulation geometry corresponds to a coherent laser beam interacting with an expanding plasma slab. In the 1D simulations, the interaction becomes incoherent, as time goes on, in a domain that spatially begins in the plasma region close to the laser light entrance, and that ends within the plasma at a frontier which moves faster than the ion acoustic wave (IAW) velocity. The higher the laser intensity, the faster moves the frontier of this spatial domain. The SBS reflectivity drops at the very moment when this domain fills entirely the plasma. Two regimes have to be distinguished. In the regimes of low laser intensity, strong sub-harmonic generation of the excited IAW is observed to take place in this moving spatial domain, so that the SBS reflectivity drop is interpreted as being due to sub-harmonic generation. In the opposite regime of high laser intensity, there is no evidence of strong sub-harmonic generation, whereas a strong ion heating is observed, so that the reflectivity drop is interpreted as being due to enhanced ion damping. In the 1D simulations the electron kinetic effects are found to be able to smooth temporally the SBS reflectivity, although the overall picture remains the same when the electrons are taken as a Boltzmann fluid. In the 2D simulations, the SBS reflectivity is observed to drop rapidly in time because of the efficient nonlinear Landau damping on the ions, as previously reported by Cohen et al. [1]. In these 2D simulations, the electron kinetic effects are found to play a negligible role as compared with the ion kinetic effects

PHASE SPACE GRANULATION AS A RESULT OF MODE-MODE COUPLING EFFECTS

No abstract availabl

Effets des interactions résonnantes ondes-particules en turbulence faible des plasmas

Dans cet article, nous passons en revue les développements récents de la théorie de la turbulence faible des plasmas lorsque la turbulence est générée par une interaction résonnante entre les ondes et les particules : ce sont d’une part les approches qui utilisent des propagateurs renormalisés et, d’autre part, la théorie de la granulation de l’espace des phases. Nous employons une méthode diagrammatique qui permet d’établir de façon systématique chacune de ces approches et de faire le lien entre elles. Nous montrons que les théories de l’élargissement de résonance n’améliorent pas de façon significative les résultats obtenus à l’aide des développements conventionnels. Nous établissons qu’au contraire le phénomène de la granulation de l’espace des phases remet en cause les hypothèses de la turbulence faible : dans certaines conditions, les agrégats émettent de façon cohérente un champ électrique induit supplémentaire qui modifie les propriétés statistiques du champ

A RECONSIDERATION OF QUASILINEAR THEORY

It is shown that even within the quasi linear framework, mode coupling terms give a zero order contribution to the growth rate.Nous montrons que même dans le cadre de la théorie quasilinéaire, les termes de couplage de modes apportent une contribution d'ordre zéro au taux de croissance