D. Pesme

Space and time behavior of parametric instabilities for a finite pump wave duration in a bounded plasma

The space and time behavior of the decay waves is computed analytically in the regime of standard parametric decay. The plasma is assumed to be homogeneous and bounded. The pump wave has a finite pulse duration. The propagation of the pump wave is taken into account, its depletion is ignored. The parametric growth is solved in terms of fluctuating initial and boundary conditions corresponding to thermal noise at equilibrium. Fluctuating source terms, representing noise emission, are accordingly retained in the coupled mode equations. The initial stage of parametric growth is investigated in detail; the time from which the asymptotic concept of absolute or convective instability applies is computed. The connection between the Manley–Rowe and flux conservation relations is discussed.

Filamentation in long scale length plasmas: Experimental evidence and effects of laser spatial incoherence

Signatures of filamentation have been observed in preformed plasmas using complementary diagnostics: time‐resolved images of the transmitted laser light, dark field imaging, and time‐resolved spectra of Raman light. This last diagnostic clearly shows the presence of small channels inside the plasma with temporal evolution in agreement with the formation of filaments. The filamentary structures disappeared when random phase plates were used in the laser beam. This result is in agreement with a theoretical analysis showing that filamentation does not grow when the speckle size is smaller than the perturbation wavelength which maximizes, in the coherent case, the filamentation growth.

The local–global analysis of the stimulated Brillouin scattering in the regime of nonlinear sound waves

The effect of ion sound wave (ISW) nonlinearities on the stimulated Brillouin scattering (SBS) in long plasmas is investigated within the framework of the Korteweg–de Vries–Maxwell equations. The nonlinear evolution of the driven ISW results in the localization of the ion density on a scale shorter than the wavelength (λs) of the resonant ISW satisfying SBS three‐wave matching conditions. Since the transverse wave amplitudes vary on a much longer scale, a local–global modeling of SBS is proposed in which this scale separation is exploited. The local part of the procedure includes a solution to the damped KdV equation with periodic boundary conditions and driven by a constant amplitude ponderomotive force. In the global part of the analysis approximate solutions for the transverse waves in long plasmas are constructed using the results from the local part. Particle‐in‐cell simulations have been performed in order to investigate the importance of kinetic effects for the local model. Numerical results obtain...

Coexistence of stimulated Raman and Brillouin scattering in laser-produced plasmas

Many different plasma instabilities and plasma modes can coexist in laser plasmas. These instabilities are usually treated as if they occur in isolation, however, more attention has recently been paid to how they interact with each other. This paper reviews three examples of experiments that have demonstrated an interaction between stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). In each case, the presence of SBS is seen to modify the behavior of the SRS process. In one case, SRS was totally suppressed by seeding SBS. The same mechanism might also be responsible for the gap in the Raman spectrum. Some theoretical and numerical studies of the interaction between SRS and SBS are also discussed, showing how increasingly detailed models can give improved agreement with experimental observations.

Stimulated Brillouin scattering in picosecond time scales: Experiments and modeling

This paper presents an experimental and theoretical study of stimulated Brillouin scattering (SBS) in laser produced plasma using a laser pump with a duration of 8–10 psec. The experiments were performed in a preformed plasma to minimize the flow velocity and have the same plasma conditions over a large range of laser intensities. The reflectivity was then compared to theoretical results over an intensity range of 1013–2×1015 W/cm2. A short pulse was used so that the SBS was in the temporally growing regime and saturation was not an issue.

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 plasma hydrodynamics, 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 n...

Evolution of the stimulated Raman scattering instability in two-dimensional particle-in-cell simulations

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 th...

Laser–plasma interaction studies in the context of megajoule lasers for inertial fusion

Laser–plasma interaction (LPI) physics is one the major issues for the realization of inertial fusion. Parametric instabilities may be driven by the incident laser beams during their propagation in the underdense plasma surrounding the fusion capsule. These instabilities may result in various effects detrimental to a good energy transfer from the laser beams to the target: the backscattering of the incident beams, the generation of energetic electrons which might preheat the fusion fuel, and the spoiling of the laser beam alignment. The control of the linear growth of these instabilities, together with the understanding of their nonlinear saturation mechanisms are therefore of fundamental importance for laser fusion. During the past few years, a series of new concepts have emerged, deeply modifying our approach to LPI physics. In particular, LPI experiments are now carried out with laser beams which are optically smoothed by means of random phase plates. Such beams are characterized inside the plasma by randomly distributed intensity maxima. Filamentation instabilities may locally increase the laser intensity maxima and deplete the electron density, leading to an intricate coupling between various nonlinear processes. One of the most striking features of this intricate coupling is the resulting ability of the plasma to induce additional temporal and spatial incoherence to the laser beams during their propagation. The increased incoherence may in turn reduce the level of backscattering instabilities.

Numerical simulation of filamentation and its interplay with SBS in underdense plasmas

We present results of a new code which models the interaction of intense electromagnetic beams with the low-frequency dynamics of the plasma fluid in two or three spatial dimensions. The light propagation is treated without the restriction of the paraxial optics approximation. The numerical scheme is based on spatial discretisation in the direction along the axis of the incident laser light whereas the transversal directions are treated spectrally. The specific cases discussed here refer to conditions found in recent experiments where filamentation was diagnosed [1]. In this study we first focus on effects originating from filamentation and/or forward stimulated Brillouin scattering (SBS), which can hardly be distinguished in the nonlinear case. We discuss then the impact of side- and backward SBS on these effects, observing the onset of an absolute filamentation instability predicted by Luther et al. [2].

Space and time behavior of backscattering instabilities in the modified decay regime

The space and time behavior of parametric backscattering instabilities is computed analytically in the so‐called modified decay regime. The plasma is assumed to be homogeneous and of finite length. The propagation of the pump wave and its finite pulse duration both are taken into account, its depletion is ignored. The parametric growth is solved in terms of fluctuating initial and boundary conditions corresponding to thermal noise at equilibrium. Fluctuating source terms, representing spontaneous emission of waves, are accordingly retained in the coupled mode equations. The initial stage of the instability is investigated in detail; the time from which the time asymptotic concept of absolute or convective instability applies is computed. Approximate expressions for the fluctuations of the waves, that are uniformly valid for any gain factor and any time, are derived.