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

XUV interferometry using high-order harmonics: Application to plasma diagnostics

In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e[minus sign/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations

Effect of the speckle self-focusing on the stationary SBS reflectivity from a randomized laser beam in an inhomogeneous plasma

The effect of laser light self-focusing (SF) in speckles on stimulated Brillouin scattering (SBS) in an inhomogeneous plasma is studied. It is found that below but near critical power SF dramatically enhances the SBS reflectivity from an individual speckle, while above the critical power the pump power depletion due to SBS prevents strong SF and limits the maximum laser intensity in a speckle. The parameters that control the SBS/SF interplay are the ratio of plasma inhomogeneity scale length to the speckle length and the product of the plasma density and the speckle cross-section. The consequences of the SF effect on the averaged SBS reflectivity of the randomized laser beam are revealed and their manifestations in recent SBS experiments with preformed plasmas are discussed. Physics of Plasmas 4(12) (1997) PACS numbers: 52.40.Nk, 52.35.Mw, 52.35.Nx, 42.65.Jx Typeset using REVT E X I. INTRODUCTION A lot of experimental and theoretical work has been devoted to the study of parametric..

Advanced Model for SBS of a Randomized Laser Beam and Application to Polarization Smoothing Experiments with Preformed Underdense Plasmas

An advanced statistical model is presented, which describes the SBS of a randomized laser beam interacting with an underdense, expanding plasma. The model accounts for the self-focusing of speckles and for its influence on the speckles SBS reflectivity in the regime where the effect of plasma heating is important. Plasma heating has an important effect on speckle self-focusing and it decreases the SBS threshold and also decreases the SBS reflectivity. The model exhibit a good agreement with the measured SBS levels at the LULI multi-beam facility for a broad range of the laser and plasma parameters and both types of beam smoothing--RPP and PS. Both the model and the experiments confirm that the PS technique allows to control the SBS level more efficiently than RPP

SBS reflectivity from spatially smoothed laser beams: random phase plates versus polarization smoothing

The reflectivity due to stimulated Brillouin backscattering (SBS) from an ensemble of independent laser speckles is investigated for different speckle statistics. Calculations are based on numerical simulations with a multidimensional code and a compact model describing the main features of speckle self-focusing. In particular, the simulations and the model are applied to speckle ensembles due to the random phase plate (RPP) and polarization smoothing (PS) techniques. A stronger SBS inhibition for PS with respect to the RPP technique is demonstrated. PACS numbers: 52.40Nk, 52.35.Mw, 52.35.Nx I. INTRODUCTION The development of laser beam smoothing techniques is currently an important element of laser fusion research. Most smoothing techniques translate the spatial intensity distribution of generic laser beams into an intensity distribution consisting of a stochastic ensemble of small-scale "speckles" or "hot spots". 1--4 Although such techniques remove large-scale intensity fluctuat..

Laser-Plasma Interaction Physics at LULI

Laser-plasma interaction physics is studied in the context of laser fusion using the six-beam laser facility at LULI. Interaction between RPP laser beams and well-characterized preformed plasmas has been performed to study various aspects of stimulated Brillouin and Raman scattering (SBS and SRS), self-focusing and filamentation. Thomson scattering of a short wavelength probe laser beam was used to provide a complete characterization of the plasma (electron temperature, density, flow velocity) and measurements of the density fluctuations associated with ion acoustic waves and electron plasma waves, with temporal, spatial, frequency and wavenumber resolution. Among the different studies, they will present results on the effect of polarization smoothing, target material, multi-species plasmas, and Langmuir decay on parametric instabilities