Ph. Mounaix

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.

Effect of the speckle self-focusing on the stationary stimulated Brillouin scattering 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.

Interaction of two neighboring laser beams taking into account the effects of plasma hydrodynamics

The interaction between two neighboring laser beams focused in a hot underdense homogeneous plasma is investigated using the non-paraxial wave coupling code KOLIBRI [S. Huller et al., Phys. Scr. T63, 151 (1996)] in two and three spatial dimensions. Both the plasma hydrodynamic evolution and the stimulated Brillouin scattering (SBS) aspects are studied in the case of strongly damped ion sound waves. The hydrodynamic effects consist in ponderomotively driven density perturbations located between the beams which may, in turn, influence strongly the light propagation through the plasma. The two beams are found to merge whenever the distance between them is smaller than or of the order of their diameter. Concerning the SBS aspect, it is found that due to interference effects between the beams, the spatial amplification of the backscattered light is asymmetric with respect to the laser axis. SBS can also enforce the hydrodynamic effects and the beam merging.

Stimulated Brillouin scattering reflectivity in the case of a spatially smoothed laser beam interacting with an inhomogeneous plasma

The stimulated Brillouin scattering (SBS) instability is investigated theoretically in the case of a spatially smoothed laser beam interacting with an inhomogeneous plasma in the regime of strong ion acoustic damping. The domain of parameters being considered corresponds to most of the present day experiments carried out with nanosecond laser pulses interacting with preformed plasmas: the characteristic length for convective amplification is assumed to be much shorter than the longitudinal correlation length of the laser field. The SBS reflectivity of one individual hot spot is analytically computed taking into account thermal noise emission and pump depletion within the hot spot. The SBS reflectivity of the whole beam is then obtained by summing up the individual hot spot reflectivities in accordance with their statistical distribution.

Stimulated Raman backscattering instability in short pulse laser interaction with helium gas

Experimental and theoretical results on the stimulated Raman backscattering (SRS) reflectivity of a short laser pulse (120 fs) interaction with an optically ionized helium gas are presented. The reflectivity is measured as a function of the gas pressure from 1 to 100 Torr. A monodimensional (1‐D) theoretical model, including the refraction induced during the ionization process, describes the dependence of the SRS reflectivity with the gas pressure and explains its maximum at around 35 Torr. In the very low pressure case (<15 Torr), the radial ponderomotive force expels the electrons out of the propagation region before the laser pulse reaches its peak intensity and significantly reduces the observed reflectivity. A 1‐D hydrodynamic calculation, included in the model, describes this density depletion and a good agreement is obtained between theory and experiments in the whole range of pressures.

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.

Strong self-focusing in quasi-stationary laser plasmas

Collective Thomson scattering imaging has been used to study the propagation and self-focusing processes taking place during the interaction of a nanosecond laser beam with a preionized gas-jet plasma. The experiments have been carried out with a laser beam power PL exceeding greatly the critical power for ponderomotive self-focusingPc. It has been found that the position of the ion acoustic waves excited by stimulated Brillouin scattering depends only weakly on the initial focal position of the interactionlaser beam. These results, together with theoretical and numerical modeling, demonstrate that in such a regime (PL/Pc≫1)self-focusing is the dominant mechanism governing the localization of the interaction processes.

Extra ion feature of Thomson scattered light in the interaction of a 600 ps laser with helium gas jet

The interaction of a 600 ps laser pulse at 0.53 μm wavelength with helium gas jet with an electron density of 8×1019 cm−3 has been studied. In this experiment the plasma parameters for density and temperature were well-defined via time-resolved Thomson scattering. Ion Thomson scattering measurements at 45° show two extra satellites which correspond to the rescattering of the Brillouin backscattered light off the thermal ion acoustic waves. Analysis of the relative amplitudes of these satellites gives a very high value of 65% of the instantaneous reflectivity at its maximum. Theoretical spectra are in good agreement with the experimental ones.

Parametric Instabilities in Picosecond Time Scales

The coupling of intense laser light with plasmas is a rich field of plasma physics, with many applications. Among these are inertial confinement fusion (ICF), x-ray lasers, particle acceleration, and x-ray sources. Parametric instabilities have been studied for many years because of their importance to ICF; with laser pulses with duration of approximately a nanosecond, and laser intensities in the range 1014 — 1015 W/cm 2 these instabilities are of crucial concern because of a number of detrimental effects. Although the laser pulse duration of interest for these studies are relatively long, it has been evident in the past years that to reach an understanding of these instabilities requires their characterization and analysis in picosecond time scales. At the laser intensities of interest, the growth rate for stimulated Brillouin scattering (SBS) is of the order of picoseconds, and of an order of magnitude shorter for stimulated Raman scattering (SRS).