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Modifications des propriétés de cohérence des faisceaux laser dans les plasmas de fusion par confinement inertiel

Abstract : Inertial con finement fusion by laser requires smoothed laser beam with well-controlled coherence properties. Such beams are made of many randomly distributed intensity maxima: the so-called speckles. As the laser beam propagates through a plasma its temporal and spatial coherence can be reduced. This phenomenon is called plasma induced smoothing. For high laser intensities, instabilities developping independently inside the speckles are responsible for the coherence loss. At lower intensities, only collective effects, involving many speckles, can lead to induced smoothing. This thesis is a theoretical, numerical and experimental study of these mecanisms. Accounting for the partially incoherent behavior of the laser beams requires the use of statistical description of the laser-plasma interaction. A model is developped for the multiple scattering of the laser light on the self-induced density perturbations that is responsible for a spreading of the temporal and spatial spectra of the transmitted light. It also serves as a strong seed for the instability of forward stimulated Brillouin scattering that induces both, angular spreading and red-shift of the transmitted light. A statistical model is developped for this instability. A criterium is obtained that gives a laser power (below the critical power for lamentation) above which the instability growth is important. Numerical simulations with the interaction code PARAX and an experiment performed on the ALISE laser facility con rm the importance of these forward scattering mecanisms in the modi cation of the laser coherence properties.
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  • HAL Id : tel-01785769, version 1

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Mickael Grech. Modifications des propriétés de cohérence des faisceaux laser dans les plasmas de fusion par confinement inertiel. Physique des plasmas [physics.plasm-ph]. University of Bordeaux, 2007. Français. ⟨tel-01785769⟩

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