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Etude de l'interaction d'un faisceau laser intense avec un plasma long et chaud dans le contexte du schéma d'allumage par choc

Clément Goyon 1
LULI - Laboratoire pour l'utilisation des lasers intenses
Abstract : Shock ignition is an alternative direct-drive scheme for inertial fusion that consists in two steps. The first one is a several nanoseconds long compression with low intensity beams. The second one is a several hundred of picoseconds stage using high intensity beams to create a converging shock leading to ignition. During the second phase, the laser beam goes through a long and hot under-critical plasma. However, the coupling of this intense pulse with the coronal plasma has not been much studied experimentally or numerically. Then, the energy absorbed as well as the role of parametric instabilities regarding reflected or transmitted intensity cannot be predicted. In this PhD dissertation, we describe an experimental study of an intense laser pulse between 2.1015 W/cm2 and 2.1016 W/cm2 interacting with millimetric plasma heated close to one keV. We begin with a theoretical description of the interaction conditions in the coronal plasma. Brillouin scattering is in strongly coupled regime, Raman instability is kinetic regime and laser intensity is above ponderomotive filamentation threshold. We recreate these interaction conditions experimentally by means of pre-heated targets which are foams or thin plastic foils. Then, we present the first measurements of time resolved backscattered spectra from the smoothed picosecond beam as well as transmitted intensity distribution through the plasma. We find that Brillouin instability can be responsible for up to 60% reflectivity in plasmas with electronic density close to critical while Raman reflectivity stays at low levels. Transmitted intensity distribution is smoothed by the propagation and its diameter increases compared to the laser focal spot in vacuum. Finally, we discuss interaction measurements in nanosecond regime to highlight the fact that parametric instabilities reduction is essential for shock ignition to be a successful scheme.
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Contributor : Clément Goyon <>
Submitted on : Monday, September 8, 2014 - 3:00:47 AM
Last modification on : Friday, July 31, 2020 - 9:22:05 AM
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  • HAL Id : pastel-01061546, version 1


Clément Goyon. Etude de l'interaction d'un faisceau laser intense avec un plasma long et chaud dans le contexte du schéma d'allumage par choc. Physique des plasmas [physics.plasm-ph]. Ecole Polytechnique X, 2014. Français. ⟨pastel-01061546⟩



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