Etude expérimentale du développement et de la saturation des instabilités paramétriques dans des conditions représentatives d’un plasma de fusion par confinement inertiel

Abstract : Experiments have been performed in the multikilojoule regime with the two French laser facilities, the LULI 2000 (2kJ) and the “Ligne d’Integration Laser” (LIL, 12kJ). High electronic temperature plasmas with large dimensions were studied. I present in the thesis some of the results on laser-plasma interaction physics and parametric instabilities. First, we have designed experiments to compare the saturation level of the Stimulated Brillouin Scattering (SBS) reflectivities between the 2ω (526 nm) and 3ω (351 nm) in the LULI 2000 in preformed CH thick foil plasmas. [1]. By choosing accurately the creation beam characteristics, we managed to create similar plasmas at the two wavelengths which have allowed making a pertinent comparison. Similar saturated SBS reflectivities were obtained which have shown that, concerning the Brillouin instability in the saturation regime, the 2ω does not present more problems that the 3ω in large plasmas. Second, we have realized experiments in the LULI 2000 to analyze the saturated Stimulated Raman Scattering (SRS) reflectivities in preformed ethylene (C2H4) gas jet plasmas at different pressures laser beams smoothed by random phase plates [2]. The jet has been characterized using an interferometer and the characterization has shown that we could reach a maximal initial electronic density of ~10 percent of the critical density at 2ω. Results have permitted to realize the first experimental validation of a scaling law established in the 90’s concerning the saturated SRS reflectivities as a function of laser/plasma parameters. A third series of experiments was conducted with the LIL laser facility where we have used a 12 kJ laser beam at 3ω in a 2.7 ns pulse to study the imprint reduction of the laser pulse with a low-density foam (7-10mg/cc) at relatively high electronic temperature (Te = 2 keV) [3]. An experimental demonstration of the supersonic ionization of the foam and of the imprint reduction of the laser pulse after propagating through low-density foam plasma has been obtained. [1] S. Depierreux, D.T. Michel et al., Phys. Rev. Lett., 103, 115001, (2009) [2] D.T. Michel et al., Phys. Rev. Lett., 104, 255001, (2010) [3] S. Depierreux, C. Labaune, D.T. Michel et al., Phys. Rev. Lett., 102, 195005, (2009)
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Michel David. Etude expérimentale du développement et de la saturation des instabilités paramétriques dans des conditions représentatives d’un plasma de fusion par confinement inertiel. Physique Nucléaire Expérimentale [nucl-ex]. Ecole polytechnique, Palaiseau, France, 2010. Français. ⟨NNT : 2010EPPX0090⟩. ⟨tel-01436787⟩

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