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Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

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An experimental study of the interaction between 10.59 μm C0₂-laser radiation and the plasma produced by that radiation on plane, massive targets in a vacuum, is described. The laser used was a TEA double discharge device, giving an in vacuo irradiance in the regime of 10¹² W/cm² with 50 ns FWHM gain switched pulses. The laser oscillator cavity was operated in both stable and unstable configurations. Particular emphasis was placed on the full diagnosis of the scattered light and experiments were, therefore, designed to investigate the spatial, temporal and spectral characteristics of this unabsorbed fraction. The irradiation conditions under which the experiments were performed were investigated by analysis of the far field energy distribution. High absorption of the incident radiation was inferred from the low reflectivity level, the fraction of the incident radiation scattered out- side the solid angle of the focusing lens being accounted for, using a novel collecting mirror, or by the use of calibrated burn paper. Thus a total reflection was measured for the first time. A simple model was developed to enable the classical absorption contribution to the total absorption to be estimated, and results indicated that inverse bremsstrahlung absorption would be an important process for the conditions pertinent to the interac- tion. The contribution of anomalous absorption and scattering processes was estimated. Predicted cold electron temperatures and density scale lengths from the model, agreed well with those determined experimentally without the need to invoke flux inhibition. The spectral composition of the backscattered radiation in the region of the fundamental was investigated, and measured spectra were successfully interpreted by considering the stimulated Brillouin scattering process, and the Doppler effect of the expanding plasma corona. In particular, the depen- dence of the parametric growth rate on electron density was investigated by numerical solution of the ion acoustic wave dispersion relation. The angular distribution of the backscattered radiation was determined using annular apertures in the backscattered beam and the distribution found to be non-isotropic. Interpretation was based upon the resonance absorption process in a non-planar plasma geometry and invoked the retro reflecting nature of the stimulated Brillouin scatter mechanism. To investigate the effect of wave refraction in the underdense plasma (exhibiting a continuous density profile), a compact and readily used ray-tracing routine was developed and applied to plasmas with planar, rippled and simple concave or convex iso-density contours.

Les chocs forts sont présents dans des phénomènes astrophysiques variés. De tels chocs sont fortement influencés par le rayonnement par son couplage avec l’hydrodynamique. Par suite, leur topologie et leur dynamique sont assez complexes. Générer de tels chocs hypersoniques en laboratoire, dans des conditions contrôlées, est ainsi un outil pertinent pour étudier l’influence du rayonnement et pour comparer aux résultats des simulations numériques. Ces chocs sont générés par des lasers intenses et par des moyens électromagnétiques. La première partie du texte est consacré à l’étude numérique et expérimentale de l’interaction de deux chocs induits par laser se propageant en sens contraires. Les expériences ont été menées sur l’installation laser kJ PALS, qui permet de former deux chocs avec des vitesses propres différentes ($\sim$ 30-55 et 10-25 km/s respectivement) dans des gaz rares à pression faible (moins de 1 bar). Des diagnostics ont été installés : interférométrie visible, spectroscopie visible à résolution spatiale et temporelle, spectroscopie XUV intégrée en temps. Nos expériences montrent une forte interaction entre les deux précurseurs radiatifs. Les paramètres physiques du plasma ont été déduits de ces diagnostics et comparés aux résultats de simulations monodimensionnelles. La seconde partie est consacrée à la conception d’une expérience où le choc est généré de façon électromagnétique. L’optimisation de ce générateur est présentée ainsi que l’environnement expérimental permettant d’étudier des chocs jusqu’à 30 km/s dans des gaz rares peu denses (1 mbar). / Strong shocks are present in various astrophysical phenomena. Such shocks are strongly influenced by the radiation through its coupling with hydrodynamics. Thus their topology and dynamics are quite complex. Generating such hypersonic shocks in the laboratory, with controlled conditions, is thus an adequate tool to study the influence of radiation and to compare them with numerical simulations. Such shocks can be generated by intense lasers and electromagnetic devices.The first part of this dissertation concerns the numerical and experimental study of the interaction of two counter propagating laser-driven shocks. The experiments, performed at the kJ PALS laser facility allowed to generate shocks with different speeds ($\sim$ 30-55 km/s and 10-25 km/s), in noble gases and low pressure (less than 1 bar). Several diagnostics were implemented: visible interferometry, time- and space-resolved visible spectroscopy, and time integrated XUV spectroscopy. Our experiment shows a strong interaction of one radiative precursor onto the second one. The physical parameters of the plasma were deduced from the diagnostics and compared with 1-D simulation results. The second part is devoted to the design of an experiment where the shock is generated electromagnetically. The optimization of this generator is presented and also the full experimental set up which allows studying shock $\sim$ 30 km/s in noble gas at $\sim$ 1 mbar.

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