Thesis (M.S.)--University of Michigan, 1976.

Thesis (M.S.)--University of Michigan, 1985.

Th.--Math.--Aix-Marseille 2, 1978.

Shock waves generated by laser-driven ablation in solids have provided a great opportunity for the study of dense plasmas. The work presented in this thesis include measurements of Hugoniot curves and the reflectivity of shocked aluminum. In these experiments, planar aluminum targets were irradiated with a 0.53µm, 2ns (FWHM) laser pulse at irradiances up to ~ 10¹⁴/cm². Temporally and spectrally resolved measurements of the target rear surface luminous emission have yielded the shock speed and temperature Hugoniot curve which showed good agreement with equation of state predictions. In addition, temporally resolved reflectivity measurements of the shocked target rear surface compared well with a theoretical model for the electrical conductivity of a dense plasma. For copper and molybdenum targets, both the luminescence and the reflectivity measurements indicated that the heating of the dense target material was dominated by radiation transport from the coronal plasma rather than shock heating. An analysis of the molybdenum results indicate that x-ray shine-through may be the dominant energy transport mechanism to the target rear surface, whereas for the copper targets the transport process appears to be much more complex. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

Commençons par rappeler que l'objectif de la thèse était double : - numérique : il s'agissait de développer un code capable de simuler la saturation de l'instabilité de diffusion Raman stimulée (SRS) via le couplage avec les ondes acoustiques ioniques dans une description fluide, en plasma homogène et inhomogène et en géométrie multidimensionnelle. La modélisation choisie est basée sur un système composé d'équations de type paraxiales pour les équations électromagnétiques et de type Zakharov pour les équations des ondes de Langmuir et des ondes sonores. Le problème a été traité dans un milieu ouvert où, contrairement à un milieu périodique, la définition des conditions aux limites se doit d'être robuste sans que les artefacts numériques introduits ne faussent les résultats physiques, notamment en plasma inhomogène où la présence de points paraboliques est en mesure de déstabiliser les instabilités absolues, ce qui ajoute une "difficulté" supplémentaire non négligeable. - physique : il s'agissait d'analyser les mécanismes de saturation non-linéaire de l'instabilité Raman et d'établir, dans la mesure du possible, des lois d'échelle. De façon à tester les potentialités et les limites de notre code, nous avons choisi de nous concentrer plus particulièrement sur le régime kLλDe ∼ 0.2 (où kL est le nombre d'onde fondamental de l'onde de Langmuir générée par l'instabilité Raman et λDe la longueur de Debye électronique). Ce régime est certainement le régime le plus difficile à étudier dans le sens où il est intermédiaire entre les régimes extrêmes (i) kLλDe = 0.1 avec la présence de la cavitation et (ii) kLλDe = 0.3 qui donne un amortissement Landau électronique fort et des effets cinétiques, et pour cette raison tous les processus sont suceptibles de se développer, comme nous l'avons effectivement observé.

Plasma-based accelerators aim to efficiently generate relativistic electrons by exciting plasma waves using a laser or particle beam driver, and "surfing" electrons on the resulting wakefields. In the blowout regime of such wakefield acceleration techniques, the intense laser radiation pressure or beam fields expel all of the plasma electrons transversely, forming a region completely devoid of electrons ("bubble") that co-propagates behind the driver. Injection, where initially quiescent background plasma electrons become trapped inside of the plasma bubble, can be caused by a variety of mechanisms such as bubble expansion, field ionization or collision between pump and injector pulses. This work will present a study of the injection phenomenon through analytic modeling and particle-in-cell (PIC) simulations. First, an idealized model of a slowly expanding spherical bubble propagating at relativistic speeds is used to demonstrate the importance of the bubble's structural dynamics in self-injection. This physical picture of injection is verified though a reduced PIC approach which makes possible the modeling of problem sizes intractable to first-principles codes. A more realistic analytic model which takes into account the effects of the detailed structure of the fields surrounding the bubble in the injection process is also derived. Bubble expansion rates sufficient to cause injection are characterized. A new mechanism for generation of quasi-monoenergetic electron beams through field ionization induced injection is presented, and simulation results are compared to recent experimental results. Finally, a technique for frequency-domain holographic imaging of the evolving bubble is analyzed using PIC as well as a novel simulation method for laser probe beam propagation. / text

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No description available.

This thesis deals with several theoretical aspects of the interaction of an intense femtosecond laser pulse with a plasma. A mechanism for the enhancement of the collisional absorption of light at high intensities is described, involving the direct excitation of collective modes of the plasma, and the importance of this mechanism for a solid-density laser-produced plasma is studied under a range of conditions. An intensity-dependent collision rate is used in a numerical calculation of the reflectivity of a steep-gradient plasma, such as might be produced by an intense femtosecond laser pulse, and the conditions required to maximize absorption at high intensities are determined. The relative contributions of field-induced ionization and collisional ionization in laser-produced plasmas are studied, and it is shown that the behaviour of a gaseous plasma is almost solely governed by the field-induced process. A model is developed to simulate the propagation of an intense femtosecond laser pulse through an initially neutral gas, and this model is used to make predictions about spectral modifications to the laser pulse. Under certain conditions the spectrum is significantly broadened and suffers an overall blue shift. Quantitative fitting of theoretical spectra to experimental results in the literature is attempted, but is complicated by associated defocusing effects in the plasma. Field-induced ionization can produce a gaseous plasma which is significantly colder, for the same degree of ionization, than a plasma produced by collisional ionization. One possible application for a cold highly-ionized plasma is in a recombination x-ray laser, and the propagation model allows the calculation of the plasma temperature, which is a crucial parameter in assessing the feasibility of such schemes.