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No description available.

No description available.

No description available.

Many laboratory and space plasma phenomena exhibit scaling, i.e., no characteristic spatial and/or temporal scale can be identified in their dynamics. This lack of a characteristic scale makes the dynamics of these systems extremely complex and intractable to analytical approaches. Their statistical features, however, appear to be simple and exhibit a degree of universality. We will explore two approaches to scaling in plasma systems, one based on avalanching sandpile model and the second one based on turbulence. The avalanching model developed here exhibits a wide range of dynamic behavior and incorporates other established models as limiting cases. A single control parameter that specifies the length scale over which the redistribution rule operates compared to the finite system size, allows us to explore different regimes of the model's dynamics close to and away from the existing fixed points. An advanced Virtual Reality visualization technique was employed to gain a better qualitative understanding of the sandpile behavior in the parameter space. This sandpile model was used to simulate features found in the fusion plasma in both low and high confinement modes. Because of the simplicity of this model, it was possible to formally characterize and explain the mechanisms underlying steep gradients formation and appearance of internal transport barriers, and to identify links to tokamak plasma behavior. The solar wind is a supersonic, super-Alfvenic flow of compressible and inhomogeneous plasma from the Sun. The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. In this case a generic and model independent method of differencing and rescaling was applied to identify self-similarity in the Probability Density Functions (PDF) of fluctuations in solar wind bulk plasma parameters as seen by the WIND spacecraft. The single curve, which we found to describe the fluctuations PDF of some quantities, is non-Gaussian. We model this PDF with two approaches-Fokker-Planck, for which we derived the transport coefficients and associated Langevin equation, and the Castaing distribution that arises from a model for the intermittent turbulent cascade. The technique was also used to quantify the statistical properties of fluctuations in the coupled solar wind-magnetosphere system. These quantitative and model-independent results place important constraints on models for the coupled solar wind-magnetosphere system.

La production de nanoparticules (NPs) par des décharges spark en phase liquide permet d’atteindre des rendements jusqu’à présent inégalés de l’ordre de quelques centièmes de milligramme par joule. Même si l’essentiel de l’énergie est dissipé dans la formation de la décharge, l’érosion des électrodes métalliques permet la production efficace de NPs. La nature des NPs formées est largement tributaire du liquide diélectrique dans lequel la décharge est réalisée. Il est ainsi possible de contrôler les nanoparticules produites en choisissant de manière ad hoc les électrodes et le liquide. Nous cherchons dans ce travail à comprendre les mécanismes qui ont lieu durant la décharge en étudiant différents cas d’élaboration de NPs soit d’alliages soit de composites. Les NPs synthétisées auront des applications dans divers domaines selon le(s) matériau(x) choisi(s). À l’aide d’un générateur d’impulsions nanosecondes, les décharges ont été créées en appliquant une impulsion de haute tension (10 kV- 200ns- 10 Hz) entre deux électrodes immergées dans de l’azote liquide. Trois systèmes principaux ont été étudiés : Si-Sn, Cu-Zn et Cu-Ag. Les tests ont été réalisés avec des électrodes pures ou avec des alliages contenant les 2 éléments en proportions variables pour améliorer notre compréhension sur la manière dont les nanoparticules sont formées. Des analyses en microscopie électronique en transmission à haute résolution (HRTEM), en spectroscopie des rayons X à dispersion d'énergie (EDX), en spectroscopie de perte d'énergie des électrons (EELS) et des analyses de micro-diffraction ont été menées pour caractériser les NPs synthétisées (morphologie, cristallinité, composition chimique, etc.). Enfin, des mesures de spectroscopie d’émission optique résolues dans le temps ont été réalisées pour disposer d’informations sur l’évolution temporelle des raies émises au cours de la décharge et ainsi sur les conditions qui prévalent dans le plasma / Discharges in liquids offer a simple way to synthesize nanoparticles at high rate and low cost. When spark discharges are ignited in a dielectric liquid, a strong heating of the electrode material occurs, producing a metallic vapor from which nanoparticles grow by condensation. Even if most of the energy is dissipated in the formation of the discharge, the erosion of the metal electrodes allows the efficient production of NPs. The nature of the NPs formed is largely dependent on the dielectric liquid in which the discharge is performed. It is thus possible to control the nanoparticles produced by choosing the electrodes and the liquid in an appropriate manner. We seek in this work to understand the mechanisms that take place during the discharge by studying different cases of elaboration of NPs either alloys or composites. The synthesized NPs will have applications in various fields depending on the material(s) chosen. Using a nanosecond pulse generator, the discharges were created by applying a high voltage pulse (10 kV-200ns-10 Hz) between two electrodes immersed in liquid nitrogen. Three main systems have been studied: Si-Sn, Cu-Zn and Cu-Ag. The tests were performed with pure electrodes or alloys containing the 2 elements in varying proportions to improve our understanding of how nanoparticles are formed. High resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS) and micro-diffraction analyzes were carried out to characterize the synthesized NPs (morphology, crystallinity, chemical composition, etc.). Finally, time-resolved optical emission spectroscopy measurements were performed to obtain information on the temporal evolution of the lines emitted during the discharge and thus on the conditions that prevail in the plasma