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Etude analytique et numérique de la réflectométrie dans un plasma fluctuant : modèles à une et deux dimensions

Abstract : Reflectometry is widely used in fusion plasmas for electron density profile measurements: the phase shift of a microwave reflected at a outoff layer depends upon the density profile. Density fluctuations may strongly disturb the profile reconstruction. It is therefore of primary importance to understand the role of the fluctuations (cutoff layer oscillation and Bragg backscattering) and to determine the correspondent phase shift. In this thesis, numerical and analytical models( one- and two-dimensional) are developed to study the effect of coherent fluctuations. After an overview of the principles of refleofometry and of the wave propagation in plasmas, the first part is devoted to the 1D numerical model ("radial" fluctuations). The spectral characteristics of the fluctuation are shown to play an important role. Analytical expressions of the phase shift are obtained under the Born approximation and are found in agreement with the numerical results. The nonlinear regime of Bragg scattering (Iarge amplitude fluctuations) exhibits phase jumps. This behavior is well reproduced by an analytical model. A second part describes the 2D code that allows for "azimuthal"fluctuations. The source is modeled as an emitting horn. The numerical results are compared with several existing theories. Only qualitative agreement is found. For fusion plasmas, the study confirms that fluctuations close to the cutoff layer play a dominant role.
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http://docnum.univ-lorraine.fr/prive/SCD_T_1997_0260_FANACK.pdf
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https://hal.univ-lorraine.fr/tel-01747387
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Submitted on : Thursday, March 29, 2018 - 10:58:22 AM
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Catherine Fanack. Etude analytique et numérique de la réflectométrie dans un plasma fluctuant : modèles à une et deux dimensions. Autre [cond-mat.other]. Université Henri Poincaré - Nancy 1, 1997. Français. ⟨NNT : 1997NAN10260⟩. ⟨tel-01747387⟩

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