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Transport turbulent d'un plasma à travers un champ magnétique : observation par diffusion collective de la lumière.

Abstract : On a toroidal plasma discharge, a collective light-scattering has been set to investigate plasma turbulence and transport. The scattering device output signal is proportional to the space Fourier transform of the plasma density. We justied the use of collective scattering as a tool to investigate large scale fluid density fluctuations thanks to a calculus taking into account the discrete nature of scattering particules, i.e. the electrons and the density fluctuations induced by their thermal movement. We found that the discrete nature of scattering particules is negligeable but possibly detectable. The time autocorrelation function of the scattered signal as a function of k is shown to be the Fourier transform of the displacement probability distribution. The steady state of the plasma is investigated thanks to Langmuir probes. We found that homogeneous plasma is obtained when adding a small vertical component to the magnetic field. The two-dimensional static form factor is measured in absolute units as a function of k. The autocorrelation function of the scattered signal, i.e. the Fourier transform of the displacement probability distribution, is investigated at dierent times as a function of k. It is found to be consistent with a Levy walk model with a characteristic exponent α close to 1. The displacement probability distribution is thus close to a Cauchy function instead of a Gaussian. This opens the way to build and check models exposed in theoretical works.
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Submitted on : Wednesday, July 28, 2010 - 4:20:53 PM
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Nicolas Lemoine. Transport turbulent d'un plasma à travers un champ magnétique : observation par diffusion collective de la lumière.. Physique [physics]. Ecole Polytechnique X, 2005. Français. ⟨pastel-00002317⟩

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