Transport non linéaire dans un réacteur Hélicon

Radiofrequencies Plasmas, widely used microelectronics, requires sufficiently low ionization rates so that the diffusion of plasma is governed by charges particles interaction with the neutral gas. Space propulsion requires higher ionization rates (few %), modifying considerably the transport of the species towards the walls. This thesis presents a theoretical and experimental analysis of these strongly ionized plasmas in the presence of an external magnetic field. Several unidimensional fluid models including neutral dynamics in various modes of pressure are studied. Their analysis, primarily analytical, highlights the presence of a neutral depletion at the center of the discharge which involves an increase in the ion flux at the wall. The power balance and the particle balance become coupled. The addition of the neutral energy balance shows that the gas heating is the principal cause of the neutral depletion. Various measurements in the diffusion chamber of an helicon reactor corroborate these theoretical forecasts. The increase in temperature of gas is quantified by LIF on argon metastables. The neutral gas density, measured by TALIF, is minimum in the center of the discharge and also presents a decrease of its mean value, very sensitive to the magnetic field. A temporal study of this phenomenon shows that the minimum of depletion in the center appears in a typical time of diffusion (milliseconds) whereas the mean density, resulting from balance between injection and gas pumping, spends more than one second to be stabilized.