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Two-Dimensional Modelling of the Hall Thruster Discharge: Final Report

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Final rept. 1 Aug 2006-1 Aug 2007

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A model of the plasma interaction with the dielectric walls of a Hall thruster has been presented. It accounts for partial thermalization of the electron population through a single parameter 34 and includes a two-population, four-parameter model for SEE. Analytical expressions are obtained for the main parameters characterizing that interaction, such as the particle and energy fluxes to the walls and sheaths, which are needed as boundary conditions of quasineutral models of the full discharge. The behavior for low thermalization is shown to differ greatly from the commonly-used, high-thermalization case. This is very relevant for Hall thrusters, where there is a growing conviction that electron thermalization is low. At low thermalization, energy losses are close to its minimum, the charge saturation limit is not attainable, and the sheath potential is small the different roles of beam and primary electrons on these characteristics have been analyzed. Significant decreases of the parallel temperature of primary electrons and, therefore, of the plasma flux into the sheath take place only at the very-low thermalization limit. The investigation of the emission model for secondary electrons has shown that the presence of a relevant fraction of elastically reflected electrons affects greatly the response. They tend to amplify the relative densities of untrapped electrons their effect on the net primary and beam fluxes comes out from the zero electrical current balance. It is reiteratively found that the role of the sheath potential fall is to adjust the primary electron flux to wall and not vice versa. Although most of the analysis is carried out for an energy-independent yield of reflected electrons, a temperature-dependent yield expression is proposed, which avoids integrals expressions at the same time that it recovers approximately the reduction of that yield with the impact energy.

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  • Electric and Ion Propulsion

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