Growth of resistive instabilities in ExB plasma discharge simulations

Two-dimensional hybrid numerical simulations of ExB discharges used in Hall thruster propulsion point to the presence of strong fluctuations attributable to resistive instabilities in the frequency range of f{approx_equal}0.1-10 MHz and the wavenumber range of {lambda}{sup -1}{approx_equal}10-500 m{...

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Bibliographic Details
Published in:Physics of plasmas 2008-01, Vol.15 (1)
Main Authors: Fernandez, E., Scharfe, M. K., Thomas, C. A., Gascon, N., Cappelli, M. A., Mechanical Engineering Department, Stanford University, Stanford, California 94305-3032
Format: Article
Language:English
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Summary:Two-dimensional hybrid numerical simulations of ExB discharges used in Hall thruster propulsion point to the presence of strong fluctuations attributable to resistive instabilities in the frequency range of f{approx_equal}0.1-10 MHz and the wavenumber range of {lambda}{sup -1}{approx_equal}10-500 m{sup -1}. Analytical analyses confirm that these resistive modes are of the convective type, become increasingly unstable at low electron mobility, and are particularly intense at high voltage. The simulations, which model cross-field electron flow via an experimentally measured mobility, exhibit large fluctuation power in a region corresponding to a strong electron transport barrier. The analysis gives an electron mobility ({mu}{sub e}) -dependent growth rate ({gamma}) scaling as {gamma}{approx}{mu}{sub e}{sup -1/2}. The predicted phase velocity of these waves is close to the ion velocity, somewhat lower than that seen in the simulations. Including the electron pressure contribution lowers the growth rate at high frequencies, and introduces a phase velocity that is shifted by {+-} the ion acoustic speed for the stable and unstable branch, respectively. Surprisingly, the phase velocity of the strong disturbances at high frequency seen in the simulations is found to be in agreement with that of the initially stable branch. Finite ionization/particle wall recombination does not change the overall conclusions at high frequencies. However, at lower f or larger {lambda}, the growth rate of the instability is dominated by the ionization rate, and the disturbances are better described as 'ionization' instabilities. The transition/competition between ionization, electron pressure, and resistive behavior gives rise to a 'quiescent frequency band' where the growth rate is found to be small, consistent with what is seen in the numerical experiments. While simple linear analysis captures much of the observed simulation behavior, comparison with limited experimental data at low frequency suggests that other effects, in particular azimuthal dynamics, are very important, and further motivate extending the hybrid simulation models to three dimensions.
ISSN:1070-664X
1089-7674
DOI:10.1063/1.2823033