Electromagnetic wave scattering experiments in hall thruster plasma plumes
We report on electromagnetic wave (EM) scattering experiments using a 17 GHz signal propagated through an electric propulsion Hall thruster plume at different radial and axial distances. The goals of the experiments were to evaluate the spectral modification of the EM signals after propagating through the plume. These spectral modifications are attributed to temporal variations in the plasma due to propagating density perturbations. The experiments were performed on a D-55 Hall thruster manufactured by TNIIMASH (Research Institute of Machine Building) and the University of Michigan 5 kW Hall thruster. Close to the exit plane on the D-55 thruster axis, the spectral response is dominated by a first sideband at approximately +/-100 kHz with respect to the 17 GHz signal. Further downstream in the axial direction, first and second sidebands are both present for the D-55. The spectral response is not distinguishable above the baseline noise beyond +/-300 kHz with respect to the signal. For the UM thruster, only first sidebands at +/-12 kHz were observed as a function of axial distance. We show that present models do not adequately predict the observed axial variations or sideband levels. A revised model that assumes a rotating Gaussian density perturbation and which is assumed generated by azimuthal drift wave oscillations is shown to allow better matching of sideband levels and their variations along the plume axis.
