Electrostatic instabilities in a low-power planar Hall thruster: Insights from PIC simulations

Abstract The planar Hall thruster (PHT) eliminates the channel wall of the traditional Hall thruster to avoid the plasma erosion-induced decrease in thruster performance and operation lifetime, which has promising application prospects for future complex space missions. However, the nature of electrostatic instabilities and the anomalous electron transport in PHTs remain unknown. In this paper, the PHT discharge is numerically investigated by a 2D-3V collision-less PIC model in the axial-azimuthal plane. The calculated oscillation dominant frequency is compared with the measured data by the ion saturation probe, and satisfactory agreements are reached. Insights into the spatiotemporal dynamics of the plasma characteristics in the PHT are provided. Short-wavelength azimuthal waves, large-scale azimuthal turbulence, and axial modes are observed during the discharge. The wavenumber-frequency power spectra reveal that the azimuthal oscillations correspond to the electron drift instability (EDI) evolving towards the ion-acoustic mode, and the axial mode is associated with the ion transit-time instability (ITTI). The excitation of ITTI is closely linked to double-layer instability driven by variations in electron transport. Besides, the relationship between the enhanced cross-field current and plasma fluctuations is established. The azimuthal instabilities dominate the formation of the cross-field electron current through the coherence between the electron density and azimuthal electric field, while the axial ITTI significantly influences transport dynamics via nonlinear interactions that generate large-scale azimuthal turbulent structures. Finally, the effect of the imposed ion current densities on the instabilities is explored. The transition to turbulence in the azimuthal mode, along with oscillations in the ion velocity, is observed at low current densities. These results provide a new physical description of the PHT from the perspective of discharge instabilities, which serves as a foundation for future operating condition selection and the development of fully predictive engineering models.