Hybrid-PIC modeling and electrostatic probe survey of Hall thrusters

Abstract

In order to study the physics of the acceleration process in Hall thrusters, an evolved hybrid particle-in-cell (PIC) numerical model was developed. A set of quasi-one-dimensional fluid equations are used for electrons, and a particle-tracking Boltzmann solver is used for the heavy species. The two solutions are linked by charge neutrality. A novel statistical approach to particle creation and tracking is employed which reduces computational time. To provide boundary conditions for the numerical model, and to verify its accuracy, an extensive set of electrostatic probe measurements were performed in the plume of an SPT- 70. Various probe geometries were used, including triple probes, to measure the time­averaged plasma parameters in the near-plume. Also, AC components of discharge current and floating potential were measured. The combined experimental and numerical results are used to examine phenomena such as the nature of low-frequency discharge oscillations, the interaction of the thruster walls with the plasma, the distribution of ion energies ejected from the thruster, and the effect on efficiency of facility background pressure. Performance results from the numerical model agree well with measurements. A formula for the change in measured efficiency versus facility pressure is also presented, based on numerical results. Widening of the ion distribution of energies is seen, and attributed to time variation of the electric field, due to oscillations. Measured low-frequency oscillations seem to be related to an ionization instability observed in the numerical model, but conclusive evidence is not found. Although the model predicts strong electric fields outside the acceleration channel, experiments do not. The discrepancy is believed to be due to localized anomalous electron conductivity caused by azimuthal asymmetry.