Mode transition of a Hall thruster discharge plasma

TLDR

A Hall thruster is a cross‑field plasma device for spacecraft propulsion, but the impact of 10–30 kHz discharge oscillations on its performance remains unresolved. The authors employ a hybrid‑direct kinetic simulation that incorporates transport of electronically excited atoms to model the Hall thruster discharge plasma. The simulation reproduces the experimentally observed mode transition, showing that increasing the magnetic field stabilizes the global oscillation by reducing electron transport, suppressing ionization, balancing Joule heating with wall and inelastic losses, and that emitted light intensity correlates strongly with discharge current.

Abstract

A Hall thruster is a cross-field plasma device used for spacecraft propulsion. An important unresolved issue in the development of Hall thrusters concerns the effect of discharge oscillations in the range of 10–30 kHz on their performance. The use of a high speed Langmuir probe system and ultra-fast imaging of the discharge plasma of a Hall thruster suggests that the discharge oscillation mode, often called the breathing mode, is strongly correlated to an axial global ionization mode. Stabilization of the global oscillation mode is achieved as the magnetic field is increased and azimuthally rotating spokes are observed. A hybrid-direct kinetic simulation that takes into account the transport of electronically excited atoms is used to model the discharge plasma of a Hall thruster. The predicted mode transition agrees with experiments in terms of the mean discharge current, the amplitude of discharge current oscillation, and the breathing mode frequency. It is observed that the stabilization of the global oscillation mode is associated with reduced electron transport that suppresses the ionization process inside the channel. As the Joule heating balances the other loss terms including the effects of wall loss and inelastic collisions, the ionization oscillation is damped, and the discharge oscillation stabilizes. A wide range of the stable operation is supported by the formation of a space charge saturated sheath that stabilizes the electron axial drift and balances the Joule heating as the magnetic field increases. Finally, it is indicated from the numerical results that there is a strong correlation between the emitted light intensity and the discharge current.