Plasma Oscillations and Operational Modes in Hall Effect Thrusters

Mode transitions have been commonly observed in Hall effect thruster (HET) operation where a small change in a thruster operating parameter such as discharge voltage, magnetic field or mass flow rate causes the thruster discharge current mean value and oscillation amplitude to increase significantly. In this study, mode transitions in HETs are induced by varying the magnetic field intensity while holding all other operating parameters constant and measurements are acquired with high-speed probes and ultra-fast imaging. Two primary oscillatory modes were identified and extensively characterized called global oscillation mode and local oscillation mode. In the global mode, the entire discharge channel oscillates in unison and azimuthal perturbations (spokes) are either absent or negligible. Downstream azimuthally spaced probes show no signal delay between each other and are very well correlated to the discharge current signal. In the local mode, signals from the azimuthally spaced probes exhibit a clear delay indicating the passage of spokes. These spokes are localized oscillations in discharge current density propagating in the E x B direction that are typically 10-20% of the mean value. In contrast, the oscillations in the global mode can be 100% of the mean discharge current density value. The spoke velocity is determined from high-speed image analysis using three methods yielding values between 1500 and 2200 m/s across a range of magnetic field settings. The transition between global and local modes occurs at higher relative magnetic field strengths for higher mass flow rates or higher discharge voltages. It is proposed that mode transitions represent de-stabilization of the ionization front similar to excitation of the well-studied Hall thruster breathing mode, which is supported by time-resolved simulations of the discharge channel plasma. The thrust is approximately constant in both modes, but the thrust-to-power and anode efficiency decrease in global mode due to increasing discharge current. New system characterization techniques are suggested that include discharge current, discharge voltage and magnetic field maps at different flow rates to identify modes of operation within a three variable parameter space.