Mode Transitions in Hall-Effect Thrusters Induced by Variable Magnetic Field Strength

Mode transitions in a 6 kW laboratory Hall-effect thruster were induced by varying the magnetic field intensity while holding all other operating parameters constant. Ultrafast imaging, discharge current, and thrust measurements were used to characterize the change in discharge channel current density and thruster performance through mode transitions. The modes are described here as global oscillation mode and local oscillation mode. In global mode, the entire discharge channel is oscillating in unison and spokes are either absent or negligible with discharge current oscillation amplitude (root mean square) greater than 10% of the mean value and can even be as high as 100%. In local oscillation mode, perturbations in the discharge current density are seen to propagate in the E ×B direction. Spokes are localized oscillations that are typically 10–20% of the mean discharge current density value. The discharge current oscillation amplitude and mean values are significantly lower than global mode. The mode transitions changed with operating conditions, where the transition between global mode and local mode occurred at higher relative magnetic field strengths for highermass flow rate or higher discharge voltage. The thrust was approximately constant through the mode transition, but the thrust-to-power ratio and anode efficiency decreased significantly in global mode. The peaks in thrust to power and anode efficiency typically occur near the transition point. Thruster performance maps should include variation in discharge current, discharge voltage, and magnetic field, known as ID − VD −Bmaps, at different flow rates to identify transition regions throughout the life of a thruster.These results are used to calculate a transition surface for use by operators to keep the thruster operating in an optimal mode.