Parametric Investigation of the Rotating Spoke Instability in Hall Thrusters

This work presents recent results of ongoing investigations into rotating spoke instabilities in Hall thrusters. Recent work by the authors demonstrated large amounts of electron current carried by spokes in the nearanode region in the H6 6-kW laboratory model Hall thruster, and raises the question of whether spokes may play a larger role in electron transport throughout the Hall thruster plume. Spokes have since been observed via high speed imaging in several Hall thrusters, including the H6, the NASA 173Mv1, the X2 dual nested channel Hall thruster, the Helicon Hall thruster, and the Busek BHT-600. While this work focuses on the H6 Hall thruster, visual examples of spoke presence in these other thrusters are also presented. A parameter study of the dependence of spoke amplitude and propagation velocity on magnetic field strength and discharge voltage in the H6 shows that the magnetic field strength has a strong influence on spoke properties. Spokes appear to be omnipresent in Hall thrusters, though with widely variable strength and stability, and appear even at very highly efficient operating conditions with high voltage and high magnetic field strength – the most prominent spoke mode in the H6 is observed at a 600 V, 65% total efficiency operating condition. In response to earlier studies that have observed spokes exclusively at low voltage or in inefficient operating regimes, a study of spoke behavior at a very low voltage operating condition links a transition to higher efficiency operation with the formation of stabilized spoke structures. Finally, we present visual evidence from high-speed imaging of spokes bridging the centrally mounted cathode to the discharge channel.