Mode transition characteristics and oscillation frequencies in Hall Effect Thrusters

Summary form only given. Mode transitions in Hall Effect Thrusters (HETs) provide valuable insight to thruster operation and suggest improved methods for thruster performance characterization. An investigation with a 6-kW HET induced mode transitions by varying magnetic field strength while holding all other operating parameters constant.¹ Two distinct modes of operation were identified: a “global” oscillation mode, where the entire discharge channel oscillated in unison; and a “local” oscillation mode, where radial spokes were observed to propagate azimuthally in the E×B direction. These thruster operational modes were characterized using discharge current monitoring, near-field plume probes and ultra-fast, all-light imaging. The criteria for transition from one mode to the other are carefully examined and quantified with a transition region defined where the thruster exhibits both modes of oscillation. An empirical relation is determined between the lower bound of the transition region and the discharge voltage and anode mass flow rate. We call this transition region in thruster parameter space the “transition surface.” Simulations have shown that the transition from local mode to global mode represents destabilization of the ionization front in the discharge channel similar to breathing mode excitation. The azimuthal spoke velocities in local mode are characterized and an empirical dispersion relation is shown that is similar to electrostatic ion cyclotron waves.² The dispersion relation is compared with different theories in literature for spoke propagation. An equation is derived to relate the global mode oscillation frequency with neutral velocity, ion velocity, and ionization rate where a comparison is made with the simple breathing mode frequency model originally proposed in 1997 and significant differences are observed with the implications discussed. Understanding mode transitio- s and plasma oscillations are critical to improving HET performance because thrust-to-power and anode efficiency decrease and cross-field electron conductivity increase during transition to global mode.