Dispersion of Low Frequency Waves in a Hall Discharge

Experimental measurements are presented for studies of low frequency ( 2), tounambiguously resolve the directions of wave propagation.I. INTRODUCTIONClosed electron-drift Hall discharges are low-pressure (-0.01 — 0.1 Pa), weakly collisional, magnetizedplasma sources that generate a relatively high velocity ionbeam suitable for use in space propulsion applications. Aparticular class of Hall plasma thrusters, the so-called"Stationary Plasma Thruster," has been used in a numberof space missions in the former Soviet Union [1]. Avariation of this Hall thruster that has an annular (co-axial) discharge channel 100 mm in diameter - the SPT-100, has a high specific impulse (1100 - 2000 sec),operates at moderately high thrust levels (55 - 150 mN),and has an exceptionally high thrust efficiency (40-60%).Because of this performance, this plasma source is nowbeing aggressively developed for use in station keepingapplications on western satellites.In a typical co-axial geometry Hall discharge, theplasma is sustained in imposed orthogonal electric andmagnetic fields. The discharge electrons are magnetized,whereas the more massive propellant ions, usually xenon,are not. Consequently, the electrostatic fields establishedby the retarded electron flow accelerate the ions to highvelocities, typically 50-60% of the discharge voltage(~100-300V). The maximum acceleration occurs in theregion between the magnetic poles, where the magneticfield is a maximum. In a co-axial geometry, the electronsare constrained to move in the closed, azimuthal E x Bdrift, with cross-field diffusion providing the necessarycurrent to sustain the discharge. An annular ceramicchannel confines the electron flow towards the anode,located at its base.It is widely known that the Hall discharge plasmasexhibit a rich spectrum of fluctuations in plasmaproperties [2]. While it is not yet known if and how thesefluctuations can impact the performance of a Hallthruster, it is believed that fluctuations in the bulk plasmaproperties are partly responsible for anomalous electrontransport across the imposed magnetic field [3]. Whilesome studies characterizing the presence and origin ofthese fluctuations and their possible control werepublished in the mid 60's to mid 70's [3]-[9], they havereceived increased attention recently [10]-[13], as there isa growing need to extend and enhance the performance ofthese thrusters for a broader range of space missions.Our research is motivated by the possibility ofenhancing Hall thruster operation by the active control ofthese fluctuations, or by the passive suppression of thefluctuations in regions of the discharge channel where areduction in electron current is desired. To do so, it isimportant that the nature of these fluctuations isadequately understood. In this paper, we present anexperimental characterization of the propagation of thesedisturbances over a range of Hall discharge operation, anddemonstrate that it is possible to affect the oscillationswithin the discharge by the insertion of biased probes.* Post-doctoral Research Associate, Student Member, AIAA* Graduate Research Assistant, Student Member, AIAA