Physical characterization of high-frequency instabilities in Hall thrusters

High-frequency instabilities (5-10 MHz) were investigated in two Hall thrusters of different scale and were shown to have the same character. Two diagnostic tools were implemented in this study: capacitive antennas, electrically isolated from plasma, and plasma immersed probes. Their corresponding signals were analyzed in terms of capacitive and charge-collecting effects. The previously suggested physical representation of these high frequency instabilities as azimuthally drifting electron density fluctuations was reinforced and significant deviations from azimuthal symmetry in the thruster plume are evidenced. Finally, data obtained by using positively biased probe evidenced lower frequency components, assigned to "ion transit time" instabilities. (c) 2006 American Institute of Physics.