Optical and electrostatic characterization of oscillatory Hall discharge behavior

Abstract

The characterization of oscillatory phenomena in Hall discharges using both optical emission measurements and electrostatic probes is presented. The instabilities of the plasma in two Hall discharges with differing geometry and operating conditions are discussed. In both cases, the measurements confirm the existence of spatially-isolated, low-frequency (5-70kHz) azimuthal drift waves when operating in the ionization regime (at low discharge voltages). These disturbances are overtaken by yet lower frequency, spatially-homogeneous disturbances when operating in the current saturation regime (at higher discharge voltages). The lower frequency disturbances are believed to be ionization instabilities, and their frequencies scale qualitatively with voltage and magnetic field in accordance with the push-pull or predator-prey model of Fife et al. (AIAA 97-3052). The optical emission-based studies of the plasma fluctuations are consistent with the results of negatively-biased ion-saturation probes, and, in addition, clearly indicate the presence of higher frequency (2-70 MHz) disturbances over a wide range of operating conditions.