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The discharge mode characteristics of cathodes may strongly influence the discharge stability and performance of electrostatic thrusters. In this article, discharge characteristics and mode transition phenomenon of the ring-cusp magnetically confined plasma bridge neutralizer (RCM-PBN) were experimentally studied using argon as the working gas. The dependences of anode current and oscillation amplitude on anode voltage, argon flow rate, heater power, and cathode-to-anode distance were investigated. Plasma properties were measured and plasma plume images were taken under different discharge modes. Two distinct discharge modes were observed during the experiments: high oscillation mode and low oscillation mode. In the high oscillation mode, the plasma plume appears dim, the anode current is low, and the oscillation level is more than 2%. While in the low oscillation mode, a spot-like structure close to the orifice is observed. The plume becomes brighter, the anode current increases, and the oscillation level decreases below 2%. The RCM-PBN was found to transition into the low oscillation mode by increasing anode voltage, flow rate, heater power and by decreasing the cathode-to-anode distance. [ABSTRACT FROM AUTHOR]

The plasma bridge neutralizer (PBN) based on a tungsten filament is a promising technique of a thermionic DC electron source where a hot filament is immersed in an inert gas flow and electrons are "bridged" from a small orifice to the ion beam. PBNs have been widely used in space propulsion and industrial applications due to their relatively simple structure and low power consumption. However, they have well-known disadvantages, namely, low emission current density and short lifetime. In this article, we propose a novel ring-cusp magnetically confined PBN (RCM-PBN) to address these issues. In the RCM-PBN, electrons are confined by a ring-cusp magnetic field, which improves the ionization efficiency and reduces the discharge chamber wall losses. Electrical insulation of the orifice plate from the chamber wall prevents a large number of electrons from being collected by the orifice plate, which greatly improves the extracted electron current. The effects of different operating parameters on the extracted electron current were studied through experiments. It was found that the increase in the extracted electron current with the extraction voltage was related to the anode spot formation. Analysis of the gas utilization factor and electron extraction cost shows that the optimal operating condition was obtained at an argon mass flow rate of 1.2 SCCM and a heater power of 45 W. At its optimum, a stable electron current of 1.1 A was extracted from the RCM-PBN with a gas utilization factor of 12.8 and an electron extraction cost of 143 W/A. [ABSTRACT FROM AUTHOR]