Spot mode transition and the anode fall of pulsed magnetoplasmadynamic thrusters

An experimentally based description of the major mechanism regulating the anode fall of a high-power, pulsed, self-field magnetoplasmadynamic thruster is presented. Plasma property data recorded to within one electron Larmor radius of the anode indicate that, with increasing current, the anode transitions from a diffuse, low-anode fall mode of operation to a mode with high-anode falls and spotty current attachment. The transition is marked by an order of magnitude increase in ion saturation current noise measured in the anode region, which is attributed to spot motion and, for the case of a smooth anode surface, is triggered by the condition at which the discharge current density to the anode exceeds the random thermal electron current density. Experiments with a roughened anode indicate that the anode fall in the spot mode serves the purpose of evaporating anode material, and comparison of anode falls measured with smooth copper, aluminum, and molybdenum anodes shows that the magnitude of the anode fall in the spot mode is dependent on anode thermal properties. The spot mode is also found to provide an explanation for anode fall saturation.