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-held magnetoplasmadynamic thruster is presented. Plasma property data recorded to within one el ectron Larmor radius of the anode indicate that, with increasing curre nt, the anode transitions from a diffuse, low-anode fall mode of opera tion to a mode with high-anode falls and spotty current attachment. Th e transition is marked by an order of magnitude increase in ion satura tion current noise measured in the anode region, which is attributed t o spot motion and, for the case of a smooth anode surface, is triggere d 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 serve s the purpose of evaporating anode material, and comparison of anode f alls measured with smooth copper, aluminum, and molybdenum anodes show s that the magnitude of the anode fall in the spot mode is dependent o n anode thermal properties. The spot mode is also found to provide an explanation for anode fall saturation.