Creation of Onset Voltage Hash by Anode Spots in a Magnetoplasmadynamic Thruster

Abstract

Experimental results are presented which quantify the evolution with rising J 2 / ˙ m (ratio of current squared to mass flow rate) of onset voltage fluctuations in magnetoplasmadynamic thrusters (MPDTs) operating with three anode materials, and an anode spot model is presented which provides a physical explanation for the properties of these fluctuations. Voltage signals taken in an MPDT operating below and above onset with anodes of copper, graphite, and lead are analyzed using the statistical measures of probability density and power spectrum. A model of voltage hash as the random superposition of many anode spotting events is used to generate voltage fluctuations with statistics similar to the observed data. The experimental fluctuation statistics evolve with rising J 2 / ˙ m first away from Gaussian, and then back toward Gaussian, with the values of skewness and kurtosis peaking at J 2 / ˙ m 110 kA 2 -s/g; this behavior is the same for all three anode materials. Non-stationarity in the statistics is shown using high-speed video to be a result of unsteady anode evaporation. The statistics of modeled voltage hash are shown to be functions of the product of the frequency of anode spotting events and their duration, with the statistics becoming more Gaussian as this product grows. Comparison of experimental and model results suggests that, above J 2 / ˙ m 110 kA 2 -s/g, anode current conduction fragments into an increasing number of anode spots.