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Scaling of Anode Sheath Voltage Fall with the Operational Parameters in AF-MPD Thrusters

Abstract

Scaling laws for the anode sheath voltage fall in applied-field MPD thrusters are derived in order to better understand the physics behind anode sheath power dissipation. A semi-empirical model is formulated and verified by comparison to experimental data on a 30 kW lithium-fed steady-state AF-MPDT obtained using a hot langmuir probe. It is found that the anode sheath voltage fall increases approximately linearly with current and applied magnetic field and is inversely proportional to mass flow rate. It is shown that, although the electrons in the anode sheath are unmagnetized, the voltage fall is attributed to plasma density reduction at the sheath edge, which is a result of increased plasma pinching at higher magnitudes of applied magnetic field. It is also concluded that thermionic emission from the anode surface leads to an increase in the anode sheath voltage fall; therefore anode material with a high work function is preferred.