Current density and Lorentz force measurements in the downstream plume of the 100 kW AF-MPD thruster SX3

Applied-field magnetoplasmadynamic thruster (AF-MPDT) is a type of spacecraft electric propulsion (EP) with input powers of up to several 100 kW, which uses an external magnetic field to accelerate a propellant and to generate thrust. They reach a high-thrust density for an EP and medium to high specific impulses (Isp), which enables their usage in interplanetary space missions and for space tugs, for example, to the Moon. At the Institute of Space Systems, the water-cooled 100 kW steady state AF-MPDT SX3 is investigated. Magnetic field measurements with a Hall-probe were performed for a wide variation of thruster operating conditions, in order to determine the local current densities and, hence, the current densities’ distribution in the plume of the AF-MPDT SX3 for thruster distances between 140 and 1000 mm. Up to 70%–80% of the discharge current was measured as current in the plume at a thruster distance of 1000 mm. At a thruster distance of 300 mm, the fraction was 90% and more for the same operation conditions. An azimuthal current, with a current density of 7.9±2.2mA/mm2, was measured with an applied field of 100 mT at a thruster distance of 403 mm. Increasing applied magnetic fields moved it further downstream. Lorentz forces, calculated from the currents in the plume, are of the order of 10% of the thrust, which was measured with a thrust balance before.