Azimuthal Ion Velocity Measurements in the 12 kW HERMeS Hall Thruster

Results of a comprehensive study of azimuthal ion motion in the cathode plume, inner and outer pole regions, and main beam of the 12 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) are presented. These laser-induced fluorescence (LIF) measurements have augmented the existing database of LIF results used to validate the physics-based simulations that will demonstrate the thruster’s life margin prior to its initial mission application as part of the Advanced Electric Propulsion System (AEPS) for the Power and Propulsion Element (PPE) of NASA’s lunar orbiting Gateway. Significant ion energy spreads, corresponding to azimuthal kinetic temperatures of ∼ 2–20 eV, were measured across the thruster’s near plume. In the cathode plume, mean azimuthal velocities of ∼ 2 km/s were measured, sufficient to produce the observed positive radial velocity component via the centrifugal effective force. Rapid azimuthal ion heating was observed on the channel centerline downstream of the thruster face, leading to ion velocity distribution functions (IVDFs) with a narrow central peak and broad wings that were fit well by a bi-Maxwellian function. Given the long classical collision mean free path in this region, a wave-based heating mechanism appears likely.