Laser-induced fluorescence diagnostics of the cross-field discharge of Hall thrusters

This article presents a review of work performed over the past ten years in France, centered on the utilization of laser-induced fluorescence (LIF) spectroscopy to diagnose the low-pressure magnetized dc discharge of a Hall thruster (HT). The latter is a gridless electric propulsion device in a crossed electric and magnetic field configuration, which is used onboard satellites and space probes for various types of maneuvers. Although the design of a HT is relatively simple, the physical mechanisms that govern thrust generation and efficiency are not yet fully understood. Characterization of the ion and atom velocity distribution function (VDF) appears to be a powerful way to obtain insights into the underlying physics. The VDF of xenon and krypton—the most common propellants—is therefore locally interrogated by means of LIF on excited levels. In this review emphasis is placed on time-averaged and time-resolved continuous-wave LIF measurements, associated quantities and recent outcomes. Results will be presented concerning a variety of phenomena: velocity vector field structuring, ion population interaction, electric field generation, ion magnetic drift, apparent atom acceleration, interaction of the plasma plume with background gas and low-frequency electric field oscillations, to name only a few.