Determination of time-resolved electron mobility in a Hall Effect Thruster via laser-induced fluorescence

Time-resolved measurements of the electron mobility in a Hall Effect Thruster are made non-invasively using laser-induced fluorescence. The ion velocity distribution function is determined in the ionization and acceleration regions of a Hall thruster throughout a cycle of a ubiquitous low frequency oscillation known as the breathing mode. Moments of the one-dimensional Boltzmann equation form a system of partial differential equations that are solved explicitly for the axial electric field, ionization rate, and density, relying on the measured distributions and a minimally-perturbative in situ probe measurement. Electron velocity is determined by simultaneously acquiring the thruster discharge current. Cumulatively, this information is used to compute the electron mobility over the course of a breathing oscillation. The calculated results show a time-averaged profile similar to that measured in other Hall thrusters. The mobility profile varies significantly over a breathing cycle, with exceptionally low mobilities emerging downstream of the acceleration region when the discharge current reaches a minimum in the breathing cycle.