Transport Physics in Hall Plasma Thrusters

An analysis of measurements supporting the presence of anomalous cross-field electron mobility in Hall plasma accelerators is presented. Non-intrusive laser-induced fluorescence measurements of neutral and ionized xenon velocities, and various electrostatic probe diagnostic measurements are used to locally determine the effective electron Hall parameter inside the accelerator channel. These values are then compared to the classical (collision-driven) Hall parameters expected for a quiescent magnetized plasma. The results indicate that in the vicinity of the anode the electron transport mechanism is consistent with that determined by elastic collisions with the background neutral xenon. However, we find that in the vicinity of the discharge channel exit, where the magnetic field is the strongest and where there are intense fluctuations in the plasma properties, the inferred Hall parameter departs from the classical value, and is close to the Bohm value of (co ce T) Qn «16. An exception to this is at the higher values of discharge voltage, where, in regions where there appears to be strong electron azimuthal shear, the value approaches that determined by classical scattering. The spatial variation in the mobility is consistent with that determined from an analysis of turbulent fluctuations in the electron density, and is found to be described by an empirical model that includes the possible effects of azimuthal electron shear in suppressing fluctuations-enhanced electron transport.