Several techniques have been developed recently for performing time-resolved laser-induced fluorescence (LIF) measurements in oscillating plasmas. One of the primary applications is characterizing plasma fluctuations in devices like Hall thrusters used for space propulsion. Optical measurements such as LIF are nonintrusive and can resolve properties like ion velocity distribution functions with high resolution in velocity and physical space. The goals of this paper are twofold. First, the various methods proposed by the community for introducing time resolution into the standard LIF measurement of electric propulsion devices are reviewed and compared in detail. Second, one of the methods, the sample-hold technique, is enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The new system is applied to study the dynamics of ionization and ion acceleration in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44-49 kHz range. A very detailed experimental picture of the common breathing mode ionization instability emerges, in close agreement with established theory and numerical simulations.

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (∼4.25 A), with vacuum facility background pressures ranging from 1.7 × 10 −5 to 8.0 × 10 −5 torr. Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal 1.7 × 10 −5 torr. Time-resolved measurements, implemented using a sample-hold scheme with 1 μs resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at 3.5 × 10 −5 torr, where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at 5.0 × 10 −5 torr. These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.

We report on the results of an experimental campaign to measure time-varying velocity distributions in the near-field of a low power Hall thruster. We employ a sample-hold technique, enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The measurements are applied to study flow field dynamics in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44-49 kHz range. A very detailed experimental picture of the nearfield emerges from these studies. The results indicate that velocity fluctuations lessen further downstream of the exit plane. Along the thruster axis, where there is a general appearance of a central jet, there is evidence of a low velocity ion population in between the periodic bursts of high velocity ions, indicative of local ionization of neutrals outside of the thruster. One possible source of this residual ionization may be background chamber gas, which is not unexpected with the limited pumping capacity of ground test facilities.