Temporally and Spatially Resolved Measurements in the Plume of Clustered Hall Thrusters

Abstract

A High-speed Dual Langmuir Probe system (HDLP) and a high-speed camera tracked 19.6-kHz waves of ionized propellant emitted by the unsteady ionization process or “breathing mode” of a cluster of Hall thrusters. Two 600-W, low-power Hall thrusters were operated together in close proximity with 200-V, 2-A xenon discharges, producing the plume examined in this study. Large amplitude plasma oscillations, with peak-to-peak magnitudes approaching 100% of the DC level, are observed in the near- and far-field plume electron density, optical intensity, and thruster discharge current. The HDLP system acquired the plume electron density in a planar grid measuring 0.31 m radially, centered on cluster centerline (with 10-mm spacing), and 0.5 m axially downstream (with 50-mm spacing). The structural evolution of the exhausted plasma density fluctuations was resolved for this entire grid at a rate of 100 kHz (using methods of Fourier analysis with the HDLP data) while a high-speed camera imaged the two discharge channels at a rate of 67,500 frames-per-second (with the discharge currents of each thruster sampled at 2 MHz). Recorded synchronously, these independent spatially and temporally resolved measurements correlated well with each other and revealed the complex nature of the interactions between the two simultaneously discharging Hall thrusters. Interactions in the near-field plume proved viable enough to encourage coupled breathing mode oscillations, in which the thrusters preferentially breathed in an in-phase “mode-0” (observed 43.4% of the run duration) or an opposite-phase “mode-1” (36.1%) manner. Comparisons of these coupled breathing mode oscillations observed in the clustered thrusters were made with single-thruster breathing mode oscillations. For both configurations, a non-dimensional, localized electron density mean fluctuation peak-to-peak magnitude of about 72% ±10% was seen at all radial and axial plume locations.