High-Speed Image Analysis and Filtered Imaging of Nested Hall Thruster Oscillations

Hall Effect Thrusters (HET) are known to exhibit many high-speed oscillations that are indicative of thruster performance. Azimuthal ionization oscillations, called spokes, have been investigated extensively but the mechanisms governing their behavior are still poorly understood. Most experimentation has been done with single-channel Hall thrusters but evidence exists that a nested-channel Hall thruster experiences partial oscillation coupling between channels. This paper details the investigation of oscillations in various configurations of the X3, a 100-kW class three-channel nested Hall thruster. A technique of using optical filters to extract plasma properties from high-speed video is examined. Separately, coupling between channels was studied by imaging multiple thruster discharge channels at once. A Photron FASTCAM SA5 recording at 75,000 fps and equipped with thin dielectric filters for strong near-infrared Xe I emission lines was used to capture high-speed video of the discharge channel plasma. The recorded video for each thruster configuration was processed with radial averaging and azimuthal binning of the discharge channel light intensity, followed by 2D Fourier transforms to produce power spectral density curves for each mode. Phase delays between modes and relative fluctuation amplitudes were also calculated to quantify coupling. The filtered imaging work revealed that plasma parameters can be extracted at low speeds but degradation in emission model accuracy prevents it from being useful at high speeds. This paper also describes a preliminary summary of the coupling that is observed through the discharge current and light emission spectra. Barring pressure effects, breathing frequency convergence is observed consistently in two-channel operation but not uniformly in three-channel operation.