Information on the ionization of propellant gases is invaluable for Hall thruster plasma studies because the ion beam is the source of thrust generation. This study proposes a new method that identifies the ionization location of Xe+ ions in Hall thruster plasmas by combining the laser-induced fluorescence spectroscopy and floating emissive probe measurements in annular and cylindrical Hall thrusters. In an annular Hall thruster, this method indicated that ionization became the largest at a few millimeters upstream of the maximum radial magnetic field, whose result agreed with a one-dimensional hybrid fluid/particle-in-cell (PIC) simulation result. On the other hand, the ionization region in a cylindrical Hall thruster was located deeper inside the discharge cavity than that of the annular thruster. In addition, a large upstream shift of the ionization region was observed with the decreasing fraction of the radial magnetic field in the cylindrical Hall thruster.

Two distinct discharge modes were observed in a 50-W-class micro-Hall thruster plasma under different operating conditions. A ball-shaped plasma with a broad plume (Mode A) was observed at low mass flow rates (less than 0.37 mg/s) over the entire operational anode voltage range (160-280 V). Raising the anode voltage beyond 200 V with the mass flow rate fixed (larger than 0.37 mg/s) produced a narrow plume and stretched core structure (Mode B). In Mode B, the thruster showed performance improvements in terms of thrust (3.8 mN vs. 3.3 mN), specific impulse (913 s vs. 800 s), and anode efficiency (28% vs. 22%), with only a 2 W difference in the anode power (61 W in Mode B and 59 W in Mode A). This suggests that operation is more advantageous in Mode B than in Mode A for the utilization of such low-power Hall thrusters. A considerable difference was observed in the axial profile of Xe II ion velocity between the two modes. Mode A exhibits an axially extended ion acceleration region outside the discharge channel, where 75% of the final ion velocity is achieved at approximately 40 mm from the thruster exit, while most of the ion acceleration occurs within 10 mm from the thruster exit in Mode B. Measurements show that the full width at half maximum of the Xe II ion energy distribution function, electron temperature, and Xe II emission intensity decreased after the plasma transitioned from Mode A to Mode B. Based on the optical emission spectroscopy, the ionization rate in the plasma plume decreased by 30-41% after the mode change, which is likely related to the reduction of the beam angle and electron current by 24% and 30%, respectively.

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A diagnostic system was developed for spectrally resolved, three-dimensional tomographic reconstruction of Hall thruster plasmas, and local intensity profiles of Xe I and Xe II emissions were reconstructed. In this diagnostic system, 28 virtual cameras were generated using a single, fixed charge-coupled device (CCD) camera by rotating the Hall thruster to form a sufficient number of lines of sight. The Phillips-Tikhonov regularization algorithm was used to reconstruct local emission profiles from the line-integrated emission signals. The reconstruction performance was evaluated using both azimuthally symmetric and asymmetric synthetic phantom images including 5% Gaussian white noise, which resulted in a root-mean-square error of the reconstruction within an order of 10-3 even for a 1% difference in the azimuthal intensity distribution. Using the developed system, three-dimensional local profiles of Xe II emission (541.9 nm) from radiative decay of the excited state 5p4(3P2)6p2[3]˚5/2 and Xe I emission (881.9 nm) from 5p5(2P˚3/2)6p2[5/2]3 were obtained, and two different shapes were found depending on the wavelength and the distance from the thruster exit plane. In particular, a stretched central jet structure was distinctively observed in the Xe II emission profile beyond 10 mm from the thruster exit, while gradual broadening was found in the Xe I emission. Approximately 10% azimuthal nonuniformities were observed in the local Xe I and Xe II intensity profiles in the near-plume region (< 10 mm), which could not be quantitatively distinguished by analysis of the frontal photographic image. Three-dimensional Xe I and Xe II intensity profiles were also obtained in the plume region, and the differences in the structures of both emissions were visually confirmed.