Three-dimensional tomographically reconstructed optical emission profiles of Hall thruster plasmas

Abstract

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.

Two-dimensional electron temperature and density profiles of Hall thruster plume plasmas using tomographically reconstructed optical emission spectroscopy

“…However, acquiring local electron information with sufficiently high spatial resolution remains a challenge owing to either the line-integrated nature of the diagnostics or insufficient signal-to-noise ratios [9]. Optical tomographic reconstruction, in contrast, can provide spatially resolved, two-dimensional emission intensity profiles and even electron information with proper population models; however, no complete sets of electron temperature and density profiles have been reported for Hall thruster plasmas [12,13].…”

“…In this study, we developed an optical diagnostic system to obtain spatially and spectrally resolved emission profiles. In addition, the local electron temperature and density of Hall thruster plasmas were obtained by employing an optical tomographic reconstruction technique and a collisional-radiative (CR) model [12,13]. Emission spectra integrated along the line of sight were measured using a single collimator and spectrometer combination and by rotating the thruster, where the inverse Radon transform technique was used to obtain the local emission intensity values for each wavelength.…”