Development of a Time-Resolved Laser-Induced Fluorescence Technique for Nonperiodic Oscillations

Abstract

Time-resolved measurements of ion dynamics could be key to understanding the physics of instabilities, electron transport, and erosion in Hall thrusters. Traditional measurements of the ion velocity distribution in Hall thrusters using laser-induced fluorescence (LIF) are time-averaged since lock-in amplifiers must average over a long time constant for a reasonable signal-to-noise ratio. Over about the past decade, at least four other time-resolved LIF techniques have been developed and applied to Hall thrusters or similar plasma devices. One limitation of these techniques is the implicit assumption of periodic oscillations in the averaging scheme. There is a need for a more general technique since Hall thrusters can operate with nonperiodic oscillations that vary unpredictably. This dissertation presents the development of a time-resolved LIF (TRLIF) technique that addresses this need. This system averages the signal using a combination of electronic filtering, phase-sensitive detection, and Fourier analysis. A transfer function is measured to map an input signal (such as discharge current) to an output signal (TRLIF signal). The implicit assumption of this technique is that the input is related to the output by a time-invariant linear system, a more general assumption than periodicity. The system was validated using a hollow cathode with both periodic and random discharge current oscillations. A series of benchmark tests was developed to validate the signal by verifying that it satisfies theoretical expectations. The first campaign with the H6 Hall thruster demonstrated signal recovery in both periodic and nonperiodic modes. Measurements of the evolution of the ion flow downstream show that kinematic compression explains the width of the ion velocity distribution only at certain phases of the oscillation. A distinct change in ion dynamics was detected as the magnetic field magnitude increased: a high-amplitude, relatively periodic oscillation in the ion velocity distribution gave way to a low-amplitude, chaotic oscillation. High amplitude oscillations of the mean ion velocity suggest that the bimodal distributions detected at many operating conditions (with time-averaged measurements) are the result of oscillations.