Deconvolution of ion velocity distributions from laser -induced fluorescence spectra of xenon electrostatic thruster plumes.

Abstract

This thesis presents a method for extracting singly-ionized xenon (Xe II) velocity distribution estimates from single-point laser-induced fluorescence (LIF) spectra at 605.1 nm. Unlike currently-popular curve-fitting methods for extracting bulk velocity and temperature data from LIF spectra, this method makes no assumptions about the velocity distribution, and thus remains valid for non-equilibrium and counterstreaming plasmas. The well-established hyperfine structure and lifetime of the 5<italic> d</italic><super>4</super><italic>D</italic>_7/2-- 6p4P05/2 transition of Xe II provide the computational basis for a Fourier-transform deconvolution. Computational studies of three candidate deconvolution methods show that, in the absence of <italic>a priori</italic> knowledge of the power spectra of the velocity distribution and noise function, a Gaussian inverse filter provides an optimal balance between noise amplification and filter broadening. Deconvolution of axial-injection and multiplex LIF spectra from the P5 Hall thruster plume yields near-field and far-field axial velocity distributions. Near-field LIF spectra provide velocity distributions that cannot be measured by probe-based methods, while far-field LIF spectra provide a basis for comparison with mass spectrometer data. Transforming far-field ion axial velocity distributions to an ion energy basis reproduces all Xe II features found in mass spectrometer data taken at the same location and conditions. Axial profiles of ion axial velocity show a zone of increasing velocity extending 20 cm downstream of the thruster exit plane, with decreasing velocity from 20 to 50 cm, and demonstrate repeatabilities within 2%. Vertical-beam LIF reveals unexpectedly strong interactions between counterflowing streams in the inward divergence region at the thruster centerline. Deconvolution of multiplex LIF spectra taken from the FMT-2 ion engine plume provides beamwise velocity distributions from 1.4 mm to 30 cm. Axial profiles of axial velocity fail to disclose the location of the neutralization plane, while radial sweeps of axial velocity show no discernible trend. Radial profiles of radial velocity show increasing divergence with radial position.