Spectroscopic characterization of FMT-2 discharge ionization processes
Discharge cathode erosion, a major failure mechanism for ion engines, is fundamentally interconnected with ionization processes. Three-component laser induced fluorescence (LIF) and emission spectroscopy measurements downstream of the FMT-2 ion engine characterize these processes with and without a keeper electrode. Emission spectroscopy provides temporally well-resolved, line-of-sight-integrated relative density for neutral (Xe I), singly-ionized (Xe II) and doubly-ionized (Xe III) xenon. LIF signal strength provides spatially well-resolved, time-integrated relative density. Correlation of both measurements with beam current provides absolute density profiles for all three species. The three-dimensional velocity distribution f(v), deconvolved from LIF spectra for Xe I and Xe II, provides three-component mean velocity and temperature profiles for Xe I and Xe II. Temperature anisotropy and f(v) skewness provide a measure of departure from thermodynamic equilibrium. These data are compared to potential hill formation and ionization models.
