Two-Photon Absorption Laser Induced Fluorescence (TALIF) Of Neutral Xenon In A Hall Effect Thruster Plasma

Abstract

TWO-PHOTON ABSORPTION LASER INDUCED FLUORESCENCE (TALIF) OF NEUTRAL XENON IN A HALL EFFECT THRUSTER PLASMA This work presents measurements of ground state neutral xenon in the plume of a 1.5 kW Hall Effect thruster (HET) using two-photon absorption laser induced fluorescence (TALIF).Neutral xenon particles in the thruster plasma play an important role in ionization processes, overall energy conversion, and life-limiting interactions with surrounding wall materials in the thruster. Therefore, a detailed understanding of the neutral particle dynamics within the plume is desired. The TALIF diagnostic technique allows for laser induced excitation of xenon from its ground state using commercially available laser systems at accessible ultraviolet wavelengths (~222 nm). The signal collected from fluorescence of the excited atoms can be used to determine the local neutral density. We present a demonstration of TALIF first in a barium oxide (BaO) hollow cathode plume at varying radial positions, then in an HET plume at varying axial positions using fiber coupled collection optics with a spatial resolution of 3.14 mm 2 . The detection limit of the TALIF measurement system 2.1 cm downstream of the thruster face was estimated to be 8 x 10 17 m -3 as determined by comparison to a known reference signal. The ability to analyze ionization and thermal characteristics using these results is also discussed.

Temporally resolved relative krypton neutral density during breathing mode of a hall effect thruster recorded by TALIF

“…The time-resolved measurements show pronounced density variations over the period of the breathing mode, and the high contrast is likely connected to the quite strong and periodic current fluctuations of the studied thruster. While demonstrated here for Kr, the same scheme should be applicable to TALIF of Xenon also [25,29,30]. The scheme presented may be applicable to other oscillations also, as long as they are quite strongly periodic, up to a frequency of ∼ 1 MHz based on raw measurements needing ∼ 100 ns and requiring a minimum of 10 phase-bins, but this should be demonstrated experimentally.…”

“…Furthermore, no common electrostatic probe is available to provide neutral particle density fluctuations in the near field without perturbing the natural plasma state. The present work extends our past use of two photon absorption laser induced fluorescence (TALIF) for reliable measurement of neutral populations [25][26][27] -by probing the ground state -but now for temporally resolved measurements relative to the breathing mode oscillation. The main advantage of TALIF for density measurements is that by using two-photon absorption we can access the relatively large energy gaps between the ground state and excited states, thereby accessing ground state transitions with available tunable laser systems typically operating in the range of ∼ 200-250 nm [25][26][27][28][29][30][31][32][33].…”

“…The present work extends our past use of two photon absorption laser induced fluorescence (TALIF) for reliable measurement of neutral populations [25][26][27] -by probing the ground state -but now for temporally resolved measurements relative to the breathing mode oscillation. The main advantage of TALIF for density measurements is that by using two-photon absorption we can access the relatively large energy gaps between the ground state and excited states, thereby accessing ground state transitions with available tunable laser systems typically operating in the range of ∼ 200-250 nm [25][26][27][28][29][30][31][32][33]. Two general disadvantages of TALIF detection are that signal-to-noise can be poor owing to small two-photon absorption cross-sections (and competing luminosity from the same emission lines) and that data is generally collected with low repetition-rate pulsed lasers (∼ 10-20 Hz).…”

Foundations of plasmas as ion sources

“…This assumption can be problematic-particularly for trying to apply LIF schemes to measure time-resolved behavior in the ion dynamics [38]. With that said, resonant two-photon excitation techniques have recently emerged as an alternative to overcome this limitation, as these schemes can directly excite the ground state of the ions of interests [39].…”