Neutral ground state particle density measurement of xenon plasma in microwave cathode by two-photon laser-induced fluorescence spectroscopy

This paper reports the first study to measure xenon neutral ground state particle density of microwave cathode by two-photon laser induced fluorescence spectroscopy (TALIF). Xenon is commonly used as a propellant in electric propulsion like Hall thrusters, ion thrusters, and their cathodes. For electric propulsion, information about neutral particles is important such as the ionization degree and the charged exchange collisions (CEX). The measurement target is XeI 5p61S06p[3/2]2, which absorbs at a wavelength of 224.29 nm and emits fluorescence of 834.7 nm. The measurement system was demonstrated for three cases: cold gas, without electron extraction, with electron extraction. From three cases, the measurement system can detect a neutral ground state particle density of 1019m−3 order without and with a plasma. In a cold gas, the neutral ground state particle density is (8.4±0.4)×1019m−3 at 0.098 mg/s. Without electron extraction, the neutral ground density decreases by ionization and excitation With electron extraction, the density varied from 0.6 to 2.3 times compared to without electron extraction depending on anode voltage. •Order of 1019 m-3 neutral density at ground state can be measured by TALIF.•Detect neutral density decrease by ionization and excitation.•Transition of neutral density is observed depending on anode voltage.