Characterization of neutral gas densities in the plume and the discharge chamber of an ion thruster using two-photon absorption laser induced fluorescence spectroscopy

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Two-photon absorption laser induced fluorescence spectroscopy represents an excellent method to accurately determine atomic and molecular ground state densities in low temperature plasma. For several species such as H, N, O, Kr, or Xe, the simultaneous absorption of two laser photons allows excitation from the ground state, while circumventing laser wavelengths in the VUV. However, compared to various published plasmadiagnostic studies on atomic H, N and O species, this method has been applied rather seldom for investigations of atom number densities in rare gas plasma discharges such as in Kr or Xe.With respect to the operation of ion sources, rare gases are commonly used to conduct processes related to ion beam sputter deposition or surface modifications within terrestrial applications, and, furthermore, as propellants of electric spacecraft propulsion devices, in particular of both gridded ion- and Hall-effect thrusters. Here, the experimental characterization of the plume, as well as of the discharge region, is indispensable to achieve a better understanding of important physical phenomena such as the emergence of a charge exchange plasma component near the thruster exit. In addition, results of these investigations are needed to optimize electric propulsion devices.In the thruster plasma, neutral gas species mainly occur due to incomplete ionization of the propellant in the discharge chamber, as part of the neutralizer flow and as background atoms in ground test facilities. In particular, charge exchange collisions between fast ions and slow thermal neutral gas atoms may cause a backflow of ionized atoms of the ion beam, which can result in critical issues such as electric charging or erosion of spacecraft or thruster components by ion impingement. Therefore, plasmadiagnostic investigations aiming at the quantitative analysis of number densities of neutral atoms are of particular interest, since neutral gas species can not be detected with standard ion beam diagnostic tools such as Faraday cups or ExB, RPA and Langmuir probes.Within the proposed project, measurements using the two-photon absorption laser induced fluorescence technique will be carried out to determine local ground state densities of neutral xenon in the plume of an ion thruster. From the measured number density distributions, neutral particle fractions emerged from charge exchange processes shall be estimated. In addition, the fraction of fast neutrals shall be measured directly at the Doppler-shifted absorption wavelength for appropriate thruster operating conditions. Moreover, experimental investigations in the discharge will be conducted mounting appropriate optical access to the chamber housing, allowing for the examination of neutral gas density distributions that are critical to the validation of numerical discharge models. Additional to the experiments in xenon, ion source resp. thruster operating with krypton as propellant will be investigated.