Laser-induced fluorescence spectroscopy on neutrals for plasma studies in Hall thrusters

Hall thrusters offer an excellent balance between specific impulse and thrust-to-area ratios. However, adapting them to new power requirements is challenging because of the complexity of plasma dynamics and material interactions. This study focuses on using a minimally intrusive diagnostic tool, laser-induced fluorescence (LIF) spectroscopy, to investigate the atomic population within an electric thruster. We developed a comprehensive LIF spectra simulator incorporating factors such as the natural gas mixture and the Doppler Effect. By leveraging the anomalous Zeeman Effect, we achieved accurate non-intrusive measurements of the local magnetic field vector, a critical parameter for the thruster design process. We propose a new data elaboration strategy that combines measurements taken with two orthogonal laser polarizations inside the vacuum chambers. This strategy was successful in decoupling the neutrals’ temperature and magnetic field values inferred from LIF spectra. An experimental campaign within a laboratory model Hall thruster operating with Krypton validated this approach and demonstrated the technique’s potential. Three different LIF excitation schemes were used, with small discrepancies in the inferred values of the model variables. The transitions starting from the level 5s′1/21 led to the best precision in the evaluation of the neutrals’ temperature and the local magnetic field. The neutrals’ drift velocity experiences an apparent acceleration until the exit of the thruster channel, which could be due to charge-exchange collisions or slow neutrals’ population depletion caused by ionization.