Bayesian analysis of triple Langmuir probe measurements for the characterization of Hall thruster plasmas

We developed a new experimental approach to characterize the plasma behavior inside Hall thrusters. The main novelty of the developed approach regards the combination of a triple Langmuir probe, which guarantees a high flexibility, and Bayesian data analysis. The triple Langmuir probe was mounted on an articulated arm that can rapidly insert the probe inside the thruster channel, providing a clear picture of relevant plasma properties along the channel centerline, from the near plume to the near-anode region. At a given operating condition of the thruster, multiple measurements were performed, changing the arrangements of the triple probe electrodes and the applied potential differences between the electrodes. Then, in order to analyze the data gathered by the triple probe, a Bayesian integrated data analysis has been adopted. This method made it possible to combine measurements from different electrode arrangements and to improve the quality of the inferred plasma parameters. Non-uniformities of the plasma sensed by the probe electrodes were taken into account within the physical model of particle collection. In order to model the interaction of the electrodes with the plasma, a parametrization of the Laframboise sheath solution was used. The developed diagnostic system, together with the integrated data analysis, proved to be a valid approach to characterize the plasma flow in Hall thrusters, offering not only a good spatial resolution of the electron temperature, plasma density, and space potential but also a consistent estimate of the measurement accuracy.