Measurement of the breathing mode oscillations in Hall thruster plasmas with a fast-diving triple Langmuir probe

We developed a novel measurement apparatus and data processing technique that allow for the quantitative reconstruction of the effects of breathing mode oscillations on the main properties of the plasma in Hall thrusters. The approach is based on the use of a triple Langmuir probe mounted on a rapidly moving arm to scan the channel centerline and was validated in an experimental campaign on a 5 kW-class Hall thruster. The probe data were sampled at high frequency during its motion, and a Bayesian methodology was used to reliably infer the plasma properties from the instantaneous voltage and current measurements. In order to model the interaction of the electrodes with the plasma, a parameterization of the Laframboise sheath solution was used. Data were collected continuously during the probe motion from the plume up to the near-anode region of the thruster, allowing for the reconstruction of the salient features of the plasma oscillations as a function of axial location. A time-frequency analysis of the measured plasma properties based on wavelets was then performed to gain insight into the evolution and phase shift of the oscillations over the investigated plasma domain. The developed diagnostic method can provide quantitative information on the instantaneous value of plasma density, electron temperature, and plasma potential along the thruster centerline with good spatial resolution and has proved to be a valid approach to investigate breathing mode oscillations in Hall thruster plasmas.