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.

In Hall propulsion, the thrust is provided by the acceleration of ions in a plasma generated in a cross-field configuration. Standard thruster configurations have annular channels with an almost radial magnetic field at the channel exit. A potential difference is imposed in the axial direction and the intensity of the magnetic field is calibrated in order to hinder the electron motion, while leaving the ions non-magnetised. Magnetic field lines can be assumed, as a first approximation, as lines of constant electron temperature and of thermalized potential. In typical thruster configurations, the discharge occurs inside a ceramic channel and, due to plasma–wall interactions, the electron temperature is typically low, less than few tens of eV. Hence, the magnetic field lines can be effectively used to tailor the distribution of the electrostatic potential. However, the erosion of the ceramic walls caused by the ion bombardment represents the main limiting factor of the thruster lifetime and new thruster configurations are currently under development. For these configurations, classical first order models of the plasma dynamics fail to grasp the influence of the magnetic topology on the plasma flow. In the present paper, a novel approach to investigate the correlation between magnetic field topology and thruster performance is presented. Due to the anisotropy induced by the magnetic field, the gradients of the plasma properties are assumed to be mainly in the direction orthogonal to the local magnetic field, thus enabling a quasi-one-dimensional description in magnetic coordinates. Theoretical and experimental investigations performed on a 5 kW class Hall thruster with different magnetic field configurations are then presented and discussed.

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.

One of the main oscillatory modes found ubiquitously in Hall thrusters is the so-called breathing mode. This is recognized as a relatively low-frequency (10–30 kHz), longitudinal oscillation of the discharge current and plasma parameters. In this paper, we present a synergic experimental and numerical investigation of the breathing mode in a 5 kW-class Hall thruster. To this aim, we propose the use of an informed 1D fully-fluid model to provide augmented data with respect to available experimental measurements. The experimental data consists of two datasets, i.e., the discharge current signal and the local near-plume plasma properties measured at high-frequency with a fast-diving triple Langmuir probe. The model is calibrated on the discharge current signal and its accuracy is assessed by comparing predictions against the available measurements of the near-plume plasma properties. It is shown that the model can be calibrated using the discharge current signal, which is easy to measure, and that, once calibrated, it can predict with reasonable accuracy the spatio-temporal distributions of the plasma properties, which would be difficult to measure or estimate otherwise. Finally, we describe how the augmented data obtained through the combination of experiments and calibrated model can provide insight into the breathing mode oscillations and the evolution of plasma properties.