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

Abstract

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.

Numerical and Experimental Investigation of Longitudinal Oscillations in Hall Thrusters

“…The use of a triple probe, despite reducing the space resolution in the perpendicular plane, allows achieving a good temporal resolution without the need to perform rapid voltage sweeping, which introduces complications in the results interpretation due to the presence of important capacitive effects, as is the case with single Langmuir probes used in previous studies (see, for instance, [51,54]). This diagnostic is described in detail in previous publications from the authors [39,40,55], therefore, only a summarized discussion is included in the following.…”

“…Triple Langmuir probes are plasma diagnostics that consist of three conductive electrodes mounted on the tip of an isolating ceramic bar. Using a proper electric arrangement of the electrodes, this type of probe allows to obtain instantaneous measurements of the plasma density n, plasma-to-ground potential V gp and electron temperature T e , [55,65]. The three electrodes are typically denominated Bias (B), Common (C) and Float (F).…”

“…The adoption of a triple probe allows gathering instantaneous measurements of plasma properties without the need of voltage sweeps, at the cost of assuming that the electrons present in the plasma have a non-drifting Maxwellian velocity distribution function (EVDF). As explained by the authors in References [39,55], the assumptions on the EVDF are acceptable to a first order, particularly in the plume, where the electron drift velocity is negligible with respect to their thermal velocity. Moreover, a set of correction parameters, introduced by Saravia et al in Reference [39], allows mitigating the impact of the electron drift on the measurements.…”

“…The present paper proposes an investigation of the breathing mode in a 5 kW-class Hall thruster, SITAEL's HT5k DM2, based on the synergic combination of a 1D fully-fluid model and experimental data. The latter comprises two different datasets: (a) the signal of the thruster discharge current acquired with an oscilloscope; (b) the measurement of the local plasma properties in the channel and near-plume carried out with a fast moving triple Langmuir probe and processed with a dedicated diagnostic method [55]. The numerical model uses standard modeling methodologies and contains physically meaningful free parameters associated with those modeling aspects that are mostly affected by uncertainty.…”

“…Concerning the outline of the paper, Section 2 describes the numerical formulation adopted to model the plasma flow in Hall thrusters, discussing the main assumptions and presenting the core equations of the model. Section 3 details the characteristics of the test item and summarizes the experimental setup and data processing technique, described at length in [55]. The comparison between the results of the calibrated plasma simulations and the experimental results is then presented in Section 4.…”