Towards a better description of LF instabilities in Hall thrusters: role of the magnetic gradients

In a Hall thruster, a magnetic field B transverse to the applied electric field E causes electron conductivity to drop, leading to an increase in the E field, and consequently to effective gas ionization and ion acceleration. The combination of these crossed fields leads to a strong electron drift current in the azimuthal E×B direction, the Hall current. As the ions are weakly magnetized, the difference between electron and ion drift velocities can lead to charge separation and the development of instabilities (azimuthal and axial, from kHz to MHz, even up to GHz, and wavelengths ranging from a few mm to several cm) such as Breathing mode instability, Ion Transit Time oscillations and Rotating Spokes. As these instabilities may control electron transport across the magnetic barrier, numerous experimental and theoretical studies have been devoted to characterizing them. However, the physics of these instabilities is complex and their consequences on anomalous transport are still difficult to quantify, leading to a largely empirical design of Hall thrusters.

Recent theoretical works have shown the role of gradients (of density, temperature and magnetic field) in the appearance of these instabilities. However, due to the difficulty of controlling these gradients, experimental work on the subject is rare and very partial. The PPS Flex thruster developed at Laplace is an original and unique tool allowing the magnetic configuration to be varied continuously. It offers a real opportunity to improve the description of these instabilities and consequently the possibility of controlling them as well as helping to give an assessment of the anomalous current. For now, dedicated diagnostics for characterizing instabilities and the thruster have been set up and are now operational at LAPLACE. Signal analysis tools (wavelet decomposition) [1] and a time readjustment technique based on signal similarity have been developed [2] and tested for PPS-1350-type magnetic configurations.

The goal of the internship is to take advantage of the development of these tools coupled with fast imaging to study experimentally the effect of magnetic field gradients on the appearance/disappearance of low frequency instabilities and on their characteristics (frequency, amplitude, location, etc.). These experimental findings will draw on the group’s expertise in theoretical instability analysis and Hall thruster simulation to provide new insights into the physics of LF instabilities.

General informations:

Working conditions: The internship will take place at the LAPLACE laboratory (University of Toulouse 3 / INP / CNRS) in the GREPHE research group (“Groupe de Recherche en Energétique et Plasmas Hors Equilibre”). Activities on propulsion are currently supported by the CNES. The Master’s student will work on the experimental test bench developed at GREPHE.

Starting date: from 01/02/2025
Application by email: please send CV, motivation letter, Master 1 transcripts. Contact: F. Gaboriau (gaboriau@laplace.univ-tlse.fr)

[1] Q. Delavière-Delion, F. Gaboriau, G. Fubiani, and L. Garrigues, Physics of Plasmas 31, 072110 (2024)
[2] Q. Delavière-Delion, F. Gaboriau, G. Fubiani, and L. Garrigues, IEPC-2024-290 paper, IEPC, Toulouse (2024)