Experimental observation of low-frequency interactions at different scales and evidence of transit time oscillations in a Hall thruster: Spectral analysis

Abstract

Hall thrusters are E×B plasma devices characterised by a large azimuthal electron current, which is involved in the formation of a wide variety of instabilities on different frequency scales (from kilohertz to gigahertz) and propagating in different directions. We have focused on a limited number of low-frequency instabilities that could be experimentally observed, such as breathing mode (BM) and ion transit time oscillations (ITTO). There are still gray areas in the understanding of the mechanisms behind these instabilities, which are often described individually, without considering an eventual exchange of information between the different types of instability. A wavelet time-frequency study of the plasma revealed the coexistence and interdependence of distinct instabilities. We suspect a possible interaction between the breathing mode and higher-frequency oscillations leading to ITTOs: it was shown that a transition occurred when the frequency of one of the BM harmonics became a multiple of the frequency of the higher-frequency oscillations. This was also accompanied by a deformation of the ion energy distribution functions.