Tracing the origins of pulsed GHz emission in Hall Thrusters: from observed effects to underlying causes

Plasma oscillations in Hall thrusters have been observed and studied at various spatial scales and frequency ranges. From low (kHz) to intermediate (MHz) frequencies, the physical phenomena at their root have been relatively well understood, even though their intricacies and interconnection are yet to be fully ascertained. At GHz frequencies, their physical origin has received, in the last decades, much less attention than their lower-frequency counterparts. Yet, an accurate identification of the underlying processes and/or instabilities at play would contribute to the explanation of microwave radiation emitted from these thrusters, which is a threat to communication systems, as well as a better understanding of their overall physics. This work is devoted to the analysis of the GHz pulsed radiated emission in a similar experimental framework to that of Mazières et al. [1] One of its first and novel aspects consists in the gathering of a large number of pulses allowing for a statistical and pulse-to-pulse analysis, on a broad enough ]0, 4] GHz frequency band for their full spectral properties to be captured. A simultaneous temporal analysis of the low-frequency oscillations of the discharge completes the picture and allows for an eventual relationship between the two processes to be investigated. The results from this procedure uncovered the existence of such a relationship. [2] In particular, the pulses were shown to exhibit different spectral patterns depending on the phase of well-known kHz oscillations of the discharge current, such as Breathing Mode and Ion Transit Time Oscillations. Such a spectral dependence is expected to give valuable insights into the physical mechanisms at the root of this emission.

[1] V. Mazières et al., Physics of Plasmas 29(7), 072107 (2022)

[2] F. Réot et al., Physics of Plasma 32(2), 023509 (2025)