Study of hollow cathode plume instabilities onset using a spatially-scanning probe

Plasma instabilities play a fundamental role in determining the lifetime of hollow cathodes and, consequently, the operative life of electric thrusters. Three main categories are identified: ionization instabilities, rotational instabilities, and turbulent ion acoustic instabilities (IAT). Ionization instabilities typically arise when the cathode operates in plume mode and decrease at lower discharge currents and mass flow rate ratios. Rotational instabilities occur in the presence of an externally applied axial magnetic field, even under spot-mode operation, while IAT develops at frequencies above 100 kHz. Several studies suggest that plasma instabilities are linked to the generation of high-energy ions, responsible for keeper surface erosion and the limit cathode lifetime. This paper presents results on the onset of plasma instabilities in a 25 A-class cathode, investigated using a fast-scanning axial probe in the cathode orifice region. Through Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) analysis, mode frequencies have been identified and their spatial localization along the centerline observed. Without an applied magnetic field, ionization instabilities have been detected at low mass flow rates, with characteristic frequencies increasing as flow decreases. In the presence of an axial magnetic field, rotational instabilities couple with turbulent ion acoustic instabilities, generating more energetic ions, and interact with ionization instabilities when the cathode operates in plume mode. CWT plots demonstrate that rotational bursts are stronger just downstream of the keeper orifice, where sputtering occurs. The dynamics of these rotational instabilities, tracked with the scanning probe, are consistent with kink-type dynamics in the plume, even at low discharge currents.