Validation of critical electron temperature for ionization instability in ampere-level hollow cathode plume

Plume oscillations in ampere-level hollow cathodes significantly impact the performance and stability of electric thrusters. In this study, a dimensionless perturbation analysis is applied for predicting the oscillation frequency and growth rate of discharge current. The correlation between the predicted and measured frequency of discharge current, as well as the theoretical growth rate and measured total spectral power, is calculated for an ampere-level hollow cathode. Based on the theoretical growth rate, the onset criterion for the electron temperature is used to characterize the ionization instability and describe the physical mechanisms behind ampere-scale hollow cathode plume oscillations. The validity of the critical electron temperature is confirmed by comparing it with the classical instability criterion. Given the strong correlation between the electron temperature instability criterion and oscillation intensity, this criterion is considered more reasonable for predicting the ionization instability. The effect of structural (anode distance, keeper to cathode orifice distance, cathode orifice diameter) and working parameters (keeper current), on the applicability and reliability of this method is systematically evaluated through a detailed analysis of instability phenomena. It should be noted that this paper is an expanded experimental validation of the critical electron temperature theory originally proposed by Georgin (2020 Ionization Instability of the Hollow Cathode Plume). Besides that, the critical electron temperature is not straightforward to apply, since it does not rely directly on macroscopic discharge parameters, such as discharge current, anode voltage, and gas flow rate. One must have access to plasma measurements in the cathode plume in order to apply this criterion, which may make it hard to use in a laboratory testing setting without plasma diagnostics. Despite this, the critical electron temperature could enhance our understanding of ionization instability and offer valuable insights for optimizing high-performance hollow cathodes.