Abstract
Hollow cathodes are widely used for spacecraft electric propulsion, however improved understanding of the cathode plasma plume is required to improve performance and lifetime. The modular hollow cathode is developed to assist in cost effective ground testing by allowing simplified component replacement in the case of failure. In this work, a Langmuir probe is employed to characterise the plasma potential, electron temperature and electron density in the external cathode plume. Additionally, optical emission spectroscopy is used to qualitatively infer the relative plume composition. We examine the plume through independent tests using xenon and krypton, adjusting mass flow rates and other operating parameters. The modular hollow cathode exhibits different modes of operation including plume, spot, and diffuse modes. The experimental observations align with the theoretical predictions using various plasma mode transition criteria. In particular, the predator-prey criterion based on the ratio between the discharge current and the mass flow rate captures most of the mode transitions. Xenon operation results in a higher current collected at the anode in comparison to krypton where the total discharge power increases. Measurements collected determine the variation of the plasma properties at a fixed position from the keeper as a function of mass flow rate, keeper current and anode voltage in a triode configuration. The plasma potential and electron temperature downstream of the cathode orifice decrease with increased mass flow rate due to a more collisional plasma. As the flow rate rises, and with it the collisionality of the cathode plasma, the trends in electron temperature align well with qualitative Xe neutral/ion line ratios. The cathode is also coupled to a low-power wall-less (external discharge) hall thruster and operated on xenon to investigate the influence of the mode of operation on the discharge current and coupling voltage.