A review of discharge instabilities in hollow cathodes

The hollow cathode instability phenomenon has multiple spatial scales and involves multiple physical regions. They are controlled by coupled physical processes within the cathode, at the orifice, and in the plasma plumes. Research relying on a single diagnostic method, limited spatial measurement points, or simplified physical models may result in ambiguous mechanism attribution and may obscure the connections among different instability modes. This motivates a systematic review that organizes hollow cathode instabilities within a unified framework of operating range, characteristic frequency, source region, diagnostic evidence, and physical mechanism. This review classifies various instability phenomena based on the discharge current range, characteristic frequency, and main occurrence regions to compare and analyze their physical causes and inter-coupling relationships. Additionally, it summarizes experimental and numerical research methods: from early simple phenomenon observations to today's advanced diagnostic measurements and numerical simulations, which can perform more quantitative analyses of the evolution of instability phenomena and their impact on plasma properties. This paper also summarizes the research progress of our group in low-power and low-flow-rate hollow cathodes. The main achievements include identifying multiple instability states, explaining their physical mechanisms, and proposing reasonable explanations through experimental diagnostics and simulations. Finally, this review identifies key future needs, including standardized source-region definitions and evidence-closure criteria, broader assessment of instability impacts beyond energetic-ion sputtering, improved synchronized diagnostics and multitime-scale models, and integrated cathode–thruster–circuit studies under transient conditions. These efforts are essential for improving mechanism attribution, cross-device comparability, and lifetime-linked stability prediction.