Coupling discharge characteristics of a low-current hollow cathode in a side-set anode configuration

The hollow cathode is a crucial component of the Hall thrusters, whose coupling discharge with the thruster body (anode) is vital for system operation. Previous studies commonly used a front-set anode configuration without neutral gas from the anode for hollow cathode discharge testing, which cannot accurately characterize the real characteristics of external hollow cathodes in Hall thrusters. In this work, a hollow cathode discharge testing system has been developed to explore the coupling discharge characteristics. Results indicate that the neutral gas from the anode impacts discharge establishment oppositely in the front-set and side-set configuration, being indispensable in the latter. In the side-set configuration, discharge establishment is sensitive to magnetic field strength, and the cathode commonly operates in the plume mode with intensified oscillations. Subsequently, optical diagnostics reveals the origin and propagation of oscillations in different frequency ranges: low-frequency oscillations (<1.5 kHz) originate from the cathode and propagate towards the anode, while medium-frequency oscillations (1.5–25 kHz) stem from the anode and propagate towards the cathode. Furthermore, the effect of operating parameters on the oscillation characteristics of anode voltage and ion/electron current at the cathode exit reveals that increasing the anode/cathode flow rates enhances and suppresses oscillations, respectively, and enhancing the magnetic field has opposing effects on medium-frequency and low-frequency oscillations. The coupling discharge establishment and stabilization are related to the electron cross-filed transport limitation imposed by the enhanced magnetic field and the ionization enhancement by the increased anode flow rate. Therefore, the strategy of matching the anode flow rate with the magnetic field strength and increasing the cathode flow rate has been proposed to suppress coupling discharge oscillation and control the cathode discharge mode. This work is beneficial for uncovering the coupling discharge mechanism of the cathode with the anode and developing stable cathode operation strategies in the side-set configuration.