Plasma kinetics: Anomalous transport process in a hollow cathode plume

In this work, the direct kinetic method is employed to investigate the anomalous transport process of current-driven instability in the hollow cathode plume. The influences of electron Mach number, gas species, and gas flow rate on the evolution of both the electron/ion velocity distribution functions and the corresponding macroscopic quantities (bulk velocity and temperature) are carefully examined. The spatial distribution of neutral particles is determined under a conical diffusion assumption, and the role of electron–neutral classical collisions in the anomalous transport process is quantitatively evaluated. The results reveal that: (i) increasing the electron Mach number enhances particle–wave interactions, thereby accelerating the formation of electron holes, high-energy ion tails, and stronger macroscopic oscillations; (ii) lighter gas species intensify particle–wave coupling, leading to a faster and stronger non-equilibrium evolution of both electrons and ions; and (iii) a higher gas flow rate suppresses nonlinear evolution by strengthening electron–neutral classical collisions, which stabilizes the velocity distributions and reduces anomalous energy transfer from electrons to ions. These findings provide new theoretical insight into the particle–wave anomalous transport mechanism and offer valuable guidance for the design and optimization of high-performance hollow cathodes.