Molecular dynamics simulation study of mechanisms of cathode spot evolution : three-stage pattern, analytical approximation, and discussions

Abstract

The consistent ionic erosion rate of cathode spots suggests the presence of considerable steady-state period throughout the spots’ evolution process. To explore the mechanism underlying this stability, this study employs the molecular dynamics (MDs) method to simulate the development of a cathode spot initiated by the leftover plasma ions. In the simulations, an individual cathode spot is surface-meshed and represented by multiple three-dimensional MD models of localised surface block coupled with plasma effects through a simplified surface layer. The simulation results, such as surface temperature and surface atom emission, reveal that stability emerges through a distinct three-stage pattern: beginning with the growth stage where surface heating is driven by leftover plasma ion bombardment, the cathode spot progresses into a transitional stage characterised by intense atomic sputtering and the formation of an atom/ion cloud above its surface, and the regulating effect of this cloud facilitates the establishment of a stationary state of cathode spot growth ultimately. On this basis, the analytical approximation of surface–plasma interaction mechanisms of cathode t evolution is explored. The three-stage evolution pattern is systematically described through a proposed analytical description which encompasses the physical processes of ion bombardment, atomic sputtering, and atom/ion cloud formation. Furthermore, the simulation results are validated through comparison with the experimental observations and other simulation studies of cathode spots, particularly in terms of the self-sustaining nature of cathode spots and the associated ionic erosion rate. The analytical approximation then served as the basis for discussions of key phenomena in vacuum arc applications such as spot ignition, extinction, and current chopping. It is shown that the model has a strong agreement with the well-established stationary and non-stationary cathode spot theories and meanwhile provides a complementary atomic perspective that extends their explanatory capability.