Formation mechanism of the complex dynamics in subnormal glow discharge systems: Mixed-mode oscillations and period-adding bifurcation.

Mixed-mode oscillation (MMO) is a type of complex dynamic behavior commonly seen in multitimescale dynamical systems. As a typical nonlinear medium, MMOs have been experimentally observed in a variety of plasma systems. However, the underlying microscopic physical mechanisms that are responsible for this complex dynamics are still unclear at present. Based on the strong nonlinear dependence of the Townsend ionization process on the electric field, the causes leading to the emergence of subthreshold small-amplitude oscillations and the corresponding period-adding bifurcation are explored with a one-dimensional, time-dependent, self-consistent plasma fluid model. An in-depth study has been conducted on the spatiotemporal evolution characteristics of plasma parameters under different dynamical states. It has been found that in the current decay phase, the localized avalanches between the cathode and the virtual anode caused by the bulk plasma play a crucial role in shaping the MMO structure within the system. These localized avalanches arise from the combined effects of rapid voltage increases and delayed bulk plasma dissipation. The findings are anticipated to deepen our understanding of the formation mechanism of the MMOs in subnormal discharge systems and provide helpful inspiration in a wide range of spatially dependent nonlinear dissipative physical systems.