Equilibrium flow-induced bifurcation between quasi-steady and bursty dynamics of ballooning mode turbulence

Edge-localized modes (ELMs) are burst-like transport events observed in the edge region of tokamak plasmas, typically driven by pressure-driven instabilities such as the ballooning mode. Understanding nonlinear dynamics of ELMs is essential for future devices such as ITER. In this study, we conduct nonlinear simulations of the resistive ballooning mode turbulence using the reduced two-fluid model that includes both the heating and the neoclassically induced equilibrium flow. In the absence of the equilibrium flow, the system exhibits the quasi-steady turbulent dynamics, while the inclusion of the equilibrium flow triggers the repetitive bursts. These bursts are triggered by the formation and decay of the intermediate-scale vortices, whose radial extent and lifetime are regulated by the flow shear through nonlinear interactions. The dependence of the burst dynamics on the flow strength is non-monotonic, indicating complex nonlinear interactions between the mean flow and turbulent vortices. These findings reveal the mechanism by which the equilibrium flow modulates the burst dynamics and the vortex size selection in the ELM-like phenomena.