Description
The formation of coherent structures and their impact on the system have always been a significant enigma in non-equilibrium physics. This constitutes a key scientific question in crucial areas such as cosmic evolution, the origin of life, and the space and terrestrial climate environment. Specifically in magnetically confined fusion, coherent structures alter plasma transport properties. They play an important role in enhancing energy confinement, improving particle confinement, and broadening divertor heat fluxes, representing a core scientific principle in critical physical processes within the pedestal region, I-mode, staircase structures, and SOL radial transport. However, a comprehensive understanding is still lacking regarding how coherent modes can spontaneously achieve high spatio-temporal correlation in a chaotic system, and what impact such highly coherent structures have on the transport of turbulent systems. The experimental observations and theoretical explanations presented here offer new scientific insights into this problem. Experiments reveal that when the plasma pressure gradient reaches a certain threshold, the system transitions from a drift-wave turbulent regime to a coherent mode-dominated regime. In this regime, the original turbulent spectrum collapses into a series of fine-scale energy level structures corresponding to distinct temporal eigenmodes, each exhibiting well-defined radial and poloidal structures. Based on two-fluid equations, the dispersion relation for drift waves can be transformed into a Schrödinger-like equation, where the local dispersion relation provides an equilibrium Hamiltonian, and the radial pressure gradient acts as a perturbation term. Consequently, when the gradient is sufficiently strong, the system transitions from localized eigenstates to global temporal eigenstates, thereby establishing non-local correlations. Furthermore, by tracing virtual test particles, it is found that coherent modes can induce a particle trapping effect, forming two topological structures: particle passing and trapping regions. Due to the conservation laws governing particle motion within the drift-wave coherent structures, effective particle transport in the gradient direction is suppressed. When the drift-wave structure undergoes non-rigid deformation, these particle invariants are broken, leading to a randomization of streamlines within the coherent mode and resulting in effective particle transport. These findings are of significant importance for understanding anomalous transport in magnetically confined fusion, the decoupling of particle and energy transport, and the interaction between fast particles and turbulence.