Turbulent homoclinic tangle and its role in the integrated pedestal and heat exhaust problem for a stationary-operation tokamak fusion reactor*

The magnetic separatrix surface is designed to provide the final confinement to the hot stationary-operation core plasma in tokamak reactors in the absence of an external magnetic perturbation (MP) or transient MHD perturbation, while diverting the exhaust heat to divertor plates. All the stationary operational boundary plasma studies and reactor designs have been performed under this assumption. However, there has been a long-standing suspicion that a stationary-operation tokamak plasma even without external MPs or edge localized modes (ELMs) may not have a stable closed separatrix surface, especially near the magnetic X-point. In this presentation, the first gyrokinetic numerical observation is reported that the divertor separatrix surface, due to Hamiltonian homoclinic tangles caused by intrinsic electromagnetic turbulence, is not a stable closed surface in a stationary-operation phase. Unlike the MP- or ELM-driven homoclinic tangles that could cause deleterious effects to core confinement or divertor plates, it is found that the micro-turbulence driven fine-scale homoclinic tangle lobes could connect the divertor plasma to the pedestal plasma in a constructive way by broadening the divertor heat-exhaust footprint and weakening the pedestal slope to the ELM-safe direction. Micro-turbulent homoclinic tangles can open a new research direction in understanding and controlling these two most troublesome and non-locally connected edge-plasma issues in a tokamak fusion reactor.