Discovery of micro-turbulent homoclinic tangle near magnetic X-point from the XGC total-f electromagnetic simulation

The outermost confinement in a diverted tokamak reactor is designed to be at the magnetic separatrix surface, which, however, can easily be distorted into wild lobe or finger structures, called homoclinic tangle, by small non-axisymmetric internal or external perturbations that could allow plasma to leak out around the magnetic X-point region. In the past, transient MHD-driven and stationary RMP-driven homoclinic tangles have been found. Recently, it has been discovered from electromagnetic edge simulations in the total-f gyrokinetic code XGC that a high-frequency, intrinsic turbulent homoclinic tangle can be formed by microturbulence even in the absence of MHD instabilities or RMPs. The turn-over frequency of this intrinsic homoclinic tangle is resonant with the electron kinetics and can easily produce an extra source of electron heat transport across the last confinement surface towards divertor plates, possibly making an intrinsic connection between the pedestal and heat-exhaust to divertor. The turbulent homoclinic tangle is found to have a non-negligible widening of the divertor heat-load width on the full-current ITER, but a negligible effect on the present tokamak plasma in a normal H-mode operation. Detailed properties of the turbulent homoclinic tangle will be presented.