Causal coupling of edge-SOL coherent structures with zonal flows

The bidirectional causal interrelation of turbulent structures in the edge region of a magnetically confined plasma is studied by means of convergent cross mapping as the causality inference method. Drift waves (DWs) inside the confinement region of the TJ-K stellarator together with zonal flows (ZFs) are interrelated with blobs in the scrape-off layer across the separatrix. Intense Langmuir-probe measurements show all three phenomena to co-exist under mutual correlation. Density perturbations related to DW turbulence are found to predominantly cause potential fluctuations as consistent with the adiabatic electron response to the perturbation. Resistivity and magnetic-curvature drive leads to a balanced, i.e, net-zero bidirectional causal coupling, reinforcing the electrostatic-potential cause of the density. Whereas DWs are verified to be causally bidirectionally coupled with ZFs, blobs are validated as a candidate for driving ZFs across the separatrix in a unidirectional causal manner. In particular, blobs are found to cause ZFs rather than vice versa, giving deeper insight into the role of blobs in the formation of edge transport barriers.