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 inside the confinement region of the TJ-K stellarator together with zonal flows 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 drift-wave 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 drift waves are verified to be causally bidirectionally coupled with zonal flows, blobs are validated as a candidate for driving zonal flows across the separatrix in a unidirectional causal manner. In particular, blobs are found to cause zonal flows rather than vice versa, giving deeper insight into the role of blobs in the formation of edge transport barriers.