Inhomogeneous mixing: from microscopic dynamics to mesoscopic staircases

Inhomogeneous mixing and the consequent mesoscopic layered structure have been observed in many physical systems, including magnetically confined fusion plasmas. Especially, in plasmas, mixing can be enhanced through turbulence spreading by intermittent coherent structures (blobs/voids), or suppressed owing to the formation of transport barriers (sheared zonal flows). Interestingly, blobs/voids and zonal flows are not independent, and they can coexist in a state of inhomogeneous mixing, often called the E×B staircase. In this paper, we first introduce recent experimental progress on the physics of blobs/voids: how turbulence spreading by blobs/voids occurs, the consequences of enhanced turbulence spreading for the scrape-off layer (SOL) power decay length and the interaction between blobs/voids and zonal flows. Then, we provide a brief review of experimental results on staircases, or more generally layered mesoscopic transport barriers. Staircases are often elusive in experiments, requiring integrating multi-diagnostic data, using high-dimensional diagnostics, or extracting hidden information from signals for their identification. Our understanding is still incomplete. This paper serves as an initial step towards applying insights gained from inhomogeneous mixing owing to blobs/voids to the understanding of a staircase. This article is part of the theme issue 'Anti-diffusive transport and layering: from subcellular to astrophysical scales'.