Inhomogeneous mixing: From microscopic dynamics to mesoscopic staircases

TL;DR

This paper reviews recent experimental advances in understanding inhomogeneous mixing in plasmas, focusing on the roles of blobs, voids, and zonal flows, and explores their connection to layered mesoscopic structures called staircases.

Contribution

It provides a comprehensive review linking microscopic blob/void dynamics to mesoscopic staircase structures in plasma transport, highlighting recent experimental progress.

Findings

Turbulence spreading by blobs/voids affects plasma edge behavior.

Interaction between blobs/voids and zonal flows influences transport barriers.

Experimental identification of staircases requires advanced diagnostics.

Abstract

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 due to the formation of transport barriers (sheared zonal flows). Interestingly, blobs/voids and zonal flows are not independent, and they can co-exist in a state of inhomogeneous mixing, often called the E x 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, utilizing high-dimensional diagnostics, or extracting hidden information from signals for their identification. Our understanding is still incomplete. This paper serves as an initial step toward applying insights gained from inhomogeneous mixing due to blobs/voids to the understanding of a staircase.