Internal transport barriers in fluids and plasmas

Turbulent flow in neutral fluids and fusion plasmas is known to have many commonalities, one example being the application of energy and enstrophy cascades. In this review, we discuss a cyclic process which may also be common to both fluids and plasmas: this includes exact coherent states (or magnetic islands), Reynolds stress-driven (zonal) flows and internal interface layers (or internal transport barriers). We connect this picture to the broader literature on layered “staircase” states in both magnetized plasmas and stratified/rotating fluids, where sharp interfaces separate well-mixed regions. We briefly review the current understanding of internal interface layers in fluids, summarize a minimal set of driving/damping relations for mean flows and transport suppression, and discuss open questions and possible research directions. The main objective is to create awareness of shared mechanisms to motivate further interdisciplinary research in this field, by both the fluid mechanics and plasma physics communities.