Recent theoretical progress in understanding coherent structures in edge and SOL turbulence

TLDR

Edge and SOL plasma transport is now understood as convective, very intermittent cross‑field transport, a paradigm that evolved from early experimental observations. This review examines the theoretical dynamics of mesoscale filaments, or blobs, that extend along magnetic field lines in the edge plasma. Blob convection arises from an effective gravity force due to magnetic curvature, producing plasma polarization and E×B drift, with additional influences from X‑point geometry, collisionality, and beta. Analytic theory and simulations show that pressure‑enhanced blobs are coherently convected toward the wall at a few hundred meters per second over centimeter‑scale cross‑fields, matching experiments and explaining observed outward transport, intermittency, non‑Gaussian statistics, and wall recycling.

Abstract

Abstract In this paper we review some theoretical aspects of the dynamics of the mesoscale filaments extending along the magnetic field lines in the edge plasma, which are often called ‘blobs’. We start with a brief historical survey of experimental data and the main ideas on edge and SOL plasma transport, which finally evolved into the modern paradigm of convective very-intermittent cross-field edge plasma transport. We show that both extensive analytic treatments and numerical simulations demonstrate that plasma blobs with enhanced pressure can be convected coherently towards the wall. The mechanism of convection is related to an effective gravity force (e.g. owing to magnetic curvature effects), which causes plasma polarization and a corresponding E × B convection. The impacts of different effects (e.g. X-point magnetic geometry, plasma collisionality, plasma beta, etc.) on blob dynamics are considered. Theory and simulation predict, both for current tokamaks and for ITER, blob propagation speeds and cross-field sizes to be of the order of a few hundred meters per second and a centimeter, respectively, which are in reasonable agreement with available experimental data. Moreover, the concept of blobs as a fundamental entity of convective transport in the scrape-off layer provides explanations for observed outwards convective transport, intermittency and non-Gaussian statistics in edge plasmas, and enhanced wall recycling in both toroidal and linear machines.