Convective transport by intermittent blob-filaments: Comparison of theory and experiment

TLDR

Blob-filaments are dense, magnetically aligned plasma structures that form near the boundary between open and closed field lines, arise from edge instability saturation, and represent a general radial transport phenomenon in confined plasmas due to curvature and ∇B forces. The paper reviews and assesses how well blob theory and simulations agree with experimental observations of blob formation, dynamics, and transport. Charge polarization from external forces causes unequal ion and electron drifts, producing an E×B drift that propels blobs radially outward across the scrape‑off‑layer.

Abstract

A blob-filament (or simply “blob”) is a magnetic-field-aligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence. Blobs become charge-polarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarization-induced E × B drift moves the blobs radially outwards across the scrape-off-layer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and ∇B forces in toroidal plasmas), blob transport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob formation, dynamics and transport and assesses the degree to which blob theory and simulations can be compared and validated against experiments.