Transport and evolution of ion gyro-scale plasma blobs in perpendicular magnetic fields

Abstract

Strongly localized concentrations or depressions of plasma density and magnetic field strength ('blobs') are ubiquitous in the edge region of tokamak fusion experiments. The existing fusion plasma literature in this area focuses primarily on blobs sufficiently large that a fluid description is appropriate. However, the blob population may include some—not necessarily easily detectable—whose characteristic lengthscales are on the order of the ion gyro-scales. This implies that a description at the fluid level is unlikely to capture the full dynamics. Here, therefore, we report hybrid (particle ions, fluid electrons) simulations of ion gyro-scale blobs, which enable us to examine the effects of finite Larmor radius on their dynamics and evolution. We find that ion gyro-scale blobs are advected with the background flow, and develop a twin-celled vortex structure. Asymmetry then arises from finite ion Larmor radius kinetics, manifesting in the size of the internal vortices, the shape of tails forming from blob ejecta, and the growth of a Kelvin–Helmholtz instability.

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ORCID for P.W. Gingell: orcid.org/0000-0003-2218-1909

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