Onset of interchange instability in a coupled core–SOL plasma

The dynamics at the edge of fusion confinement devices is driven by interchange instabilities and involves the motion of plasma across two regions—the “core region” and the scrape-off layer (SOL)—distinguished by whether field lines are, respectively, closed or connected to the wall. Motivated by this phenomenon, we present an extensive linear stability analysis of a two-layer plasma model encompassing the coupled interactions between the region with closed field lines and the SOL. We focus on the effect of varying the particle diffusivity and ion viscosity, revealing the significant variation in the spatial structure of the critical modes. In addition, we have investigated the dependence of the stability threshold on the ratio of the width of the region with closed field lines to that of the SOL; this dependence is strong when the ratio is sufficiently small, but becomes insignificant once the ratio is of order unity.