Analysis of high-field side plasma instabilities in tokamak edge

Balanced double-null configurations are of general interest for boundary plasma physics, and they have been proposed for some future designs. Experimental observations demonstrate absence of plasma fluctuations in tokamak high-field side scrape-off layer in a balanced double-null configuration (Smick et al., 2013), and it is commonly assumed that plasma instabilities are suppressed on high-field side in the edge plasma due to the stabilizing effect of magnetic curvature. At the same time, the experimental evidence points to extremely steep plasma density profiles on high-field side, which should provide a strong instability drive. In the present study, the drift-resistive-ballooning mode instability model is investigated analytically and numerically to determine the characteristics of plasma instabilities, turbulence, and transport in tokamak scrape-off layer on high-field side. • Experimental evidence points to quiescent conditions on high-field side (HFS) tokamak scrape-off layer (SOL) in balanced double-null (DN) configuration. • At the same time, in DN the HFS SOL radial plasma profiles are very steep, which should provide a strong instability drive. • Analysis of HFS SOL plasma instabilities is presented, based on drift-resistive-ballooning mode (DRBM) model. • DRBM linear analysis and nonlinear numerical simulations demonstrate that some level of plasma turbulence should be present. • The amplitude of HFS fluctuations may still be small due to marginal stability type constraints. • For quantitative comparison with the experiment, apparently a more complete model will be needed.