Noncurvature-driven modes in a transport barrier

Transport barriers that form in both the edge and interior regions of high temperature magnetically confined discharges are characterized by steep plasma gradients, strong E×B and diamagnetic flows, and varying levels of magnetic shear. This study addresses the linear stability of such configurations in the context of a simple slab model using both analytic calculations as well as numerical simulations from the gyrokinetic GS2 code. Three linear modes of potential importance are found: the Kelvin–Helmholtz instability, the tertiary mode, and a nonlocal drift wave instability. Each mode is unstable only in the presence of nontrivial spatial variations in either the E×B flow and∕or the plasma gradients. The strongest conclusion of this study is that the drift wave mode may be an important driver of anomalous transport in the edge region of magnetic confinement devices. Two other weaker conclusions that warrant further study are as follows: (1) the Kelvin–Helmholtz instability may be associated with edge-localized modes or edge transport and (2) the tertiary mode can potentially limit the radial growth of a transport barrier.