The eigenmode equation describing ballooning collisionless drift instabilities is analyzed both analytically and numerically. A new branch of eigenmodes, which corresponds to quasi-bound states due to toroidal coupling effects such as ion ∇B drifts, is shown to be destabilized by electron Landau damping for typical tokamak parameters. This branch cannot be understood by the strong coupling approximation. However, the slab-like (Pearlstein-Berk-type) branch is found to remain stable and experience enhanced shear damping.
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