Collisionless two-fluid theory of toroidal ηi stability

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Abstract
A collisionless two‐fluid theory based on a 14‐moment generalization of the ‘‘double‐adiabatic’’ equations is developed to lowest order in the Larmor radius parameter, and applied to derive the toroidal ηi stability boundary for all values of the ratio of the density gradient scale length divided by the field curvature length. The present model is an improvement over existing two‐fluid models in view of the collisionless nature of the ηi instability, while retaining the advantage over kinetic theory of the practicability of mode‐coupling simulations. The linear stability boundary, linear growth rate, and real frequency agree fairly accurately with drift‐kinetic theory.