Anisotropy effects on toroidal ηi stability from collisionless two-fluid theory

Ion temperature and temperature gradient anisotropy effects on the toroidal branch of the local ηi instability are analyzed using collisionless two-fluid theory. For isotropic gradients there is good agreement with the Braginskii model for the thresholds even for anisotropic ion temperature, if one averages over the degrees of freedom. But during rapid parallel or perpendicular heating anisotropy in both Ti and ηi is expected to occur. For parallel heating, upper ηiparallel thresholds several times the lower threshold occur at moderate Tiparallel/Tiperpendicular for moderate to large εn, with low growth rates in the intermediate regime. For perpendicular heating lower thresholds are found but are readily finite-Larmor-radius stabilized. Comparison with kinetic theory shows a sizable ηi range for which the instability is non-reactive with low growth rate. Growth rates and thresholds tend to agree better with kinetic theory for k√Tiperpendicular/(miωci2)<0.5 than the Braginskii model. The perturbo-theoretic limitations of both two-fluid theories are delineated. copyright 1996 American Institute of Physics