Collisionless microinstabilities in configurations with periodic magnetic curvature

Abstract

In view of investigating the stability of a closed (toroidal) configuration in the high temperature collisionless regime, a two-dimensional model simulating the effects of magnetic curvature variation, magnetic shear and particle trapping is adopted. Use is made of the Vlasov equation including finite Larmor radius and wave-particle resonance effects. Low frequency electrostatic modes are considered. Then two types of wave having the same periodicity L as the magnetic curvature, or localized in a region where curvature is unfavourable, are found. One has the frequency of the known drift wave and the other (the flute-`gravitational' wave) has frequency determined by the average favourable curvature along the lines of force. The latter wave is stabilized by imposing that L be sufficiently short as to ensure good ion communication making ion Landau damping effective. The former one by imposing that L make the effects of longitudinal ion sound wave prevail over the effects of ion inertia on their transverse motion. If the lines of force are not closed or if they are closed but their length is much larger than L, drift waves with wavelength larger than L have to be considered. In the first case they can be stabilized by shear, in the second case, waves with transverse wavelengths short enough as to make the effects of transverse inertia prevail over those of longitudinal ion inertia, remain unstable. The influence of trapped particles is investigated, finding that it contributes to reducing growth rates. Stability conditions are given for the most significant cases observing that, for non-hydromagnetic types of mode, they are easier than those obtained for the collisional regime. It is recalled that while no wave localized in a region of unfavourable curvature was found in the high temperature collisional regime, a wave driven by the known drift mechanism but localized over distances shorter than L can be found in the collisionless regime.