Linear-stability criteria for drift and curvature-driven modes

In this paper an investigation on the linear stability of a locally Maxwellian and collisionless plasma with respect to general electrostatic perturbations of the drift type is proposed. The representation used for the perturbation contains as a special subclass the ballooning mode representation. General stability theorems are found in two asymptotic limits, corresponding to the regimes typical of the collisionless trapped particle and universal drift modes, respectively. In both cases at most two unstable modes can exist, which are related both to the presence of temperature gradients and to unfavourable curvature of the magnetic field. Here a comparison is made with the stability predictions in a sheared slab plasma, for drift waves, and it is found that only in particular limits similar results are recovered in toroidal configurations. Finally analytic stability criteria are derived for the regime of the trapped-particle mode and an application is given for a large aspect ratio with circular cross-section and for radially localized perturbations.