Electrostatic micro-instabilities in axisymmetric toroidal plasmas

A careful comparison of the different terms of the ion gyrokinetic equation is made for two ranges of poloidal mode numbers: kx ai nearly equals ε and kxai nearly equals 1, respectively, where the small parameter ε is defined as the ratio of any characteristic equilibrium radial length-scale to the connection length; the shear parameter is assumed to be of order unity. Postulating different characteristic values for the radial mode numbers, the dissipative and collisionless trapped electron modes are identified on the one hand and the ion temperature gradient mode on the other hand. The followed rigorous scaling and expansion procedures have brought us to the conclusion that the radial component of the ion magnetic drift velocity is responsible for an increase of the usual shear damping of non-overlapping electron modes if the wave frequency is larger than the ion transit frequency, and to an even larger ion transit resonance damping in the opposite limit. Annihilation of shear damping for overlapping electron modes, owing to their coupling through the ion magnetic drift frequency, requires two conflicting conditions to be simultaneously satisfied (and is therefore questionable) unless the shear parameter is smaller than unity. Finally, micro-instabilities which are edge relevant are briefly described. 36 refs