Theory of nonoverlapping low frequency modes in axisymmetric toroidal plasmas

Abstract

The theory of low frequency instabilities in axisymmetric toroidal plasmas is presented from the point of view of the two-fluids equations, assuming the standard drift wave ordering. Attention is focused on the limit in which neighboring rational surfaces (corresponding to a fixed toroidal mode number) are sufficiently far apart that mode overlapping can be neglected. Owing to field line bending, poloidal side bands m±1,… coexist with the primary mode m, enhancing noticeably the role of the parallel ion dynamics. The electron and ion branches are investigated systematically under those conditions. It is found that the radial widths of the eigenmodes increase with respect to the slab values; the shear damping rate of the electron branch, respectively, the growth rate of the ion branch increase correspondingly. Other interesting new results are obtained concerning, in particular, the frequency, the growth rate and the poloidal asymmetry of the ion mode fluctuations. It is mentioned that those appear to be directly relevant to various experiments and, in particular, to internal transport barriers. Shortcomings of the standard ballooning formalism are pointed out.