Existence of ion temperature gradient driven shear Alfven instabilities intokamaks

The existence of unstable ion temperature gradient driven Alfven eigenmodes (AITG) is demonstrated in tokamak plasmas, which are ideally stable with r espect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of numerical solution s of a set of one-dimensional integral equations along the ballooning coord inate (quasi-neutrality and parallel Ampere's law). Furthermore, theoretica l analyses of the eigenmode dispersion relation, which is given in a compac t analytical form in the small ion orbit width limit (compared to the radia l wavelength), provide a basis for explaining the general properties of the modes. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be not too far away from ideal MHD marginal stability. (C) 1999 American Institute of Physics. [S1070-664X(99)93705-9].