Existence of discrete modes in an unstable shear Alfven continuous

In this letter, we demonstrate the existence of unstable ion temperature gr adient driven Alfven eigenmodes in tokamak plasmas, which are ideally stabl e with respect to magnetohydrodynamics (MHD). Conditions for the destabiliz ation of such modes are quantitatively discussed on the basis of theoretica l analyses of the mode dispersion relation, which is given in a compact ana lytical form. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be sufficientl y close to ideal MHD marginal stability.