Axisymmetric Alfven modes induced by energetic particles in tokamak plasmas

Axisymmetric Alfven modes in the presence of energetic ions are studied using the MHD-kinetic hybrid model for tokamak plasmas. The general dispersion relation of continuous spectra is derived for the coupled m = 0, +/- 2 poloidal harmonics. The dispersion relation is analyzed for the single pitch angle slowing down distribution of well circulating energetic particles. In addition to the conventional axisymmetric Alfven modes existing in the pure MHD description, axisymmetric Alfven modes are also induced by the energetic ions if the pressure ratio between the energetic ions and the magnetic energy density is larger than a critical value, and the transit frequency of the energetic ions is in resonance with the emerging Alfven modes. The existence of global Alfven eigenmodes is also discussed. Published under an exclusive license by AIP Publishing.