Nonlinear evolution of Alfven eigenmode and excitation of geodesic acoustic mode

Full text: Nonlinear evolution of toroidal Alfven eigenmode (TAE) destabilized by energetic particles was investigated with the energetic-particles and magnetohydrodynamics hybrid computer simulation. It is demonstrated that the wave-particle trapping causes the saturation of the TAE instability for low or moderate amplitude δ Br / B ≤ 10-3. For stronger energetic-particle drive of the instability, however, the MHD nonlinearity becomes important and reduces the saturation level of the instability. The suppression of the saturation level arises from both the n=0 harmonics and the higher-n harmonics. The n=0 harmonics generation was theoretically analyzed. It was also found in the simulation results that geodesic acoustic modes (GAM) are excited in the TAE evolution through the MHD nonlinearity. The frequency and the spatial profile of the pressure and velocity fluctuations are in good agreement with theory of GAM. The mechanism of the GAM excitation will be discussed. (author)