TH/3-1Ra · Nonperturbative effects of energetic ions on Alfvén eigenmodes
Y. Todo1), N. Nakajima1), K. Shinohara2), M. Takechi2), M. Ishikawa2), S. Yamamoto3)
1) National Institute for Fusion Science, Toki, Japan
2) Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka, Japan
3) Institute of Advanced Energy, Kyoto University, Uji, Japan
Abstract: Nonperturbative effects of energetic ions on Alfvén eigenmodes both in the linear property and in the nonlinear evolution were investigated using a particle- magnetohydrodynamic hybrid simulation code . For a JT-60U experiment, where the fast frequency sweeping (FS) mode was observed, a new kind of energetic particle mode (EPM) was found near the plasma center at frequency close to the central frequency of the fast FS mode. Frequency and spatial profile of the new EPM depend on spatial profile of energetic ions. This is a nonperturbative effect of energetic ions on the linear property. We also found nonperturbative effects of energetic ions on the nonlinear evolution. Two types of nonlinear evolution take place depending on the initial energetic ion pressure. When reduced distributions of energetic ions are considered for the initial condition, frequency shifts both upward and downward at a rate of frequency sweeping close to that of the fast FS mode. The change in the EPM spatial profile is small. The frequency shifts are comparable to the linear growth rate of the instability. This suggests that the saturation mechanism of the instability is the particle trapping which gives rise to the frequency shifts of the bounce frequency. The time evolution can be explained by the perturbative approach where the EPM spatial profile is assumed fixed. On the other hand, when a classical distribution is taken for the initial condition, a large redistribution of the energetic ions occurs, leading to frequency downshift, an appreciable change in the EPM spatial profile, and breakdown of the perturbative approach. In this paper, we report the nonperturbative effects of energetic ions on the EPM in JT-60U. Numerical analysis results of toroidal Alfvén eigenmodes (TAE) in LHD are also presented.
Keywords: Alfvén eigenmodes, Alfvén modes, energetic particles, helical system, tokamak,