Dr. Hao Wang
National Institute for Fusion Science, Japan
Time: Monday, April 16, at 4pm
Location: Rm. 413, Bld. 11#, Yuquan Campus
Simulations of energetic particle driven geodesic acoustic mode with two different models
The energetic particle driven geodesic acoustic modes (EGAMs) in a 3-dimensional LHD equilibrium data are simulated using MEGA code. MEGA is a hybrid simulation code for energetic particles (EPs) interacting with a magnetohydrodynamic (MHD) fluid. In the present work, both the conventional and new models of MEGA are employed. In the conventional model, only the EPs are described by the kinetic equations, while in the new model not only the EPs but also the thermal ions are described by them. Using MEGA with a conventional model, very good validations are obtained. The poloidal velocity oscillation is a combination of m/n = 0/0 (strong), 1/0 (medium), and 2/10 (weak) components. The chirping primary mode and the associated half-frequency secondary mode are firstly reproduced with the realistic input parameters and 3-dimensional equilibrium. The bulk pressure perturbation and the EP pressure perturbation of the secondary mode are in anti-phase and cancel each other out. Thus, the frequency of the secondary mode is low. The secondary mode is excited by the EPs not by the nonlinear MHD coupling, and this excitation is confirmed by the analysis of resonant particles in the phase space. In addition, it is found that the transition between low frequency EGAM and high frequency EGAM is decided by the slope of EP velocity distribution. Using MEGA with an extended model, the EGAMs are reproduced, and the mode numbers and mode frequencies are consistent with theory and experiment. Also, the bulk ion heating during the EGAM activity is reproduced by simulation for the first time. The heating power is close to the value evaluated from the experiments.