Simulation study of interaction between energetic particles and magnetohydrodynamic modes in the JT-60SA inductive scenario with a flat q≈1 profile

Abstract

Interactions between energetic particles (EPs), an internal kink mode, and other magnetohydrodynamic (MHD) instabilities in the inductive scenario of JT-60SA are simulated with MEGA, a global EP-MHD hybrid code. For this scenario, it was predicted by TOPICS, an integrated transport code that the internal kink mode can be unstable and the sawtooth relaxation results in a flat safety factor (q) profile with q ≈ 1 for r/a ≤ 0.6. In this equilibrium, it is found in the simulation results that the stability of the internal kink mode depends strongly on the bulk plasma pressure gradient. In the n=1 simulations, the pressure-driven internal kink mode is dominant. In the presence of co-passing EPs generated by the N-NB, these EPs transfer energy to the internal kink mode; however, the EP driving rate is much lower than the driving rate from the bulk plasma pressure gradient. The mode's frequency is less than 1 kHz because the toroidal and poloidal orbit frequencies of the co-passing EPs are approximately equal within the q=1 surfaces. In the multi-n simulations, where n≤8 modes are retained, the most unstable modes are high n interchange modes with poloidal number m=n whose linear growth rates exceed that of the pressure-driven internal kink mode observed in the n=1 simulations. The overlapping of these modes creates a stochastic magnetic field, leading to stronger EP and bulk plasma pressure redistributions than those observed in the n=1 simulations. During the nonlinear phase, the transition from the high m=n modes to the low m=n modes is observed where the dominant mode is the m/n=1/1 mode with an internal kink-like structure. These low m=n modes are generated by the nonlinear coupling of the high m=n modes. The EP kinetic effect has a minor contribution to the dynamics of these nonlinearly generated m=n modes.