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Description
We report the analogous experimental observation of the nonlinear mode coupling preceding sawtooth crashes on HL-2A and HL-3 tokamaks. In this experiment, nonlinear mode coupling between m/n = 1/1 internal kink mode (where m and n are poloidal and toroidal mode number respectively) and energetic particle modes (EPMs) is conclusively identified by MEGA analysis, and the mode coupling is specified as characterized as the interaction between the n = 1 kink mode and n = 2 EPM to generate n = 3 sideband. Furthermore, remarkable fast ion loss following crashes is also discussed. Numerical simulations successfully reproduce the process through a comparison of a series of cases with different mode numbers. The results show that although the initial transport of fast ions is induced by the n = 1 internal kink mode, the dramatic subsequent loss is due to the nonlinear mode coupling process. These findings demonstrate the impact of nonlinear interactions on the confinement of energetic particles (EPs). Active control strategies should therefore be developed for multi mode nonlinear coupling to mitigate the potential loss of alphas in future fusion devices.