Nonlinear simulations of energetic particle modes in tokamak plasmas with reversed magnetic shear

Effects of energetic particles (EPs) on m/n = 3/1 magnetohydrodynamic (MHD) instabilities have been investigated via hybrid simulations for reversed magnetic shear configurations. The simulation results shown that a global instability, energetic particle modes (EPMs), can be excited by the strong energetic particle drive with the frequency lying into the Alfven continuum. In the linear growth stage, EPs not only induce radial broadening of mode structures but also enhance toroidal mode coupling when EP beta is large enough. In the nonlinear saturation stage, the good flux surfaces are still kept over the entire plasma area in a long time evolution of EPMs. To clarify nonlinear saturation of the mode, effects of the energetic particle beta values, the width of the particle distribution, the beam energy and the initial pitch angle on the saturation amplitude of the EPMs are discussed in detail. Furthermore, MHD nonlinearity is found to reduce the saturation level of the EPM compared to linear MHD case, while the high-n harmonics have less impact.