Kinetic effects of energetic particles on resistive MHD stability

We show that the kinetic effects of energetic particles can play a crucial role in the stability of the $m/n=2/1$ tearing mode in tokamaks (e.g., JET, JT-60U, and DIII-D), where the fraction of energetic particle ${\ensuremath{\beta}}_{\mathrm{frac}}$ is high. Using model equilibria based on DIII-D experimental reconstructions, the nonideal MHD linear stability of cases unstable to the $2/1$ mode is investigated including a $\ensuremath{\delta}f$ particle-in-cell model for the energetic particles coupled to the nonlinear 3D resistive MHD code NIMROD [C. C. Kim et al., Phys. Plasmas 15, 072507 (2008)]. It is observed that energetic particles have significant damping and stabilizing effects at experimentally relevant $\ensuremath{\beta}$, ${\ensuremath{\beta}}_{\mathrm{frac}}$, and $S$, and excite a real frequency of the $2/1$ mode. Extrapolation of the results is discussed for implications to JET and ITER, where the effects are projected to be significant.