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 ${\beta }_{\mathrm{frac}}$ 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 $\delta f$ 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 $\beta$, ${\beta }_{\mathrm{frac}}$, 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.

• Received 16 September 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.135001