Nonlinear simulations of energetic particle-driven instabilities interacting with Alfven continuum during frequency chirping

Toroidal Alfven eigenmodes excited by energetic particles (EPs) and their transition to energetic particle modes are investigated using a three-dimensional nonlinear kinetic-magnetohydrodynamic hybrid simulation code. Both the symmetric chirping inside the Alfven continuum gap for upward and downward directions and the asymmetric chirping affected by the Alfven continuum are found in the simulations depending on the EP drive and the mode damping. For weak drive and damping, symmetric frequency chirping is observed with a chirping rate consistent with the Berk-Breizman theory (Berk et al 1997 Phys. Lett. A 234 213). For moderate drive and damping, upward chirping is dominant because downward chirping is interrupted by interaction with the Alfven continuum. For strong drive and damping, two branches of frequency chirping are found for different poloidal harmonics. The dominant chirping is downward into the Alfven continuum and the other is an upward chirping inside the Alfven continuum gap. The creation and the propagation of holes and clumps are found in EP phase space, which is qualitatively consistent with the Berk-Breizman theory.

Automated summary

Toroidal Alfven eigenmodes excited by energetic particles and their transition to energetic particle modes were studied using a three-dimensional nonlinear kinetic-magnetohydrodynamic hybrid simulation code. The simulations revealed symmetric and asymmetric chirping patterns as well as the creation and propagation of holes and clumps in EP phase space under different drive and damping conditions.