Energetic Particle Mode Dynamics in Tokamaks

Abstract. Energetic Particle Modes [1] (EPM) are strongly driven oscillations excited via wave-particle resonant interactions at the characteristic frequencies of the energetic ions [1, 2], ωtE, ωBE and/or ωdE, i.e., respectively the transit frequency for circulating particles and the bounce and precessional drift frequencies for trapped ions. A sharp transition in the plasma stability at the critical EPM excitation threshold has been observed by nonperturba-tive gyrokinetic codes in terms of changes in normalized growth rate (γ/ωA, with ωA = vA/qR0), real frequency (ωr/ωA) and parallel wave vector (k‖qR0) both as α = −R0q2β ′ [3, 4] of the thermal plasma and that, αE [3, 5, 6], of fast ions are varied. The present work further explores theoretical aspects of EPM excitations by spatially local-ized particle sources, possibly associated with frequency chirping, which can radially trap the EPM in the region where the free energy source is strongest. Results of a nonperturbative 3D Hybrid MHD Gyrokinetic code [5] are also presented to emphasize that nonlinear behaviors of EPM’s are different from those of Toroidal Alfvén Eigenmodes (TAE) [7] and Kinetic TAE (KTAE) [8] and that particle losses and mode saturation are consistent with the mode-particle pumping model [9] (particle radial convection). Results of theoretical analyses of nonlin-ear EPM dynamics are also presented and the possible overlap with more general nonlinear dynamics problems is discussed. 1. EPM Excited by Ion Cyclotron Radio Frequency (ICRF) Heating