  A novel 2D nonlinear dynamical paradigm is constructed to interpret the fast
and repetitive frequency chirping and amplitude oscillation of Alfv\'en
eigenmodes excited by energetic particles in fusion plasmas as observed in
global gyrokinetic simulations. In this non-perturbative paradigm of the
collisionless phase-space dynamics, wave-particle resonant interactions cause
the phase-space structure to continuously twist and fold, leading to the
repetitive excitation and decay of the Alfv\'en eigenmode. The radial
(perpendicular to the dominant wave-particle interaction) dependence of the
mode amplitude and toroidal precessional drifts of the energetic particles
leads to the 2D dynamics of wave-particle interactions, which is found to be
responsible for the repetitive process of formation and destruction of the mode
structure.
