Quasilinear RBQ Simulation of Intermittent Behavior of a Single TAE

The oscillatory, intermittent behavior of a single unstable toroidicity-induced Alfvén eigenmode (TAE), observed in DIII-D experiments [Van Zeeland et al., Nucl. Fusion, 2021] and driven by suprathermal beam ions, is investigated using simulations. A key objective of this study is to validate the quasilinear Resonance Broadened Quasilinear (RBQ) code [Gorelenkov, Duarte, Phys. Plasmas, 2019]. We demonstrate that a realistic, time-dependent beam ion distribution function used in RBQ simulations can reproduce the observed intermittency of TAEs in the nonlinear regime. The numerically computed TAE growth rate is found to be critical for capturing the first burst of oscillation within a few milliseconds following the onset of the neutral beam injection blip. This initial oscillation exhibits a characteristic quiet period lasting few milliseconds, consistent with experimental observations and reproduced in the simulation. The overall time evolution and saturation level of TAE amplitude are shown to be sensitive to additional pitch-angle scattering, which may be associated with convective flows due to AE or microturbulence.