Validation and verification of quasi-linear modeling using a single unstable Alfvén mode in DIII-D

The oscillatory behavior of a single unstable Alfvénic mode observed in DIII-D experiments [Van Zeeland et al, NF'21] excited by suprathermal beam ions is investigated numerically. The goal of such study is to validate the quasilinear methodology used in recently developed Resonance Broadened Quasilinear code RBQ [Gorelenkov et al., PLA'21]. Another goal is to verify RBQ against the fully nonlinear Vlasov code BOT. The excited mode is in the TAE gap, identified using the ideal MHD code NOVA. The comparison helps to understand multiscale intermittencies and oscillations observed in experiments and predicted in simulations [Berk, PRL'92, Gorelenkov et al., PLA'21]. An interplay between the growth, damping rates and the effective scattering frequency in RBQ simulations exhibits a predator-prey behavior of the mode amplitude evolution. Since experiments suggest that the mode remains near marginal stability throughout its evolution, a self-consistent quasilinear framework is employed that embodies collisions and that replicates the same saturation levels predicted by nonlinear theory [Duarte et al, PoP’ 19]. Additionally, beam blip experiments like these exhibit a rapidly changing drive that needs to be factored into simulations.