Nonlinear Fishbone Dynamics in Spherical Tokamaks

Authors: F. Wang, G.Y. Fu

Abstract: Linear and nonlinear kinetic-MHD hybrid simulations have been carried out to investigate linear stability and nonlinear dynamics of beam-driven fishbone instability in spherical tokamak plasmas. Realistic NSTX parameters with finite toroidal rotation are used. The results show that the fishbone is driven by both trapped and passing particles. The instability drive of passing particles is comparable to the trapped particles in linear regime. The effects of rotation are destabilizing and a new instability region appears at higher qmin (> 1:5) values with qmin being the minimum of safety factor profile. In nonlinear regime, the mode saturates due to flattening of beam ion distribution, and it persists after initial saturation while mode frequency chirps down in such a way that the resonant trapped particles move out radially and keep in resonance with the mode. Correspondingly, the flattening region of beam ion distribution expands radially outward. A substantial fraction of initially non-resonant trapped particles become resonant around the time of mode saturation and keep in resonance with the mode as frequency chirps down. On the other hand, the fraction of resonant passing particles is significantly smaller than that of trapped particles. Our analysis shows that trapped particles provide the main drive to the mode in the nonlinear regime.

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