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.
Submitted to: Nuclear Fusion
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