Saturation of fishbone instability by self-generated zonal flows in tokamak plasmas

Global gyrokinetic simulations of the macroscopic fishbone instability in a DIII-D tokamak plasma find that the fishbone can generate zonal flows that dominate its nonlinear saturation, in contrast to the conventional picture of fishbone saturation via wave-particle interaction. The saturation mechanism is identified in energetic particle phase space where zonal flows prevent, through Doppler-shift, coherent structures from persisting or drifting when the mode frequency down-chirps. A gyrokinetic treatment is then found essential for realistic simulation of fishbone modes. This novel saturation mechanism is supported by quantitative agreement between simulation results and experimental measurements of the fishbone saturation amplitude and neutron emissivity drop.

Moreover, the zonal flows’ shearing rate exceeds the linear growth rates of unstable drift-waves which may suppress thermal plasma turbulent transport. The fishbone-induced zonal flows are therefore likely responsible for the formation of the ion transport barrier observed experimentally in this DIII-D discharge during fishbone bursts. Finally, gyrokinetic simulations of an ITER baseline scenario similar to this DIII-D discharge show that the fishbone induces insignificant energetic particle redistribution, similar to past studies, but produces zonal flows that may well suppress turbulent transport. Destabilizing benign fishbones in ITER burning plasmas is therefore possibly a way to enhance fusion performance.