  Gyrokinetic and kinetic-MHD simulations are performed for the fishbone
instability in the DIII-D discharge #178631, chosen for validation of
first-principles simulations to predict the energetic particle (EP) transport
in an ITER prefusion baseline scenario. Fishbone modes are found to generate
zonal flows, which dominate the fishbone saturation. The underlying mechanisms
of the two-way fishbone-zonal flows nonlinear interplay are discussed in
details. Numerical and analytical analyses identify the fishbone-induced EP
redistribution as the dominant generation mechanism for zonal flows. The zonal
flows modify the nonlinear dynamics of phase space zonal structures, which
reduces the amount of EPs able to resonate with the mode, leading to an early
fishbone saturation. Simulation results including zonal flows agree
quantitatively with DIII-D experimental measurements of the fishbone saturation
amplitude and EP transport, supporting this novel saturation mechanism by
self-generated zonal flows. Moreover, the wave-particle mode-locking mechanism
is shown to determine quantitatively the fishbone frequency down-chirping, as
evident in GTC simulation results in agreement with predictions from analytical
theory. Finally, the fishbone-induced zonal flows are possibly responsible for
the formation of an ion-ITB in the DIII-D discharge. Based on the low EP
transport and the large zonal flow shearing rates associated with the fishbone
instability in gyrokinetic simulations of the ITER scenario, it is conjectured
that high performance scenarios could be designed in ITER burning plasmas
through fishbone-induced ITBs.
