Numerical simulations of fishbones driven by fast ions in negative triangularity tokamak

It is shown that discharges with negative triangularity have lower turbulence induced transport and better energy confinement, so the tokamaks with negative triangularity are recognized as a better choice for future fusion devices. In order to explore the features of the energetic particle driven instabilities with negative triangularity, the kinetic- magnetohydrodynamic hybrid code M3D-K has been applied to investigate the linear instability and nonlinear evolution of the fishbone driven by energetic ions with different triangularity. Based on EAST like parameters, it is found that negative triangularity destabilizes the ideal internal kink mode, but stabilizes the fishbone instability. Nonlinear simulations show that the fishbone instability with negative triangularity is hard to saturate without fluid nonlinearity. The possible explanation is that the orbits of fast ions locate more centrally with negative triagularity, so the energy exchange between energetic ions and the fishbone is more efficient than that with positive triangularity. These simulation results demonstrate that considering the fishbone driven by energetic particles, the negative triangularity does not confer a obvious advantage over the positive triangularity.