Speaker
Description
Energetic particles (EPs) are essential for sustaining burning plasmas through self-heating, but they can also drive global instabilities that degrade EP confinement, making quantitative prediction of EP transport crucial for achieving high fusion performance. Among these instabilities, fishbone modes are particularly important because their macroscopic mode structures can induce significant EP transport. Since the level of EP transport is largely determined by the nonlinear saturation amplitude of the fishbone mode, understanding its saturation mechanism is a key issue. The conventional saturation mechanism is associated with resonant wave-particle interaction, which flattens the EP distribution gradient in phase space. However, recent gyrokinetic simulations have suggested that fishbone-induced zonal flows may play a dominant role in the nonlinear saturation of fishbone modes.
In this work, nonlinear global gyrokinetic simulations are performed using the optimized GTC code to simulate fishbone dynamics, including a more self-consistent treatment of zonal fields. In particular, by improving the zonal-field equations to include the contribution from the zonal electron density perturbation, we find that the saturated zonal-flow amplitude is significantly reduced compared with simulations in which this contribution is neglected. As a result, the self-generated zonal flow does not appear to be the dominant saturation mechanism of the fishbone mode in our simulations. When the zonal fields, including zonal flow and zonal current components, are artificially suppressed, the fishbone mode can still reach nonlinear saturation with a saturation amplitude comparable to that obtained in the fully self-consistent simulation. These results indicate that the dominant fishbone saturation mechanism is more likely associated with nonlinear EP dynamics, such as resonant phase-space redistribution, while fishbone-induced zonal fields play a subdominant role under the present conditions.