arXiv subjects

Yuehao Ma

Publications and source records attributed to Yuehao Ma.

4 recordsLinked to original sources

arXiv2026

First Observation of Fishbone-Driven Zonal Flows with Fine Reversed Structure in Tokamak Plasmas

We present the first direct experimental observation of fishbone-driven zonal flows in the core of the EAST tokamak. In contrast to the global pattern predicted by previous models and simulations based on the energetic-particle-expulsion mechanism, the observed flows exhibit a fine-scale, radially reversed structure inside the q = 1 rational surface. The flow rises faster and saturates earlier than the fishbone within a single burst, indicating that a beat-driven nonlinear process dominates the early stage rather than the energetic-particle-expulsion mechanism. Global nonlinear gyrokinetic simulations quantitatively reproduce the observed radial profile and reveal that this structure arises from the cancellation of comparable but opposite contributions from thermal ions and electrons. This cancellation mechanism is not captured in previous theoretical frameworks. These findings establish that the fishbone can generate sheared flows with a distinct radial topology, offering a promising pathway for regulating turbulence and improving core confinement.

physics.plasm-ph↗

arXiv2025

Electromagnetic turbulence in EAST plasmas with internal transport barrier

In this study, global nonlinear electromagnetic gyrokinetic simulations are conducted to investigate turbulence in the Internal transport barrier (ITB) region of the EAST tokamak discharge with weakly reversed magnetic shear. Linear simulations reveal two dominant ion temperature gradient (ITG) modes: a higher frequency mode at the $q=1$ surface, which dominates in the electrostatic limit, and a lower frequency mode near the $q_{\min}$ surface, which prevails under the experimental $\beta$ (the ratio of plasma pressure to magnetic pressure). Finite $\beta$ effects effectively suppress higher frequency ITG modes, and once $\beta_i$ on axis exceeds 0.5\%, this ITG mode is no longer dominant, and the ITG mode near $q_{\min}$ surface becomes the primary instability. Therefore, electromagnetic effects play a crucial role in stabilizing ITG modes, and in causing the transition between the most unstable mode at different radial positions. The linear growth rate of the unstable mode in the electrostatic limit is approximately 1.25 times higher than that of the dominant mode in the electromagnetic case. However, in the electromagnetic nonlinear regime, the thermal ion heat conductivity is reduced by at least a factor of 4. This reduction primarily results from nonlinear electromagnetic effects enhancing the shearing effect of zonal flows, thereby further suppressing microturbulence. Finally, energetic particles exert a slight stabilizing effect on ITG turbulence due to dilution and finite $\beta$ contributions. It is emphasized that the electromagnetic effect on ITG with weak magnetic shear should be included to accurately calculate the transport coefficients.

physics.plasm-ph↗

arXiv2025

Cross-scale Interaction between Microturbulence and Fishbone in Fusion Plasmas

Global gyrokinetic simulations are performed for the first time to investigate cross-scale interactions between electromagnetic ion temperature gradient (ITG) turbulence and fishbone instability in tokamak plasmas. The investigation of fluctuation response in the multiscale simulation including both instabilities indicates a strong impact of fishbone on ITG turbulence. Detailed analysis reveals that fishbone-driven zonal radial electric fields at nonlinear saturation significantly suppress electromagnetic ITG turbulence, reducing ion thermal transport close to the neoclassical level. The simulation results agree well with experimental observations that turbulence suppression during fishbone bursts. These findings advance understanding of multiscale interactions that enhance thermal confinement in fusion plasmas.

physics.plasm-ph↗

arXiv2021

Formulation and verification of multiscale gyrokinetic simulation of kinetic-MHD processes in toroidal plasmas

A comprehensive gyrokinetic simulation model has been implemented in the global toroidal gyrokinetic code (GTC) and verified for studying low-frequency waves and turbulence in magnetic fusion plasmas by treating all kinetic-MHD processes on an equal footing. A theoretical framework has been formulated to unify various methods for efficiently solving the electron drift kinetic equation in multiscale simulations by separating electron responses into analytic and non-analytic parts based on the smallness parameter of electron-to-ion mass ratio. The model can be reduced to the ideal MHD model with both the linear dispersion relation and the nonlinear ponderomotive force in theory and simulation. The model is used for the verification and validation of simulating internal kink modes in the DIII-D tokamak with accurate calculations of equilibrium parallel current and compressible magnetic perturbation. A large simulation database has been generated to train a surrogate model to predict the kink instability. Statistical analysis shows that the radial location of safety factor q=1 flux-surface and the plasma beta inside the q=1 surface are the most important parameters for predicting the kink instability.

physics.plasm-ph↗