18th Technical Meeting on Energetic Particles in Magnetic Confinement Systems

Simulation of excitation of zonal structure by reversed shear Alfvén eigenmode with MEGA code

Not scheduled

20m

Poster Effects of Energetic Particles in Magnetic Confinement Fusion Devices

Speaker

Shizhao Wei (National Institute for Fusion Science)

Description

Large-scale fusion devices such as ITER, as well as possible fusion reactors in the future, are expected to operate at steady state with a substantial non-inductive current fraction, which generally renders a reversed shear scenario. At this time, energetic particles (EPs), especially fusion alpha particles in the core region, will easily excite reversed shear Alfvén eigenmodes (RSAE). RSAE can affect the transport of EPs in the system in turn, then affect the heating and confinement of the system plasma. Therefore, the study of nonlinear excitation, evolution and saturation of RSAE may be of great significance for future fusion research.
There have been theoretical studies on the modulation instability of toroidal Alfvén eigenmode (TAE) [1] and RSAE [2] using gyrokinetic theory; and simulation studies on the nonlinear modulation of the TAE by zonal structure (ZS) [3]. There are still unexplained discrepancies between theoretical and simulation studies. Some recent work about the excitation of ZS through beat driven mechanism [4] may explain some of the differences.
This work uses the MEGA code to simulate the nonlinear excitation of ZS through beat driven mechanism and RSAE being modulated by ZS. Based on the physical characteristics of RSAE itself, the results obtained are partially different from those of the TAE simulation. The results also show the growth rate and amplitude are the same as the theory described. But there are still differences between theory and simulation, like the magnitude of magnetic field disturbance. We hope that subsequent work can make the two get more unified results.

[1] L. Chen, F. Zonca. Nonlinear excitations of zonal structures by toroidal Alfvén eigenmodes. Physical Review Letters, 2012,109:145002.
[2] S. Wei, T. Wang, N. Chen, Z. Qiu. Nonlinear reversed shear Alfvén eigenmode saturation due to spontaneous zonal current generation. Journal of Plasma Physics, 2021, 87: 905870505.
[3] Y. Todo, H. Berk, B. Breizman. Nonlinear magnetohydrodynamic effects on Alfvén eigenmode evolution and zonal flow generation. Nuclear Fusion, 2010, 50: 084016.
[4] L. Chen, Z. Qiu, F. Zonca. On beat-driven and spontaneous excitations of zonal flows by drift waves. Physics of Plasmas, 2024, 31: 040701.