Description
Nonlinear behavior of shear Alfvén waves (SAWs), particularly bursting events and their self-consistent interactions with energetic particles, is a key issue in magnetic confinement fusion research. Recurrent bursting SAWs can drastically degrade plasma confinement, which are often observed in present-day tokamaks and stellarators supplied by neutral beam injection heating [1,2]. On the other hand, SAWs during ion cyclotron resonance frequency heating (ICRH) typically maintain steady amplitudes [3,4], despite theoretical predictions suggesting the possibility of a bursting state [5]. This implies that the strong velocity space diffusion caused by the RF wave electric field plays a significant role in determining the wave nonlinear states.
This work reports the first comprehensive simulations of ICRH-driven tokamak plasmas on the slowing-down timescale using the kinetic-MHD hybrid code MEGA, where SAW-induced fast-ion transport is self-consistently included during the high-energy tail formation. Both ICRF-induced bursting and non-bursting toroidal Alfvén eigenmodes (TAEs) are observed. We will first present conditions that determine whether TAEs exhibit bursting behavior or maintain steady amplitudes. Then, the mechanism that triggering the burst will be illuminated. The comprehensive simulations show that the resonance overlap in phase space plays a pivotal role in triggering such bursting events. Finally, a strategy for mitigation/suppression of TAE bursts utilizing an additional ICRF antenna will be proposed, which is also known as the phase-space engineering of energetic ions in MCF.
References:
[1] Wong, K. L., et al. Phys. Rev. Lett., 66, 1874 (1991).
[2] Osakabe, M., et al. Nucl. Fusion, 46, S911 (2006).
[3] García-Muñoz, M., et al. Phys. Rev. Lett., 104, 185002 (2010).
[4] Kazakov, Ye O., et al. Nat. Phys, 13, 973 (2017).
[5] Breizman B.N., Berk H.L. et al., Phys. Plasmas 4, 1559 (1997).