Description
In the magnetic confinement fusion devices, there are multiple fast ion species such as fusion product α-particles and fast deuterons produced by neutral beam injection (NBI). These fast ions may induce instabilities such as Alfvén Eigenmodes (AEs) which enhance fast-ion transport and losses. The study of fast ion driven instabilities is one of the important topics for fusion reactor.
To clarify the relationship between instabilities and fast ion transport in the multiple fast ion species plasma, experiments with the combined injection of hydrogen and deuterium beams were conducted in the Large Helical Device (LHD). The AE bursts caused by the fast protons and fast deuterons were observed in the experiments [1].
In the plasma with multiple fast ion species, a complex synergetic effect of multiple fast ion species may arise on the instabilities and fast ion transport, because the interaction with the instabilities may differ for different species. For example, one fast ion species may drive an instability while another species may stabilize it. The stabilizing fast ion species may be transported by the instability driven by another fast ion species. Computer simulation is a useful tool for investigating the synergetic effect. A hybrid simulation code for nonlinear MHD and energetic particle dynamics, MEGA, can investigate AE instabilities including the fast ion source, collisions, and losses[2,3].
In this study, simulations of the LHD experiments with the fast protons and fast deuterons are performed using MEGA code where NBI, collisions, and fast ion losses are taken into account. In the MEGA simulations, AE modes with frequencies that are consistent with those observed in the LHD experiment[1] could be reproduced. The additional AEs were destabilized by multiple fast ion redistributions, and then, the loss rates of both fast proton and fast deuteron significantly increased.
[1] S. Kamio, et al, Plasma and Fusion Research, 16, 2402044 (2021)
[2] Y. Todo, et al., Nucl. Fusion 55, 073020 (2015).
[3] R. Seki, et al., Nucl. Fusion, 59, 096018 (2018).
