Speaker
Description
In magnetic confinement fusion devices, there are multiple fast-ion species such as fusion product (i.e., α particles) and fast deuterons produced by neutral beam injection. These fast ions may induce MHD instabilities such as Alfvén Eigenmodes (AEs) which can enhance fast-ion transport and losses. The study of fast-ion-driven MHD instabilities is one of the important topics for fusion reactors.
In order to clarify the relationship between instabilities and fast ion transport in the plasma with multiple fast ion species, experiments with the combined injection of hydrogen and deuterium beams were conducted in the Large Helical Device (LHD). In these experiments, recurrent AE bursts caused by the fast protons and fast deuterons were observed [1].
Computer simulation is an effective approach for investigating instabilities driven by multiple fast-ion species. A hybrid simulation code for nonlinear MHD and energetic-particle dynamics, MEGA, has been developed to simulate recurrent bursts of fast-ion-driven MHD instabilities, including sources, collisions, and losses [2]. In MEGA simulations for LHD experiments with fast protons and fast deuterons, AEs with frequencies consistent with experimental observations were successfully reproduced. In addition, it was shown that the synergistic effects of multiple fast-ion species enhance fast-ion transport and losses in LHD [3].
On the other hand, in the previous simulation conditions, AEs could not be destabilized only by fast deuterons. The synergistic effects of multiple fast-ion species on AEs and fast-ion transport remain unclear when the fast-ion pressure gradients vary, particularly in cases where the fast-deuteron pressure gradient becomes sufficiently large to excite AEs.
In this study, MEGA simulations of AEs driven by multiple fast-ion species are performed in the LHD magnetic field configuration when the pressure gradients of fast deuterons and fast protons are varied. In the LHD, as the fast deuteron pressure gradient increased, the amplitude of the initial AEs became larger. In addition, the amplitudes of the additional AEs associated with the redistribution of fast protons and fast deuterons also increased, accompanied by enhanced fast-ion transport. The energy transfer between the AEs and fast ions will be evaluated and the dependence of AEs driven by multiple fast-ion and its transport on the fast-ion pressure gradients will be shown.
[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., Proceedings of 30th IAEA Fusion Energy Conference, Chengdu (2025).