Abstract: Fast-ion transport by Alfvén eigenmodes (AEs) is a key issue for fusion reactor development. In DIII-D tokamak experiments, stiff transport of fast-ions increased with neutral beam (NB) injection power when AE amplitudes exceeded a threshold [1]. To investigate the universality of this phenomenon, we performed NB power scan experiments in the Large Helical Device (LHD), where the stellarator configuration enables better isolation of fast-ion physics from equilibrium changes.
Our previous study [2] demonstrated fast-ion profile stiffness correlated with AE amplitude in LHD. Figure 1 summarizes key observations: (a) AE amplitude increases linearly with NB power beyond Pt-NB ≈ 1.6 MW, consistent with DIII-D trends [3]; (b) neutron emission rate Sn shows reduced growth compared to Fokker-Planck simulations, indicating confinement degradation; (c) FIDA measurements (663-665 nm, 98-166 keV) directly reveal stiffening of the fast-ion density profile.
In this presentation, we provide an overview of LHD fast-ion research capabilities (NBIs, FIDA, FIDASIM, NPA diagnostics), followed by detailed discussion of the profile stiffness results. Importantly, we present new analyses beyond our published work, including updated interpretations of Fig. 1 data and exploration of alternative degradation mechanisms that may contribute alongside, or instead of, AE-driven transport at high NB power. Future research directions will also be discussed.
[1] W. W. Heidbrink et al., PRL 99, 245002 (2007).
[2] S. Kamio et al., J Fusion Energ. 44 50 (2025).
[3] C. S. Collins et al., PRL 116, 095001 (2016).
Figure 1: Injection NB power dependencies of (a) AE amplitude, (b) neutron emission rate Sn, and (c) fast-ion profiles.