Observation of a Fast-Ion Profile Stiffness Due to the Alfvén Eigenmode

Fast-ion transport driven by Alfvén eigenmodes (AEs) is one critical issue facing fast-ion confinement in magnetic fusion device. In the DIII-D tokamak experiment, stiff transport of fast-ions increased with increasing neutral beam (NB) injection power when the amplitudes of multiple interacting AEs exceeded a certain threshold. These experiment results are supported by simulation studies that predict monotonically degrading fast-ion confinement and profile stiffness with increasing beam power. To investigate the universality of the fast-ion profile stiffness dependence on AE amplitude, an experiment was performed at the Large Helical Device (LHD) to scan the injection current of the NB and vary the AE amplitude. Under the experimental conditions, the AE amplitude increased linearly with NB injection power. The blue shifted FIDA intensity between 663-665 nm, corresponding to the energy range of 98-166 keV in the ctr-direction, was used for estimating the radial profile of the fast-ion density. There is direct evidence of stiffening of the fast-ion profile and degradation of the fast-ion confinement. This is consistent with the experimentally observed reduction in the expected neutron emission rate. We have demonstrated that under AE-prone confinement conditions, even if the fast-ion source increases due to NB injection, they experience enhanced transport by AEs and do not increase in density.