A study on the TAE-induced fast-ion loss process in LHD

Characteristics of fast-ion losses induced by toroidal-Alfvén eigenmodes (TAEs) are investigated over wide parameter ranges of Large Helical Device (LHD) plasmas to reveal the fast-ion loss process. To study fast-ion losses, a scintillator-based lost-fast ion probe is used, and an increment of fast-ion loss flux due to TAEs from the neoclassical orbit loss level (ΔΓfast ion) is measured. The dependence of ΔΓfast ion on the TAE magnetic fluctuation amplitude (b_θTAE) changes from a linear to a quadratic and finally a third power with an increase in the magnetic axis shift. It is found that the dependence of fast-ion loss flux on TAE magnetic fluctuation amplitudes changes at a certain fluctuation level in a fixed configuration. Experimental results show that in the small b _θTAE regime, ΔΓfast ion is proportional to b _θTAE, whereas ΔΓfast ion increases with the square of b_θTAE in the larger b_θTAE regime. A simulation by orbit-following codes that incorporate magnetic fluctuations with frequency chirping-down due to TAEs suggests the change in the fast-ion loss process from a convective (ΔΓ fast ion ∝ b _θTAE) to a diffusive character as b_θTAE increases.