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

Characteristics of fast-ion losses induced by toroidal-AlfvLen 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 (ΔΤfast ion) is measured. The dependence ΔΤfast ion of on the TAE magnetic fluctuation amplitude (bθTAE) 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 bθTAE regime, ΔΤfast ion is proportional to bθTAE, whereas ΔΤfast ion increases with the square of bθTAE in the larger bθTAE 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 (ΔΤfast ion ∝ bθTAE) to a diffusive (ΔΤfast ion ∝ bθTAE²) character as bθTAE increases. [ABSTRACT FROM AUTHOR]