Fast-ion transport during repetitive burst phenomena of Toroidal Alfvén Eigenmodes in the Large Helical Device

Alfvén instabilities induced fast-ion losses have been directly observed for the first time by a newly developed scintillator lost ion probe (SLIP) in the Large Helical Device (LHD). The SLIP can measure the pitch angle and gyro radius of escaped fast ions toward loss region. Neutral beam driven Alfvén Eigenmodes (AEs) are excited under the reactor relevant conditions: the ratio of fast ion (beam) speed vb and Alfvén speed vA is more than 0.3 ~ 4.0. The beta value for fast ions is considered roughly to be ~ 10%. Non-linear phenomena related to Alfvén instabilities are observed under such conditions. During repetitive Toroidal Alfvén Eigenmode (TAE) bursts, synchronized fast ion losses are observed by SLIP. From the orbit calculation the measured fast ion with pitch angle of 130 degrees and beam energy of 150 keV surely pass through the locations of TAE gaps. The orbit analysis found that the observed fast ions interact strongly with the excited TAEs. This result becomes the first experimental evidence of radial transport of fast ions predicted theoretically during TAE activities. In addition, from the correlation between stored energy degradation and fast-ion loss rate, it is found that fast-ion losses induced by TAE activities with low toroidal mode numbers categorize two phenomena without and with fastion loss enhancements, which indicate the fast-ion redistribution and loss.