Energetic-Ion-Driven Global Instabilities Observed in the Large Helical Device and Their Effects on Energetic Ion Confinement

摘要

This paper reviews various global instabilities destabilized by tangential neutral beam injection (NBI) in the Large Helical Device (LHD) plasmas. These global modes are toroidal Alfv?n eigenmodes (TAEs), which are also observed in tokamak plasmas, and helicity-induced Alfv?n eigenmodes (HAEs) which are observed only in three-dimensional plasmas such as LHD plasmas. Moreover, reversed magnetic shear Alfv?n eigenmodes (RSAEs) are observed in a reversed magnetic shear (RS) plasma in the LHD, where the sign of the magnetic shear changes from positive in the plasma central region to negative in the plasma peripheral region. The RSAEs exhibit a characteristic frequency sweeping due to temporal evolution of the rotational transform profile. In the RS plasma, the energetic-ion-driven geodesic acoustic mode (GAM) is also excited. The GAM interacts nonlinearly with the RSAEs and generates a multitude of frequency sweeping modes through a three-wave-coupling process. The TAEs and GAM exhibit various types of nonlinear evolution, that is, pitchfork splitting and rapid frequency chirp-up and/or chirp-down. The linear and nonlinear characteristics of these energetic-ion-driven global instabilities in the LHD are compared with those observed in tokamak plasmas. TAE bursts having rapid frequency chirp-down induce redistribution and/or loss of energetic ions. Future important issues are briefly described.