Contribution of the Large Helical Device Plasmas to Alfvén Eigenmode Physics in Toroidal Plasmas

In the large helical device (LHD) having three dimensional configuration, Alfven eigenmodes (AEs) destablized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-monotonic rotational transform (l/2 pi) profiles. In a plasma with monotonic l/2 pi-profile, core-localized toridicity-induced Alfven eigenmode (TAE) as well as global one are often observed. With the increase in the averaged toroidal beta value, defined as the ratio of total plasma pressure to toroidal magnetic pressure, core-localized TAE with low toroidal mode number becomes global. In a relatively high beta plasma with monotonic l/2 pi-profile, two TAEs with different toroidal mode number often interact nonlinearly and generate another modes through three wave coupling. In a plasma with non-monotonic l/2 pi-profile generated by intense counter neutral beam current drive, reversed shear Alfven eigenmode (RSAE) and geodesic acoustic mode (GAM) excited by energetic ions were observed for the first time in a helical plasma. Nonlinear coupling was also observed between RSAE and GAM.