Further observations of neutral beam driven global Alfvén eigenmodes in Wendelstein W7-AS

The low shear in W7-AS magnetic configurations leads to global Alfven eigenmodes (GAEs) of both helicities. Thus, low frequency GAEs with the same helicity as toroidal Alfven eigenmodes (TAEs) and elliptical Alfven eigenmodes (EAEs) in tokamaks exist in W7-AS; GAEs that are usually forbidden in tokamaks. Observations of diagnostic signals reveal a rich spectrum of neutral beam injection (NBI) driven GAEs in W7-AS in the frequency range 0 ≤ fGAE ≤ 500 kHz. These GAEs are excited via sideband resonances with NBI ions at low plasma densities and via fundamental resonances at higher plasma densities. A transition in the mode of excitement occurs when the increasing plasma density brings the central local Alfven velocity down below the maximum tangential injection velocity of the NBI ions.