Further observations of neutral beam deiven global alfven eigenmodes in wendelstein W7-AS

摘要

The low shear in W7-AS magnetic configurations leads to global Alfven eigenmodes (GAEs)for both helicities. Thus, low frequency GAEs with the same helicity as toroidal Alfven eigen- modes(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≤f_GAE≤500 kHz. These GAEs are excited via sideban resonances with NVI ions at low plasma densities and via fundamen- Tal resonances at highest plasma densities. A transition in the mode of exicitement occurs when the Increasing plasma density brings the central local Alfven velocity down below the maximum tangential Injection velocity of the NBI ions.