Neutral beam driven global Alfvén eigenmodes in the Wendelstein W7-AS stellarator

Abstract

During neutral beam injection coherent magnetohydrodynamic activity occurs at low and medium \ensuremath{\beta}, which is driven by the energetic beam particles. As interpretation is given in terms of marginally stable global Alfv\'en eigenmodes (GAE), which are destabilized by Landau resonances of the far circulating particles. Toroidicity induced gaps for toroidal Alfv\'en eigenmodes do not occur in W7-AS but GAE modes in gaps below the shear Alfv\'en continua can be excited. This is investigated within a gyrofluid model, which has been used sucessfully for tokamak cases previously.