Collisionless damping of the geodesic acoustic mode (GAM) in helical magnetic configuration is investigated by a drift-kinetic simulation. We utilize the neoclassical transport code 'FORTEC-3D', which solves the drift-kinetic equation based on the δf method, to study how the magnetic configuration affects the collisionless damping of GAMs. In a recent analytical study, in which a simplified expression for helical magnetic field was used, it was found that the helical ripples strengthen the damping rate of the GAM oscillation. In this paper, the change in the damping rate is investigated in detail for realistic magnetic field configurations of the Large Helical Device. It is found that the GAM damping is faster and the frequency is lower as the magnetic axis is shifted inwards. This result suggests the possibility of controlling both the neoclassical transport level and the GAM oscillation, or the zonal flow, in helical plasmas. The collisional effect on GAM damping is also investigated, however, it is found that the GAM oscillation in helical plasmas is insensitive to the collisional effect because the enhancement of collisionless damping by the magnetic field ripples is very strong.
Potential fluctuation associated with the energetic-particle-induced geodesic acoustic mode in the Large Helical Device