Global Simulation of the GAM Oscillation and Damping in a Drift Kinetic Model

Collisionless damping of the geodesic acoustic mode (GAM) is investigated by a drift kinetic simulation. The main subject of the study is to analyze how the magnetic configuration and the finite-orbit-width(FOW) effect of the ion drift motion affect the collisionless damping of GAM. We utilize the neoclassical transport code ”FORTEC-3D”, which solves the drift kinetic equation based on the delta-f method, to study these issues. In recent analytical study on GAM and zonal flow it is found that the FOW effect and the helical components of magnetic field spectrum change the damping rate of the GAM oscillation. We inspect the change of the damping rate in our simulation. First, the dependence of the damping rate on the FOW effect is investigated. We find that the collisionless damping becomes faster as typical banana width becomes wider. On the other hand, the damping rate in helical magnetic configuration is mainly determined by the effect of helical ripples. It is found that the sideband components, which appear as the axis moves inward, make the GAM damping faster. This result suggests the possibility of controlling both the neoclassical transport level and the GAM oscillation, or zonal flow, in helical plasma. The collisional effect on the GAM damping is also investigated in banana and plateau regimes