Elongation effect on beta-induced Alfvén eigenmode

Beta induced Alfvén eigenmode (BAE) can be an important candidate for ion loss in burning plasmas. Elongation effect on BAE has been investigated by the gyrokinetic eigenvalue code DAEPS in this work. We construct a shaped equilibrium model by modifying local s − α model with which the capability of the DAEPS code has been extended to study the elongation effect. It is discovered that the BAE growth rate first increases with elongation factor κ, reaches a maximum and then decreases. This trend occurs for many different values of η i. We find that, in the weak or moderate elongation region, the BAE instability is reactive type and mainly determined by the fluid/MHD effects, namely the combination of stabilizing field line bending term and destabilizing interchange drive in the vorticity equation. However, in the strong elongation region, the BAE instability becomes dissipative and is mainly driven by the wave–particle resonance effect embedded in δW k since the fluid driving damps away. It is also discovered that the wave–particle resonance decreases with elongation in this region, which is due to the decrease of the geodesic curvature with elongation and leads to the decrease in the growth rate of BAE.