Simulation of two fluid and energetic particle effects in stellarators

MHD and resistive MHD are inadequate to understand the stability of stellarators properly. Ideal MHD ballooning mode theory predicts β limits substantially below the values that can be expected in experiments. Resistive MHD is even more pessimistic, predicting that many stellarators are completely unstable. Including two fluid effects, ideally and resistively stable stellarator equilibria can be obtained. It may be possible to completely stabilize ballooning modes. The two fluid computations are done with a realistic value of the Hall parameter, the ratio of the ion skin depth to the major radius. Hybrid gyrokinetic simulations with energetic particles indicate that global shear Alfvén TAE modes can be more stable in stellarators than in tokamaks. Computations in a two-period compact stellarator obtained a predominantly n = 1 toroidal mode with the expected TAE frequency. The TAE modes are more stable in the two-period compact stellarator than in a tokamak with the same q and pressure profiles. The cause for the stabilization is believed to be the increased damping rate due to 3D geometry. Simulations were performed with the M3D extended MHD code.