Session RP1 - Poster Session VIII.
POSTER session, Thursday afternoon, October 30
Fran Hill Southeast Exhibit Hall, ACC
Stellarator steady states and stability obtained from two-fluid nonlinear relaxation are very different from MHD predictions and may allow higher beta. Numerical simulations of the NCSX high beta stellarator with the M3D two-fluid code show that a strong equilibrium pressure gradient near the plasma edge (``H-mode'') creates a region where diamagnetic drifts and related effects drive a steady state poloidal flow on fast MHD-like time scales, well before the neoclassical parallel viscous stress acts to reduce v_i\theta. The flow in the strong \nabla p region is similar over a large range of p_e/p_i, in the ømega_*i-direction with magnitude significantly smaller than an average diamagnetic velocity based on (p_e+p_i)/2. The flow robustly stabilizes high mode number resistive ballooning/interchange modes that limit MHD stability in simulation, but generally not in experiment. In addition, electron two-fluid effects can increase magnetic island growth rates on interior rational surfaces. These effects increase with \beta and p_e/p_i, suggesting that the best stellarator confinement may exist in hot-ion operation. The beta limit may be a gradual one due to the overlap of large, although contained, magnetic islands that destroy the effective plasma confinement, rather than catastrophic instability.