Physics design of a high-beta quasi-axisymmetric stellarator

Key physics issues in the design of a high-beta quasi-axisymmetric stellara tor configuration are discussed. The goal of the design study is a compact stellarator configuration with aspect ratio comparable to that of tokamaks and good transport and stability properties. Quasi-axisymmetry has been use d to provide good drift trajectories. Ballooning stabilization has been acc omplished by strong axisymmetric shaping, yielding a stellarator configurat ion whose core is in the second stability regime for ballooning modes. A co mbination of externally generated shear and non-axisymmetric corrugation of the plasma boundary provides stability to external kink modes even in the absence of a conducting wall. The resulting configuration is also found to be robustly stable to vertical modes, increasing the freedom to perform axi symmetric shaping. Stability to neoclassical tearing modes is conferred by a monotonically increasing i profile. A gyrokinetic delta f code has been u sed to confirm the adequacy of the neoclassical confinement. Neutral beam l osses have been evaluated with Monte Carlo codes.