Advances in the physics of plasma confinement in stellerator magnetic configurations.

Stellarators use three-dimensional magnetic field shaping to provide stable plasma confinement without the need for driven plasma currents or stabilizing feedback systems. The maximum plasma β (plasma pressure normalized to magnetic pressure) that can be confined in the W7-AS stellarator was explored by varying the shape of the magnetic field, the heating power, and the magnetic field strength. The maximum volume-averaged β achieved in quasi-stationary plasmas was 3.4%. In general, β-limiting instabilities were not observed. Low mode-number magnetic instabilities were sometimes observed, but they saturated at low levels that did not strongly degrade confinement or prevent access to higher β-values. The achieved β was nearly constant for 0.8 < B < 1.2 T, for fixed plasma shape, indicating a soft-saturation of plasma confinement. Comparisons with theoretical predictions will be presented. Advances in numerical modeling and optimization have enabled combining these favorable stellarator characteristics with the low-aspect ratio and good confinement of advanced tokamaks. A compact quasi-axisymmetric design has been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4.4. It is calculated to be passively stable to the ballooning, kink, vertical, Mercier, and neoclassical tearing modes for β > 4% without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β = 4% (the rest is from the coils). The quasi-axisymmetric magnetic field produces low damping in the toroidal direction, adequate fast-ion confinement, and thermal neoclassical transport similar to equivalent tokamaks. The coils have been designed to provide good flux surfaces and flexibly investigate the predicted MHD stability and transport properties.