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The confinement characteristics of some related quasi-axisymmetric stellarator (QA) designs are compared to gain more physical understanding of what causes the differences in their transport. From this, some general rules are found for features of QA magnetic geometries that are deleterious or helpful for confinement. [ABSTRACT FROM AUTHOR]

The microinstability properties of nine distinct magnetic geometries corresponding to different operating and planned stellarators with differing symmetry properties are compared. Specifically, the kinetic stability properties (linear growth rates and real frequencies) of toroidal microinstabilities (driven by ion temperature gradients and trapped-electron dynamics) are compared, as parameters are varied. The familiar ballooning representation is used to enable efficient treatment of the spatial variations along the equilibrium magnetic field lines. These studies provide useful insights for understanding the differences in the relative strengths of the instabilities caused by the differing localizations of good and bad magnetic curvature and of the presence of trapped particles. The associated differences in growth rates due to magnetic geometry are large for small values of the temperature gradient parameter η≡d ln T/d ln n, whereas for large values of η, the mode is strongly unstable for all of the different magnetic geometries. [ABSTRACT FROM AUTHOR]

Presents the magnetohydrodynamics stability of external kink modes and vertical modes in compact stellarators. Stabilization of vertical mode; Derivation of a simple stability criterion; Results of the systematic parameter study.

Recent progress in the theoretical understanding and design of compact stellarators is described. Hybrid devices, which depart from canonical stellarators by deriving benefits from the bootstrap current which flows at finite beta, comprise a class of low aspect ratio A<4 stellarators. They possess external kink stability (at moderate beta) in the absence of a conducting wall, possible immunity to disruptions through external control of the transform and magnetic shear, and they achieve volume-averaged ballooning beta limits (4%-6%) similar to those in tokamaks. In addition, bootstrap currents can reduce the effects of magnetic islands (self-healing effect) and lead to simpler stellarator coils by reducing the required external transform. Powerful physics and coil optimization codes have been developed and integrated to design experiments aimed at exploring compact stellarators. The physics basis for designing the national compact stellarator will be discussed. [ABSTRACT FROM AUTHOR]