Compact quasiaxisymmetric stellarators, a near axisymmetric theory

Researchers have developed a new theoretical framework for compact quasiaxisymmetric (QA) stellarators, simplifying their design by treating them as nearly axisymmetric devices. This approach models plasma equilibrium using ideal magnetohydrostatics, accounting for finite plasma currents and pressure.

The theory identifies localized 'ridges' where field lines become collimated, similar to X-points in tokamaks. These ridges offer potential for divertor designs without requiring a rational rotational transform, a common constraint in stellarator divertors.

Unlike X-points, which must encircle the entire torus, these sharp ridges are typically localized to specific regions of the flux surfaces. The new perturbative treatment, which expands on deviations from perfect axisymmetry, analytically describes key features of compact QA devices, including the tendency for ridges to form on the inboard side.