Quasisymmetry

This article is about the concept in magnetic confinement fusion. For mathematics terms, see Quasisymmetric.

In magnetic confinement fusion, quasisymmetry (sometimes hyphenated as quasi-symmetry) is a type of continuous symmetry in the magnetic field strength of a stellarator.^[1] Quasisymmetry is desired, as Noether's theorem implies that there exists a conserved quantity in such cases. This conserved quantity ensures that particles stick to the flux surface, resulting in better confinement and neoclassical transport.

A flux surface and coils of NCSX, a quasi-symmetric stellarator.

It is currently unknown if it is mathematically possible to construct a quasi-symmetric magnetic field which upholds magnetohydrodynamic force balance, which is required for stability. There are stellarator designs which are very close to being quasisymmetric,^[2] and it is possible to find solutions by generalizing the magnetohydrodynamic force balance equation.^[3] Quasisymmetric systems are a subset of omnigenous systems. The Helically Symmetric eXperiment and the National Compact Stellarator Experiment are designed to be quasisymmetric.^{[citation needed]}