Quasi-isodynamic stellarator

A quasi-isodynamic stellarator is a type of stellarator that satisfies the property of omnigeneity, avoids the potentially hazardous toroidal bootstrap current, and has minimal neoclassical transport in the collisionless regime.
Wendelstein 7-X, the largest stellarator in the world, was designed to be roughly quasi-isodynamic.
In contrast to quasi-symmetric fields, exactly QI fields on flux surfaces cannot be expressed analytically. However, it has been shown that nearly-exact QI can be extremely well approximated through mathematical optimization, and that the resulting fields enjoy the aforementioned properties.
In a QI field, level curves of the magnetic field strength on a flux surface close poloidally, and not toroidally, causing the stellarator to resemble a series of linked magnetic mirrors.