Quasi-isodynamic stellarator

A quasi-isodynamic (QI) stellarator is a type of stellarator (a magnetic confinement fusion reactor) that satisfies the property of omnigeneity, avoids the potentially hazardous toroidal bootstrap current, and has minimal neoclassical transport in the collisionless regime.^[1]

Wendelstein 7-X, the largest stellarator in the world, was designed to be roughly quasi-isodynamic (QI).

In contrast to quasi-symmetric fields,^[2] exactly QI fields on flux surfaces cannot be expressed analytically.^[3] However, it has been shown that nearly-exact QI can be extremely well approximated through mathematical optimization,^[4] and that the resulting fields enjoy the aforementioned properties.

In a QI field, level curves of the magnetic field strength ${\displaystyle B}$ on a flux surface close poloidally (the short way around the torus), and not toroidally (the long way around), causing the stellarator to resemble a series of linked magnetic mirrors.