Quasi-isodynamic stellarator explained

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

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.

Notes and References

1. Helander . Per . Nührenberg . Jurgen . February 2009 . Bootstrap current and neoclassical transport in quasi-isodynamic stellarators . Plasma Physics and Controlled Fusion . en . 51 . 5 . 055004 . 10.1088/0741-3335/51/5/055004. 2009PPCF...51e5004H . 11858/00-001M-0000-0026-F470-B . 250779652 . free .
2. Boozer . Allen H. . 1983 . Transport and isomorphic equilibria . Physics of Fluids . en . 26 . 2 . 496–499 . 10.1063/1.864166. 1983PhFl...26..496B .
3. Cary . John R. . Shasharina . Svetlana G. . September 1997 . Omnigenity and quasihelicity in helical plasma confinement systems . Physics of Plasmas . en . 4 . 9 . 3323–3333 . 10.1063/1.872473 . 1997PhPl....4.3323C . 1070-664X.
4. Goodman . Alan . Camacho Mata . Katia . Henneberg . Sophia . Jorge . Rogerio. Landreman . Matt. Plunk . Gabriel. Hakan . Smith. Mackenbach . Ralf. Beidler . Craig. Helander . Per. September 2023 . Constructing precisely quasi-isodynamic magnetic fields . Journal of Plasma Physics . en . 89 . 5 . 905890504 . 10.1017/S002237782300065X . 253708037 . 1. free . 2211.09829 . 2023JPlPh..89e9004G .