A magnetic confinement fusion device that uses carefully shaped external coils to confine plasma without relying on an internal plasma current, offering a path to steady-state fusion energy.
A stellarator is a toroidal magnetic confinement device in which the rotational transform—the twist of magnetic field lines around the torus needed to confine charged particles—is produced entirely by external electromagnetic coils. This distinguishes it from the tokamak, where the rotational transform arises primarily from a toroidal plasma current.[1]
In any toroidal confinement system, charged particles experience vertical drifts due to the curvature and gradient of the magnetic field. These drifts would carry particles out of the device unless the field lines twist helically around the torus. In a stellarator, this helical structure is imposed by the geometry of the external coils themselves, shaped in complex, non-planar three-dimensional forms.[2]
The stellarator concept was invented by Lyman Spitzer Jr. at Princeton University in 1951, predating the tokamak by several years. Early stellarators suffered from higher-than-expected transport losses. By the late 1960s, Soviet tokamak results showed markedly superior confinement, and most laboratories shifted to the tokamak line. Stellarator research continued at reduced scale in Germany, Japan, Spain, Australia, and the United States.[1]
The renaissance began in the 1980s with computational optimisation of the three-dimensional magnetic geometry. The most ambitious product is Wendelstein 7-X (W7-X) in Greifswald, Germany, the world’s largest optimised stellarator.[3]
W7-X began plasma operations in December 2015. It achieved verified results including electron temperatures exceeding 20 million °C, pulse lengths beyond 100 seconds, and energy confinement times consistent with design predictions.[4]
The Large Helical Device (LHD) in Japan achieved ion temperatures above 10 keV and sustained plasma operation exceeding 45 minutes. The Helically Symmetric Experiment (HSX) at Wisconsin demonstrated the first quasi-helically symmetric configuration.[2]
Stellarators offer inherent steady-state capability and absence of disruptions. However, complex three-dimensional coil geometry is difficult and costly to manufacture with required precision. Confinement of fast ions and alpha particles in non-axisymmetric fields remains an active research area.[3]