HSX stellarator. (Credit: University of Wisconsin-Madison)

A device for thermonuclear fusion research that uses a combination of differently shaped coils which together create a desired helical magnetic field to confine the hot plasma. To hold the particles in the donut-shaped magnetic container, they must circulate in spirals, once on the inside and once on the outside of the torus. The stellarator creates a suitable magnetic field by combining helical, toroidal and poloidal coils. There are many variations that use different combinations and shapes of coils (classical, heliac, heliotron, torsatron, and helias). In the most modern modular stellarators, an assembly of specially twisted coils creates the magnetic field. The first stellarator in the shape of a three-dimensional figure “8” was built in 1953 by Lyman Spitzer at Princeton University. There are over ten stellarators operating around the world. The largest are Wendelstein 7-X in Germany, the Large Helical Device (LHD) in Japan, and the Helically Symmetric eXperiment (HSX) in the USA.