Modular coil from Wendelstein 7-AS stellarator. (Credit: Wikimedia Commons)

Specially distorted circular magnetic coils, which create the major part of the magnetic field in the stellarator. The shape of the modular coils is calculated by a computer program that is given the shape and characteristics of the magnetic field that the coils are to create. The program must also take into account that the designed coils can be manufactured. In order to limit the number of unique modular coil shapes that will need to be produced, modular stellarators have multiple (three to five) symmetry where the same coil configuration is repeated. Sometimes the same coil shape is used upside-down. Modular coils are used for example in the Wendelstein 7-X stellarator.