Coils and plasma in the Wendelstein 7-AS modular stellarator. (Credit: IPP)

A stellarator that uses a set of specially shaped modular coils to create a twisted helical magnetic field. The first modular stellarator was Wendelstein 7-AS, launched in 1988. It was the first stellarator to achieve an enhanced confinement mode, the so-called H-mode. The shape of the coils of a modular stellarator is designed by a computer program, which tries to shape them to achieve an optimal magnetic field. The fabrication and installation of such coils is very demanding in terms of precision, but they allow the modular stellarator to achieve very good plasma parameters.