Modular coils are external magnetic field coils used to produce (part of) the confining field of a stellarator. The design of such coils starts from a certain magnetic configuration with specific desirable physical properties (see Stellarator optimization). This magnetic configuration is produced by a certain current distribution flowing on an external control surface, i.e., the surface in which the external field coils will be located.
The shape of the discrete modular coils attempts to reproduce this ideal, continuous current distribution on the control surface with a certain precision. The modular coils are deformations of circular coils. Their shapes are allowed to be complex while staying within engineering limits. The number of differently shaped coils is kept as small as possible to facilitate coil construction and assembly.
- D.A. Spong, D.J. Strickler, S.P. Hirshman, J.F. Lyon, L.A. Berry, D. Mikkelsen, D. Monticello, A.S. Ware, Stellarator flexibility options with variable modular coil currents, Stellarator News 88 (2003)