The magnetic field structure in a modular stellarator

The numerically predicted magnetic surface structure within the separatrix of the interchangeable module stellarator (IMS) [IEEE Trans. Plasma Sci. PS‐9, 212 (1981)] has been measured experimentally with the use of an electron beam. The results show nested, well‐formed surfaces with a rotational transform profile corresponding to the predicted profile. In the area outside the separatrix, regions of localized magnetic flux emergence from the coil volume (modular divertors) have been seen using an electron beam as well as during plasma operation, and correspond to regions of localized particle flux emergence from between the modular coils.