There are strip wound cores, hence there is no air-gap in the flux path or the core can be built with distributed air-gap controlling the value of the (permiability).
The strip wound cores have minimum losses and have power ideal from small toroidal transformers to utility level distribution transformers. At sigma we have technology to design efficient magnetics in different configurations.
The cores are available in different BH curve fitting configurations. For example square are ( cobalt based ) which can be used to switch core.