A New Degree of Freedom in Stellarator Coil Design

Reversed toroidal field (RTF) coils introduce a new degree of freedom in stellarator coil design. While the total plasma-linked current is fixed by Ampere’s law, compensating RTF currents allow the modular-coil current to exceed this value without altering the net linkage. This redistribution decouples key coil constraints and modifies the external coil geometry without significantly compromising the confining magnetic field. As a result, the achievable coil-plasma separation can be increased in the configurations studied, potentially enabling more compact stellarator designs.