Optimization of the Planar Coil Stellarator

Thea Energy is developing a system of planar coils to generate stellarator magnetic fields. This work details the optimization of the two types of planar coils that are used in the design: the plasma encircling coils, and the shaping coils. The plasma encircling coils generate toroidal flux, much like the toroidal field (TF) coils in a tokamak. Encircling coil geometries are optimized to generate the majority of the equilibrium's magnetic field. An array of dipole-like shaping coils is added to the encircling coil set to shape the magnetic field. Five unique shaping coil geometries are used in the array to leverage an economy of scale. The array is located on a winding surface between the plasma boundary and the encircling coils and is sufficiently offset from the plasma boundary to leave enough room for tritium breeding/neutron shielding. Large sectors of the blanket and shaping coil array can be extracted between the planar encircling coils during maintenance. Both sets of encircling and shaping coils are constrained to be planar and convex, enabling tension winding of the coils. We demonstrate that reasonable magnetic field errors of less than 1% can be achieved by optimizing both types of planar coils, subject to realistic engineering constraints.