Design of the superconducting encircling coils for Helios, the planar coil stellarator

The “Helios” stellarator, developed by Thea Energy Inc., is a fusion power plant that employs an all-planar coil system to generate an optimized, quasi-symmetric magnetic field for plasma confinement, significantly simplifying manufacturing and maintenance compared to traditional stellarator designs. This paper details the preconceptual design, analysis and modelling of the primary field-generating coil set: twelve large plasma encircling superconducting coils. These coils, which provide the main toroidal field, are complemented by an array of smaller planar shaping coils for precise and localized field correction, which are detailed in a companion paper. The encircling coils utilize a high-current, low-turn architecture, and are manufactured from a high-temperature superconducting (“HTS”) cable. The planar geometry confers unique manufacturing advantages for the stellarator: the HTS cables require bending in only one direction and eliminates the need for twisting. A magnetostatic finite element model of the most extreme encircling coil in the array revealed a peak field on coil of 20.5 T, an operating current of 48 kA, inductance of 4.5 H, stored energy of 5.2 GJ, and required an HTS cable with 230 tapes in parallel. The field distribution on coil enables the amount of HTS tapes per cable to be optimized, reducing the quantity of HTS required by nearly 30%. The Lorentz forces are managed by a robust mechanical support system, comprising winding pack cases, inter-coil trusses, and a central support structure, which limits HTS tape strain to less than 0.4%. The von Mises stress of the structure is <800 MPa and optimizable to <600 MPa, enabling the use of widely available stainless steel. The design and analysis presented here validates the technical feasibility of planar encircling coils, a critical component of a compelling new path for stellarator development.