Stellarators mainly utilize magnetic fields from external coils to confine the plasma, enabling steady-state operation while avoiding instabilities like disruptions raised from plasma currents. Similar to tokamaks, the fusion power of stellarators is proportional to . High-temperature superconducting (HTS) coils can withstand high currents and thus offer significant advantages in enhancing fusion power or reducing the machine size. However, HTS materials, particularly Rare Earth-Barium-Copper Oxide, present unique electromagnetic and mechanical properties that pose new challenges for designing stellarator coils. To address these challenges, we developed a new code, FOCUS-HTS, built on its predecessor, FOCUS. FOCUS-HTS can model coils as either filaments or finite-build shapes using the Fourier representation or cubic B-splines. In addition to standard physics and engineering targets, FOCUS-HTS can optimize tape strains, electromagnetic (EM) forces, and critical current densities. Developed in Python with automatic differentiation, the code allows easy interfacing and GPU acceleration. For demonstrations, FOCUS-HTS has been used to reduce the EM force of the W7-X coils and design HTS coils for a precisely quasi-axisymmetric stellarator.
This paper presents a new code called FOCUS-HTS that can design stellarator coils using high-temperature superconducting (HTS) materials. HTS coils offer advantages in enhancing fusion power or reducing machine size, but pose new challenges. FOCUS-HTS can model coils as filaments or finite-build shapes, and optimize for physics, engineering, and HTS-specific targets like tape strain and critical current density.