Non-insulated, non-planar HTS coils for the EPOS and CSX stellarators

The EPOS (Electrons and Positrons in an Optimized Stellarator) and CSX (Columbia Stellarator Experiment) experiments are table-top-sized high-temperature superconducting (HTS) stellarators in development. Both will use quasi-axisymmetric configurations to achieve stable plasma confinement, with EPOS focusing on a cold e+e−plasma (0.1-10 eV, ~10 L) and CSX on an electron-ion plasma (5-30 eV, ~100 L) with a low aspect ratio achieved using interlocking coils. The magnets are designed as 3D-printed metal structures with channels for winding HTS tape (ReBCO), accounting for HTS strain and torsion constraints using SIMSOPT [1, 2, 3]. Developing final coil configurations involves iterative testing and various manufacturing methods, including continuous double-pancake winding under constant tension, cooling, and powering the coils. Both use cryogenic test-stands, with coldhead cooling and high-current power supplies to test frame materials and HTS cooldown. For the EPOS hardware test campaign, non-planar test coils are cooled to 20 K, with the end goal of a high-field coil with 2 T on axis and to operate multiple coils simultaneously. CSX has tested planar and non-planar double-pancake coils in liquid nitrogen, with upcoming experiments testing a 0.1-T coil at 15 K, aiming to achieve 0.5 T on axis for a high-field coil. Results from these tests will guide further development.

[1] C. Paz-Soldan, “Non-planar coil winding angle optimization for compatibility with non-insulated high-temperature superconducting magnets,” Journal of Plasma Physics, vol. 86, no. 5, p. 815860501, 2020. doi:10.1017/S0022377820001208

[2] P. Huslage, D. Kulla, J.-F. Lobsien, T. Schuler, and E. V. Stenson, ‘Winding angle optimization and testing of small-scale, non-planar, high-temperature superconducting stellarator coils’, Superconductor Science and Technology, vol. 37, no. 8, p. 085010, Jul. 2024, doi: 10.1088/1361-6668/ad5382.

[3] Simsopt development team, Simsopt: A python framework for stellarator optimization, 2021 https://zenodo.org/badge/247710081.svg