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Description
The transition from low-temperature superconductors (LTS) to high-temperature superconductors (HTS) has opened new possibilities for toroidal field coil design in fusion applications. These next-generation HTS coils represent a significant departure from traditional D-shaped LTS coils, introducing unique operational dynamics and challenges. A key innovation is the adoption of HTS stacks, where hundreds of stacked HTS tapes are soldered together to form robust, high-current coils. While these coils offer resilience to small heat loads or quenches, they introduce complications during rapid charging, as currents may redistribute unevenly among the tapes due to current-sharing effects.
This study explores the behaviour of these HTS toroidal field coils under various operational scenarios, with a focus on understanding the interplay between charging rates, current sharing, and loss mechanisms. A hybrid modelling approach is employed, combining circuit modelling with finite element method (FEM) simulations using the T-A formulation. This methodology captures the critical influence of ramp rates, joint resistance, turn-to-turn resistance, and superconductor saturation on coil performance and energy efficiency.
A particular emphasis is placed on understanding current-sharing phenomena in large-scale stacked coils containing up to several hundreds of HTS tapes. The simulations and models demonstrate how rapid charging can induce significant current redistribution, potentially leading to increased losses and non-uniform heating. Addressing these challenges is essential for ensuring the reliability and scalability of HTS toroidal field coils in fusion devices.
Additionally, the work includes a comparative analysis of square and D-shaped coil geometries, examining their relative advantages and drawbacks under fixed constraints such as central magnetic field or transport current. This analysis provides valuable insights into optimizing the coil design for specific operational requirements, considering both geometric and electromagnetic factors.
This study highlights the distinct characteristics of HTS-based toroidal field coils, particularly the critical role of current-sharing dynamics in stacked coils. By integrating advanced modelling techniques and exploring innovative geometries, the findings contribute to the development of more efficient, stable, and scalable superconducting coil systems for next-generation fusion technologies.
