In magnetic confinement fusion, conventional designs, such as those for ITER, JT-60SA, and LHD, employ Low-Temperature Superconducting (LTS) wires, and the related technology has matured. Among the current fusion reactor designs, especially those conducted by start-up companies, most employ High-Temperature Superconducting (HTS) magnets. Various types of large-current HTS conductors are being developed worldwide, and may be categorized according to their characteristic structures. The background and present status of HTS magnet design and large-current conductor development are reviewed; their prospects and issues are also discussed.

Helical Fusion Co., Ltd. （HF） is a Japanese fusion startup that aims to implement steady-state helical fusion reactors in society. By boldly incorporating new technologies such as high-temperature superconducting magnet coils and liquid metal blankets, HF bypasses issues that are difficult to solve; this will enable the early realization of an attractive fusion reactor and accelerate the social implementation of fusion energy. HF is currently in the individual demonstration phase and is working on developing each new technology. In the future, HF aims to realize the first fusion power generation plant, the fusion pilot plant （FPP）. Before the FPP, HF will construct the final experimental device, a scaled-down version of the FPP, and then conduct an integrated demonstration in which all the new technologies work simultaneously. Once all new technologies have been established, construction of the FPP will begin immediately, with completion and start of operations targeted for the 2030s. The FPP will demonstrate the worldʼs first net power generation and long-term steady-state operation. If the FPP is successful, the world will begin to implement the First of a Kind commercial fusion reactor in the 2040s and beyond. This paper introduces the new technologies that HF is currently developing; it also shows the current state of research and development for the new technologies.

Kyoto Fusioneering Ltd. (KF), a Japanese fusion start-up established in 2019, is advancing commercial fusion energy through an engineering-focused approach. Its core efforts focus on three systems: plasma heating with gyrotrons, energy conversion using SiC composite blanket modules, and tritium fuel cycle technologies. The Fusion by Advanced Superconducting Tokamak (FAST) project—Japanʼs first privately led fusion power generation demonstration by a tokamak reactor was launched in 2024, with KF as project leader. The FAST project aims to integrate the key components of a nuclear fusion reactor into a compact fusion tokamak (major radius 2–3 m) using HTS (REBCO) magnets to achieve high magnetic fields and long plasma durations (~1,000 s). The project targets high neutron wall loading (300–1,000 kW/m²) to enable realistic power extraction from DT fusion. The FAST roadmap includes completing the conceptual design by 2025, starting construction by 2030, and commencing initial operations by 2035.

The high-purity niobium material used in superconducting cavities is an ingot produced by electron beam melting; it is a polycrystalline with a grain size of 10–200 mm. Niobium sheets sliced from ingots contain large grains (LGs). Superconducting cavities made from LG niobium have the advantages of a high maximum acceleration gradient, Q value, and low manufacturing cost. Large-numbered tensile testing at room temperature using two kinds of LG niobium sheets with RRR392 and RRR189 was conducted; the tensile strengths were 79.2 and 83.3 MPa, respectively, about half that of ordinary fine grain (FG) niobium. The variation of strength was significant owing to crystal orientation. The minimum tensile strength was estimated based on material strength studies to apply the LG cavity to the High-Pressure Gas Safety Act, and the allowable stress for vessel design was derived, which were 12 and 15 MPa, respectively, less than half that of FG niobium. The strength estimation method shown here can be applied with approximately 50 tensile testing results; it is also simple and versatile, and does not require crystal orientation measurement.