The first ITER toroidal field coil (TFC) has been successfully manufactured by the Japanese Domestic Agency in January 2020. The ITER TFCs are the largest Niobium Tin (Nb3Sn) superconducting magnets in the world; each is enclosed in an austenitic stainless-steel case with a height of 16.5 m and total weight is 310 tons (Knaster et al 2008 IEEE Trans. Appl. Supercond. 18 495–498). A set of 18 TFCs will be installed around vacuum vessel to function as a plasma confinement magnet system. The responsibility to procure 18 TFCs and 1 spare coil is shared between European Domestic Agency and Japanese Domestic Agency (Bellesia et al 2020 IEEE Trans. Appl. Supercond. 30 4202205; Sborchia et al 2008 IEEE Trans. Appl. Supercond. 18 463–466). To hold a common magnetic and geometrical properties among all the TFCs, tight tolerances of sub-millimeter order are defined on each TFC. The fabrication of those massive magnets with such tight tolerances involved some major technical challenges. These technical challenges were solved by pre-assessment and process qualification through some qualification trials. As a result, techniques established to solve those challenges were implemented to the TFC manufacturing, leading to the successful completion of the first TFC. The details are described in the paper.

The ITER Toroidal Field (TF) coil is a D-shaped superconducting magnet. A set of 18 TF coils forms a donut shape when assembled around the ITER vacuum vessel. The magnetic property of a coil is characterized by a current center line (CCL). To serve their function as plasma containment magnets, severe requirement of φ2.6 mm cylindrical tolerance is defined for the critical portion of the TF coils. In previous study, the manufacturing tooling and procedure have been developed and applied for manufacturing of Winding Packs (WP) and TF Coil Case (TFCC) subassemblies. In integration of a WP into a TFCC, predetermined CCL of the WP shall be controlled and transferred to reference points of the TFCC. For precise control of the CCL positions, deformations of the WP and the TFCC must be controlled. Also, the precise tracking of the CCL position required some techniques to evaluate the CCL positions even after the WP is completely covered by the TFCC. Techniques have been developed through welding trials and structural simulation analysis. Those techniques are applied to TF coil production and two TF coils have been completed successfully.

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A current center line (CCL) was precisely controlled with markers called "CCL positional plates (CCLPPs)" during the ITER toroidal filed (TF) coil integration. CCLPPs with the same x and y coordinates (CCLPP pair) are attached on both sides of the TF coil Winding Pack (WP). The CCLPP insufficiently fixed has to be replaced with new one to ensure the accuracy of CCL. On the other hand, the replacement possibly leads to some issues, such as, dropping the CCLPP into the CC and damaging the WP. Thus, unnecessary replacement should be avoided. We have therefore implemented a double check process to confirm soundness of the CCLPPs. First, change of the distance of CCLPP pair (∆W CCLPP ) is evaluated (Check A). However, ∆W CCLPP would be influenced by the WP deformation. Thus, the Check A possibly judges the fixing condition of the CCLPP as "unusual" even if it has no issue. This may lead to the unnecessary replacement of the CCLPP. To avoid the unnecessary replacement, the local displacement of each CCLPPs (∆r CCLPP ) is also evaluated (Check B). In this paper, we define the criteria for "unusual" ∆W CCLPP and ∆r CCLPP based and verify its validity for the TF coil integration by considering the measurement error and geometrical configuration of the CCLPPs. In the case of one Toshiba TF coil, the double check process confirmed that CCLPPs were insufficiently fixed, and they were successfully replaced to the new ones. Thus, it was demonstrated that the proposed criteria indicated the appropriate action to keep the CCLPP soundness. The method proposed in this paper would be powerful tool to inspect the soundness of the CCLPP which is important for the CCL control not only in the ITER TF coil, but also in the future fusion magnets.