The ITER Toroidal Field Coil Structure (TFCS) consists of the coil case (CC) and inter-coil structure (IS) that connects the TF coils (TFCs). TF coil integration is conducted to integrate the CC and winding pack (WP). The National Institutes for Quantum and Radiological Science and Technology (QST), serving as the Japanese domestic agency (JADA) in the ITER project, is responsible for procuring nine TF coils, including the TFCS and TFC integration. This commentary introduces the papers relating to the TFCS and TFC integration in this special issue of TEION KOGAKU.

The toroidal field coil case (TFCC) is a large structure having a D-shape with a height of 16.5 m and a width of 9 m. The TFCC is composed of four subassemblies called as AU, BU, AP, and BP. Although the subassemblies are large welding structures, it is required for the interfaces to satisfy severe dimensional tolerances that are on the order of millimeters. The AU and BU each have U-shaped cross-sections and are assembled in two and three segments, respectively, by welding. In the case of butt welding for U-shaped cross-sections, larger angular welding deformation is commonly generated. In order to minimize the welding deformation, the authors developed a control method through welding trials using small and full scale mock-ups. For the AU subassembly, a combination method using manual TIG welding and monitoring the welding angular deformation was proposed. For the BU subassembly, a procedure using automatic TIG welding with a U-shaped groove shape and reinforced jigs while monitoring angular deformation during welding was developed. After the establishment of these welding technologies, the welding of AU and BU began. As the result of AU and BU welding in series production, welding deformation has been controlled well and machining duration rationalized, thereby decreasing amount of extra material based on manufacturing results. As of August 2020, a total of 19 AU and BU weld have been successfully completed.

For the ITER, 18 of the world’s largest Toroidal Field (TF) coils will be installed. The components that connect each TF coils are called Inter-coil structure components. Inter-coil structure components will be cooled down to 4 K and exposed to radiation during ITER operation. These components must ensure huge magnetic force while insulating the TF coils. In this study, the authors developed glass fiber reinforced plastic (GFRP) having a compressive strength property that minimizes degradation even in a radiation environment. The compressive strength of this GFRP is demonstrated to satisfy the required value. The authors also manufactured a customized Ni-based superalloy (Alloy718) bar from a standard product. The mechanical properties at room temperature and 4 K were obtained, and it was confirmed that these properties exceed the requirements. The Inter-coil structure components used for the interface require tight tolerance, so an alumina coating is applied on the surface of stainless steel. Next, the authors tested the alumina coating to see if it deteriorated after a thermal cycle. It is reported the optimizing the component manufacturing process requires an alumina coating and high dimensional accuracy. This views and opinions expressed herein do not necessarily reflect those of the ITER organization.

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.

The ITER Toroidal Field (TF) coil is composed of a Winding Pack (WP) and a TF coil case (TFCC). In the manufacturing of a TF coil, the gap between the WP and the TFCC is filled with radiation resistant Triglycidyl-p-aminophenol (TGPAP) resin. Vacuum Pressure Impregnation (VPI) is adopted. The selected resin system displayed two potential problems: high viscosity and cracking after cure. A series of production optimizations have been performed to develop techniques to apply the selected resin for the TF coil production: crack countermeasure, narrow gap injection, and pressure control. For crack countermeasure, the addition of fiberglass tape or sheet layer was found to be effective in preventing fragmentation of cracked resin. Since the cracked resin would not harm the TF coil quality as long as it stays in the original position, addition of confining fiberglass layers solves the problem. In narrow gap qualification tests, resin injection into a 2 mm wide space was observed with proper selection of fiberglass layer addition conditions. The pressure qualification test showed that resin cured without additional pressurization can satisfy the compression strength requirements. From those results, techniques for the TF coil production have been developed, and with the implementation of those techniques the gap-filling of the first TF coil in Japan was successfully completed in 2019. Since then, two more TF coils have completed the gap-filling process with some improvements.

Over the last half-century, the most fundamental measurement of AC voltage has been done by comparing the Joule heating of an unknown AC signal to that of a reference direct DC voltage using thermal voltage converters (TVCs). However, the accuracy of AC-DC difference measurements of the TVC is limited by the accuracy of the model describing the AC-DC difference of the reference TVC. To satisfy the requirements for improved AC voltage metrology, national metrology institutes are developing quantum standards based upon the Josephson effect. These quantum-based AC voltage standards have significant advantages over a conventional measurement method in terms of accuracy and versatility. This article reviews the fundamental principle of AC voltage measurements with a conventional method based on a TVC and application of the AC-programmable Josephson voltage standard system using a sampling technique for the measurement of the AC-DC difference at low frequency.