Lessons learned from European and Japanese production of ITER toroidal field coils

In 2023, the manufacturing of all the ITER TF coils has been completed 15 years after the sign of procurement arrangements in 2008. This paper has been jointly submitted by F4E and QST to recollect the lessons learnt in the production of the two parties along the last 15 years.


Introduction
ITER has 18 toroidal field (TF) coils plus one spare each of which is a D-shaped Nb 3 Sn superconducting magnet of approximately 17.7 m in height, 9.2 m in width and 310 tons in weight as shown in figure 1 [1,2].A current of 68 kA flowing in the TF coils produces a peak magnetic field of 11.8 T on the coils and 5.3 T at the tokamak major radius of 6.2 m to confine the plasma.To withstand the electromagnetic load of 600 MN during operation, a winding pack (WP) is assembled with thick stainless-steel structure.As shown in figure 2, WP consists of 7 double pancakes (DPs), which are 5 regular DPs and 2 side DPs.DP is composed of TF Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.conductor, radial plate (RP), cover plate (CP) and their electrical insulations.Major parameters of TF coils are listed in table 1.

Strategies of TF coil manufacturing
The ITER TF coil procurement is divided into procurements of TF conductor, TF structure and TF coil as shown in figure 3. The TF conductor employing Nb 3 Sn superconductor is manufactured by China, Europe, Japan, Korea, Russia, and America after qualifying each TF conductor in SULTAN test facility at Swiss Plasma Center and central solenoid model coil (CSMC) test facility at QST [3][4][5][6][7].Then, they are shipped to Europe and Japan where the TF coils are manufactured.The structures procured in Japan are also sent to Europe and Japan.Europe and Japan are responsible for 10 TF coils and 9 TF coils, including 1 spare, respectively Then, the manufactured TF coils are transported to the ITER construction site in the south France where they will be assembled as a backbone of ITER.For TF coil manufacturing, in Europe, there are 3 different kinds of contracts for RP manufacturing, WP manufacturing and insertion including cold test of WP.In Japan, there are 2 contracts to manufacture TF coils with 2 lines with different suppliers, which manufacture 5 TF coils, including 1 spare, and 4 TF coils, respectively.Each contract is to manufacture RP/WP manufacturing and insertion.As shown in figure 4, procurement arrangements (PAs) were signed in 2008 for European and Japanese TF coils.Before starting a series of production, prototyping and qualification were performed to develop the manufacturing procedures and demonstrate their technical feasibilities.Then, actual  manufacturing of TF coils started at around the beginning of 2014 in both parties.Simultaneously, first TF coils manufactured in February 2020.In 2023, the manufacturing of all the ITER TF coils has been completed 15 years after the sign of PAs.
In WP manufacturing as shown in figure 5, TF conductor, received from conductor suppliers, wound to D-shape as a DP on a double spiral trajectory.The wound conductor is heattreated at temperature up to 650 • C to react the Nb 3 Sn [8].In parallel, RP is manufactured to fit the wound and heat-treated conductor.The conductor, after heat treatment, is insulated and inserted into a RP groove.CP is laser-welded on RP and insulated around the DP.Each supplier has prepared the winding machine, the heat-treatment furnace, the transfer equipment, the insulation machine, and the impregnation equipment for the manufacturing of the DP of 14 m in height and 9 m in width.
In insertion as shown in figure 6, a WP is inserted into structures.The structures are closure welded to assemble them.Gap between the WP and the structures are filled with a resin.To meet stringent tolerance of less than several mm, especially interface between each TF coil, TF coil are finally machined [9].Finally, Factory Acceptance Test, which consists of dimensional inspection, electrical test and so on, is performed before starting packing and shipment.Europe performed insertion process horizontally.Japan performed it vertically without final machining.Since the main difference between them is gravity, it affects the deformation during the insertion and the welding.In order to achieve good plasma confinement, the homogeneity of the TF coils is important.Therefore, current center line (CCL), which is defined as the barycentre of the as-built conductors inside the WP, shall be met in severe requirements even the manufacturing strategy is different.

Winding, heat-treatment, RP manufacturing techniques
An only option was Nb 3 Sn for ITER TF coils at design phase as a superconductor since the maximum magnetic field is 11.8 T in coil.The heat-treated conductor is insulated and inserted into a RP groove while trying to avoid applying more than 0.1% strain since Nb 3 Sn is brittle and it would permanently degrade when experiencing higher strains.In order to insert the TF conductor into the RP groove, the difference in trajectory lengths of the winding and RP groove should be controlled within 230 ppm, which is approximately 7 mm over a turn of 34 m, as shown in figure 7. It is important to note that during heat treatment the length of the conductor changes due to the reaction of Nb 3 Sn.In order to achieve the required accuracy, a laser optical measurement was utilized, achieving accuracies better than ± 0.01% for the DP winding.In addition, accurate prediction of the change in the winding trajectory due to heattreatment was utilized, by winding the DP on a reduced trajectory such as to precompensate the change in length experienced during the heat treatment.Thanks to this approach, all the 133 DP windings could be inserted into their respective RPs as shown in figure 8.

Electrical insulation with radiation resistant resin
The electrical insulation of the ITER TF coil should withstand a voltage of 19 kV up to a maximum fast neutron fluence of 10 22 n m −2 and an absorbed dose of 10 MGy.The electrical insulation consists of polyimide layer and glass cloth filled with a radiation resistant resin for mechanical  (CTD) for the ITER TF coil.No experiences were for large magnets using cyanate-ester resin before manufacturing the TF coil although there was only for palm-size.Since cyanateester has high reaction heat during curing, there is a risk of thermal runaway.In addition, TF coil has long and narrow gaps between the RP and the conductor.Therefore, they require R&D for the impregnation and curing techniques for the TF coil manufacturing.
Through R&Ds of the cyanate-ester and epoxy mixed resin properties, a viscosity of less than 50 mPas and a pot-life of more than 100 h were chosen to be suitable for the resin to fill out the long and small gaps between the RP and the windings.Allowable volume of the resin was determined to avoid the thermal runaway of the resin.In addition, since overlaps of polyimide tape without interleaving the glass cloth disturbs the resin flow, a bonded glass/polyimide tape has been developed to achieve a good adhesion of the combined materials.Qualification results verified no voids within the resin layer as shown in figure 9.

Deformation control during welding in TF structures
After manufacturing basic segments as shown in figure 10, TF structures are manufactured by welding and machined to achieve the severe tolerance.Conventional method requires larger extra thickness, which requires longer machining time.Although both side welding technique is employed to compensate the welding deformation, it requires that longer handling of structures to turn them around.Therefore, a control system was newly developed to mitigate angular distortion using one-side welding and hydraulic jack as shown in figure 11 [18].As the result shown in figure 12, welding distortion with newly developed procedure have satisfied to allow 3 mm extra thickness.

Current center line (CCL) positioning for error field mitigation
CCL positioning is important to achieve good plasma confinement and stability, by minimizing the magnetic error field, as well as to ensure toroidally symmetric heat fluxes on the first wall panels on the high field side of ITER.For the ITER TF coils, the required tolerances of the CCL are Φ2.6 mm in the inboard region as shown in figure 13.Through every manufacturing step, the CCL has been measured and positioned properly.In particular, the CCL position is also measured after insertion of WP in the stainless-steel case, through holes drilled in the structure to allow laser tracker measurement as shown in figure 14.As the result, all the TF coils have satisfied the requirements of CCL positioning as shown in figure 15 although Europe and Japan approaches are different in posture.

Final machining with high accuracy
The structures are manufactured and assembled within an accuracy of several mm.In the final machining, the required  tolerance is around 1 mm at the interfaces with adjacent TF coils and other components.The most severe surface tolerance of ± 0.2 mm is applied to the two 7 m-long wedged surfaces of the straight leg.In a dimensional inspection after closure welding, a deformation over the extra-material thickness exceeding tolerances has been found in specific interface parts, namely the Intermediate Outer Intercoil Structure (IOIS) [9].The deviations on those interfaces could potentially impact assembly of the TF coils.Therefore, Japan and Europe compared results of the welding deformation for their first TF coils in these areas and found that they had the common trends.Since the shape of all the TF coils are identical, it was concluded that the TF coils can be assembled as long as they share the same interface deviation trends.As a consequence, the TF coil reference as-built drawing for assembly was changed after confirming that no clashes with other components take place because of  these manufacturing deviations.Since the first comparison of Europe and Japan manufactured TF coils, results and experiences have been shared between two parties and no further major deviations have been observed.

Conclusion
Solving the technical challenges above, the manufacturing of all the 19 TF coils have been completed in 2023.Figure 16 shows the TF coil in Europe and the final 9th completed TF coil in Japan.Although Europe and Japan have different approaches, TF coils manufactured both by the European and Japanese suppliers meet the requirements of ITER TF coil.Techniques developed for the winding, the heat-treatment, the transfer, the impregnation, the welding, and the assembly satisfied the required tolerance.In 2023, the manufacturing and the delivery of all the 19 TF coils have been completed 15 years after starting Nb 3 Sn strand manufacturing in 2007.

Figure 1 .
Figure 1.ITER TF coil and assembly between winding pack and structures.

Figure 2 .
Figure 2. Winding pack (WP) and double pancake (DP)in the inboard part of ITER TF coil.

Figure 4 .
Figure 4. Strategies and results of TF coil manufacturing in Europe and Japan.

Figure 6 .
Figure 6.Manufacturing steps for insertion of WP into structures.

Figure 7 .
Figure 7. Tolerance of heat-treated winding to insert into RP groove.

Figure 8 .
Figure 8. Results on conductor and RP groove trajectories.

Figure 10 .
Figure 10.Basic segments of TF structure.

Figure 11 .
Figure 11.Control system to mitigate angular distortion using one-side welding.

Figure 12 .
Figure 12. Results on angular distortion of welding for TF structure sub-assemblies.

Figure
Figure European and Japanese TF coils.

Table 1 .
Major parameters of ITER TF coils.