Completion of All Nine ITER Toroidal Field Coils in Japan

The fabrication of nine ITER toroidal field coils (TFCs) has been completed successfully in Japan. When a set of 18 TFCs are installed around the ITER vacuum vessel, it generates a plasma-confinement magnetic field. To achieve the required plasma-confinement quality, the error field should be limited to 50 ppm of the nominal toroidal field. This error field requirement is propagated to each component of the magnet system as manufacturing and assembly tolerances. For each TFC, this requirement is given in terms of current center line (CCL) tolerances. Even though the TFC is a gigantic module of 9 m wide and 16.5 m high, the positional tolerances allowed for its CCL is 2.6 mm in diameter at the straight portion and ±3.0 mm in lateral direction at the curved portion. On the other hand, limiting TFC assembly misalignments is also important in the minimization of the error field. Therefore, sub-millimeter profile tolerances are defined on the important interfaces of TFCs while the manufacturing of a TFC involved welding of 40 to 100 mm thick austenitic stainless steel. The completion of all nine TFCs is realized by the techniques developed through discussions and trials along the fabrication process. High accuracy, three-dimensional measurements with laser trackers were introduced to record the fabrication outcomes in each step and to feedback the data to the next step. Based on the measurement results accumulated in the process, the manufacturing results of all nine TFCs were evaluated in the view of its CCL quality and interface tolerances.