Application of High-Precision Assembly Technology for Large Structures by Laser Beam Welding

Abstract

Abstract Toroidal Field coils (TF coils), which produce strong magnetic field in ITER, are used in order to confine the high temperature and high density plasmas stably for fusion reaction. The conductor made of Nb3Sn strands transform a superconducting conductor by heat treatment. The superconducting conductor is inserted into the D-shaped Radial Plate (RP), which have the structure with spiral grooves on both side for it, and then, insulating tapes are wound step by step around the superconducting conductor in RP as taking up the superconducting conductor. After insertion of the insulation-treated conductor into RP, Cover Plates (CPs) are put on RP spiral grooves and seal welded. After welding of CPs on RP of both sides, that the D-shaped Double Pancake (DP) is completed. The accuracy affects the TF coil performance and the deviation from ideal shapes and locations of the coils generates so called ‘error field’. In order to start up and confine the plasma stably, the allowable amount of the error field is about a few Gauss (Gauss : 1 × 10-4 T)[1]. Therefore, the dimensional accuracy of the 1/10000 order such as the profile within ±3mm for the circumference length and the flatness within ±3mm in the whole area of the D-shaped DP (14m × 9m) of approx. 37 m is required by ITER design. In addition, the TF coil is a huge component (with a weigh of about 300 ton) to withstand the high electromagnetic force of hundreds of MN. It is an essential requirement for ITER design to achieve the accuracy of more than 1/10000 even though machining after welding is not allowed. Laser beam welding technology for large structure with thick plate of Mitsubishi Heavy Industries was applied the TF coils to minimize and avoid large welding deformation in the conventional welding process such as TIG welding to meet the ITER requirements.