Room Temperature Magnetic Determination of the Current Center Line for the ITER TF Coils

The ITER tokamak includes 18 superconducting D-shaped toroidal field (TF) coils. Unavoidable shape deformations as well as assembly errors will lead to field errors, which can be modeled with the knowledge of the current center line (CCL). Accurate survey during the entire manufacturing and assembly process, including transfer of survey points, is complex. In order to increase the level of confidence, a room temperature magnetic measurement of the CCL on assembled and closed winding packs is foreseen, prior to insertion into their cold case. In this contribution, we discuss the principle of the CCL determination and present a low frequency ac measurement system under development at PSI, within an ITER framework contract. The largest current allowed to flow in the TF coil at room temperature and the precision requirements for the determination of the CCL loci of the coil are hard boundaries. Eddy currents in the radial plates, the winding pack enclosures, and possibly from iron in the reinforced concrete floor of the assembly hall are more subtle to bring under control quantitatively, and will limit the highest frequency that can be used to measure reliable values of the field gradients. Our objective is to measure the CCL loci by combining magnetic data obtained with printed circuit board flux coils and survey data gathered around the perimeter of the D-shaped coil.