The Determination of the Current Center Line of the TF Coils of ITER

Abstract

Perturbations of the axial symmetry of the poloidal magnetic field in the order of of the toroidal magnetic field (5.3 T) might induce the occurrence of magnetic islands which could degrade plasma confinement and eventually lead to disruption. A weighted parameter of the lowest harmonics, the `3-mode' index, is used as the figure of merit of ITER confinement capability with respect to possible magnet geometrical deformations and assembly errors. Three sets of correction coils will ensure that error fields are kept below the Three Mode Error Index (TMEI) during operation. Fabrication tolerances and assembly misalignments of the TF coils beyond specified values might become a relevant contributor to possible error fields in the magnetic configuration. An accurate determination of the coil current center line (CCL) is essential as the reference for survey operations during fabrication and final installation phases. The CCL will be defined geometrically as the barycenter of the conductor grooves of the winding pack, which will consist of seven, stacked D-shaped radial plates. However, it also seems feasible to reconstruct the CCL of the winding pack from magnetic measurements. A current of 1 kA circulating in the winding pack at room temperature will generate a magnetic field gradient of 0.07 mT/mm at 500 mm from the winding pack center. With an array of probes operated in flip mode and accurately mounted with respect to the WP fiducial marks the CCL can be determined with accuracy < +/-1 mm. The paper will evaluate the accuracy of both geometric and magnetic procedures for current center line definition.