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Geometrical deformations and assembly errors in the ITER Toroidal Field (TF) coils will lead to magnetic field perturbations, which could degrade plasma confinement and eventually lead to disruption. Extensive computational studies of the influence of coil deformations and assembly errors on plasma behavior have given the basis for definition of the geometric tolerance of the Current Center Line (CCL) of the winding pack of the TF coil. This paper describes an analysis method to establish the feasibility to measure the magnetic CCL locus of the final winding pack (WP) with accuracy better than 1 mm. The proposed method is based on arrays of gradient coils accurately mounted with respect to the WP fiducial marks and datum surfaces. The magnetic measurements will be performed at defined locations around the WP perimeter to characterize accurately the CCL locus. The analysis emphases the robustness and sensitivity of the method versus the measurement location and the TF coil 3D geometrical deformation. The analysis and proposed measurement techniques will be described in detail. [ABSTRACT FROM PUBLISHER]

Abstract: Error fields in Tokamaks are small departures of the exact axisymmetry of the ideal magnetic field configuration. Their reduction beyond a threshold value by the error field correction coils is essential since sufficiently large static error fields lead to discharge disruption. The error fields are originated not only by coils fabrication and installation alignment tolerances, joints and busbars but also due to the presence of ferromagnetic elements. The start of plasma current flattop with relatively low plasma density is considered as a critical state of the 15MA scenario for the onset of locked modes causing disruptions. A figure of merit, the ‘3-mode’ criterion, based on the lowest error field harmonics [(1,1); (2,1); (3,1)] was chosen to assess the error fields expected in ITER. Analysis performed last years in an independent way by CREATE (EU) and Efremov Inst. (RF) groups has allowed a deep understanding of the error fields induced by all the possible sources. Both groups were successfully benchmarked with the estimation of the error fields induced by a given deformed shape of a TF coil obtaining a perfect match of the results up to 2 orders of magnitude smaller than the defined threshold of B 3-mode/B to <5×10−5. Three different sets of independent variables based on a 3D rigid body movement of the coils have been provided for the 6 CS modules, 18 TF coils and 6 PF coils tolerances, which have allowed a clear understanding of the weight of each of the variables in the induced error fields. The results obtained in 2008 with a realistic set of magnets tolerances concluded that the system of correction coils provides effective suppression of the error fields with margin to correct possible impact of other sources. In particular, it has been shown that superconducting joints, feeders and busbars play a secondary effect; however the radial position of the TF coils and the tilt and radial shift of the CS stack would have a relevant influence. The ensuing recent sets of variables studied aimed at deepening the understanding of the operational limits. The present paper addresses the impact of the fabrication tolerances and installation misalignments of the magnet system; the impact of ferromagnetic inserts and busbars as well as the design of the correction coils will be thoroughly covered elsewhere (Amoskov et al., in press ). [Copyright &y& Elsevier]

Perturbations of the axial symmetry of the poloidal magnetic field in the order of 10-4 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. [ABSTRACT FROM AUTHOR]

The toroidal magnetic field of ITER is provided by 18 Nb3 Sn superconducting coils that provide 5.3 Tin the Tokamak magnetic centre thanks to a nominal current of 68 kA. Two major procurement packages will be integrated to accomplish the final assembly of the coils: 1) the winding pack which houses the double pancakes in 7 stacked radial plates (RP) and 2) the TF coil case which houses the winding pack (WP) and is the main structural component of the ITER machine. 19 TF coil cases (18+1 spare) will be entirely procured by Japan, whereas the WP and the TF coil assembly will be shared between Japan (9) and Europe (10). The insertion process of the WP inside the coil case, VPI and final closure welding is presented with an emphasis on the R&D still remaining. An appropriate metrology strategy and correct benchmarking of the WP current center line (CCL) in the coil case external walls is essential to allow the correction of the error fields during operation. The TF coils will be assembled and pre-aligned in pairs together with their corresponding VV sector and craned to the Tokamak hall hanging from 2 independent hooks. The integration of the IF coils in the Tokamak primary datum system will have to bc accomplished within the maximum tolerances defined by the error field analysis. As-built 3D data collected during manufacture will be used to ensure the correct positioning of the CCL against the Tokamak datum system and ease the alignment process. Shimming will be used in the gravity support for vertical alignment and in the upper and lower OIS for toroidal alignment. A subsequent survey of the `as-installed' TF coils will determine the final Tokamak Datum System. [ABSTRACT FROM AUTHOR]

The current design of the ITER Toroidal Field coils and structures, the main critical design and manufacturing issues, and the status of the procurement arrangements for these components, which will be released to the ITER parties in early 2008 to start the manufacturing contracts, are described. Some qualification and R&D work still required in preparation for the manufacture are also mentioned. [ABSTRACT FROM AUTHOR]