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Controlling the geometry of the machine during the assembly process will be a very demanding activity that can only be achieved by managing variation at each stage. Accurate dimensional control will ensure that the exacting tolerances required for machine operation are achieved, and by the end of the process metrologists will have recorded the exact as-built geometry of the machine—an invaluable source of information for future maintenance, modifications or revision.


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1-3 mm
assembly tolerances
Tokamak Complex
fiducial nests
0.2-0.4 mm
network uncertainty range
pillars on site
The ITER metrology team has developed a vast matrix of reference "targets," or fixed points, on the ITER site, inside the Tokamak Complex and Assembly Hall, and inside the Tokamak pit for use in measuring and aligning tokamak components and systems.

The primary survey network for the ITER site provides a global coordinate system for civil engineering works and the positioning of the buildings. It is based on 11 geodetic pillars that have been measured in relation to one another and to a wider GPS system, and which now provide a fixed reference base (or datum) from which more localized measurements can be made and calculated.



Inside of the Tokamak Complex a vast matrix of reference "targets," or fixed points, provides the physical manifestation of the global coordinate system. Metrologists will be able to access the overall coordinate system from any location by measuring a small number of local reference points. The accuracy of the network has been measured within a global uncertainty range of 0.2 to 0.4 mm, allowing installation contractors to align components to sub-millimetre accuracy.

Inside of the concrete bioshield, target nests covering the full height of the walls are used to derive the vertical datum axis for machine assembly. As the pit becomes more crowded and lower pit wall targets are progressively hidden from view, targets in the port cell galleries will maintain lines of sight and play an important role in measurement tasks.



As machine assembly progresses, the as-built geometry of core components serves as a reference for further alignment activities. The cryostat base will be aligned using the target nests of the Tokamak Pit Network; the toroidal field coils and cryostat lower cylinder will be aligned in relation to the as-built position of the cryostat base; and the coordinate frame of the 18 toroidal field coils will serve as reference for the final alignment of the vacuum vessel, the remaining magnet systems and the in-vessel components.

Metrology will continue to play an important role after the assembly of the core machine, as in-vessel components in large numbers are installed during Assembly Phase II. Manufactured in plants around the world and shipped to ITER, several thousand components must be customized during assembly to achieve the demanding tolerances allocated to in-vessel systems. As a single example, the assembly of the blanket modules inside the vessel requires a survey of approximately 4,400 components; each one needs to be individually customized to deliver the required alignment.