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Measurements for ITER's primary survey network have an accuracy of 1 mm. David Wilson adjusts equipment on one of the eleven survey monuments positioned around the ITER platform.
David Wilson is a man with a challenging job. During the assembly of the ITER device, he'll be responsible for ensuring that the alignment of the principal components and ancillary systems—both in relation to one another and to their global position within the Tokamak Building—is carried out precisely. How, exactly, is precisely defined in the context of a machine with components weighing hundreds of tonnes and standing twenty metres tall?

"For many of the machine's largest components, we're looking at assembly tolerances in the order of 1 to 3 millimetres," says David, who is the alignment and metrology lead engineer for ITER. "The size and weight of the components and the fact that we're aligning the components along both a horizontal and a vertical axis makes aligning and assembling the ITER machine very demanding."

Advances in optical metrology techniques (from the Greek metron for measurement) over the last two decades have created sophisticated tools that will play an important role in the success of assembly operations for ITER. From laser trackers that deliver real-time measurements within 0.01mm/m accuracy; to photogrammetry cameras capable of processing thousands of images and recreating the as-built, 3D position of components; and laser scanners that create digital representations of component surfaces ... extensive use of all these metrology instruments will be required to successfully manage ITER assembly.

David's first measurement task for ITER was the creation of a fixed reference base from which to gauge all future measurements: in 2010, he oversaw the installation of eleven instrument stations on and around the ITER platform. These eleven geodetic pillars were measured in relation to one another and in relation to a wider GPS system, creating ITER's primary survey network. "This first network," David explains, "provided the coordinate system for civil engineering works and the positioning of the buildings."

A similar reference, or datum, network will be installed within the machine pit and adjacent galleries once the Tokamak Building is completed. Exploiting the most reliable (i.e., unmovable) surface available—the concrete bioshield—David and his future team will establish a matrix of reference "targets," or fixed points for use in measuring and aligning tokamak components. A survey of the key building characteristics (bioshield, port cells, penetrations ...) will determine the optimum position within the pit from which to commence assembly of the machine.

Dimensional control of the components for ITER begins at suppliers' factories, where measurements will be made during, and on completion of, the manufacturing processes. Metrology engineers will re-inspect the principal components once they arrive at ITER and continue to control them throughout the assembly process. "By recording the as-built geometry of the machine, we will have an actual record of ITER assembly, one that takes into account all of the variations with respect to design," says David. "This will be an invaluable source of data for future modifications or revisions of the machine."

Assembly operations will progress from bottom to top; during the first assembly tasks, the empty machine pit will provide a stable platform for metrology instruments and a clear area for alignment activities. "One of our major challenges, however, will be the progressive congestion inside of the Tokamak Building," stresses David. "Instrument positioning will become more challenging and tailored solutions will need to be provided. We need to ensure that lines of sight remain available throughout the assembly of the principle tokamak components ... even when there is no longer any room to swing a cat!"

A huge amount of measurement data will be compiled during assembly of the vacuum vessel, as the nine vessel sectors are aligned and welded first as pairs, then as triplets, then into one vessel. These measurements will ensure that the required tolerances are maintained, despite variation from design parameters that may occur as a result of sector manufacture, alignment precision, measurement uncertainty, or weld shrinkage. "We need to develop and qualify our measurement processes today, as ITER components are under construction all over the world," stresses David. Using software tools developed specifically for ITER, a survey simulation of 9,000 points based on CAD data (Computer Aided Design) was carried out to determine achievable accuracy.

At every stage of ITER assembly, as the "as-built" data accumulates, David and his team will adjust alignment criteria. Optimization algorithms will be used to achieve the best possible configuration for machine operation, especially in relation to the as-built magnetic axis of the machine and the alignment of components in relation to this axis.

"Achieving assembly tolerances in ITER will mean pushing the boundaries of survey accuracy while at the same time managing a huge amount of as-built data," says David. "One should never say never, but I think it safe to say that twenty years ago, ITER couldn't have been built to the tolerances that have been requested today."

Thanks to David Wilson for his contribution to this article.

For more information on metrology activities for ITER, visit the ITER website.


SFA showing its professional approach, even when welcoming a visitor.
When running an international enterprise like ITER, with colleagues all over the world, it is indispensable to have regular, personal interaction. ITER Director-General Osamu Motojima set off to Korea during the first week of October on this very mission: visit some of the manufacturing facilities where ITER components are taking shape and meet Myeun Kwon, former Director of the KSTAR Research Centre and newly appointed president of the Korean National Fusion Research Institute (NFRI), and Gyung-Su Lee, Chairman of the ITER Management Advisory Committee (MAC).

The first stop was SFA Engineering, an automation equipment company with vast experience in nuclear fusion. In March 2010, SFA signed a contract with the Korean Domestic Agency for the design of 18 custom assembly tools.
 
The following day, a visit to Hyundai Heavy Industries (HHI) and its sub-supplier Ilsung was on the agenda. Both companies are located in Ulsan, South Korea's seventh-largest metropolis. HHI, the largest ship-building company in the world, will manufacture sectors 1 and 6 of the nine-sector ITER vacuum vessel. As subcontractor to HHI (contract signed in August 2010) Ilsung will be responsible for manufacturing the equatorial and lower ports for these sectors.

In order to verify the design and manufacturing feasibility of the vacuum vessel sectors, HHI has fabricated three types of full scale mock-ups: The inboard segment mockup (VISM) for electron beam welding process optimization and the inner wall shielding assembly trial; the vacuum vessel upper segment mockup (VUSM) for process optimization, assembly feasibility and a welding distortion study between outer shell and flexible support housings; and the vacuum vessel lower segment triangular support mockup (VLTM) for a study of fabrication feasibility.

In addition to assembly tooling and the manufacture of two vacuum vessel sectors, Korea also has procurement responsibility for the AC/DC converters for the ITER switchyard. Director-General Motojima made a final stop to Dawonsys, under contract with the Korean Domestic Agency to perform prototype R&D and testing. Dawonsys provided the power systems for the superconducting magnets as well as the high-voltage power system for the KSTAR tokamak.

All assemblies—from the output of DC busbars, to the input of AC busbars, to a full-scale, six-pulse central solenoid converter unit—were engineered and integrated in the prototypes. Essential parameters such as temperature rise at rated current, good current sharing among paralleled thyristors, fuse coordination, and the soundness of the converter structure over the electromagnetic stress at fault condition were confirmed by the prototype converter.

The contract for AC/DC converters signed on 12 August between the Korean Domestic Agency and Dawonsys is expected to run for seven years, and covers the design, fabrication, site assembly and installation, and technical assistance during the integrated circuit test. The converters for ITER's toroidal field and central solenoid coils, vertical stability coils and correction coils (including master controllers, a dummy load, and spares) are deliverables on the contract.

"This was a very nice occasion to confirm the progress of real manufacturing and the large potential of the involved Korean institutions", DG Motojima summarized his impressions.

We'd like to thank Junho Ko from ITER Korea for his contribution to this article.

The four-hectare switchyard is composed of seven 400 kV bays equipped with double busbars that will supply the power to the steady state and pulsed power networks.
A short distance to the southwest of the construction projects on the ITER platform, the RTE (Réseau de Transport d'Electricité) switchyard, called Prionnet, is coming out of the ground at high speed. Here, electricity from the French high voltage lines will be converted to lower voltage and distributed to the ITER scientific facilities.

The four-hectare switchyard is composed of seven 400 kV bays equipped with double busbars that will supply the power to the steady state and pulsed power networks.

At the end of the month, a pylon weighing 120 tonnes and standing approximately 43 metres tall will be erected between the temporary ITER Headquarters building and the construction platform to carry the two high-voltage lines coming from the French "Tavel-Boutre" line.

Tools and heavy equipment are stored in a dedicated area in the RTE switchyard, including some 12 km of copper cable that will be buried in the soil to earth the power installations above and ensure personal safety.


Representatives from CN-DA, ASIPP, and the ITER Organization participating in the signing ceremony in Beijing this week for the ITER magnet feeders.
A contract award ceremony in Beijing this week marked the transition from design to manufacture for ITER's magnet feeders. ITER's 31 magnet feeders will convey and regulate the cryogenic liquids to the magnets for cooling purposes and also connect the magnets to their power supply.

On 16 October, contracts were signed between the Chinese Domestic Agency (CN-DA) and the Institute of Plasma Physics at the Chinese Academy of Science (ASIPP) for feeder process qualification and for the manufacture of four prototypes. Representatives from CN-DA, ASIPP, and the ITER Organization participated in the signing ceremony.

Luo Delong, head of the CN-DA, expressed his appreciation for the design, R&D, and quality system improvement efforts made by ASIPP in the last few years through financial support from the Chinese Ministry of Science and Technology; these efforts will significantly benefit the implementation of the contract. ASIPP will now ensure that the manufacture of the magnet feeders for ITER can be completed according to the strict quality and schedule requirements of the ITER Organization.

Director LI Jiangang, on behalf of the ASIPP, expressed his appreciation for the consistent support from the CN-DA and made a commitment that ASIPP will spare no effort to fulfill its contractual responsibilities through close cooperation with the CN-DA and the ITER Organization.