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Toroidal field coils

First ITER magnet arrives this year

7 Jan 2019 - ITER Japan

A major milepost is projected for 2019 as the first of ITER's powerful, high-field magnets is scheduled to arrive from Japan. Let's take a look behind the scenes at the last-stage fabrication activities that are mobilizing the expertise and skill of heavy industry specialists under the responsibility of Japanese QST, the National Institutes for Quantum and Radiological Science and Technology.

QST_firstTF_coldtest_396.JPG
The first Japanese winding pack was cold tested in October in this purpose-built cryogenic chamber at Mitsubishi. In a final step before shipment to ITER, the winding pack will be inserted in its structural case.

Eleven years after completing the signatures on documents specifying technical and quality control requirements for the supply of nine toroidal field coils, the Japanese Domestic Agency is overseeing the last, spectacular sequences on its first production unit.

The toroidal field coils are the ITER magnets responsible for confining the plasma inside the vacuum vessel using high-performance, internally cooled superconductors called CICC (cable-in-conduit) conductors. Following the completion of the single largest superconductor procurement in industrial history, fabrication of the final coils is proceeding in Japan (9 toroidal field coils plus 10 coil structures to be sent to Europe) and Europe (10 toroidal field coils). Each coil is made up of a superconducting winding pack and surrounding stainless steel coil case.
 
The list of applicable superlatives is long—the toroidal field coils are the largest and most powerful superconductive magnets ever designed, with a stored magnetic energy of 41 GJ and a nominal peak field of 11.8 T. Together they weigh in at over 6,000 tonnes including superstructure, representing 60 percent of the magnetic array on the machine and over one-fourth of the Tokamak's total weight. They require 4.57 km of conductor per coil wound into 134 turns in the central core, or winding pack, of the magnet. And they have required the longest procurement lead-time of any ITER component, with six out of seven ITER Members involved in the production of 500 tonnes of niobium-tin superconducting strand (100,000 km) required for the toroidal field superconducting cables.
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In the insert, the different elements of the toroidal field coils are shown: the superconducting winding pack (in green), and the inner (BP, AP) and outer (BU, AU) coil case sub-assemblies. QST—with the cooperation of Mitsubishi Heavy Industries, Ltd., Mitsubishi Electric Corporation, Hyundai Heavy Industries, and Toshiba Energy Systems & Solutions Corporation—will supply 9 toroidal field coils (assembly of winding packs and coil structures) plus another 10 coil structures to the ITER Project.

The first winding pack to come off the assembly line in Japan is currently undergoing final inspection by the industrial consortium Mitsubishi Heavy Industries/Mitsubishi Electric Corporation. The final sequence of testing involved high voltage tests, helium leak tests, and finally cryogenic tests, during which the winding pack is inserted into a cryostat (see top photo) and cooled to 80 K (-193 ˚C) to confirm leak tightness. With the successful end of cold testing, the winding pack is now undergoing post-cold-test helium leak tests and high voltage tests and will soon be ready for assembly with its toroidal field coil case. Five other winding packs are in various stages of production.

The 200-tonne case assemblies are also in series production. After successful fitting tests early last year, two have been delivered to Europe for insertion activities and a third will arrive this month; another completed production unit will remain at Mitsubishi for the assembly of the Japanese coil that is due at ITER in 2019. The fitting tests are the most delicate stage in the coil case manufacturing process, demonstrating that sub-assemblies manufactured and welded at different factory sites can be successfully paired with gap tolerances as strict as 0.25 to 0.75 mm along 15-metre weld grooves.

Please see the gallery below for a full update on manufacturing progress.

Testing the fit
Fitting tests are performed for AU/BU case structures at Hyundai Heavy Industries (HHI) in Ulsan, Korea. The inboard AU sub-assembly was manufactured by Mitsubishi Heavy Industries (MHI) and shipped to HHI to be matched with the BU sub-assembly there. The complete structure weighs approximately 200 tonnes.
Tolerances of less than 1 mm
Following the insertion of the winding pack into the coil case, the case sub-assemblies will be closure welded. Case manufacturers are responsible for preparing the weld bevels between each sub-assembly to very precise tolerances (less than 1 mm); during fitting tests, these bevels are carefully verified. Tolerances of less than 1 mm along huge 16-metre components with welding lines of more than 15 metres are extremely demanding.
Two halves, two suppliers
"Imagine trying to put a jetliner into narrow hanger that is only 1 mm wider (0.5 mm on both sides) than the plane," says Kenji Gomikawa, an ITER structural engineer who was on hand to witness recent fitting tests in Japan. "That is the type of challenge we are facing as we move huge 100-tonne subassemblies to fit together precisely. An added challenge is that the two halves of the coils cases are manufactured by different suppliers at different sites (AU: Mitsubishi Heavy Industries, BU: Hyundai Heavy Industries and Toshiba Energy Systems & Solutions Corporation). But we did it!"
Rehearsing insertion
At Mitsubishi, the structural case destined for the first winding pack was fit-tested vertically because—unlike their European counterparts (see https://www.iter.org/newsline/-/3201)—Japanese contractors will be performing the insertion of the winding pack into the coil case with the inboard segment "AU" below and the outboard segment "BU" positioned overhead. Tolerances of less than 1 mm were achieved. (August 2018)
Cover plate lines up
At Hyundai Heavy Industries (HHI) in Ulsan, Korea, BU/BP outboard structures are matched during fitting tests. HHI is contractor to the Japanese Domestic Agency for nine sets of outboard structures.
Within target
Fitting test for AU/BU structures at Toshiba Energy Systems & Solutions Corporation in Yokohama, Japan. After more precise positioning, the gap of 10 mm in this picture narrowed to meet the target of 0.5 mm.
Not even discernible
At recent tests at Toshiba, the gap between the AU/BU sub-assemblies could not even be felt. Each half of the coil case measures 16 metres in length and weighs approximately 100 tonnes.
Preparing for the last act
Technicians at Toshiba inspect the gap on the closure weld bevel after the curved cover plate (BP) was fitted on the BU sub-assembly. Once the winding pack is inserted, the cover plate will be positioned and welded.
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