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Manufacturing milestone achieved in Europe

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The first step in the fabrication of the full-size, superconducting prototype of a toroidal field coil double pancake has been successfully carried out in Europe. Winding was completed at the beginning of August at the ASG premises in La Spezia, Italy.

The European Domestic Agency, Fusion for Energy, is responsible for procuring ten toroidal field coils (and Japan, nine). These D-shaped coils will be operated with an electrical current of 68,000 amps in order to produce the magnetic field that confines and holds the plasma in place. Toroidal field coils will weigh approximately 300 tons, and measure 16.5 m in height and 9.5 m in width.

Each one of ITER’s toroidal field coils will contain seven double pancakes. These double pancakes are composed of a length of superconductor, which carries the electrical current, and a stainless steel D-shaped plate called a radial plate, which holds and mechanically supports the conductor through groves machined on both sides along a spiral trajectory.

The first stage of toroidal field coil manufacturing—the winding of the double pancakes—is the most challenging. It consists of bending the conductor length along a D-shaped double spiral trajectory. As the conductor must fit precisely inside the radial plate groove, it is vital to control its trajectory in the double pancake and in the groove of the radial plate with extremely high accuracy. The trajectory of the conductor, in particular, must be controlled with an accuracy as high as 0.01 percent.

For this reason, the winding line employs a numerically controlled bending unit as well as laser-based technology to measure the position and the dimensions of the conductor. The winding takes place in an environment with a controlled temperature of 20 °C +/-1 C, at an average speed of 5 m of conductor per hour.

For the European commitments to ITER, a consortium made up of ASG (Italy), Iberdrola (Spain) and Elytt (Spain) will manufacture the full-size, superconducting prototype as well as the production toroidal field coil double pancakes in the future.

The next steps in the manufacturing process are: heat-treatment of the double pancakes at 650 °C in a specially constructed inert atmosphere oven, electrical insulation; and finally the transfer of the double pancakes into the grooves of the stainless steel radial plates. After assembly and the application of electrical insulation on the outside of the radial plate, the module is finally impregnated with special radiation-resistant epoxy resin to form the prototype double pancake module.

Work on the module is scheduled to be completed by the beginning of next year, in time to allow for the prototype to be tested at -77 K in order to assess the effect of the low temperature. The module will then be cut in sections in order to analyze the impregnation of the insulation.

Read the detailed article on the F4E website here.

2nd batch of Russian TF conductors en route to Italy

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The superconductors for the ITER magnet system are among the longest-lead production items for the project; the first five Procurement Arrangements concluded by the ITER Organization between late 2007 and mid-2008 concerned the conductors for the toroidal field magnet system.

The Russian Domestic Agency is responsible for 20 percent of toroidal field conductor procurement and 14 percent of poloidal field conductor procurement. Production is ongoing according to the schedule of the Procurement Arrangements.

On 25 June, the second batch of toroidal field conductor unit lengths started on their way from the premises of the Kurchatov Institute in Moscow to the city of La Spezia, Italy, where the winding of ten toroidal field coils will take place.

Demonstrating the attachment of Russian industry to fulfill its contractual obligations on time, two 415-metre production lengths of niobium-tin (Nb3Sn) conductor for toroidal field side double-pancakes were loaded onto trucks at the Institute. This latest shipment follows the delivery of four conductor unit lengths to Europe in October 2012, including a copper dummy and a 100-metre qualification length.

Seven similar units lengths have passed all of the tests stipulated in the Procurement Arrangement and meet ITER Organization requirements; they will, in turn, be shipped as well.

12 minutes to understand TF coil manufacturing

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The magnets responsible for confining the ITER plasma—the eighteen D-shaped toroidal field coils—will form an impressive superstructure within the ITER machine: at approximately 6,000 tons (coils plus cases), they will represent over one-fourth of the Tokamak’s total weight.

In two new videos produced by the European Domestic Agency, we are taken inside a vast manufacturing facility in La Spezia, Italy, where preparations are under way for the fabrication of ten toroidal field coils (nine plus one spare) that are part of the European contribution to ITER.

From winding through heat treatment and on to insertion into radial plates, the toroidal field coil manufacturing process is complex and exacting, requiring unprecedented levels of tolerances and performances. In the videos, experts from the ASG consortium* and Europe speak of the technical challenges, the specialized tooling, and the qualification work underway.

You can see the two 6-minute videos on F4E’s website.

*ASG consortium: Iberdrola Ingeniería y Construcción SAU, ASG Superconductors SpA and Elytt Energy SL

Winding can begin in Italy

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The European winding line for toroidal field coils in La Spezia, Italy is now ready. This impressive line—40 metres long, 20 metres wide, 5 metres high—has made it possible to carry out winding trials that have never been done before on a line of this scale and with such precision: recently, the first full-size double pancake turn was successfully completed with the large dummy conductor that had been delivered in May.

The toroidal field winding facility is located on the premises of ASG, supplier to the European Domestic Agency and part of a European consortium that includes Iberdrola and Elytt.

The winding line in La Spezia will have the task of winding niobium-tin superconducting cables into the characteristic shape of ITER’s toroidal field coils—a D-shaped double spiral called a double pancake. The spooled cable will be delivered in a single 760-metre length weighing seven tons.

The first task of the winding line will be to unspool and straighten the cable, after which the cable will be cleaned and sandblasted. The continuous, 760-metre length will then shaped into the 12m x 9m double pancake and heat treated at over 650°C in a specially constructed inert atmosphere oven. Finally, following electrical insulation, the double pancake will be transferred into the grooves of the stainless steel radial plates to form a double pancake module.

So that the double pancake fits precisely into the radial plate grooves, it is vital to control the accuracy of the conductor’s trajectory in the double pancakes. The winding line is thus required to achieve precision in bending the conductor on the order of a few tens of parts per million—a very demanding target considering its large dimensions. Successful results of the first trial winding of a full-size turn demonstrated that the winding line is, indeed, capable of achieving the required precisions.

After insertion into the radial plates, each double pancake module will be impregnated with epoxy resin, stacked in groups of seven, and jointed electrically to form „winding packs.” These winding packs will be inserted into stainless steel cases that, in turn, are welded together to form the completed toroidal field coil.

For the moment, the winding line will continue to undergo testing. In total, 70 superconductor lengths are needed to produce the European contribution to ITER’s toroidal field magnet system (ten toroidal field coils); Japan will contribute the other nine toroidal field coils.

Europe’s superconductor lengths will be produced by five different suppliers. Each specific supplier’s conductor will have slightly different mechanical behaviour; therefore, testing will be carried out in the winding line during the next few months on prototypes from each supplier before the start of real production. Final qualification, to take place in the autumn, will consist of winding a real (not a dummy) superconducting cable into a full-size double pancake prototype.

Another large technical area is also in its final installation phase in the ASG premises: a large inert atmosphere oven measuring 48 x 20 x 5 metres that will be used to carry out the heat treatment of the double pancakes. The oven has been dimensioned to heat treat up to three double pancakes at a time. After the successful completion of leak testing—carried out to verify the capability of the furnace to keep the concentration of impurities during the heat treatment below the required threshold of tens of parts per million—the oven is now in the final installation phase. Workers are completing the assembly of external components (electrical connections, sensors, piping, fans and vacuum pumps) and the final testing should start at the end of July.

With the completion of the winding line and the oven, Europe can report that the principal and most complex elements for the production of the toroidal field coils are now in place.

An eagerly anticipated dummy

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After driving through the night, the oversize truck pulls up in the early May dawn at the ASG facilities in La Spezia, Italy. The special delivery, a wooden square box with 5-metre dimensions, contains a large spool around which the eagerly anticipated dummy of a 760 m long copper conductor is wound.

The dummy is a mockup of the ITER conductors. These conductors will each be used in the toroidal field coils to carry 68,000 amps of electrical current in order to produce the magnetic field which confines and holds the plasma in place. In total, 19 superconducting conductor lengths (each measuring 760 m) and 8 conductors (each measuring 415 m) will be produced.

Although the final components will consist of superconducting materials, the dummy is made only of copper strands which have been plaited together (cabled) and inserted into a jacket in order to form a round conductor with a diameter of 44 mm. Nonetheless, the dummy package weighs an impressive 13 tons. Because of its large dimensions, it is only transportable during certain hours of the night after other traffic has been cleared.

The dummy was manufactured for the European Domestic Agency F4E by ICAS, an Italian consortium consisting of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Criotec, and Tratos Cavi. The next steps of the process will be undertaken by ASG, part of the Iberdrola consortium (which includes Iberdrola and Elytt), F4E’s toroidal field coil supplier and the company to which the dummy was delivered. The copper dummy length will be used for the commissioning of the toroidal field coil winding line.

In recent months, two additional toroidal field lengths made from superconducting strand were manufactured, thus completing the qualification phase during which both tooling and manufacturing procedures are verified. These conductor lengths are expected to be shipped to La Spezia by the end of the summer.

On May 15, the fabrication of the first production toroidal field conductor length was completed at Criotec: this length is the first conductor which will be inserted into the ITER machine. In the coming two years, 26 additional toroidal field lengths will be fabricated and supplied by ICAS.

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