Manufacturing milestone achieved in Europe

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.

3,000 sensors for detecting the quench

A robust detection system is under development to protect the ITER magnets in case of quenches—those events in a magnet’s lifetime when superconductivity is lost and the conductors return to a resistive state.

When cooled to the temperature of 4.5 Kelvin (around minus 269 degrees Celsius), ITER’s magnets will become powerful superconductors. The electrical current surging through a superconductor encounters no electrical resistance, allowing superconducting magnets to carry the high current and produce the strong magnetic fields that are essential for ITER experiments.

Superconductivity can be maintained as long as certain thresholds conditions are respected (cryogenic temperatures, current density, magnetic field). Outside of these boundary conditions a magnet will return to its normal resistive state and the high current will produce high heat and voltage. This transition from superconducting to resistive is referred to as a quench.

During a quench, temperature, voltage and mechanical stresses increase—not only on the coil itself, but also in the magnet feeders and the magnet structures. A quench that begins in one part of a superconducting coil can propagate, causing other areas to lose their superconductivity. As this phenomenon builds, it is essential to discharge the huge energy accumulated in the magnet to the exterior of the Tokamak Building.

_To_57_Tx_Magnet quenches aren’t expected often during the lifetime of ITER, but it is necessary to plan for them. „Quenches aren’t an accident, failure or defect—they are part of the life of a superconducting magnet and the latter must be designed to withstand them,” says Felix Rodriguez-Mateos, the quench detection responsible engineer in the Magnet Division. „It is our job to equip ITER with a detection system so that when a quench occurs we react rapidly to protect the integrity of the coils.”

„A quench is not an off-normal event,” confirms Neil Mitchell, head of the Magnet Division. „But we need a robust detection system to protect our magnets, avoid unnecessary machine downtime, and also as a safety function to discharge large stored energy and avoid damage to the first confinement barrier—the vacuum vessel.”

Quench management will be a two-fold strategy in ITER: first quench detection, then magnet energy extraction. The time between detection and action has to be short enough to limit the temperature increase in the coil and avoid any damage. „We have on the order of 2-3 seconds to detect a quench and act,” says Felix.

The primary detection system—called the investment protection quench detection system—will monitor the resistive voltage of the superconducting coils (there is also a secondary detection system, see box below). Why the voltage? „Whereas during superconducting operation the resistive voltage in a coil is practically zero, a quench would cause it to begin to climb,” explains Felix. „By comparing voltage drops at two symmetric windings for instance, the instruments will detect variations of only fractions of a volt.”

Above a threshold level, these variations trigger a signal that is sent to the central interlock control system. In order to avoid unnecessary machine downtime, specific signal processing is required within the quench detection system to discriminate the resistive voltage from the inductive one due to the variations of the magnetic field—that is, to distinguish „true” signals from „false.”

_To_58_Tx_”The Tokamak environment will be a very noisy one for our instruments—that’s one of the challenges of quench detection in ITER,” says Felix. „The difficulty will be to cull out false triggers while at the same time not allowing a real quench to go undetected,” says Felix. „We have tried to build enough redundancy into the system so as to minimize false signals. We don’t want to discharge the coils and lose machine availability if we don’t have to.”

If a quench is confirmed, the switches on large resistors connecting coil and resistors are thrown open and the magnetic energy of the coil is rapidly dissipated, avoiding any damage to the coils. For the toroidal field coils that have the largest amount of stored energy, 41 GJ, achieving total discharge can take about one a half minutes.

To detect the start of a quench in any part of the magnet system, voltage measuring instruments (over 3,000 sensors) will be integrated at regular distances onto ITER’s coils, feeding bus bars, and current leads. Following the Manufacturing Readiness Review for coil instrumentation last December, the Magnet Division is currently in the phase of preparing over 20 individual tenders (~EUR 25 million). The instruments imply a variety of components and technologies to compensate inductive signals. Much process and material development has gone into the design of these systems. 

In addition, an R&D collaboration has been underway at the superconducting Korean tokamak KSTAR since 2009 to learn more about compensating the electromagnetic fields. ITER is collaborating with the KSTAR magnet team to gather information on the electromagnetic signals picked up by the superconducting cables during plasma disruptions. This data will assist the ITER team in designing compensation systems to separate the electromagnetic noise of a disruption from a quench.
„Quench detection in ITER is the most challenging around,” concludes Felix, who has approximately 25 years of experience in the field. „At the Large Hadron Collider (LHC), for instance, we were working with faster detection times. But in ITER, there will be a tremendous amount of interference for the instruments to sort through—electromagnetic noise, swinging voltages, couplings, perturbations. At ITER, we are also dealing with higher current, bigger common mode voltages, and larger stored energy. We’ll be pushing quench detection and protection to the limit of technology today.”

Toroidal field coils: strand production passes 400 tons

„Toroidal field strand procurement is going rather well,” reports Arnaud Devred, who heads the Superconductor Systems & Auxiliaries Section at ITER. „We are on schedule.”

Manufactured by suppliers in six ITER Domestic Agencies—China, Europe, Japan, Korea, Russia and the USA—production of niobium-tin (Nb3Sn) superconducting strand for ITER’s toroidal field coils began in 2009 and has now topped 400 tons.

That’s more than 80,000 kilometres of strand—enough to go around the world twice at the Equator.

Worldwide capacity has had to ramp up significantly to meet the Project’s demand. There are eight qualified suppliers for ITER, including three that are new to the market (one in China, one in Korea and one in Russia). In 2011 and 2012, these eight suppliers, together, turned out over 100 tons annually.

„One hundred tons per annum represents a spectacular increase in the worldwide production of this multifilament wire which was estimated, before ITER production, at a maximum of 15 tons per year,” says Devred. „As you would expect, the price has come down, and this 'surge’ in production for ITER may well open up new markets.”

Eighteen toroidal field coils will be produced for ITER plus a nineteenth (a spare). That’s approximately 420 tons of strand, give or take a bit of spare material planned by each Domestic Agency. The production curve will begin to flatten in 2013 (see graph above) as contracts are brought to a close in several Domestic Agencies.

Devred estimates the market value of the toroidal field strand procurement at over EUR 200 million.

„It has been very satisfying to see this procurement unfold and to watch our international collaboration develop at every step in the process,” says Devred. „In addition to the sheer scale of this procurement, what is also remarkable is the quality control and quality assurance that we have been able to set into place.”

Four of the ITER suppliers are using a production technique called internal tin, while another four are using a bronze process. „It has been up to us to demonstrate that we can control both types of production within technical requirements,” explains Devred, „We weren’t sure of ourselves since this is the first time there has been such a large-scale production of internal tin. Test data shows that we can do it effectively.”  

Quality testing for ITER calls for statistical process control on critical parameters, systematic low-temperature measurements on strands, and regular low-temperature measurements on full-size conductors (25 percent of toroidal field conductor unit lengths are tested). This testing data is stored, like manufacturing data, in ITER’s conductor database, which is currently fed by approximately 150 users, including suppliers and Domestic Agencies. Some 350,000 individual objects are stored in this web database—created to monitor the quality assurance/quality control processes of the conductor Procurement Arrangements.

Devred credits the „early days” with setting up the processes and systems that are proving to work today for conductor procurement: before the signature of the first ITER Procurement Arrangement, the specifications for ITER conductors were written by a committee made up of worldwide experts in large conductor procurement. Very tight quality control was developed that imposes many control points at each stage of fabrication verified by the Domestic Agencies and the ITER Organization. „I believe this will be the key to our final success," says Devred. "I am confident that what is coming off of the manufacturing lines is as good as can be made.”

Read more on how strand is produced in Newsline 140.

China delivers first load to ITER

It’s a long road from the Institute of Plasma Physics (ASIPP) in Hefei, China to the ITER site in southern France: 500 kilometres by route to the port of Shanghai; some 10,000 nautical miles from Shanghai to the port of Marseille-Fos; another hundred kilometres for the last leg of the journey, from Fos to the ITER site.

The distance was covered by trucks and the container ship Lyra in 38 days. Three crates that had been loaded at Hefei on 25 April were delivered to the Poloidal Field Coils Winding Building on Monday 2 June, three days ahead of schedule.

The crates contained the first batch of ITER items delivered by ITER China to the European Domestic Agency Fusion for Energy (F4E): 737 metres of dummy conductor, in three lengths, to be tested in a mockup of poloidal field coil number five (PF5). The 25-ton load was also the first ITER item to enter the large on-site winding facility.

„The conductor will be used to test the whole fabrication process,” explained Neil Mitchell, head of the ITER Magnets Division, as the crates were being unloaded and inspected. „This copper conductor will be wound, tested for tolerance, insulated, impregnated under vacuum and formed into a 'double pancake’ in the same way the actual superconducting niobium-titanium conductor will be handled by F4E at a later stage. What matters here are the mechanical properties, which are similar in both the copper dummy and the actual superconductor.”

An ITER load, even when it’s only a dummy component destined for mockup testing, is not an ordinary load. The transport crates were equipped with several monitoring devices to record movement and accelerations throughout the journey; other systems monitored the pressure inside the conductors, which are filled with pressurized inert gas, to confirm that there was no contamination by damp or salt.

Verifying that the crates (and hence the conductors) had not suffered during the 38-day voyage was the first thing that Piergiorgio Aprili, from F4E, and Chen Huan and Sun Yana, representing the Chinese transporter Sinatrans, did upon the delivery of the load. The integrity of the accelerometers was verified—proof that no major shock had occurred—and the crates’ „Black Box” was recovered for later analysis.

Compared with the oversized components that will soon be delivered to ITER this week’s 25-ton load appeared quite modest. However, for all those involved—ITER China, Sinatrans, F4E, the ITER Organization and the logistics service provider Daher—this was an important moment. „We have already managed several shipments of small components to and from the ITER Domestic Agencies,” explained Daher’s Laurence Prudhomme who oversaw the unloading operations, „but this is the first real heavy load we’re dealing with.”

Beginning early next year, the loads will change in scale: several hundred tons, by then, will be routine.

View an example of accelerometre data collected during the transportation of TF Coil conductor from Hefei to the Japanese port of Kobe in November 2012.

First hardware afloat from China

On Thursday 25 April, the morning silence at the Institute of Plasma Physics (ASIPP) in Hefei, China, was broken by the noise of a high powered trailer. Inside the superconductor shop of ASIPP, workers were busy preparing to load the 737 metres of dummy conductor for ITER’s Poloidal Field Coil number five (PF5)—this represents the first delivery from China to the ITER construction site in France.
According to the Procurement Arrangement signed between the Chinese Domestic Agency and the ITER Organization, China will fabricate 64 conductors for ITER’s poloidal field coils, including four dummy conductors for cabling and coil manufacturing process qualification. ASIPP is responsible for all the poloidal field conductor fabrication in China. The fabrication of the PF5 dummy was completed in by ASIPP in 2011.
„This is the very first batch of ITER items to be shipped from China to the ITER site in Cadarache," said Luo Delong, Deputy Director-General of ITER China. Before, conductors for the toroidal field coils had been shipped to Japan and Europe. "This milestone is a further step for the ITER project. According to our schedule, we will now start massive production of conductors this year. Our goal is that all procurement items from China be supplied consistent with the ITER schedule and with ITER quality requirements.”

According to the shipment schedule the PF5 dummy conductors, which left Shanghai on 30 April, will arrive at the ITER site on 5 June.

Russian TF conductor successfully tested in SULTAN

Having recently celebrated its fifth anniversary, the ITER Project has moved steadily from negotiations to real manufacturing, and from dummy testing to production of the tokamak’s construction elements.

One of the first systems to be manufactured in line with the ITER Organization (IO) Integrated Schedule Plan is the superconductor for the ITER magnet system. Russia has demonstrated high stability and reliability during this process, fulfilling all its obligations in time. This has not only been acknowledged by the IO experts, but also by the international superconductor community.

The Russian Toroidal Field (TF) conductor with bronze route strands  was tested in the SULTAN facility by Centre de Recherches en Physique des Plasmas- Ecole Polytechnique Fédérale de Lausanne (CRPP-EPFL) in late September — early October 2012. This is the fourth Russian sample to be tested in SULTAN but the first sample containing two sections of conductor made of real production length which will be used to manufacture real TF coils for the machine. The left section of the conductor was cut from side Double Pancake  pre-production conductor (Phase III) while the right section was made from first production (Phase IV) regular Double Pancake.

The results obtained with the the TFRF4 (Toroidal Field Russian Federation # 4) sample show very good agreement with results of the two last samples TFRF2 and TFRF3, which demonstrated the relatively good stability of the conductor during electromagnetic cycling, as well as its good durability during the warm-up/cool-down procedure.

Testing the TFRF4 sample was a very important milestone which completed the pre-production phase of the TF conductor procurement process. This means we can now proceed to the final production stage. At the same time, it opens the way to start shipping the real conductors to the coil manufacturer so they can be used to make coils for the ITER tokamak.

ITER "conductor community" meets in Moscow

The traditional International Conductor meeting was held in Moscow on 10-13 September, 2012. The regular meeting was attended by representatives from the ITER Organization, experts from the ITER Domestic Agencies of Europe, China, Japan, Republic of Korea, Russia and USA, as well as specialists from the DAs’ suppliers.

Such meetings are particularly important since the ITER magnetic system, with conductors forming its core, is one of the ITER tokamak’s key elements. The manufactured conductors, which are designed to withstand super high current in continuous mode, have to meet the IO’s strict requirements.

At the moment, 10 out of the 11 conductor Procurement Agreements, are either well into the production phase or are completing the qualification/pre-production phase. This is particularly true for the Toroidal Field conductors, where 75 percent of the required Nb3Sn strands and one third of the cable-in-conduit conductor unit lengths have been completed. Also, a technical solution has been found for the Central Solenoid conductors that are being implemented by the ITER Japanese partner.

„This is a clear indication  that the ITER project is moving ahead and is able to keep schedule”, says the meeting’s Chair Arnaud Devred, ITER Superconductor Systems and Auxiliaries Section Leader.

In Devred’s opinion, „in spite of the difficulties of coordinating work with about 30 suppliers and six DAs around the world, the ITER conductor community has always tried to work in a cooperative and synergetic manner, and the conductor meetings have always been a great opportunity for sharing experience and tackling difficult interface issues.

The conductor meeting is also an opportunity to showcase the work done in the Russian Federation and for the DAs involved in coils procurement to visit the conductor production facility”. Russia is responsible for the procurement of 22 kilometres of conductors, destined for Toroidal field (TF) coils, and 11 kilometres destined for the Poloidal field (PF) coils of the ITER magnet system. TF coils include more than 90 tons of superconducting Nb3Sn strands; PF coils include 40 tons of Nb-Ti strands.

Arnaud Devred highly praised the progress achieved by the Russian suppliers saying that „The Russian Domestic Agency has now entered full TF conductor and PF cable production. It is a proactive partner, eager to play collectively and to assume its role within the ITER collaboration”.

The next regular meeting is planned for March 2013 in Cadarache.

Radial Plate prototype takes to the sea

The Russian captain has answered the question more than a thousand times but he obviously likes to tell the story: the Echion, his 3,000-ton cargo ship, owes her name to one of the Argonauts, the ancient Greek heroes who accompanied Jason on his quest for the Golden Fleece.

The Echion docked last Saturday at the Mediterranean port of La Seyne-sur-Mer, 65 kilometres east of Marseille. The load she was to take delivery of sat just on the other side of the road — a 21st century highly sophisticated piece of equipment in a 19th century steel and red brick hall that Gustave Eiffel (of Eiffel Tower fame) designed a century and a half ago.

The component was produced by Constructions Industrielles de la Méditerranée (CNIM), under a contract awarded by F4E, the European Domestic Agency for ITER, three years ago. Another European company, the Italian SIMIC S.P.A, also produced a Radial Plate Prototype using different technologies.

This approach enables the Domestic Agencies in charge of procuring the actual ITER components to select the best solution before the industry launches into series production — the 19-Coil (plus spare) TF system in ITER will require the manufacturing of 134 Radial Plates, 70 to be procured by Europe, 64 by Japan.

Radial Plates are D-shaped stainless steel structures with grooves machined on both sides, into which insulated superconductor cables are inserted at a later stage. They are 112 mm thick, weigh between 5.5 and 9.7 tons and measure 8.7 by 13.8 metres.

Each of the ITER TF Coils contains seven radial plates, five „regular” and two „side” plates arranged in „double pancakes”.

Once the Echion reaches Italy’s Ligurian sea, the radial plate prototype will be delivered to the ASG Superconductor plant at La Spezia where the 450 metres of conductor will have to be shaped according to the groove trajectory, then heat treated, electrically insulated and finally inserted into the grooves.

Loading operations at La Seyne began on Monday 3 September in the middle of the night. While traffic was closed off on the narrow coastal road that runs between CNIM and the port installations, the transportation frame containing the radial plate was delicately lifted and transported to a waiting area close to the Echion.

Early in the morning, operations resumed. Two cranes hoisted the 35-ton load onto the ship and carefully deposited it at the bottom of the cargo hold. Steel clamps were then welded directly onto the hold’s floor so that the load would be perfectly immobilized for the duration of the voyage to La Spezia.

By noon, the Echion was ready to sail. The Russian captain and his seven-man crew were optimistic: the weather report anticipated calm seas and clear sky for the  24 ahead — the time it would take to reach destination.

Click here to view a video of the operations.