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.”

Packed for India, then China

In a nondescript warehouse some 30 kilometres from the ITER site, instrumentation components destined for the Tokamak’s magnet systems are being prepared for a long journey.

Carefully arranged in their cardboard boxes, dozens of components—cables, connectors, sensors, signal conditioners—are being taped, wrapped into thick heat/humidity insulation aluminium foil and placed into a robust wooden crate.

The crate is going to India, where an ITER Organization contractor will install about 20 different types of electronic components into three cubicles and make sure that everything is operational. Once completed and tested, the cubicles will be shipped to ITER China to be used for the tests of prototype current leads, which must be qualified before actual series production begins.

For the components shipped on this occasion, the Magnet Division has relied on the help of CODAC Division engineers who have prepared a cubicle including a sub-system responsible for the investment protection during the tests.

„This place acts like a buffer,” explains ITER Coil Instrumentation engineer Felix Rodriguez-Mateos. „This is where we store the instrumentation components that we have developed, or bought off the shelf when industry has developed a solution that we consider satisfactory. The components are verified and reconditioned before being sent to the Domestic Agency in charge of their qualification or integration into prototypes and mockups.”

Contrary to the large majority of ITER components that are procured and delivered to the ITER Organization „in-kind” by the Domestic Agencies, the totality of magnet instrumentation (for feeders, coils and structures) is provided by the ITER Organization by way of „fund procurements.”

The ITER Organization buys (or develops) the needed components, has them installed by a contractor or directly by the Domestic Agency concerned. It is then the Domestic Agencies’ responsibility to validate the assembly procedures in prototypes or mockups prior to entering actual production. (In a later phase, in a lab installed for ITER, the ITER Organization will test assembly procedures for the systems it is responsible for.)

The complex logistics involved in sending the component-packed crates around the world are handled by the DAHER Group as part of their framework contract with the ITER Organization. „We never send anything before every problem, customs-related or other, is solved,” says DAHER’s Ines Bollini, who is present every time a crate leaves the warehouse. „There can be no improvisation…”

Every other week or so, a crate leaves the warehouse for a foreign destination. Its content is as important for ITER success as the giant components being manufactured throughout the world.

Management Advisory Committee meets in Barcelona

For the second time in its history, the ITER Council Management Advisory Committee (MAC) convened for an extraordinary session in order to assess the status of the ITER project schedule and the implementation of corrective actions.

The meeting took place from 18-19 March at the headquarters of the European Domestic Agency in Barcelona in the attendance of high-level representatives of the ITER Organization and seven ITER Members.

Since the last special MAC meeting held in August 2012, the ITER Organization has worked closely with Domestic Agencies to complete the integration of Detailed Work Schedules (DWS)—detailed schedules that exist for every component or system. The IO and DAs completed the integration of the remaining DWS, namely Main Vacuum Vessel, IC Antenna, PF Coils and TF Structure, which will allow for monitoring of the schedule.

MAC requested that the Unique ITER Team continue to make significant efforts to take action focusing on super-critical milestones and to take all possible measures to keep to the Baseline schedule. The ITER Organization and Domestic Agencies are committed to doing their best to implement this request.

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.

PF Coils Building’s alarms now integrated into CODAC

On 2 October, the European Domestic Agency Fusion for Energy (F4E) andthe ITER Organization have successfully concluded the first site acceptance test of the Control, Data, Access and Communication (CODAC) integration of the Poloidal Field (PF) coils building controller.

Due to the impressive size and weight of the PF coils, ranging from 10 to 24 metres and weighing up to 400 tonnes, a specific building was constructed to assemble them on the ITER site.

The F4E CODAC team and the Site, buildings and power supplies project team worked together to achieve this result in collaboration with OMEGA and INEO.

The main objective of this activity was to integrate the local PF coils building alarm monitoring system into the overall site alarm system which will be in place for the building construction activities over the next eight years.

The system handles more than 2,000 signals generated by the PF coils building subsystems responsible for heating, ventilation, air-conditioning, cooling water, heating water, electrical distribution, cranes and fire detection. Any alarm generated by those systems will be visible on any location through the CODAC network.

The excellent collaboration between the F4E and the ITER IO CODAC teams, along with the technical support received from ITER IO towards the development of the PF coils building interface, made this joint initiative a success.

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.

V-Day "kissing" on the winding line at ASIPP

The winding line for ITER’s correction coils  located at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) in Hefei, China has been busy these days with commissioning tests. Commissioning for this 44-metre-long, 15-metre-wide, 4-metre-high winding line began in July 2012.

Part of the commissioning process includes the winding of two 2×2 turn coils, one bottom-type correction coil and one side-type correction coil. On 23 August, the winding of the 2×2 turn bottom correction coil was completed and the coil was moved to the table for temporary storage.

The winding mould for ITER correction coils, assembled in three parts, was designed by ASIPP supplier JUNENG. The mould is aligned with structural adjustments built into the winding table that were made by ASIPP supplier KEYE Company.  The two side winding mould extensions are not needed to create the BCC coils.

In preparation for the next stage of commissioning—winding the larger side-type correction coil, the winding mould extensions were "kissed" together on 24 August, which is only one day later than the Chinese Valentine’s Day (7 July on the lunar calendar). Over the next few days the mould will be measured and any necessary adjustments made; it will then be ready for the  winding of a 2×2 turn side correction coil.

Both suppliers have been able to successfully coordinate with ASIPP and with one another, delivering quality work as well as expertise  to the winding line.

With the winding of the 2×2 turn bottom correction coil complete, ASIPP has achieved an important commissioning milestone. It hopes to complete the 2×2 turn side correction coil commissioning test in September, thereby laying a solid foundation for the winding qualification.

An eagerly anticipated dummy

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.