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.”

Korean contract advances neutral beam ports

The Korean Domestic Agency signed an important contract in July for the fabrication of neutral beam port in-wall shielding with Korean supplier Hyundai Heavy Industries Co., LTD (HHI). Through this contract, installation of the in-wall shielding into the port stub extensions will begin in mid-2015 with fabrication completed by early 2016. Hyundai Heavy Industries is also manufacturing two sectors of ITER vacuum vessel as contractor to the Korean Domestic Agency, as well as seventeen equatorial ports and the nine lower ports

The vacuum vessel’s neutral beam ports are composed of a connecting duct, port extension, and port stub extension. The spaces between the inner and outer shells of the port extension and port stub extension are filled with preassembled blocks called in-wall shielding. The main purpose of in-wall shielding is to provide neutron shielding for the superconducting magnets, the thermal shield and the cryostat.

In order to provide effective neutron shielding capability with the cooling water, 40-millimetre-thick flat plates (steel type 304B4) are used in almost all areas of the volume between port shells.

In-wall shielding is composed of shield plates, upper/lower brackets and bolt/nut/washers. Pre-assembled 368 in-wall shielding blocks will be assembled into the neutral beam port extension and port stub extension during port fabrication, while 160 field joint in-wall shielding blocks will be assembled after field joint welding on the ITER site. The total net weight of all neutral beam in-wall shielding approximates 100 tons.

Ki-jung Jung, Director-General of the Korean Domestic Agency, commented during the signature: „ITER Korea takes very seriously the demands of the vacuum vessel schedule and quality requirements by ITER.”

Green light for ITER’s blanket design



After three days and 29 presentations, a comprehensive design review with probably the largest participation in the history of the ITER project was completed last week. More than 80 experts from the ITER Organization, Domestic Agencies and industry attended the Final Design Review of the ITER blanket system.

„The development and validation of the final design of the blanket system is a major achievement on our way to deuterium-tritium operation—the main goal of the ITER project,” Blanket Integrated Product Team Leader (BIPT) and Section Leader Rene Raffray concluded at the end of the meeting, obviously relieved at the success of this tremendous endeavour. „We are looking at a first-of-a-kind fusion blanket which will operate in a first-of-a-kind fusion experimental reactor.”

The ITER blanket system provides the physical boundary for the plasma and contributes to the thermal and nuclear shielding of the vacuum vessel and the external machine components such as the superconducting magnets operating in the range of 4 Kelvin (-269°C). Directly facing the ultra-hot plasma and having to cope with large electromagnetic forces, while interacting with major systems and other components, the blanket is arguably the most critical and technically challenging component in ITER.

The blanket consists of 440 individual modules covering a surface of 600 m2, with more than 180 design variants depending on the segments' position inside the vacuum vessel and their functionality. Each module consists of a shield block and first wall, together measuring 1 x 1.5 metres and weighing up to 4.5 tons—dimensions  that not only demand sophisticated remote handling in view of maintenance requirements during deuterium-tritium operation, but also an approach to attaching the modules which is far from trivial when considering the enormous electromagnetic forces. 

The first wall is made out of shaped „fingers.” These fingers are individually attached to a poloidal beam, the structural backbone of each first wall panel through which the cooling water will be distributed. Depending on their position inside the vacuum vessel, these panels are subject to different heat fluxes. Two different kinds of panels have been developed: a normal heat flux panel designed for heat fluxes of up to 2 MW/m2 and an enhanced heat flux panel designed for heat fluxes of up to 4.7 MW/m2.

The enhanced heat flux panels are located in areas of the vacuum vessel with greater plasma-wall interaction and they make use of the hyper-vapotron technology which is similar to that used for the divertor dome elements. All panels are designed for up to 15,000 full power cycles and are planned to be replaced at least once during ITER’s lifetime. A sophisticated R&D program is currently under way in Japan for the development of remote handling tools to dismantle and precisely re-position the panels.  

Due to the high heat deposition expected during plasma operation—the blanket is designed to take a maximum thermal load of 736 MW—ITER will be the first fusion device with an actively cooled blanket. The cooling water is fed to and from the shield blocks through manifolds and branch pipes. Furthermore, the modules have to provide passage for the multiple plasma diagnostic technologies, for the viewing systems, and for the plasma heating systems.

Because of its low plasma-contamination properties, beryllium has been chosen as the element to cover the first wall. Other materials used for the blanket system are CuCrZr for the heat sink, ITER-grade steel 316L(N)-IG for the  steel structure, Inconel 718 for the bolts and cartridges, an aluminium-bronze alloy for the pads that will buffer the electromechanical loads acting on the segments, and alumina for the insulating layer. 

The procurement of the 440 shield blocks is equally shared between China and Korea. The first wall panels will be manufactured by Europe (50%), Russia (40%) and China (10%). Russia will, in addition, provide the flexible supports, the key pads and the electrical straps. The assembly of the blanket is scheduled for the second assembly phase of the ITER machine starting in May 2021 and lasting until August 2022. The work will be performed with the help of two in-vessel transporters working in parallel.

In assessing the work presented at the Final Design Review, Andre Grosman, deputy head of Magnetic Fusion Research Institute at CEA and chair of the review panel, enthusiastically commended the BIPT for its achievements since the Preliminary Design Review in December 2011 which were „beyond the expectation of the panel.” He added: „We have singled out the continuity and benefit of the work done by the ITER Organization and the Domestic Agencies within the BIPT framework with a sharing of risk and information among all stakeholders.”

The panel nevertheless pointed out some remaining issues, including a few challenging issues that need to be addressed at the project level. But thanks to the excellent quality of work performed by the BIPT, the ITER blanket design can today be called „approved.” The BIPT can now turn its focus to addressing the feedback received at the Final Design Review, applying the final touches to the design, and preparing for the Procurement Arrangements, where fabrication is handed over to the Domestic Agencies, starting at the end of 2013.

ITER is well underway


The Eleventh ITER Council convened last week at the ITER site for a two-day meeting that brought together the high-level representatives of the seven ITER Members.

As approximately 100 people took their places in the solemn setting of the new Council Room, Director-General Osamu Motojima welcomed the participants, adding, „I would like to take this opportunity to thank the Members, in particular Europe, the Host Party, and Agence ITER France for providing the project with the ITER Organization Headquarters building where staff is nearly fully installed.” 

The Council noted the strong measures that have been taken by the ITER Organization and the Domestic Agencies to realize strategic schedule milestones and to develop new corrective measures for critical systems such as buildings, the vacuum vessel, the cryostat, and the superconducting magnets. Delegates urged further corrective actions to improve schedule execution and to seek additional savings.

Delegates welcomed the integrated project management approach proposed by the ITER Organization to enhance collaboration between the ITER Organization and the Domestic Agencies, an approach, according to Director-General Motojima, to „cooperate even more closely for the implementation of ITER.”

The ITER Council also celebrated the recent major licensing milestone for ITER, the strong pace of construction activities at the ITER site, and the manufacturing activities well underway in all ITER Members.

The next ITER Council meeting is scheduled to take place in Japan in June 2013.

Click here to view the photo gallery of the Eleventh ITER Council
 
Read the Press Releases in English and in French.

"ITER is well underway"


The Eleventh ITER Council convened last week at the ITER site for a two-day meeting that brought together the high-level representatives of the seven ITER Members.

As approximately 100 people took their places in the solemn setting of the new Council Room, Director-General Osamu Motojima welcomed the participants, adding, „I would like to take this opportunity to thank the Members, in particular Europe, the Host Party, and Agence ITER France for providing the project with the ITER Organization Headquarters building where staff is nearly fully installed.” 

The Council noted the strong measures that have been taken by the ITER Organization and the Domestic Agencies to realize strategic schedule milestones and to develop new corrective measures for critical systems such as buildings, the vacuum vessel, the cryostat, and the superconducting magnets. Delegates urged further corrective actions to improve schedule execution and to seek additional savings.

Delegates welcomed the integrated project management approach proposed by the ITER Organization to enhance collaboration between the ITER Organization and the Domestic Agencies, an approach, according to Director-General Motojima, to „cooperate even more closely for the implementation of ITER.”

The ITER Council also celebrated the recent major licensing milestone for ITER, the strong pace of construction activities at the ITER site, and the manufacturing activities well underway in all ITER Members.

The next ITER Council meeting is scheduled to take place in Japan in June 2013.

Click here to view the photo gallery of the Eleventh ITER Council
 
Read the Press Releases in English and in French.

Tore Supra ready to go WEST

On the other side of the CEA fence, in Cadarache, sits a large tokamak which played an important role in the definition of ITER. Tore Supra, a CEA-Euratom device which began operating in 1988, was the first tokamak to successfully implement superconducting magnets and actively-cooled plasma-facing components.

Over the past twenty-four years, Tore Supra has explored the physics of long-duration plasma pulses, reaching a record of 6.5 minutes in December 2003.

In 2000-2002, Tore Supra was equipped with a new carbon-carbon fibre (CFC) „limiter” — the equivalent of the divertor in ITER — capable of withstanding an ITER-relevant heat load of 10 MW per square metre.

This project, named CIEL for Composants Internes Et Limiteurs, demonstrated that, while CFC performs very well in terms of power handling and compatibility with the plasma, its use results in substantial erosion caused by the physico-chemical reactions between the carbon of the limiter and the hydrogen (deuterium) in the plasma. Further experiments in JET have confirmed these observations.

Now, there are not many options when it comes to choosing the material of a divertor. Fifty years of experience in tokamak technology have narrowed them to two: it’s either CFC or tungsten, their respective advantages or disadvantages depending on the plasma regimes they are exposed to. (More here).

In ITER, it was originally planned to begin operations with a CFC divertor and replace it with a tungsten one before the start of nuclear operation (deuterium + tritium) in 2026. After years of discussions, panels and reviews, a new plan was established and ITER is now considering doing without the first-phase CFC divertor.

Indeed, substantial cost reductions would be achieved by installing a tungsten divertor right from the start and operate it well into the nuclear phase. This solution would also provide for an early training, during the non-nuclear phase of ITER operation, on how to operate with a tungsten divertor.

The ITER Members, however, have not yet reached a unanimous position on this issue.

Whatever ITER decides eventually, the tungsten option must be explored and this is what Tore Supra’s WEST project (W Environment in Steady-state Tokamak, where „W” is the chemical symbol of tungsten) is about.

„ITER success is CEA’s top priority,” says Alain Bécoulet, the Head of CEA-IRFM (Institut de Recherche sur la Fusion Magnétique) which operates Tore Supra. „By installing an ITER-like full tungsten divertor in Tore Supra, we can turn our platform into a test-bench on ITER critical path. We can thus contribute to reducing the risk and to saving time and money for ITER. WEST is not something we would add to Tore Supra like we did with CIEL. It’s more like Tore Supra becomes WEST to serve ITER.”

The CIEL project provided IRFM with a strong experience in cooperating with the industry. Adapting Tore Supra to accommodate a full tungsten divertor — 500 components with a total of 15,000 tungsten tiles — is a challenge the Institute is ready to take on. (All carbon will have to be taken out of the device; in-vacuum vessel magnetic coils will need to be installed in order to modify the plasma shape from circular to „D-shaped” and heating systems will have to be adapted to the new configuration.)

The formal decision to go WEST is due to be taken by CEA at the end of 2012; Bécoulet is optimistic: partners are showing interest and „customers” other than ITER appear eager to utilize the future test bench as well. „All fusion machines, present and projected,” he says „are expected to go tungsten.”

Bringing a timely answer to ITER interrogations means that Tore Supra, which Bécoulet calls „a technological jewel”, should prepare to go WEST early in 2013 and be ready for the first experiments in 2015.

Click here to view an animation of the WEST project.


Reflecting on San Diego

A large gathering of fusion scientists such as the 24th IAEA Fusion Energy Conference held in San Diego on 8 — 13 October, offers a unique vantage point to assess the progress of fusion research worldwide.

The 400 papers and posters that were presented throughout the week — some 70 being ITER-related — clearly demonstrated that, in many critical areas, researchers are reaching a much better understanding of the phenomena that control plasma behaviour — and we all know that this fundamental knowledge is essential to the success of our project and, beyond, to the future of fusion energy.

One of the highlights of the meeting was the presentation of JET’s results: now equipped with an ITER-like wall, JET, together with ASDEX Upgrade, is answering critical questions on how tungsten would affect plasma performance in  ITER. The results on fuel retention on JET are encouraging and we are awaiting further results from JET on how to optimize plasma performance.

The synergy between exploratory experiments on ASDEX Upgrade and follow-up experiments on JET has been valuable.  This has implications for the role of satellite experiments in the member states when ITER comes into operation. 

The superconducting tokamak facilities, EAST, KSTAR and JT-60SA, clearly see how they could contribute to ITER. It was also pleasing to hear that the SST-1 tokamak at IPR has been reassembled and that commissioning of the superconducting magnets is underway.

We were all very interested in the new data on disruptions and runaway electrons that the teams working at JET, Alcator C-ModDIII-D and T-10, as well as other machines, have accumulated. Innovative techniques are being developed to minimize the impact of the runaway beams on the plasma-facing materials on DIII-D, which is good news for our community. The effectiveness of a second gas valve to reduce the asymmetry in the radiated power during a disruption was studied on C-Mod.

The disruption observations from JET indicate that there are significant differences between a carbon divertor and a tungsten divertor.  The plasma current decays more slowly and the vertical position also evolves more slowly. This is probably related to the decreased radiated power –  however, the duration of the halo currents is greater.

An intriguing result from analysis of NSTX disruptions is that 98% of disruptions can be flagged with at least 10 ms of warning with only 6% of false positives.

These results combined with other results on disruption mitigation give greater confidence that a sufficiently reliable disruption mitigation system can be developed for ITER, though further work is needed.

An increasing body of fundamental knowledge is also being accumulated in the field of ELM control, which will lead us to re-evaluate how to either mitigate the impact of, or preferably suppress, ELMs by means of the in-vessel coils. ASDEX Upgrade, DIII-D and MAST are strongly contributing to this exploration and understanding as well as theorists from around the world. In addition, interesting new data using other techniques, including pellet injection, to control ELMs were shown by DIII-D, EAST and KSTAR.

Increasingly detailed analysis of stability and confinement characteristics of H-mode plasmas are advancing in impressive fashion: the conference heard of the remarkable agreement between the predictions of the „EPED” code and measurements of limiting pedestal pressure in many tokamak experiments.

An interesting new analysis of the influence of isotopic mass on the confinement characteristics of H-mode plasmas was also reported from JT-60U, and striking results from nonlinear 3D modelling of individual ELMs with the JOREK code were also presented. An extensive review of recent data on the dependence of the H-mode power threshold carried out by the ITPA will also give us food for thought.

On the first day of the conference, ITER Director-General Osamu Motojima gave a status report on ITER in the first technical session of the meeting.  Later that day, there was a session devoted to ITER Physics, Scenarios and Heating and Current Drive Technology.  These two sessions provided an excellent introduction and overview about the status of the project and recent scientific and technical achievement.

At a town meeting that was held on the following night after a full day of meetings, both of us respectively presented the "The ITER Research Plan”,  and "Burning Plasma Research on ITER”” to a large audience.  There is clearly interest in what ITER will do during the operations phase.

David’s presentation related ITER’s plans to achieve burning plasma conditions to some of the recent work highlighted at the meeting. Based on successfully achieving burning plasma conditions; Rich’s presentation described how ITER would be able to advance our understanding of deuterium-tritium burning plasmas far beyond what we achieved on JET and TFTR during the 1990s.  The presentation described not only the progress that both JET and TFTR had made but also the outstanding scientific issues.

The results described above are not a comprehensive summary of the meeting but merely some casual observations of interesting results to stimulate people to read the conference proceedings and articles in Nuclear Fusion.

Better, maybe, than a dramatic announcement of a revolutionary breakthrough, the conference provided a clear and reassuring image of a community that is marshalling its facilities and intellectual resource to make fusion happen and in the process, addressing key issues of importance to ITER.

See more pictures here. 



Larsen & Toubro Ltd will manufacture ITER Cryostat

The ITER cryostat will be the world’s largest high-vacuum pressure chamber ever built. On 17 August, the contract for the manufacturing of the 3,800 ton steel-structure was signed with the Indian company Larsen & Toubro (L&T) Ltd.

The cryostat forms the vacuum-tight container surrounding the ITER vacuum vessel and the superconducting magnets – essentially acting as a very large refrigerator. It will be made of stainless steel with thicknesses ranging from 50 mm to 250 mm. The structure will have to withstand a vacuum pressure of 1 x 10 -4 Pa; the pump volume is designed for 8,500 m3. Its overall dimensions will be 29.4 meters in diameter and 29 meters in height. The heavy-weight will bring more than 3,800 tons onto the scale – making it the largest vacuum vessel ever built out of stainless steel.

The cryostat will have 23 penetrations allowing internal access for maintenance, as well as over 200 penetrations—some as large as four metres in size – providing access to the vacuum vessel for cooling systems, magnet feeders, auxiliary heating, diagnostics, and the removal of blanket and parts of the divertor. Large bellows are used between the cryostat and the vacuum vessel to allow for thermal contraction and expansion in the structures.

India, being one of the seven Members of the ITER project, is in charge of procuring the cryostat. On 17 August, Shishir Deshpande, Project Director ofITER-India and Anil Parab, Vice President of the L&T Heavy Engineering division, signed the contract for manufacturing of the ITER cryostat.

The design of the ITER cryostat represented a huge international endeavour involving engineers and technicians from both the ITER Organization and the Indian Domestic Agency. „The cryostat is an essential part of the ITER machine. Seeing this huge component taking shape in the factory is certainly important and encouraging news. It means that the ITER project has entered a decisive phase,” ITER Director-General Osamu Motojima said. 

The cryostat will be manufactured by the Heavy Engineering division of L&T at its Hazira plant, near Surat in Western India, in the state of Gujarat. It will be dispatched in 54 modules to the ITER site in Cadarache, as it cannot be transported in its entire size. Pre-assembly of the cryostat modules will be done in a temporary workshop at the ITER site and then transported to the tokamak pit where they will be welded together by using the advanced „narrow groove all position gas tungsten arc welding technique”.

Mr. M.V. Kotwal, Member of L&T board and President of L&T Heavy Engineering stated: "L&T is proud to be part of this mammoth global collaborative research to build a greener planet.”