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ITER NEWSLINE 169
Superconductivity was first observed by Heike Kamerlingh Onnes and colleagues Cornelis Dorsman, Gerrit Jan Flim, and Gilles Holst on 8 April, 1911, at Leiden University in the Netherlands. The four scientists measured the sudden loss in resistance of mercury when the temperature was lowered below 3 K. Two years later, Onnes was awarded the Nobel Prize for "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium."
Superconductivity had not been predicted in advance by the physics community. Onnes expected to find resistance dropping to zero as the temperature decreased, but other physicists of the time disagreed. Thomson (Lord Kelvin) expected resistance to become infinite at 0 K. In 1912, Onnes demonstrated an application of superconductivity by creating a persistent current flowing around a ring.
The background to these discoveries reveals much about the constraints that have governed practical uses of superconductivity. In those early days, the problem was achieving low temperatures; the measurement of material properties at these temperatures followed on from this. There was intense competition between Dewar (at the Royal Institution in London) and Onnes and Olszewski (at the University of Cracow) to achieve the lowest temperature on Earth. Dewar led the race by liquifying hydrogen at 20 K in 1898, until Onnes liquified helium at 4.2 K in 1908. Advances in technology preceded the physics; Onnes was able to discover superconductivity through his cryogenic technology. At Leiden University, he had established a well-respected department, attracting good workers in the field and working closely with industry. His cryoplant was based on equipment manufactured by Linde.
It was not for another 50 years that applications in superconductivity began to be developed. In 1961, the first commercial NbTi superconductor was produced by Westinghouse (US). The arrival of the first practical superconductors coincided to within a decade with the first successes in the magnetic confinement of plasmas. It was quickly obvious that for fusion reactors, superconductivity was going to be indispensible.
One of the first superconducting plasma confinement devices was reported at the 1st Magnet Technology Conference in 1965 (image 1). The first superconducting tokamak, T-7, begin operating in 1979 at the Kurchatov Institute in Moscow, and is still operating as the HT-7 Tokamak at the ASIPP in China. Triam, at the Kyushu University in Japan, was the first tokamak to use Nb3Sn superconductors in the late 1980s. As usual, fusion reactor studies ran ahead of the technology.
By the mid-1970s, the 5th Magnet Technology Conference had a separate session for Fusion Magnets and the first design studies for superconducting fusion reactors. Preceding this, the 3rd MT conference in 1970 presented a proposal for a superconducting stellarator called W7X (the construction of the Wendelstein 7-X stellarator nears completion in Germany).
100 years after the discovery of superconductivity and 50 years after the first commercial applications, we have arrived at the construction of the ITER magnets, which will use 500 tonnes of Nb3Sn and 250 tonnes of NbTi, cooled with supercritical helium flowing at kilograms/second. The ITER magnets have dimensions about two orders of magnitude larger than the first superconducting device in 1965. ITER will also make use of the latest high temperature superconductors as part of the current leads that pass current to the coils.
The basic lessons for ITER from Kamerlingh Onnes are not just the discovery of superconductivity, but also the development and application of sophisticated technology in collaboration with industry, supported by an efficient working environment and the best workers in the field.
This week, representatives from the ITER Organization and the six Domestic Agencies engaged in the production of ITER's magnets gathered together at the ITER Headquarters in Cadarache to assess the status of the magnet conductor production.
The conductors are a core component of the magnets and one of the longest lead-time items for the ITER Project. As summarized by Arnaud Devred, ITER's Superconducting Systems Section Leader, the production of strands for both the toroidal and the poloidal field magnets made out of Nb3Sn and NbTi, respectively, is ongoing in all six Domestic Agencies. "As of today, 40 percent of strands required for the toroidal field magnets have been produced, which equals 165 tonnes of superconducting wire. All Domestic Agencies have successfully qualified their domestic suppliers and started to register the manufactured strands into the Conductor Database."
The panel this week also reviewed the latest test results of a conductor sample for the central solenoid, which recently showed some unexpected behaviour. Required to withstand 60,000 current pulses during plasma operation, the conductor test performed in November 2010 at the SULTAN facility in Switzerland revealed unacceptable degradation after only 6,000 pulses.
In order to assess and solve the problem, experts assembled at the Château de Cadarache this week agreed on a multiple-step approach: a series of tests will be performed over the next weeks and months aiming to find out whether the unsatisfactory performance is a result of the sample configuration, the sample preparation, or the conductor design itself. "These tests will be performed in parallel in order to save time and to enable the start of procurement," says Devred. "The SULTAN test serves as a risk mitigation strategy for launching production, but the true validation of the performance in conditions similar to the operating environment will be made by a long length of conductor." This so-called central solenoid insert test will occur during the second half of 2013, when 50 metres of central solenoid conductor will be tested in the central solenoid model coil based in Naka, Japan that provides test conditions very similar to those in ITER.
Current leads are the components that transmit the large currents from room-temperature power supplies to very low-temperature superconducting coils. The current leads for the ITER Tokamak have come a long way: from the original 60 kA proposals from the Japan Atomic Energy Research Institute (JAERI) and European partners (KIT's "demonstrator"), to the first prototypes fabricated and tested in China at the Chinese Academy of Sciences, Institute of Plasma Physics (ASIPP), and on to the presentation of the final design at the ninth High Temperature Superconductor (HTS) working group that met at Cadarache this week.
The HTS working group, which brings together experts from institutes in Japan (NIFS), Europe (KIT, CERN), China (ASIPP) and the ITER Organization, has been supporting the development of the HTS current leads for ITER since 2008. The current lead designs presented this week were fully endorsed by the working group, an endorsement crowning not only the ITER Organization's recent efforts, but also the significant investment by ASIPP/China in the fabrication and testing of four current lead pre-prototypes over the last three years.
It did not take long after the discovery of High Temperature Superconductors (HTS) in the 1980s for engineers to realize that HTS would permit significant reduction in the heat conducted into cryogenic systems through current leads. This is a particularly pertinent discovery for ITER because of the expense of removing heat at the liquid-helium temperatures at which the ITER magnets are operated.
HTS current leads use a short segment of HTS that can sustain much higher current-densities than even good conductors such as copper, allowing the reduction of the material cross-section and the related heat conduction by about tenfold. In ITER, where 60 current leads transfer a staggering 2.7 MA (MegaAmperes) into—and out of—the cryostat, conventional current leads without HTS would conduct approximately 1W/kA into the cryogenic system. This represents approximately 20 percent of the total heat extraction capacity of the cryoplant that will be installed in ITER.
The factor 2 increase in the cost of current leads due to addition of (still relatively expensive) HTS material is more than offset by the cost savings for cryoplants and power savings during operation. Most importantly, HTS current leads—in a similar way to superconducting magnets—contribute to the positive energy balance of the ITER Tokamak. They are thus a minor, but crucial element of a tokamak ... that is, if you can call a 3-metre-long object that weighs half a ton and carries 68 kA "minor."
For the design of the ITER HTS current leads, the ITER Organization has chosen a safe approach. Due to the unprecedented scale of these leads both in current (68 kA for the TF coil feeders) and voltage (30 kV), an effort has been made to adopt proven concepts wherever possible. Also, the design had to be completed within a tight schedule and with limited resources. For this effort, the HTS working group played a crucial role, bringing together experience from other projects that use superconductivity on a large scale, such as EAST, LHC, LHD and W7X. Major features of the design are derived directly from the successful CERN development undertaken for the more than 1000 HTS current leads that were built for the LHC.
More news will follow when the ITER current lead designs are qualified in ASIPP/China in mid-2012.
How do you build a better magnet? This coming September, over 700 of the world's foremost experts in all aspects of magnet technology will gather in Marseille for the 22nd meeting of the biennial Magnet Technology conference (MT-22). First established in 1965, the Magnet Technology conference is the world's largest gathering dedicated specifically to advancing the science and technology of magnet applications, from the MRI machines that allow for non-invasive examination of the human body to the powerful, high-current superconducting cables that will contain, shape, and drive the ITER plasma.
Because of the international nature of the magnet community, the conference location rotates between North America, Europe, and Asia. This year, which happens to celebrate the 100th anniversary of the discovery of superconductivity, is Europe's turn, and the ITER Organization has agreed to host the event, as ITER is a critical project pushing the boundaries of existing magnet technology in a wide variety of areas.
Abstract submission for the conference recently closed; the organizers received a record number of abstracts highlighting both the overall interest in the field and also interest in the ITER project. "This shows the vitality of our discipline in all the topics of the conference, at the moment when we are celebrating the centenary of Superconductivity but also the fifty years of Applied Superconductivity which is the main focus of our Conference," said Jean-Luc Duchateau of CEA, the scientific program chair for MT-22.
In parallel to the technical sessions of the conference, ITER and CEA are organizing a scientific and industrial exhibition to be held on site at the conference venue, to allow the scientists and engineers involved in magnet technology research to interact with the companies and organizations who are responsible for building the actual hardware that comprises the world's most sophisticated and advanced magnet systems. Registration for the conference opens on 1 April, 2011, and more information on all aspects of the conference taking place at the Parc Chanot in Marseille may be found on the MT-22 web site at www.mt22.org. A special evening will be dedicated to celebrating the 100th anniversary of superconductivity and selected papers will be published in a dedicated issue of IEEE Transactions on Applied Superconductivity.
In order to achieve superconductivity, the niobium-titanium (NbTi) and niobium-tin (Nb3Sn) conductors inside ITER's magnets will have to be cooled down with supercritical helium in the temperature range of 4 Kelvin (-269°C)—a process that requires substantial amounts of energy that impact the net energy gain. The efficiency of future fusion power plants could be drastically increased if superconductors could be operated at higher temperatures (> 65 K) using affordable liquid nitrogen, for example, instead of supercritical helium as coolant.
"Targeting a future commercial fusion machine, it may be very demanding to avoid liquid helium cooling for the coil system," Walter Fietz from the Karlsruhe Institute of Technology (KIT) in Germany writes in an article for Fusion Engineering and Design. "This would require less refrigeration power and allow omitting the radiation shield of the coils, resulting in a less complex cryostat and a size reduction of the machine."
"Having a material at hand that can transport currents without losses, that would be a dream," says Jean-Luc Duchateau from CEA who developed the superconducting tokamak Tore Supra. There are many materials being tested in labs around the world. At KIT in Karlsruhe, scientists have been experimenting for many years with a material that holds all the promises for successful application in the harsh environment of a fusion reactor: Yttrium Barium Copper Oxide, a crystalline chemical compound abbreviated as "YBCO". The material's operating temperature is in the range of around 50 K and its physical behavior in high magnetic fields brings it very close to Jean-Luc Duchateau's dream come true. The downside, however, is that so far it has not been possible to produce reliable strands out of YBCO.
In order to coordinate international efforts, a workshop is being organized at KIT on 26-27 May to further investigate options of HTS for high current and high fields for DEMO and future fusion applications. The workshop's flyer can be downloaded here .