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- Successful demonstration of a key technology for the ITER Poloidal Field Coil -
Test Coil Using Nb-Ti Superconducting Conductor Attained 52 kA at 6.4 T
Sep. 5, 2008
Japan Atomic Energy Agency (JAEA), the ITER Organization, and Fusion for Energy (F4E) with the support of the European Commission have jointly tested a prototype Niobium(Nb)-Titanium(Ti) superconductor for the ITER Poloidal Field (PF) coils. The test coil using the Nb-Ti conductor achieved stable operation at 52 kA and 6.4 Tesla, emulating the operating conditions of the PF coils in the ITER tokamak.
The ITER PF coils require an operation of a large conductor current of more than 50 kA, which is about 70% larger than any previous experience. The results gave the researchers complete confidence that this conductor would fulfill the extremely demanding performance needed for the ITER PF coils, demonstrating a key technology required for ITER.
The world’s largest superconducting test facility at JAEA Naka Fusion Research Establishment in Naka city was selected for the tests because of its technical characteristics and the availability of world class expertise. A team, consisting of experts from JAEA, F4E and the ITER Organization, performed the tests with participation from Russian and United States Domestic Agencies.
The test coil with an outer diameter of 1.5 m and weighing 6 tons was manufactured by an international collaboration: the Russia Domestic Agency produced the 0.73-mm diameter Nb-Ti superconducting strands and bundled them into a cable consisting of 1,440 strands; the European Domestic Agency assembled the cable into a steel jacket to make the final conductor. Winding of the conductor, insulating the turns and bonding them together to form a coil was also performed in Europe.
To reach such a large conductor current requires many individual strands. However, in such a cable, two competing and contradictory requirements have to be resolved: to maintain AC losses (heat generation in a superconductor) within acceptable levels under operating conditions while maintaining good electrical contact between Nb-Ti strands to give equal distribution of the current. Based on the knowledge from JAEA's research, a solution was to apply nickel coating to the strand surface with a thickness of 2 microns to provide an appropriate contact resistance between strands for low AC losses and stable operation. The test coil was operated stably up to at least 52 kA at 6.4 T and 4.5 K and the AC losses of the conductor were confirmed to be below one half of the target figure.
Extremely powerful superconducting magnets will be used to maintain the stability of the fusion plasma inside ITER. Demonstrating that the superconducting strands for these magnets would perform to design specification was one of the most important technological milestones for the project.
Based on these achievements, the Chinese, European and Russian Domestic Agencies will start procurement of the ITER PF conductor.
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