February 12, 2016, at the Cold Test Facility at CEA-Saclay, the first toroidal field coil of JT-60SA - manufactured by General Electric/Alstom under the supervision of the CEA - became the first of 28 superconducting coils of JT-60SA – already manufactured or still being processing - to reach the superconducting state.
The first toroidal field coil while being lowered Inside the Cold Test Facility cryostat
A total of 20 of toroidal field coils, including two spares, are in production in Europe, 10 are being built by General Electric / Alstom, Belfort, France, under the supervision of the CEA and 10 by ASG, Genoa, Italy, under the supervision ENEA.
The first of the 20 toroidal field coils were delivered by Alstom/General Electric, December 15, 2015 at the Cold Test Facility, built at CEA-Saclay for testing, in nominal operating conditions, performance and robustness of these coils.
To be operated with a sufficient safety margin, the superconducting toroidal field coils of JT-60SA have to be cooled at a temperature of -268°C (i.e. 4.5 degrees above absolute zero temperature, denoted 4.5 Kelvin). This cool-down is achieved by means of the circulation in the coil of a helium flow gradually approaching its liquefaction temperature and monitored by a cryogenic refrigerator. The transition to the superconducting state is reached, meanwhile, without magnetic field, at about -264°C (about 9 Kelvin). For a superconducting coil achieving the superconducting state corresponds to the vanishing of its electrical resistance and to its ability of sustaining a high electrical current without heat dissipation.
The first toroidal coil in the test cryostat before closure, cooldown and testing
Thus, the cooling down the first toroidal field coil, with a total mass of about 15 tons, was initiated in February 1, 2016. Approximately 12 days were therefore needed for the coil to reach the superconducting state which has actually been observed on February 12 around 3:00 am. From that moment, the full qualification program of the coil has been engaged. First, the control of the hydraulic properties of the helium cooling system has been thoroughly checked. Second, the performance test, which consists in circulating into the coil an electrical current at the nominal intensity of 25,700 Ampere, was validated. Third, the fast discharge test, which aims at verifying the proper behavior of the coil in the event of a request for a fast shutdown of the full current intensity in less than one minute, was successfully performed. Finally, the ultimate "quench" test allowing the measurement of both the operating temperature margin and the check of the robustness of the coil against a major incidental event, was successfully passed on Friday, February 19. This test consists, while the nominal current (25 700 amperes) is flowing through the coil to deliberately induce and record a controlled temperature rise from the nominal operating temperature up to the resistive transition temperature. In this situation, the safety system protecting the coil shall automatically trigger, in less than 0.1 seconds, a fast discharge request that transfers the magnetic energy stored into the coil to a set of dedicated dump resistors. At this stage, any malfunction can have irremediable consequences for the integrity of the coil. Thus this ultimate test reflects the confidence both in the design and manufacture of the coil and in the mastery of the magnet safety system protecting the operation of the cold test facility.
The experimental results obtained this February 19th, 2016, have shown that the actual "quench" temperature of the coil is above 7.5 Kelvin showing an extra margin with respect to the estimated minimum quench temperature.
Thus, all the relevant operating parameters of the first toroidal coil have been measured, verified and validated. As for the quench temperature, they all have exhibited additional operating margins above the estimations of the different international actors involved in this great collective success both in terms of design, manufacturing or testing.
The design of the toroidal field coils, started in 2007, was jointly performed by CEA, ENEA, F4E and JAEA. The components constituting the coils were provided by companies working under the control of these institutions. Thus, the superconducting strand was produced by the Japanese company Furukawa, under control of F4E who also supervised the production of the conductors made by the Italian company ICAS. ENEA supervised the production by the Italian company Walter Tosto of the stainless steel casings in which the superconducting electrical winding is inserted and gives it its mechanical strength. Finally CEA supervised both the production of coils made by Alstom / General Electric in Belfort and the design and construction of Cold Test Facility at CEA-Saclay, in partnership with SCK-CEN. CEA also has the duty of performing the cryogenic tests of 20 toroidal field coils that are or will be produced by the summer of 2017.
The results obtained these last days successfully crowned nearly 10 years of research and development conducted in an exceptional international environment. With this technological achievement, a major milestone was reached for the JT-60SA project.
The first coil will be warm up and then equipped still in Saclay, with its external mechanical structure before being prepared for final shipment to Japan.
The second toroidal field coil was delivered at Saclay from Belfort on March 10 and the first Italian coil from Genoa followed. Deliveries and cryogenic tests will succeeded one another at a steady pace in Saclay to allow shipping and delivery to Japan of the last toroidal field coils by the end of fall 2017.
Last update : 08/09 2016 (507)
