Review of Nb3Sn conductors for ITER

The magnetic system of ITER is a key component of the machine, not only from the cost point of view, it amounts for about 30% of the machine cost, but also from the performance point of view since any magnet limitation in this field will undoubtedly limit the capability of the whole machine. The ITER magnets experiencing high magnetic field (11.5-13 T range), namely the Central Solenoid (CS) and the Toroidal Field (TF) system, will make use of complex conductors using Nb3Sn as superconducting material. This superconductor has superior capability compared to the NbTi alloys used in usual superconducting magnets such as MRI magnets, but shows sensitivity to strain. The two ITER model coils (CSMC and TFMC) tested in 2000-2002 have given confidence in the industrial feasibility and the operating capability of the ITER magnets, as both were able to reach their nominal operating point without trouble. However, later analyses of these coil test results in terms of temperature margin have shown a degradation compared to expectations drawn from the properties of the basic strand composing their conductors. Taking advantage of the progress made by the superconductor industry since the model coil fabrications, the ITER Nb3Sn conductors have been re-designed using higher performance (advanced) strands to recover the original designed temperature margin without changing the conductor envelopes. Two types of action were then initiated among the ITER Participant Teams: first a short R (and) D program was launched to understand the source of degradation which has been suspected to be the bending of the strands inside the cable under the electromagnetic load, and second, more pragmatically, new strands were ordered to industrial companies, to test them, and then to build and test full-size conductors using these strands. To get rapidly experimental results, the European Union fabricated and tested by mid 2006 two full-size conductor samples using the TFMC conductor design but with advanced Nb3Sn strands. A third conductor using the ITER TF conductor design should be tested at the end of 2006. The test results of the first two full-size EU conductors can be compared to the properties of their basic strand and to the test result of the TFMC conductor, they can also be assessed with regard to the expected performance of the ITER conductors. This work gives the opportunity to discuss the way to extrapolate strand and full-size short sample test results to the ITER coils. (author)