Review of Nb3Sn conductors for ITER

The ITER magnets experiencing high magnetic field (>11 T), the central solenoid (CS) and the toroidal field (TF) system, will make use of complex conductors using Nb3Sn as superconducting material. This superconductor offers the required capability in temperature and magnetic field but shows sensitivity to strain. The two ITER model coils, the CS model coil (CSMC) and the TF model coil (TFMC) tested in 2000–2002 have given confidence in the industrial feasibility and the operating capability of the ITER magnets. However, careful analyses have revealed a degradation in terms of temperature margin. Taking advantage of the progress made by the superconductor industry since the model coil fabrications, the ITER Nb3Sn conductors have been redesigned using higher performance (advanced) strands to recover the original designed temperature margin without changing the conductor envelopes. An R&D program was launched among the ITER participant teams (PTs) to understand the source of degradation which was found to be likely the bending of the strands inside the cable under the electromagnetic load. Meanwhile, new strands were ordered to industrial companies, to manufacture and to test full-size conductors. The European Union PT tested by mid-2006 two full-size conductor samples with advanced Nb3Sn strands but using the TFMC conductor design, they showed performance much below expectation with regard to strand performance and to ITER TF conductor design. Overall performance is not much above the TFMC conductor sample performance. This poor performance has been attributed to more sensitivity to deformation of advanced strands. Nevertheless, tests of real ITER TF conductor samples are needed to draw final conclusions. The first samples should be tested at the end of 2006 and in 2007. However, one must already be prepared to redesign the ITER conductors as well as to modify and adapt the strand specifications, using the new tools (test facilities, mechanical models) developed since the model coil tests. In addition, the way to extrapolate strand and full-size short sample test results to ITER coils is not straightforward and has to be agreed among the ITER team and the ITER PTs.