Risks and benefits of Incoloy 908

The central solenoid and toroidal field coils of ITER are designed with cable-in-conduit Nb3Sn superconducting conductors jacketed with Incoloy 908. This material, developed to limit the critical current degradation of the Nb3Sn strands, shows excellent mechanical properties at 4 K, particularly under cycling. Nevertheless, it is a crack prone material when heat treated in the presence of oxygen, which requires careful control of the atmosphere during manufacture. A programme of extensive mechanical testing of samples and manufacture of a pancake with incoloy conductor jacket was carried out in Europe. The origin of cracks in the incoloy jacket after heat treatment was investigated.