Optimisation of the reaction heat treatment cycle of internal tin and bronze Nb3Sn strands for ITER

The heat treatment cycles of Nb3Sn superconductor strands are mainly designed to achieve the highest possible critical currents and are, therefore, very long (up to nearly 600 h including ramping times). Such long interruptions would delay the manufacturing cycle of large superconducting magnet coils considerably. Therefore, it was of interest to investigate if the same or nearly the same performance of the strands could be achieved with shorter heat treatment cycles. Strand material used for the ITER CS and TF model coils from two European suppliers were submitted to various heat treatments and subsequently characterised with respect to critical current, hysteresis losses, residual resistance ratio (RRR) and magneto-resistance. The measurements were supplemented by metallurgical and microscopic investigations. This paper summarises the promising results opening the possibility to shorten the total heat treatment time for the ITER TF coils by approximately 1 year.