Metallographic autopsies of full-scale ITER prototype cable-in-conduit conductors after full cyclic testing in SULTAN: III. The importance of strand surface roughness in long twist pitch conductors

As part of the ITER conductor qualification process, 3 m long cable-in-conduit conductors (CICCs) were tested at the SULTAN facility under conditions simulating ITER operation so as to establish the current-sharing temperature, T-cs, as a function of multiple full Lorentz force loading cycles. After a comprehensive evaluation of both the toroidal field (TF) and the central solenoid (CS) conductors, it was found that T-cs degradation was common in long twist pitch TF conductors while short twist pitch CS conductors showed some T-cs increase. However, one kind of TF conductors containing superconducting strand fabricated by the Bochvar Institute of Inorganic Materials (VNIINM) avoided T-cs degradation despite having long twist pitch. In our earlier metallographic autopsies of long and short twist pitch CS conductors, we observed a substantially greater transverse strand movement under Lorentz force loading for long twist pitch conductors, while short twist pitch conductors had negligible transverse movement. With help from the literature, we concluded that the transverse movement was not the source of T-cs degradation but rather an increase of the compressive strain in the Nb3Sn filaments possibly induced by longitudinal movement of the wires. Like all TF conductors this TF VNIINM conductor showed large transverse motions under Lorentz force loading, but T-cs actually increased, as in all short twist pitch CS conductors. We here propose that the high surface roughness of the VNIINM strand may be responsible for the suppression of the compressive strain enhancement (characteristic of long twist pitch conductors). It appears that increasing strand surface roughness could improve the performance of long twist pitch CICCs.