Evaluation of bending strain dependence of critical current of Nb3Al conductor for coils with react-and-wind method

In order to clarify the limitation of bending strain in conductor to fabricate the toroidal field coils of DEMO reactor by react-and-wind (R&W) method, the effect of bending strain on the critical currents (Ic) of Nb3Al cable-in-conduit (CIC) conductor was investigated using the newly developed apparatus which can bend cooled sample from outside of cryostat. The Nb3Al strand manufactured by jelly roll process was 0.74 mm in diameter with the copper/non-copper ratio of 4.0. The CIC conductor sample which consisted of 54 Nb3Al strands and 27 copper wires inserted into a stainless steel conduit was 12.4 mm in diameter and 2.3 m in length. The CIC conductor was wound into a torsion coil spring shape with 4.5 turns and 150 mm in diameter. The bending strain and external magnetic field ranges for Ic measurement were from 0% to 0.6% and from 9.2 T to 11.3 T, respectively. It was clearly shown that the Ic decrease of the CIC conductor by bending is considerably low compared with that of strand, indicating the relaxation of intrinsic strain in Nb3Al filaments in CIC conductor due to the cabling effect. The Ic was not decreased up to 0.4% in bending strain and slightly decreased to 96% of initial Ic at 0.55% in bending strain. This indicates that the R&W method with bending strain of 0.6% is fully applied to the coil fabrication method. The furnace size for heat-treatment for R&W method becomes half of wind-and-react (W&R) method.