Bending Tolerance of React&Wind Nb3Sn Conductors for Fusion Magnets

The react-and-wind technique for manufacturing of large Nb3Sn fusion magnets (RW) is very attractive because of the superior performance of the Nb3Sn with lower thermal strain compared to the wind-and-react technique. The procedure for magnet winding is also drastically simplified for the RW approach. The flat cable, made of a large number of Nb3Sn strands, is heat treated on a spool with constant radius. Then the cable is unspooled to assemble the conductor with stabilizer/steel jacket and wound on a spool to be shipped to the winding factory, where the conductor is unspooled and wound in the final geometry - either round shape for Central Solenoid or D-shape for Toroidal Field Coils. The bending strain must be controlled during the handling from heat treatment to the final magnet in order not to exceed the irreversible strain limit. For design purposes, it is assumed so far that a bending strain ±0.3% during handling is acceptable. In this work, the bending tolerance of a 63 kA RW fusion conductor is investigated by monitoring the performance in the SULTAN test facility after bending/straightening at decreasing radii till a degradation of the current sharing temperature performance, Tcs, is observed. The experimental assessment of the bending tolerance during handling is a major instrument for dimensioning of the cable thickness and heat treatment radius of the RW conductors for fusion.