Theoretical and experimental analysis of Nb₃Sn strand buckling in large scale CIC conductor

The assessment of the performance of toroidal field (TF) coil of ITER has been progressing. Unpredictable strand buckling was observed by the destructive investigation of the conductor. The buckling direction was perpendicular to the Lorentz force (LF), and the mechanism of it was due to the thermal shrinkage caused by the difference of thermal contraction between strand material and conduit. Our previous work utilized a 2-dimensional string model and demonstrated that the observed 2 mm stand bending could have led to strand bending if the total amount of slide at the contact cross over was assumed to be 53 μm. To verify this estimation, we fabricated a device for the measurement of friction force between strands under constriction force comparable to the LF several hundred kN/m. Our results for Cr-coated 0.89 mm diameter strand surrounded by bare Cu strand indicate that thermal contraction stress applied to strand of 45 N would be sufficient to overcome the static friction force when the contraction force reduced to tenth of maximum LF. The mechanism of slide motion could be divided into two processes: separation of the inner wall of the conduit and the separation from other strands due to a gradual reduction of LF. (author)