Application of the prebending strain effect on CuNb/Nb₃Sn superconducting coils fabricated by a react-and-wind method

We have found that prebending treatment, which is repeated bending load at room temperature, greatly enhances the critical current, upper critical field and critical temperature of practical Nb₃Sn superconducting wires. In this paper, we focus on the application of the prebending strain effect to practical superconducting coils fabricated by a react-and-wind method. To demonstrate the prebending strain effect on the react-and-wind coil, we prepared two kinds of CuNb /Nb₃Sn superconducting monolayer coils. For one of the coils, the superconducting wire of 1.0 mm diameter was repeatedly bent by using ten fixed pulleys before the winding process, resulting in a prebending strain value of 1.0%. The final winding diameter of both coils was 200 mm and the number of turns was 49. In the compressive stress condition, the critical current of the coil with 1.0% prebending strain was 296 A at 11 T, which was larger than that of the coil with 0% prebending strain. These values were much larger than the critical current of the witness sample. The results indicate that the prebending treatment enhanced the coil critical current. In the case of the hoop stress condition, the coil critical currents were approximately 230 A at 11 T for both coils. The maximum electromagnetic hoop stress was calculated to be 360 MPa. The short sample tensile test results qualitatively explained the critical current deterioration of the coil. Furthermore, the 360 MPa hoop stress did not deteriorate the critical current irreversibly.