Enhancement of the E(J, B) power law characterization for superconducting wires from electrical measurements on a coil
Résumé
The design of superconducting applications requires the characteristics knowledge of the wire used, in particular the E(J) law and its field dependence. These are usually measured on short samples. However, this method has two major drawbacks. First, the measured voltages are only of a few microvolts, therefore very difficult to obtain. Secondly, tests are not performed under conditions of the subsequent use of the conductor e.g., wire is not wound. To overcome these problems, we propose an original characterization method. The principle is to wind a test coil made from a few meters of superconducting wire and then measure its V(I) characteristic. A computer program was developed to obtain the wire characteristics from this measurement. For this, the superconductor has been modeled by a power law model E(J, B) which takes into account the effects of the magnetic flux density B on the critical current density Jc and on the exponent n. A sub-function allows to calculate the theoretical V(I) curve of the coil for a given set of model parameters, taking into account the self magnetic field of the coil. An optimization function is then used in order to find parameters of the model which better fit the experimental curve. Therefore, the wire is characterized in its terms of use. In addition, the measured voltages on the coil are higher than for a short sample, thus easier to obtain. Furthermore, the self field of the coil is used to determine the field dependence of the E(J, B) law. Moreover, this method can be used to quantify the degradation of the wire's characteristics, according to the winding process e.g., the bending radius and the tensile strength.