Electromagnetic evaluation of the collective behavior of 720 twisted strands for the TF model coil experiment

The TF model coil experiment is aimed at simulating the magnetic behavior of the toroidal field system of ITER. This coil will operate in DC current mode. Even in these conditions, the theoretical critical currents of the cable, which should be the sum of the critical current of the 720 superconducting strands, may not be reached. The intrinsically nonperfect current distribution in the connection area can affect the conductor performances in the peak field region, only 1.2 m away. This has a strong influence on the voltage-current characteristic and can generate a quench before the theoretical critical current is reached. The theoretical critical current chart is calculated as a function of temperature and field. This calculation is based on a precise characterization of the strand at different temperatures and fields, coupled to quality assurance for each of the billets. A redistribution process is activated by the saturation of the overloaded strands leading to current transfer into underloaded strands. The interstrand contact resistance and the field pattern affect this process.