Analysis of current and voltage distribution in the first Japanese qualification sample of an ITER TF conductor

The critical currents of the Japanese ITER TF conductors made through an internal-tin and bronze process were preliminarily estimated to be about 5.7 K, which approximates the design value of 5.7 K, and about 6.1 K, respectively, at 68 kA and 11.8 T using a short conductor sample. To investigate the influence of the current distribution in the sample conductor, a simulation was performed using a lumped circuit model of a cable and static electrical field model for jackets. The simulation results show that a large, non-uniform current distribution is established due to magneto-resistance of the copper in the joint and an imbalance of contact resistance of the strands to the copper and by poor soldering between the copper shoes, results which make a precise evaluation of the critical current performance difficult. The analytical results indicate that the current sharing temperature of the internal-tin and bronze process conductors is expected to be 6.0 K and 6.7 K, respectively, when the current distribution is uniform. In addition, solder filling of the joints makes the current distribution uniform due to the normal resistance in the high field zone, and the current sharing temperature can be estimated as almost the same as when the current distribution is uniform. The other possible solution is to use a thin copper plate with a low RRR to reduce the influence of magneto-resistance and any non-uniformity in contact resistance. Conductor performance is under-estimated in this case because the non-uniform current distribution still remains. However, the reduction in the estimated current sharing temperature is expected to be a few hundred milli-Kelvins, which seems acceptable as a margin in a qualification trial.