Segregation of copper in the ITER cable and ramp rate limitation

The quench of a superconducting cable containing a defect and subjected to a current increase is studied. The cable is represented by a model which consists of a strand with a local decrease in the critical current I_c and the rest of the cable lumped together. It is assumed that the quench takes place when the electric field in the defect reaches the instability level. An important feature of the process is the noticeable portion of current expelled from the defect to the rest of the cable prior to the quench. It is shown that given the strong inequality in the strand currents during the ramp, the influence of the defect on the quench current is proportional to I_c⁴. This criterion is applied to the problem of copper separation in the triplets of the ITER cables.