Current distribution measurement in the busbars of the ITER TF Model Coil

The ITER Toroidal Field Model Coil (TFMC) was tested in 2001 at FZK (Karlsruhe, Germany). The BB1 busbars were instrumented with Hall probes to measure the current distribution across their cable sections. The BB1 busbars are pieces of conductor, about 7 m long, connecting the coil terminals to the BB2 busbars (going to the current leads). The cable is composed of 1152 twisted NbTi strands grouped into six main subcables (petals), each wrapped with a steel foil. The experimental setup is described as well as the method used for calculating the current distribution which leads to estimate the current unbalance among the petals. We observed a slight but monotonous evolution of the steady current distribution as current increases. We also analyzed the transient evolution between inductively and resistively driven current distributions, during a current plateau following a ramp. Assuming the inductive distribution to be uniform, leads finally to a low absolute nonuniformity (within ±6%) of the steady current distribution among the petals. The transient behavior is also discussed through a simple theoretical model.