Optimization of the niobium-tin inserts for the 80 kA current lead to be used in the TOSKA facility for the ITER toroidal field model coil test

One essential tool in the construction of the forced-flow cooled 80 kA current lead to be used in the TOSKA facility at FZK for the ITER TFMC tests is the use of niobium-tin inserts in the lower temperature region of the heat exchanger allowing the operation of the lead with minimum He mass flow rate in a wide current range. Due to the fact that there is only limited space for the superconductor inserts in the 80 kA current lead, it was necessary to look for niobium-tin strands having a very high critical current at low magnetic fields and high temperatures. It was decided to use internally stabilized bronze routed strands because of the absence of an outer barrier which may hinder the current transfer from the copper of the current lead to the superconductor filaments. During the test measurements, it was found that the current transfer from the external copper to the superconductor was a second important criterion for the strand choice. A series of measurements were done using superconductor strands embedded in profiles made of different copper materials. The result of the test measurements are presented which were the base for the decision which strand material is appropriate for the inserts.