World-Wide Benchmarking of ITER ${\rm Nb}_{3}{\rm Sn}$ Strand Test Facilities

The world-wide procurement of Nb₃Sn and NbTi for the ITER superconducting magnet systems will involve eight to ten strand suppliers from six Domestic Agencies (DAs) on three continents. To ensure accurate and consistent measurement of the physical and superconducting properties of the composite strand, a strand test facility benchmarking effort was initiated in August 2008. The objectives of this effort are to assess and improve the superconducting strand test and sample preparation technologies at each DA and supplier, in preparation for the more than ten thousand samples that will be tested during ITER procurement. The present benchmarking includes tests for critical current (I_c), n-index, hysteresis loss (Q_hys), residual resistivity ratio (RRR), strand diameter, Cu fraction, twist pitch, twist direction, and metal plating thickness (Cr or Ni). Nineteen participants from six parties (China, EU, Japan, South Korea, Russia, and the United States) have participated in the benchmarking. This round, conducted with a bronze-route Nb₃Sn strand, involved samples prepared by a common laboratory (CERN) and sent out to the participants ("IO-prepared samples") and also samples prepared by each individual participant ("self-prepared samples"). I_c samples prepared and measured by CERN were found to have an average I_c of 188.7 A and a standard deviation of 1.8 A (1.0%), while those same set of samples measured by all the participating labs in round-robin fashion were found to have a standard deviation of 2.1 A. Self-prepared samples had an average I_c of 188.1 A, and showed a lab-to-lab standard deviation of 3.5 A. The results demonstrate significant progress in the world-wide capability to accurately and reproducibly measure Nba₃Sn critical current over the past decade. Future benchmarking efforts will include an annual cross-check of supplier and DA facilities, and also a round of internal t- - in Nb₃Sn samples to assess each contributor\´s sample-preparation techniques. A separate round of NbTi benchmarking is also envisioned.