Characterization of Nb–Ti Strands in the Process of Industrial Production for the ITER Poloidal Field Coils

Abstract

More than 100 tons of Nb-Ti superconducting strands were produced by JSC “ChMP” (RF) for the ITER poloidal field coils (PF1&6) in the period from 2010 to 2014. The strands were manufactured based on the NT-47 alloy with a titanium content within 46.0-48.5 wt%, using the technology developed at Bochvar Institute. Measurements of the temperature and field dependence of the critical currents were carried out at the Low Temperature Physics Department of Atominstitut, Technische Universitat Wien, on strands stemming from the 30%, 60%, and 90% stages of industrial production. The facility for variable temperature critical current measurements was upgraded in order to provide direct measurements of the critical current up to 300 A at relatively high temperatures (T > 5 K) in a gas-cooled mode and up to 900 A in liquid helium. Unique results regarding the field and temperature dependence of the critical current in the range from 2 to 12 T and from 3.5 to 7 K, respectively, are presented and discussed based on singleand two-component pinning models. On the basis of mathematical models, the critical temperature at which the transition to the resistive state occurs (T cs ) was evaluated for the operating conditions (6.4 T and 33 A) of the poloidal field coils. Statistical analysis revealed that both models adequately describe the experimental Ic(T, B) data; however, a comparison of the models based on the Fisher test shows a statistical difference.