Study of the Temperature and Field Dependence of the Critical Currents in Nb-Ti Strands for the ITER Poloidal Field Magnet System

Commercial multifilamentary Nb-Ti strands produced by the Russian Federation DA for the ITER poloidal field coils PF1 and PF6 were characterized regarding the critical current versus magnetic field dependence from 2 to 10 T over a wide range of temperatures from 2.5 to 7 K. The data were collected on several Nb-Ti strands at the stage before production and after 30% of the production for the ITER program. J_c(B, T) fits were determined using parameterization models with single pinning and two-pinning components. The validity and limitation of these estimations will be discussed. The pinning force curves are presented in the temperature range from 2.5 to 4.2 K and indicate a shift of the pinning force peak to lower fields with increasing temperature. The current sharing temperature T_cs at the operating current (33 A) and field (6.4 T) was determined from both parameterization models. The results of T_cs show good reproducibility for each set of Nb-Ti strands. The good agreement between experimental and fit results over very wide temperature ranges can be used for the analysis of experimental data on the PF conductor performance and for the design of future superconducting magnets.