Critical-Current Measurements on ITER Nb3Sn Strands: Effect of Temperature

Transport critical-current (I_c) measurements were made on commercial multifilamentary Nb₃Sn strands at temperatures (T) from 4 to 17 K and magnetic fields (H) from 0 to 14 T. Samples investigated were taken from the stage 1 pre-production strand for the central solenoid of the International Thermonuclear Experimental Reactor (ITER) project. Specimens were mounted on a three-turn Ti-6Al-4V (percent by mass) mandrel, which was a shorter version of the standard ITER critical-current mandrel. The measurements covered the range of critical currents from less than 0.1 A to over 700 A. To verify the accuracy of the variable-temperature measurements, we compared critical-current values obtained on a specimen that was immersed in liquid helium at 5 K to those measured on the same specimen in flowing helium gas at the same temperature. This comparison indicated that specimen temperature was controlled to within 60 mK during the measurements. The critical-current data presented include electric-field versus temperature (E-T) characteristics, I_c(T) at constant H, and extrapolated effective upper critical field as a function of temperature. These data are part of what is needed for the ITER strand characterization.