HIGH-FIELD FLUX-PINNING AND THE UPPER CRITICAL-FIELD OF NB-TI SUPERCONDUCTORS

Nb-Ti wires of different composition and metallurgical state have been studied near the upper critical field (H-c2) in an attempt to separat e H-c2, the irreversibility field (H) and the resistively measured tr ansition field (H-r). For optimized multifilamentary Nb-47wt.%Ti wire, we find that H=10.2 T, H-c2=10.8 T and H-r=11.8 T at 4.2 K. The tran sitions were studied by extended electric field vs. current density cu rves, magnetization and small current transport measurements. We found a strong correlation between magnetization, bulk flux pinning (F-p) e xtrapolation and extended E-J curves for the value of H, all giving H =10.2 T. We have also shown that the maximum in H-r occurs for Nb-44w t. %Ti.