Pinning Properties of Commercial Nb-Ti Wires Described by a 2-Components Model

Abstract

Abstract-We report on the magnetic and transport characterization of different NbTi commercial strands, carried out at variable temperature and magnetic field. From the critical current densities extracted from transport measurements and magnetization cycles we were able to calculate the normalized bulk pinning forces. The curves show good temperature scaling throughout the explored temperature range, and the reduced pinning force can be described by a simple two-components model system. The extension of the 2-components description of the pinning force to an expression for the critical current density gives a very good agreement with experimental measurements over the whole explored B, T range. The model works for all investigated samples, which are different in size, Cu:nonCu ratios, filament diameters and layouts. These results suggest that pinning mechanisms in conventional NbTi strands should be revised, since Nb-Ti composition gradients and grain boundaries seems to play a not negligible role.

Test Results of a NbTi Wire for the ITER Poloidal Field Magnets: A Validation of the 2-Pinning Components Model

“…5 reports the normalized pinning force curves, measured between 4.2 K and 8.5 K, as function of the reduced field , being the irreversibility field. A relevant temperature non-scaling is observed, opposite to what was deduced in the past on different wires [7], [11], characterized in a more restricted temperature range (only up to 6.5 K). The corresponding values for , as determined by magnetization cycle closure points, and the pinning force maxima , are reported in Table III.…”

“…As discussed in [11], we believe that the two possible types of strong pinning defects might be identified with clusters of ribbons [21], dominating at low reduced fields, and to Nb/Ti concentration gradients present around them [22], responsible for the behavior at high reduced fields.…”

“…Motivated by these considerations, we recently formulated the 2-pinning components model [11], based on the assumption that two pinning mechanisms are dominantly acting in these systems, that allowed us to accurately describe the performances of technological strands for fusion over a complete magnetic field range, and in the temperature range 4.2 K to 6.5 K. However, the model, as formulated up to now, could not take into account a possible temperature non-scaling of the reduced pinning force curves, whereas recent extensive tests carried out on a strand produced for the ITER PF magnets up to 8.5 K, revealed the expected temperature non-scaling. The fitting model has thus been upgraded, in order to take into account the observed phenomenology.…”

Techno-Economic Optimization of the NbTi DTT Feeders

“…Three alternative layouts (B, C, D layouts in Table I) for the CS feeders were developed and analyzed on top of the baseline layout (layout A in Table I) already presented in [10]. The alternative layouts are developed based again on the scaling from the critical current in [11], but with a progressively reduced number of SC strands 𝑁 𝑆𝐶 and increasingly higher number of segregated Cu strands 𝑁 𝐶𝑢,𝑠𝑒𝑔𝑟 . While the first stages of the cabling stages are kept unchanged, with the classical twisting of 3 triplets of (SC/Cu) wires, as well as the last stages consisting of 6 petals, the intermediate stages are modified from the original 5 × 5 pattern (layout A) to 3 × 5 (layout B), 4 × 5 (layout C) and eventually 3 × 3 (layout D).…”

“…A first design of all feeders has been already performed in DC condition, checking that the temperature margin to current sharing Δ𝑇 𝐶𝑆 at the nominal current is > 2K for all feeders [10]. This first design step was quite straightforward, since it was based on a wellestablished scaling for the strand critical current [11], considering the nominal operating temperature of 4.5 K. In DC condition, the temperature computed along the feeders turned out to be totally acceptable in view of the short length of the feeders and of a small effect of the radiative load from the thermal shield, and Joule losses foreseen in the joint to the coil cables.…”