Growth Rate and Morphology of Nb3Sn Layers in ITER-Type Bronze-Processed Wires Under Different Diffusion Annealing Regimes
The Nb3Sn-based bronze-processed multifilamentary composites have been fabricated at the Bochvar Institute of Inorganic Materials, Moscow, Russia, and studied by electron microscopy to determine the relationships between their structural parameters and annealing duration at different temperatures (575 °C, 650 °C, 750 °C). The structure of superconducting layers during their formation is characterized by thickness, average size of equiaxed grains, and by the fraction ratio of columnar and equiaxed grains. It was found that at high diffusion annealing temperatures (above 650 °C), the superconducting layers grow much faster, but the average size of equiaxed Nb3Sn grains even under a short exposure (10 h) is much larger than after a long low-temperature annealing. It is shown that to ensure the best current-carrying capacity of Nb3Sn-based wires in high magnetic fields, it is necessary to take into account different trends: on the one hand, the possibility of approaching the Nb3Sn phase stoichiometry and providing its maximum amount and on the other hand to avoid the undesirable growth of grains resulting in a lower pinning force.
Influence of two-stage annealing treatment on critical current of bronze-processed multifilamentary Nb3Sn superconducting materials
Abstract The influence of changes of volume fraction of Nb3 Sn, grain size and upper critical magnetic field due to two-stage annealing treatment (low temperature annealing to form fine grains + high temperature annealing to raise upper critical magnetic field) on overall critical current and critical current density of the bronze-processed Nb3Sn compound were studied at the magnetic field of 3–15 T. When annealing temperature was low (773–923 K) and the volume fraction of Nb3 Sn was low in the first stage annealing, the second stage annealing could raise the overall critical current over the whole range of applied magnetic field of 3–15 T due to increase in upper critical magnetic field Hc2 and due to increase in the volume fraction of Nb3Sn accompanying with reduction in Sn concentration in the bronze matrix, which played a role to reduce residual strain in Nb3Sn, leading to high Hdu,c2 although the loss in pinning force arose from the coarsening of the grains. When the annealing temperature was high (973 K) and the Nb3Sn was formed until the Sn in the bronze was consumed in the first stage, the second stage annealing could not raise the critical current due to increase in grain size and due to no effective increase in Hc2. The critical current density at low magnetic fields below several Testas was reduced by the second stage annealing due to increase in grain size but that at high fields was raised due to increase in high Hc2. The reverse two-stage annealing treatment (high temperature annealing in the first stage + low temperature annealing in the second stage) reduced the Hc2 slightly with increasing second stage annealing temperature and time. The critical current density at low magnetic fields was determined mainly by the grain size but that at high fields was determined by the combination of the upper critical field and grain size. The present results could be accounted for by the empirical equation based on the Suenaga's speculation, indicating that the global pinning force is proportional to the product of inverse grain size and (1 − h) 2 h 1 2 taken from the Kramer's equation where h is the reduced magnetic field.
The Results of Russian ITER TF Conductor Sample Test in SULTAN
In the framework of the ITER Qualification Tests the Russian sample RFTF2 was tested in SULTAN facility. The sample is made of two identical ITER TF-type cable-in-conduit conductor sections. Bronze route strands were used for the conductor fabrication. The main characteristics of the conductor and the strand are presented as well as the sample assembly details. The test was performed in accordance with the ITER International Organization specified program. In order to evaluate the conductor performance, the current sharing temperature was measured at specified electromagnetic load cycling steps. Few voltage-current characteristics were measured with the aim of n-value estimation. Both conductor sections of the RFTF2 sample showed identical performance: the current sharing temperature is 6.4 K, that not only satisfies, but substantially exceeds ITER requirement of 5.7 K. The tests of RFTF-2 conductor sample showed that electromagnetic cyclic load as well as warm-up had negligible effect on the conductor performance. The results of the SULTAN sample test are compared with strand performance.