Pingxiang Zhang

Optimization of Bronze Processed

Operated under high magnetic field (>;11 T), the toroidal field (TF) system consists of complex conductors using Nb3Sn as superconducting material. According to the strand requirements for the ITER TF conductor, bronze processed Nb3Sn strands with high superconducting performance have been manufactured using high tin content bronze at WST. The studies on filament diameter, diffusion barrier, sample heat treatment and annealing temperature have been carried out to enhance Nb3Sn strand properties. The results of these investigations have been presented. The result indicates that the different filament diameter has a weak influence on the critical current (Ic) value for the different reaction degree. In the manufacturing process the annealing temperature should be kept below 500°C to realize a high n value. The continuous Nb3Sn layer formation on the internal surface of the barrier causes a considerable increase of hysteresis loss when single Nb barrier is used. There is almost no difference of Ic, hysteresis loss and grain morphology for two heat treatments: ITER heat treatment cycle B and WST heat treatment cycle.

{\hbox {Nb}}_{3}{\hbox {Sn}}

The multifilamentary Nb3Sn strands with a diameter of 0.82 mm and different local area ratios of Nb to copper (LAR) have been successfully fabricated by an internal tin process in Western Superconducting Technologies Co., Ltd. (WST). The heat treatments have been completed in a vacuum condition. The Critical Current (Ic) and hysteresis loss of the strands are measured after different heat treatments according to ITER requirement. The maximum Ic is about 285 A (4.22 K, 12 T, 0.1 ¿V/cm) and the hysteresis loss is 290 ~ 410 mJ/cm3 (4.22 K, ±3 T) per strand unit volume. In this paper, the Ic and the hysteresis loss of the strands with different LARs and different heat treatments are investigated. It is a reference for fabricating the qualified Nb3Sn strands for ITER project in large scale in WST.

Strand for ITER at WST

As one of low temperature superconducting materials, multifilamentary Nb₃Sn strand is an ideal choice for high-field superconducting magnets (>10 T). The International Thermonuclear Experimental Reactor (ITER) has special requirements for the Nb₃Sn strands: high critical current density and low hysteresis loss. Multifilamentary Nb₃Sn strand with high performance has been developed by bronze process. The highest non-Cu J_cn(12 T, 4.22 K, 0.1 ¿V/cm) value of 812 A/mm² has been obtained. The hysteresis loss of the strand is 380 mJ/cm³ with 650°C ×100 h. The properties of the Nb₃Sn strand can meet the specifications proposed by the ITER project.

Investigation of Superconducting Properties of

ITER toroidal field (TF) systems consist of 18 independent coils that are around the torus, whose primary function is to confine the plasma particles. The TF coil conductor is a cable-in-conduit conductor (CICC) made up of superconducting, Nb3Sn-based strands mixed with pure copper strands. As the only supplier in China, Western Superconducting Technologies Company, Ltd. (WST) will supply TF Nb3Sn strands using internal tin route for ITER, and over 6,000 km of Nb3Sn strands have been produced in the past four years. Main performance of Nb3Sn strands, including critical current, n value, wire diameter, Cu/non-Cu ratio, hysteresis loss and RRR are reported and analyzed in this paper.

{\rm Nb}_{3}{\rm Sn}

Three kinds of Nb3Sn strands with different designs were made by bronze route. Studies were carried out on bronzeto-NbTa-volume ratio, filament diameter, annealing, hot isostatic pressure process, diffusion barrier, and heat treatment. It is important for strand designs to achieve a good performance. Moreover, bronze-to-NbTa-volume ratios of 2.4 and 2.8 were applied in the strand designs. Strands with lower bronze ratio means that more Nb3Sn layer fraction formed, while, at the same time, it is provided with a high critical current density (Jc). Jc increases with the reduction of filament diameter, when there is the same bronze ratio. In addition, Nb and Ta were used as diffusion barrier materials. A strand that used Nb barrier has several times hysteresis loss to a strand that used Ta barrier. When it comes to the intermediate processes, such as annealing and hot isostatic pressure, they could affect the homogeneous deformation and properties of strands. It is important to keep a low annealing temperature to realize a high Jc and n-value. As for the hot isostatic pressure process, it is helpful for uniform cross section. Furthermore, different heat treatments were also carried out, while analysis was also conducted on the microstructure.microstructure.

Strands by Internal Tin Process for ITER

Multistep heat treatments are required to produce the superconducting Nb3Sn in the International Thermonuclear Experimental Reactor toroidal field coils; however, deviations in the temperature and dwell time during heat treatment of the big conductors are unavoidable, and these could affect the performance of the Nb3Sn strands. To investigate the influence of heat treatment tolerances, both internal-Sn- and bronze-process-type Nb3Sn strands were heat treated with different cycles. For the internal-Sn process strands, the critical current density Jcn increases as the temperature increases from 630 °C to 650 °C and remains unchanged at 670 °C for 100 h. The Sn content in the filament increases with increasing temperature, and the grain sizes significantly increase from an average of 130-202 nm from 630 °C to 670 °C. For both the internal-Sn process strands and bronze route strands, Jcn seldom changes when the duration at 650 °C is increased from 100 to 200 h. Despite these changes, this study shows that Nb3Sn strands are not very sensitive to small heat treatment variations at 650 °C, and a variance of ±5 °C is acceptable for both types of Nb3Sn strands.

Investigation of

The optimization of critical current density in NbTi superconductors involves a complex schedule of heat-treatments, which precipitate normal conducting alfa-Ti precipitates and reduce the dimensions of these precipitates to the optimum size for flux pinning. Final drawing strain is the most important parameter for the optimization of the critical current density. In this paper, high homogeneity Nb47Ti alloy was applied for the fabrication of the NbTi superconductors. The NbTi samples were having the same intermediate stage heat-treatment schedule just varied in the final drawing strain ranging between 4 to 5.5. The critical current of these samples were tested at 4.2 K under different magnetic fields ranging between 2 to 8 Tesla. Based on the results, it was found that the optimal final strain for the NbTi/Cu samples was about 4.8. Jc decreased abruptly after the maximum value has been passed. The effect of the final drawing strain on the Jc in the low magnetic field region was more significant than which in the high magnetic fields.