Huixian Gao

New Progress of Nb 3 Sn Strand Production for ITER in WST

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.

Study and Manufacture of Nb 3 Sn Strands by Bronze Route

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.

An Investigation Into the Heat Treatment Tolerance of WST Nb 3 Sn Strands Produced for Massive Fusion Coils

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.

Study of NbTi Superconducting Strands for JT-60SA Toroidal Field Coils

JT-60SA is a fusion experiment designed to support the operation of ITER and to investigate how best to optimize the operation of fusion power plants that are built after ITER. The toroidal field (TF) coils uses the Cable-in-conduit Conductor (CICC) with NbTi superconducting strands. The TF coils will operate at with 25.7 KA @5.65 T, 4.9 K. Western Superconducting Technologies Co., Ltd. (WST) has studied two types of NbTi superconducting strands for the JA-60SA TF coils. The strands choose the internal CuNi alloy for the internal resistive barrier, which is expected to calibrate and control the inter-strands resistances and to better current distribution between strands in CICC. The strand samples were tested in the range of 4 11 T magnetic field at 4.22 K for critical current, and 5.7 K 6.4 K at 5.65 T for current sharing temperature. This paper presents the results on critical current(Ic), critical current density (Jc), current sharing temperature (Tcs), and hysteresis loss of the strands.