Development of Superconducting Coils using (Ba, Na)Fe₂As₂ Round Wires with Large Critical Current

We report the fabrication of 10 m-class (Ba, Na)Fe₂As₂ long round wires and coils using these round wires. Round wires are fabricated by powder-in-tube (PIT) method and hot-isostatic-press (HIP) technique. To increase I_c keeping the diameter of the wire constant, the cross section area of the superconducting core is enlarged by changing the dimensions of metal sheaths. At 4.2 K under a high field of 100 kOe, J_c of a short segment of HIP wire reaches 42 kAcm^-2, which is comparable to the highest J_c of IBS round wires of 44 kAcm^-2. Furthermore, I_c of the HIP wire at 4.2 K under 10 kOe and 100 kOe reaches 95 A and 54 A, respectively, which is about twice the value of the previous report. We also fabricated a small demonstration superconducting coil using 12.5 m long PIT wires by HIP method. I_c of the whole coil is ∼60 A under the self-field, and magnetic field at the center of the coil reaches 2.6 kOe. The coil generated the magnetic field of 43 Oe/A, which is almost consistent with the calculated field from the coil dimensions. (paper)