Development of Superconducting Coils using (Ba, Na)Fe₂As₂Round 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 Ic 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, Jc of a short segment of HIP wire reaches 42 kAcm-2, which is comparable to the highest Jc of IBS round wires of 44 kAcm-2. Furthermore, Ic 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. Ic 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.