Abstract Body

The authors conducted a design study on a 1-MJ class mobile superconducting magnetic energy storage (SMES) system for on-site eigenvalue measurement of electrical power systems. The SMES coil consists of a force-balanced helical coil (FBC) for mobility and weight saving because the FBC can minimize the mass of the structures to support the induced electromagnetic forces.

The authors consider the use of MgB₂ superconducting wires from the viewpoint of reducing the refrigeration power requirement and the manufacturability of high-current conductors such as Rutherford cables. However, the bending strains imposed on the MgB₂ strands during the fabrication of Rutherford cables degrade the wire's critical current [2]. In addition, because the FBC has three-dimensional complex windings, the critical current of the MgB₂ strand degrades due to the bending strain caused by the complexity of the helical coil trajectories. Therefore, it is necessary to investigate the allowable strain of MgB₂ strands to fabricate Rutherford cables and helical coils.

In this research, the authors use the 0.67-mm diameter MgB₂ strands, fabricated by Hitachi, Ltd. [3]. The allowable strain after heat treatment of 0.67-mm diameter MgB₂ strands was confirmed to be 0.19% [4]. However, the Rutherford cables are generally fabricated before the heat treatment. Therefore, as a first step in this study, the authors evaluated the critical current characteristics of MgB₂ strands under the bending strains expected during the fabrication of Rutherford cables.

In this measurement experiment, the MgB₂ strands samples are cooled using a two-stage GM refrigerator. Figure 1 illustrates the cooling system and the experimental setup for evaluating the critical current properties of the MgB₂ strands. The cooling temperature ranges from 10 K to 20 K, controlled by a film heater placed on the second stage of the GM refrigerator. Four slots with different radii of curvature are cut into the surface of the sample plate. Four MgB₂ wire samples are set in the slots and heat-treated while applying the bending strain. These wire samples are connected in series. After heat treatment, the sample plate and the wire samples are set on the second stage of the GM refrigerator. The critical current is measured by the four-terminal method.

Based on the experimental results, this work plans to evaluate the allowable bending strain of the MgB₂ wires and to discuss the design considerations for the MgB₂ Rutherford cables and helical coil.

References

[1] S. Nomura, et. al., "Operation scenario of mobile SMES for on-site eigenvalue measurement of electric power system," IEEE Trans. Appl. Supercon., vol. 28, no. 3, Jun. 2018, Art. No. 441807.
[2] T. Yagai, et. al., "Development of design for large scale conductors and coils using MgB₂ for superconducting magnetic energy storage device," Cryogenics, vol. 96, pp. 75-82, 2018
[3] H. Tanaka, et. al., "Performance of MgB₂ Superconductor Developed for High-Efficiency Klystron Applications," IEEE Trans. Appl. Supercon., vol. 30, issue 4, Jun. 2020, Art. No. 6200105.
[4] H. Tanaka, et. al., "Influence of Sintering Conditions on Bending Tolerance at RT and Ic of In Situ MgB₂ wire," IEEE Trans. Appl. Supercon., vol. 29, issue 5, Jun. 2019, Art. No. 8401104.

Acknowledgment

This work was supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research(B) (Grant Number 22H01477).

Figure 1. Schematic illustration of the cooling equipment (a) and the critical current measurement of curved MgB₂ samples.

Keywords: Superconducting magnetic energy storage (SMES), MgB₂, Bending strain, critical current test, GM refrigerator