Speaker
Description
The authors have designed a 1-MJ-class mobile superconducting magnetic energy storage (SMES) system. The SMES coil features a force-balanced helical coil (FBC) to reduce weight, as the FBC minimizes the mass required to support the induced electromagnetic forces.
In this study, the authors propose using 0.67-mm diameter MgB2 strands, manufactured by Hitachi, Ltd., for winding the FBC. Initially, the FBC was designed with an MgB2 Rutherford cable to enhance the current capacity of the winding. However, the bending strains experienced by MgB2 strands during the fabrication of Rutherford cables can degrade the critical current of the superconducting wire. To address this issue, the authors suggest an alternative approach: winding the FBC with a stranded cable that induces smaller bending strains of MgB2 strands.
The proposed stranded cable consists of 12 MgB2 strands twisted around a central copper strand with a diameter of 1.92 mm. Unlike Rutherford cables, this design eliminates the need for significant deformation of the MgB2 strands at the edges, thereby reducing bending strains. As a result, the impact of deformation on critical current degradation is minimized, enhancing the overall current capacity of the cable.
Numerical analysis indicates that the bending strains of the MgB2 strands in a stranded cable with a twist pitch of 80 mm are less than 0.5%. This suggests that the allowable current for the stranded cable will be higher than 1200 A at 20 K.
In this presentation, the authors will report on the critical current experiments conducted on MgB2 strands that were bent prior to heat treatment and will discuss the critical current characteristics of MgB2 strands under bending strains before heat treatment at first. During these experiments, the MgB2 strand samples were cooled using a two-stage GM refrigerator, maintaining the temperature at 20 K. The critical current was measured using the four-terminal method.
Additionally, the authors will present experimental results on the critical current of MgB2 stranded cables with varying twist pitches and bending conditions. Based on these experimental findings, the study aims to evaluate the allowable bending strain for MgB2 wires and discuss design considerations for MgB2 cables and the force-balanced helical windings.
