Development of MgB<sub>2</sub> Rutherford Cable for Large-scale SMES Coils Cooled by Liquid Hydrogen

Abstract

To realize the practical application of Superconducting Magnetic Energy Storage (SMES) systems cooled by liquid hydrogen, MgB2 is promising as a material for SMES coils in terms of reducing cost in the future and enabling high critical temperature above the temperature of liquid hydrogen. Major technological development issues to realize the application of SMES coils are fabrication of the Rutherford superconducting cable with a kA-class current capacity and fabricating the coils using both Wind and React (W&R) and React and Wind (R&W) methods. Since there is little data regarding the allowable bending strain of the MgB2 strand before heat treatment, an experiment was conducted and it was determined to be approximately 4 % before the fabrication of the cable. The Rutherford cable was designed and developed based on analysis of the space curve theory because the complex bending strain is exposed to the MgB2 filaments during the cable and coil fabrication processes. However, the critical currents measured using cables developed according to the design were degraded approximately 30%. This degradation can be explained as being due to local deformation of the strands caused by compression at the cross-over point between the strands of the conductor. The broken sections of the Nb barrier, which were observed by reviewing SEM images of the strand cross-section, can cause degradation of the critical current. Two double pancake coils, which were primarily wound using the Rutherford cables and manufactured applying both the R&W and W&R methods for the purpose of comparing fabrication processes, were connected in series to excite the coils. As a result, an operating current of 600 A was successfully achieved.