Theoretical Investigation of Current-Sharing Properties of Three-Strand Parallel Conductors Composed of REBa₂Cu₃O_y Superconducting Tapes

摘要

For nuclear magnetic resonance superconducting magnets wound with REBa2Cu3Oy (REBCO, where RE = rare earth elements such as Y, Gd, Eu, and so on) superconducting tapes, the current capacity of the tapes needs to be enhanced from the viewpoint of magnet protection. Thus, the introduction of the configuration of parallel conductors was proposed. The constituent tapes should be insulated and transposed for low ac loss and uniform current sharing among the tapes. However, when the tapes have different values of critical current, I-c, and their n-values, the tapes will not have uniform transport currents. In this paper, the current-sharing properties of three-strand parallel conductors composed of REBCO tapes, which were wound into coils, were theoretically investigated by accounting for the spatial distribution of the magnetic field, i.e., magnetic field and field angular dependences of I-c. Here, it was assumed that the I-V characteristics are expressed by the n-value model. Equivalent circuit equations were derived and numerically solved. As a result, it was revealed that at a frequency higher than 0.01 Hz, the current flow is almost uniform even if there is a variation of 10% in I-c and the n-value among the tapes. On the other hand, in the case of dc, the current sharing depends on the dispersion of I-c and the n-value. In this paper, a simple solution for the control of the contact resistance at the terminals among the strands was proposed.