電流再配分を伴うＣＩＣ型超電導撚線導体の安定性評価法

Abstract

Cable-in-conduit conductors (CICCs), composed of multi-strand superconducting cables cooled with supercritical helium, are essential to superconducting magnets that require large current capacities, low AC losses and high rigidity for fusion machines. A conductor cabled by several hundreds of superconducting strands with superficial chrome plating, oxide or CuNi result in the problem of unbalanced current distribution among the strands. This is caused by small differences in the self-inductance of each strand as well as an electrical resistance difference several times larger at the portion of electrical contacts between each strand and the current leads. This unbalanced current decreases the minimum quench energy (MQE) under the case of uniform current distribution. The MQE of CIC multi-strand cables under unbalanced current distribution is greatly affected by the electrical resistance among the strands. This means that the stability is influenced by the current-sharing process among strands. A higher margin of stability is induced by increasing current sharing and more rapid current transfer to the neighboring strands rather than thermal diffusion to helium. The most suitable parameter to govern this process is the impedance between strands. A simplified electrical circuit model simulating the current-sharing process among strands was proposed to estimate stability. The circuit parameters, that is the impedance, governing the sharing process were measured easily using short samples of CIC cables at 4.2K. The experimental results of stability tests show that the CIC has an extensive stability margin according to lower impedance between strands. We also confirm that the proposed method is applicable to estimate the stability of CIC.