Enhancement of current density in superconducting fusion magnets by a copper-clad stainless-steel conduit

1.热点温度是管内电缆导体设计的重要指标之一。一般来说，束中作为稳定剂的铜的数量是由允许的热点温度决定的，这与导体中具有高电流密度相矛盾。在本研究中，提出了一种铜包导线。在淬火过程中，束和铜包层之间的电流共享可以减小束内的电流。这降低了热点温度。为了评估淬火电缆导管导体的温升，我们使用两种实验技术测量了电缆束和导管之间的热接触电导。首先，在冷却到液氮温度的短导体上施加电流。在升温过程中，由束和导管之间的温差得到的电导几乎与温度无关。在第二个实验中，测量了室温下表面压力下的电导。结果表明，电导受表面压力的影响较大。环境气体电导率的影响相对较小。从这些结果中，我们得出结论，有必要测量与预期电磁力等效的表面压力下的电导。因此，热接触电导的估计在设计阶段被认为是困难的。利用零维热平衡方程分析了铜包层对模拟导体的影响。模拟导体具有直径为50mm的圆形截面，在不锈钢导管外包有1 mm厚的铜包层。分析表明，即使管束与管道之间存在较高的热阻，也有可能降低热区温度。

1.The hotspot temperature is one of the most important design criteria for cable-in-conduit conductors. In general, the amount of copper, as a stabilizer, in a bundle is determined by the allowable hotspot temperature, which contradicts to have a high current density in the conductor. In this study, a conductor with a copper-clad conduit is proposed. Current sharing between the bundle and copper cladding can reduce the current in the bundle during a quench. This reduces the hotspot temperature.2.To evaluate temperature rise in a quenching cable-in-conduit conductor, we measured the thermal contact conductance between the bundle and the conduit using two experimental techniques. First, a current was applied to a short conductor cooled to liquid nitrogen temperature. The conductance, obtained from the temperature difference between the bundle and the conduit during temperature rise, was nearly independent of temperature. In the second experiments, conductance was measured under surface pressure at room temperature. The results confirmed that the conductance is strongly affected by the surface pressure. The effect of the conductivity of ambient gas was relatively small. From these results, we conclude that it is necessary to measure the conductance under a surface pressure equivalent to the expected electromagnetic force. The estimation of the thermal contact conductance is therefore considered difficult at the design phase.3.The effect of copper cladding was analyzed for simulated conductors using zero-dimensional heat-balance equations. The simulated conductors have a circular cross-section with a diameter of 50 mm and a 1-mm-thick copper cladding outside a stainless-steel conduit. The analyses demonstrated the possibility of reducing the hotspot temperature even though high thermal resistance exists between the bundle and conduit.