Investigation on Irregular AC Losses in Large Superconducting Coils

发现钢管内电缆超导体的不规则交流损耗不能从短导体样品试验结果中估计出来。电流回路必须不规则地在电缆中形成，随着长时间常数而衰减，从而增加了交流损耗。我们提出了一种形成长回路的机制。该电缆由若干分段子电缆组成。如果子电缆表面的一股与相邻子电缆表面的另一股接触，则两股应在所有分段电缆节距的LCM距离处再次相遇，从而形成一对长环路。这些回路的时间常数基本上被描述为它们的电感与接触电阻的比值。为了估计长LCM回路的时间常数τ=L/R，我们测量了LHe中两股链之间的交叉接触电阻。试验结果表明，交叉接触电阻约为50 μΩ。结果表明，时间常数在0.1 s左右，比实测的5 ~ 100 s时间常数短。这是因为与线接触面积（cm数量级）相比，交叉接触面积（弹性极限约为10 μm）非常小。为了更详细地研究接触条件，我们制作了由81股组成的电缆，其中孔隙率为40%，并确定了电缆中所有股的三维位置。发现有些回路有几个大于10mm的线接触。这一事实表明，应该形成具有长时间常数的循环。

It is found that irregular AC losses in cable-in-conduit superconductors cannot be estimated from short conductor sample test results. Current loops, which must be irregularly formed in the cable, decay with the long time constants, and hence enhance the AC losses.We propose a mechanism forming the long loops. The cable is composed of several staged sub-cables. If one strand on the surface of a sub-cable contacts with the other strand on the surface of the adjacent sub-cable, the two strands should encounter each other again at LCM distance of all staged cable pitches, and thereby result in forming a pair of a long loop. The time constants of these loops are fundamentally described as ratios of their inductances to their contact resistances.In order to estimate the time constants τ=L/R of the long LCM loops, we measured the cross contact resistance between the two strands in LHe. The test results showed that the cross contacting resistances were about 50 μΩ. The results suggest that the time constants are around 0.1 s, which is shorter than the measured time constants of 5 to 100 s. This stems from the fact that the cross contact area, which is estimated about 10 μm in the elastic limit, is very small compared with the line contact area, which is the order of cm.In order to investigate the contact conditions in more detail, We fabricated a cable composed of 81 strands with 40% void fraction, and identified 3-dimensional locations of all strands in the cable. It is found that some loops have several line contacts with more than 10mm. This fact demonstrates that the loops with long time constants should be formed.