A solution for transverse load degradation in ITER Nb3Sn CICCs: verification of cabling effect on Lorentz force response
A solution for transverse load degradation in ITER Nb3Sn CICCs: verification of cabling effect on Lorentz force response
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DOI：
10.1088/0953-2048/21/5/054011
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发表时间：
2008-03
影响因子：
3.6
通讯作者：
A. Nijhuis
中科院分区：
文献类型：
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作者：
A. Nijhuis
本文介绍了专为ITER型导管内电缆导体开发的新型横向电磁负载优化模型（TEMLOP）的最新结果。国际热核实验堆（ITER）用的Nb3Sn cciccs随着电磁载荷的增加，其性能明显下降。复合材料的热收缩差异不仅影响临界电流（Ic）和温度裕度，而且电磁力在Nb3Sn层中引起显著的横向链接触和弯曲应变，导致局部丝裂和永久性退化。2006年5月提出的先验TEMLOP预测的最基本特征是，通过增加后续布线阶段的节长和减少空隙率，可以大大、直接地改善CICCs的严重退化。这些纠正措施给予更多的支持股，充分减少应变，因此避免细丝损坏在股交叉点在电缆。这是第一次提出增加缆索扭距，当时没有实验证据。2006年秋季制造的全尺寸欧洲TF导体原型样品（TFPRO-2）根据这一新的见解进行了调整，并于2007年4月在苏丹进行了实验验证，以验证预测。结果非常突出：首次实现了基于单链特性的Nb3Sn CICC导体的性能，具有高n值且无退化迹象。作为输入，除了电缆的性能外，该模型还直接使用单轴应力应变、周期性弯曲和接触载荷下的单股电缆的测量数据。本文介绍了用TARSIS装置获得的用于制造TFPRO-2的ITER OST链的最新测试结果。根据这些最新的钢绞线结果，该模型证实，不仅钢绞线弯曲会导致退化，而且，根据钢绞线和电缆的布局，钢绞线接触应力也可以发挥关键作用。TEMLOP表明，所提出的ITER参考TF导体布局中，第一级三组扭转节距为45 mm的扭转节距方案和空隙率实际上是最坏的情况。研究还表明，缩短间距可以改善性能，但这需要每米复合导体使用更多的Nb3Sn材料。然而，现在已经有实验证明，所提出的改变使ITER TF导体的工作裕度恢复到预期的链性能。ITER TF导体规格正在调整中，由于不再需要补偿退化，因此可以在导线设计上节省大量费用。
We present the latest results of the novel model for transverse electromagnetic load optimization (TEMLOP) especially developed for the ITER type of cable-in-conduit conductors (CICCs). The Nb3Sn CICCs for the International Thermonuclear Experimental Reactor (ITER) showed a substantial degradation in their performance correlated with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current (Ic) and temperature margin, but electromagnetic forces cause a significant transverse strand contact and bending strain in the Nb3Sn layers, resulting in localized filament cracking and permanent degradation. The most essential feature of the a priori TEMLOP predictions presented in May 2006 is that the severe degradation in CICCs can be improved greatly and straightforwardly by increasing the pitch length in subsequent cabling stages and by reducing the void fraction. These corrective measures give more support to the strands, sufficiently reduce the strain, and therefore avoid filament damage at the strand crossover points in the cables. It was the first time that an increase of the cable twist pitches has been proposed and no experimental evidence was available at that time. A full-size European prototype TF conductor sample (TFPRO-2), manufactured in autumn 2006, was adapted according to this new insight and tested in April 2007 in SULTAN for experimental validation of the predictions. The results were outstanding: for the first time an Nb3Sn CICC conductor achieved the performance that can be expected based on the single-strand properties, with high n value and no sign of degradation. As input, besides the cable properties, the model directly uses the measured data from single strands under uni-axial stress and strain, periodic bending and contact loads. The recent test results of the ITER OST strands used for the manufacture of the TFPRO-2 obtained with the TARSIS set-up are presented. With these most recent strand results, the model substantiates that not only strand bending is causing degradation but, depending on the strand and cable layout, the strand contact stress can also play a critical role. TEMLOP demonstrates that the twist pitch scheme and void fraction, of the proposed ITER reference TF conductor layout with a first-stage triplet twist pitch of 45 mm, turns out to be practically a worst-case scenario. It is also shown that shorter pitches can lead to an improvement but this requires more Nb3Sn material per metre composite conductor. However, it has been experimentally proven now that the proposed changes recover the ITER TF conductor operational margin up to the expected strand performance. The ITER TF conductor specification is being adapted now and it becomes possible to gain significant savings on the strand design, as degradation no longer needs to be compensated for.