Coupling current control in Rutherford cables wound with NbTi, Nb3Sn, and Bi:2212/Ag

Calorimetric measurements of AC loss have been performed on Rutherford cables wound with NbTi, Nb₃Sn, and Bi:2212/Ag strands, respectively. For the NbTi cables Ni and stabrite coatings had been applied, while for the Nb₃Sn and Bi:2212/Ag cables the strand surfaces were just bare Cu or Ag, respectively. Most of the cables contained resistive cores: ribbons of kapton, or titanium (NbTi), stainless steel (NbTi and Nb₃Sn), and nichrome-80 (Bi:2212/Ag). In all cases the cores were found to lead to a strong suppression of the face-on (field normal to the broad cable face) coupling current loss; to such an extent that even the Bi:2212/Ag cables, which would otherwise be severely cross-sintered, show the possibility of acceptable coupling loss. Effective crossover interstrand contact resistances are calculated and normalized to a standard cable specification (‘LHC inner') for the purpose of intercomparison.     

Segregation of copper in the ITER cable and ramp rate limitation

The quench of a superconducting cable containing a defect and subjected to a current increase is studied. The cable is represented by a model which consists of a strand with a local decrease in the critical current I_c and the rest of the cable lumped together. It is assumed that the quench takes place when the electric field in the defect reaches the instability level. An important feature of the process is the noticeable portion of current expelled from the defect to the rest of the cable prior to the quench. It is shown that given the strong inequality in the strand currents during the ramp, the influence of the defect on the quench current is proportional to I_c⁴. This criterion is applied to the problem of copper separation in the triplets of the ITER cables.     

Current distribution in multistrand superconducting cables — experimental methods and results

It is widely acknowledged that current non-uniformity is a major source of reduction of quench currents in superconducting multistrand cables at non-steady state conditions. Recently we performed several experimental studies on the current non-uniformity in CICC and AC cables. In this paper we present the summary of the experimental methods used. Indirect methods with use of local magnetic field sensors can provide general information about non-uniformity inside CICCs and other large size cables. Indirect methods may be used in real superconducting devices. Direct measurements of the current in each strand provide exact information about the current distribution, but they need special sample preparations. Because no method is perfect, the best idea is to use them as complementary to each other in the study of a certain type of a cable. The results obtained from the measurements of the current non-uniformity in CICC and AC cables by both methods are briefly discussed.     

Current carrying capability of HTS Roebel cable

We have developed a numerical technique to estimate the current carrying capability of HTS Roebel cable composed from coated conductor strands. The influence of self-field on the critical current density is studied computationally for a Roebel cable using a realistic field and angle dependence behaviour of critical current. The computations are carried out for N/2 (number of strands/strand width in mm), and N/5 Roebel cable for N = 2–15. The local current distribution in each strand satisfies the self-consistent criteria J = J_c(B(J)) except for a small region where the current density is set to zero to maintain the condition of equal currents in all strands. The variation of critical current with vertical and horizontal separation between the strands is also investigated. Results are compared with the measured values of critical current for some of our cables. The comparison shows an error of up to 10% which we attribute mostly to the model not accounting for the spread in I_c values of the constituent strands.     

Stability measurements on cored cables in normal and superfluid helium

The relative stability of LHC type cables has been measured by the direct heating of one of the individual strands with a short duration current pulse. The minimum energy required to initiate a quench has been determined for a number of cables which have a central core to increase the effective inter-strand cross-over resistance. Experiments were performed in both normal helium at 4.4 K and superfluid at 1.9 K. Conductors in general are less stable at the lower temperature when measured at the same fraction of critical current. Results show that the cored-cables, even when partially filled with solder or with a ‘porous-metal' filler exhibit a relatively low stability at currents close to the critical current. It is speculated that the high inter-strand electrical and thermal resistance inherent in these cables may affect the stability at high currents.     

CFETR馈线导体设计与热稳定性分析

CFETR馈线导体设计采用NbTi超导线材管内电缆(CICC)结构,其中超导电缆主要由6个花瓣形状的电缆和一个中间铜芯绞制而成,而花瓣形状的电缆由NbTi超导线材与铜线沿顺时针方向通过4级绞制而成,导体外部采用不锈钢铠甲结构。计算分析了运行工况下导体结构的热稳定性参数。结果显示,在95.6kA的最高载流下,分流温度低于6K,热点温度低于250K,满足设计要求。     

14T全身超导MRI磁体的技术挑战——大规模应用强场超导磁体未来十年的发展目标之一

本文首先综述了大规模应用的超导磁体,依赖并推动铌三锡Nb3Sn导线技术进步,向更强磁场发展的趋势.着重分析了超高场14 T全身MRI磁体的挑战性技术.选择青铜Nb3Sn导线,采用Nb3Sn线圈和NbTi线圈相结合的混合结构,对14 T全身MRI磁体进行了电磁概念设计和热稳定性及失超保护仿真分析,并简要阐述了14 T全身MRI磁体在应力、接头和匀场方面的关键问题.根据分析结果认为:1)Nb3Sn导线是14 T全身MRI磁体需要面临的首要挑战性问题—作为最佳选择的青铜Nb3Sn导线,其现有产品的性能指标离14 T全身MRI磁体的要求尚存在有一定的差距;2)14 T全身MRI磁体的失超保护涉及线圈的铜超比设计、运行电流同线圈电感的协调配置、被动保护的分段策略和主动保护的失超触发控制以及主动屏蔽结构磁体在失超过程中的逸散磁场限制等多个十分复杂的环节,是最具挑战性的综合性技术.     

AC losses and stability on large cable-in-conduit superconductors

The cable-in-conduit superconductors are preferred for applications where the AC losses and stability are a major concern, e.g., fusion magnets and SMES. A review of coupling currents loss results for both NbTi and Nb₃Sn cable-in-conduit conductors (CICC) is presented and the AC loss relevant features are listed, with special emphasis for the role of the interstrand resistance and strand coating. The transient stability approach for CICCs is discussed and the analytical models are quoted as well as the relevant experimental database. The likely spectrum of transient disturbance in CICC is reviewed and the need to account for interstrand current sharing in the design is outlined. Eventually a practical criterion for the interstrand resistance is proposed to link the stability and AC loss design.     

Considerable rise in the stability of combined superconductors doped by intermetallic compounds with an extremely high low-temperature specific heat

The influence of doping intermetallics based on rare-earth elements and ceramics with an extremely high low-temperature specific heat on the stability of combined NbTi superconductors against electromagnetic disturbances is studied experimentally and theoretically. The objects of investigation are standard NbTi conductors (0.85 mm in diameter) in a copper matrix that are soldered to copper wires containing high-specific-heat dopants. CeCu₆, HoCu₂, CeAl₂, and PrB₆ intermetallics (at 4.2 K, their specific heat is on average 150 times that of copper) or Cd₂O₂S ceramics (at 4.2 K, its specific heat is higher than that of copper by 600 times) are introduced into combined superconductor samples in the form of fine powder. The powder is placed into the copper wire either as one thick (0.6 mm in diameter) strand or as 19 thin (0.1 mm in diameter) strands. An undoped reference sample contains a solid conductor. Samples with a transport current placed in an external transverse magnetic field are subjected to longitudinal magnetic disturbances causing pulsed heating of the samples by eddy currents. The disturbance time is varied in a wide range: from 50 μs to 1.2 ms. To compare the critical energy densities of the disturbances in differently shaped samples, a mathematical technique is developed that is based on analytical solution of the equation of electromagnetic diffusion into the sample. It is found that the critical energy density of doped samples is much higher (by several times) than that of the reference sample. Upon direct cooling of the samples by liquid helium in a vertical channel (the most intense heat removal under steady-state conditions), the effect of stability improvement for the doped samples persists. Moreover, it is shown that the high-specific-heat dopants, raising the heat needed to warm the sample, restrict the heat flux into the liquid and thereby increase the energy removed by the coolant during unsteady heat transfer.     

NbTi/Bi2223复合超导体制备工艺的研究

采用合适的工艺制备NbTi/Bi2223复合超导体,制备完成后在液氮温度下,对复合带中的高温带Bi2223/Ag进行IC测试,证明这种工艺不会对Bi带造成破坏,工艺可行。在进行IC测试时,需要注意NbTi/Bi2223复合超导体的换流长度,因此计算了NbTi/Bi2223复合超导体在液氮温度下的换流长度。     

The similarities in loss creation in LTS and HTS cables

The coupling current losses represent an essential contribution to AC losses in most practical superconducting conductors. The origin of this loss type is well known, being caused by induced currents in different loops consisting of superconducting and non-superconducting parts. However, the ‘current pattern' in different conductor types (strands, flat or round cables, more complicated cable structures, CICC) varies appreciably. These differences are mainly due to geometrical effects (size and shape of filaments and/or strands, their spatial distribution, conductor aspect ratio, demagnetization effects). Although the general knowledge about AC losses in low temperature cable structures is by far not complete (mainly at higher frequencies, in inhomogeneous fields and for inhomogeneous cable structure), an attempt is made to summarize those results which can be adopted to high T_c conductors and some remarks are made about new features of AC losses in these conductor types.     

中心螺管模型线圈交流损耗计算及测试

中心螺管模型线圈选用 Nb Ti/ Cu复合超导材料 ,超导股线直径 0 .85 m m,Cu截面积与 Nb Ti截面积比为 1.38;经过 4级绞缆和成型 ,导体尺寸为 17.4 mm× 17.4 m m,中心冷却孔直径 4 .1m m。线圈用超临界氦冷却 ,流体进口温度为 3.8- 4 .2 K,压力 6 bar,每个流道流量 2 .2 g/ s。用量热法对中心螺管模型线圈进行了交流损耗测试并与计算值相比较 ,3次交流损耗电流分别是 :30 0 A/ s速度励磁和放电 ,最大电流 5 k A ,交流损耗测量值为每脉冲 78.3J;30 0 A / s速度励磁和放电 ,最大电流 7k A ,交流损耗测量值为每脉冲 14 0 .7J;4 0 0 A/ s速度励磁和放电 ,最大电流 7k A,交流损耗测量值为每脉冲14 5 .4 J;计算值分别为每脉冲 98J,1385 J,14 2 J;两者较好相符。     

内封光纤高温超导CORC©缆线电磁特性仿真研究

内封光纤高温超导 CORC?? (Conductor on Round Core) 缆线在传统 CORC?? 缆线结构的基础上, 将光纤嵌于铜芯骨架内, 不仅能够保持原有集束缆线表面形貌的完整性, 还能通过外置分布式光纤传感解调设备, 实现对缆线失超的快速检测. 然而, 改变缆线结构有可能引起缆线电磁特性发生变化, 继而影响高场磁体的设计与应用. 本文以高温超导CORC?? 缆线为研究对象, 采用有限元仿真技术研究了直流供电的内封光纤CORC?? 缆线在正常运行工况下的电磁场分布特性, 通过对比普通CORC?? 缆线, 分析了内封光纤对CORC?? 缆线电磁性能产生的影响. 研究结果表明, 两种缆线在正常运行工况下的电流密度及磁通密度分布特性较一致, 初步论证了内封光纤高温超导CORC?? 缆线在高场磁体应用的可行性.     

用于ITER的NbTi超导股线临界电流测试方法

介绍了一种用于ITER的NbTi超导股线在不同温度下临界电流测试的方法和装置。该装置包括一孔径为70mm、最高磁场高达16T的背景磁体以及变温杜瓦等系统。对NbTi超导线变温测试设计了二元电流引线。用该系统得到了其NbTi超导股线在不同磁场不同温度下的临界电流,用“Luca”定标律拟合了测试结果,并对测试结果进行了分析。     

小型NbTi磁体的失超传播特性研究

针对小型实验NbTi超导磁体,通过建立螺线管线圈正常态区域传播的数学模型来定性描述其三维传播特性,并就磁体失超的实验波形做深入分析,阐述绕组匝间及层间的热传导效应对磁体失超传播速度的影响,掌握实验磁体失超过程中的内阻变化曲线,为磁体的失超信号检测与保护奠定良好的理论基础。     

超导股线Nb3Sn的性能测试研究

随着超导磁体的不断开发应用,对高临界电流密度超导材料的需求不断增长,以金属间化合物为基体的超导材料Nb3Sn具有特殊的实际意义,其制成的导体临界性能高于NbTi导体,Nb3Sn股线也是ITER磁体的关键组成部分.为了选择超导磁体合适的运行参数以及确定其稳定运行的范围,了解其超导特性是必要的.根据磁体设计所用标准,磁体运行时性能与股线的性能密切相关.本文介绍了一种测试Nb3Sn股线临界性能的方法,实验采用四引线法进行,测试中对样品提供了一个垂直方向的背景磁场,其大小可从0 T变化到16 T,实验时样品置于变温杜瓦内,温度调节通过控制进入变温杜瓦的氦气量来实现,可使温度变化小于0.01 K.对测试结果运用Summer定理进行了拟合并加以分析.     

二元液态金属Cu-Ni和Ag-Cu凝固过程的分子动力学模拟研究

用Quantum Sutton-Chen多体势对Ag6Cu4和CuNi液态金属凝固过程进行了分子动力学模拟研究.在冷却速率2×1012到2×1014K/s范围内,CuNi总是形成fcc晶体结构,而Ag6Cu4总是形成非晶态结构.考虑到CuNi及AgCu中原子半径之比分别为1.025和1.13,那么模拟结果证实了原子的尺寸差别是非晶态合金形成的一个主要影响因素.此外采用键对及原子多面体类型指数法对凝固过程中微观结构组态变化的分析,不但能说明二十面体结构在非晶态合金形成和稳定性中所起的关键作用,又有助于对液态金属的凝固过程、非晶态结构特征的深入理解.     

热等静压对NbTi/Cu多芯超导线加工过程的影响

采用传统方法生产的NbTi/Cu多芯超导线在生产芯数较多的超导线时存在填充系数较大的问题,这直接影响了芯丝的整齐排布,所以组装完成后加入热等静压过程.研究了经过热等静压和未经过热等静压的NbTi/Cu多芯超导线的加工性能.经过试验证明,经过100～150MP,2小时热等静压处理的NbTi/Cu多芯超导线坯锭在加工过程中具有良好的加工性能,提高了超导线复合坯锭的成品率;而未经过热等静压处理的NbTi/Cu多芯超导线坯锭加工性能较差,坯锭内部缺陷较多.     

一种实验用交流超导磁体的研制

该文介绍了用于探测线圈法测量超导材料交流损耗的一种交流超导磁体的设计、加工与绕制。超导磁体设计采用成熟的螺线管磁体技术,磁体骨架为低温环氧树脂加工而成,所用超导线为NbTi/Cu复合超导绞线。为了满足磁体的稳定性,每两层超导线间留出了一条冷却通道。所绕磁体的磁场强度和均匀度都满足设计要求。