Theoretical analysis for the mechanical behavior caused by an electromagnetic cycle in ITER Nb3Sn cable-in-conduit conductors

The central solenoid（CS）is one of the key components of the International Thermonuclear Experimental Reactor（ITER）tokamak and which is often considered as the heart of this fusion reactor.This solenoid will be built by using Nb3Sn cablein-conduit conductors（CICC）,capable of generating a 13 T magnetic field.In order to assess the performance of the Nb3Sn CICC in nearly the ITER condition,many short samples have been evaluated at the SULTAN test facility（the background magnetic field is of 10.85 T with the uniform length of 400 mm at 1%homogeneity）in Centre de Recherches en Physique des Plasma（CRPP）.It is found that the samples with pseudo-long twist pitch（including baseline specimens）show a significant degradation in the current-sharing temperature（Tcs）,while the qualification tests of all short twist pitch（STP）samples,which show no degradation versus electromagnetic cycling,even exhibits an increase of Tcs.This behavior was perfectly reproduced in the coil experiments at the central solenoid model coil（CSMC）facility last year.In this paper,the complex structure of the Nb3Sn CICC would be simplified into a wire rope consisting of six petals and a cooling spiral.An analytical formula for the Tcs behavior as a function of the axial strain of the cable is presented.Based on this,the effects of twist pitch,axial and transverse stiffness,thermal mismatch,cycling number,magnetic distribution,etc.,on the axial strain are discussed systematically.The calculated Tcs behavior with cycle number show consistency with the previous experimental results qualitatively and quantitatively.Lastly,we focus on the relationship between Tcs and axial strain of the cable,and we conclude that the Tcs behavior caused by electromagnetic cycles is determined by the cable axial strain.Once the cable is in a compression situation,this compression strain and its accumulation would lead to the Tcs degradation.The experimental observation of the Tcs enhancement in the CS STP samples should be considered as a contribution of the shorter length of the high field zone in SULTAN and CSMC devices,as well as the tight cable structure.

Metallographic investigationof first full-sizehigh-Jc Nb_(3)Sn cable-in-conduit conductor aftercyclicloadingtests

In order to verify the feasibility of applying high-Jc Nb_(3)Sn strand in fusion magnet,a full-size cable-in-conduit conductor(CICC)with short twist pitch(STP)cable pattern was manufactured and tested in SULTAN facility at SPC,Switzerland.Three levels of cyclic electromagnetic(EM)load were applied on the sample stepwise,no visible decrease of current sharing temperature(TcsT was observed until the EM load increased to 80 kA×10.8 T,after that the Tcs decreased dramatically with the EM cycles,which suggested that irreversible deformation,causing a change in the strain state,or even damage has occurred in the superconducting strands.For investigating the reason which caused the conductor performance degradation,the tested conductor was dissected for metallographic observation.Eight segments which subjected to different EM loads were extracted from one of the legs,the geometric feature changes of the cable cross-sections were analyzed and compared.A good correlation was found between the decrease of the Tcs and deformation of the cable cross section.A mass of cracks were found on the sub-elements of strands in the segment which subjected to highest EM load,but the amount of crack is much lower in other segments.Combining the analyses,it is speculated that the critical EM load which causes irreversible degradation is between 850 kN/m and 870 kN/m for this conductor.The results could be a reference in high-Jc Nb_(3)Sn CICC design.

Superconducting Busline Design for HT-7U Tokamak

Superconducting (SC) tokamak HT-7U has seven pairs of buslines connecting toroidal/poloidal coils and the current leads. These SC buslines (SCBLs) share a common cryostat and are made of the cable in conduit conductors (CICCs) arranged as a decoupling configuration. In order to reduce the heat loads conducted from the seven current leads with a capacity of 15 kA during the magnets cooldown, the buslines with a much lower thermal conduction were employed in comparison with the current leads, and a special cooling loop was designed.

Nb_3Sn超导股线热应变分析模型研究

ITER聚变反应堆上Nb3Sn超导股线是由脆性金属间化合物构成的,由于其间各组分不同的热胀冷缩系数和不同的弹性模量,使得股线从热反应温度或热处理温度到室温、以至到低温运行时,会在Nb3Sn复合物中产生热应变。为了研究Nb3Sn的热应变和构建相应的数学模型,进行了加热期间局部应变测量以及室温下的拉力测试。根据Nb3Sn股线不同组分应力/应变曲线所显示的弹塑特性,对它们进行量化分析。测试分析发现,在室温下沿轴向加载到Nb3Sn超导丝的局部应变是压缩应变,高温下是拉应变;另外用数值分析计算方法可以再现依靠温度的轴向热应变。