International Thermonuclear Experimental Reactor Nb3Sn Research Articles

Buckling Behavior of Nb3Sn Strand Caused by Electromagnetic Force and Thermal Mismatch in ITER Cable-In-Conduit Conductor

Recent experiments on the international thermonuclear experimental reactor Nb3Sn cable-in-conduit conductor have shown degradation in current-sharing temperature ( ${T_{cs}}$ ) over several thousand electromagnetic (EM) cycles. The destructive investigations on these specimens have found that the Nb3Sn strand buckled and cracked on the surface of the long twist pitch cable at the low EM load side. In order to discuss the strand buckling behavior, we propose a theoretical model to analyze the influences of twisting pitch, stainless steel tape width (wrap width), coverage rate, wrap stiffness, and friction factor on the buckling behavior of the Nb3Sn strand. The cases of Nb3Sn strand with and without EM loading are discussed, respectively. Moreover, a slip of the cable within jacket, which often reinforces the plastic deformation of the strand, is also considered in this study.

Influence of heat treatment excursion on critical current and residual resistivity ratio of ITER Nb3Sn strands

Heat treatment is critically important to the performance of Nb3Sn superconducting strands. For very large Nb3Sn magnet coils, such as the International Thermonuclear Experimental Reactor (ITER) central solenoid (CS) coils, heat treatment carries the risk of temperature and time excursion, which may result in performance degradation. Therefore, it is prudent to study the effect of possible excursion on Nb3Sn performance. In this study, Nb3Sn strands used for ITER CS coils are heat treated at different temperatures for different times. Their critical current, residual resistance ratio and hysteresis losses are measured. It is found that in the range we studied, critical current and hysteresis losses do not change significantly. Residual resistance ratio, however, decreases with increasing heat treatment temperature and time. This is attributed to the diffusion of metallic elements from the plated Cr layer to the copper stabilizer. Based on a model of metallic elements diffusion, a numerical code is developed to predict residual resistance ratio as a function of heat treatment temperature and time.

Comparison and Analysis of Twist Pitch Length Test Methods for ITER Nb3Sn and NbTi Strands

A twisted multifilamentary structure is needed for Nb3Sn and NbTi strands to be used in the International Thermonuclear Experimental Reactor (ITER) magnets. The Cable-In-Conduit Conductor (CICC) for the coils of ITER relies on twisted, multifilament, Nb3Sn and NbTi based composite strands. As important parameters for the superconducting strand design and production, the twist pitch length and the direction of strands must meet the requirements according to ITER Procurement Arrangement (PA) and this must be verified. The technical requirements include 15 mm ± 2 mm for twist pitch length and right hand twist for direction. The twist pitch and the twist direction can be measured on straight sections of strand, which is recognized by the repetition of filament bundles or by the angle of the filaments. Several test methods and results were described and compared in the present paper. The accuracy, the uncertainty and the feasibility of different methods were analyzed and recommended measurement methods were proposed for ITER strands verification.

Local strain exerted on Nb3Sn filaments in an ITER strand

As part of an international project to benchmark facilities for measuring the strain dependence of critical current in International Thermonuclear Experimental Reactor (ITER) Nb3Sn strands, direct measurement of local strain exerted on Nb3Sn filaments was attempted at cryogenic temperature by means of a pulsed neutron technique. The lattice axial strain increased linearly with a slope close to unity against applied strain, while the thermal axial strain was −0.22% at 8.5 K. As a result, the force-free strain was evaluated to be 0.22–0.23%. This key parameter should provide an accurate estimate of the peak location of critical current versus applied strain. The lattice transverse strain decreased linearly as a function of applied strain with a slope of 0.33–0.34. The lattice strains of the Nb and Cu components were also measured and their behavior was analyzed by computing diffraction elastic moduli based on micromechanics theories. The stress–strain curve calculated according to the rule of mixtures described quite well the macroscopic curve measured for the present ITER Nb3Sn strand.

A simple phenomenological model for characterizing the coupled effect of strain states and temperature on the normal-state electrical resistivity in Nb3Sn superconductors

To establish a general scaling relation for the effects of strain tensor on the critical current density in Nb3Sn, the normal-state transport properties of strained Nb3Sn superconductors are invaluable, which are directly relevant to the superconducting state. The three-dimensional (3D) strain- and temperature-dependent electrical resistivity of Nb3Sn in the normal state is explored, and a phenomenological model is suggested. The numerical simulation by the model proposed in this paper shows predicted normal state electrical resistance behavior, which in good agreement with the experimental data. Moreover, the dependence of the strain induced variation of the electrical resistivity of normal conducting Nb3Sn on temperature with the largest change occurring at the temperature of 40 K (closer to the martensitic transformation temperature) can also be predicted. The model is helpful for identifying the scaling relation for the critical current density in the International Thermonuclear Experimental Reactor Nb3Sn strands and understanding the origin of strain sensitivity in Nb3Sn conductors.

The effect of axial and transverse loading on the transport properties of ITER Nb3Sn strands

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5 mm. The strand axial tensile stress–strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load. The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

A three-dimensional strain model for the superconducting properties of strained International Thermonuclear Experimental Reactor Nb3Sn strands

The critical current density parameterization for the International Thermonuclear Experimental Reactor (ITER) Nb3Sn production accounting for the three-dimensional (3D) nature of strain is significant for the analysis of the performance of magnet systems. A simple model for the 3D strain dependence of the critical current density in ITER Nb3Sn strands is proposed in this paper. It is on the basis of the strain-induced variation in the electronic density of states at the Fermi surface. The validity of the proposed scaling law is verified by making comparisons with experimental data acquired on the bare/jacketed Nb3Sn wire. The derived 3D model in axial form is capable of adequately describing the anomalies in the strain function of the Nb3Sn wires jacketed with AISI 316 L stainless steel, which arise from a multi-dimensional strain effect, while none of the existing research models can describe such trend. The model provides a new way of identifying the scaling relation for the critical current density in Nb3Sn strands.

Local strain and its influence on mechanical–electromagnetic properties of twisted and untwisted ITER Nb3Sn strands

It is important to evaluate the local strain exerted on superconducting filaments in Nb3Sn strands, because it influences both superconducting and mechanical properties, in particular for the ITER (International Thermonuclear Experimental Reactor) project. The local strain in the twisted and untwisted Nb3Sn strands was directly measured at room temperature as well as at low temperatures by means of quantum beam techniques. The local strain consists of thermal strain and lattice strain. The latter changes as a function of external strain. The interrelation between the force-free strain and the intrinsic strain showing a maximum critical current was considered on the basis of the present experimental data as well as the recent theory. The thermal strains along both directions parallel and transverse to the strand axis were numerically evaluated. Their evaluated results could explain well the observed values, when To is the recovery temperature of pure Cu. The force-free strain along the axial direction is deduced to be distributed among grains with different crystal orientation with respect to the axial direction. It is suggested that this fact affects the definition of intrinsic strain.

Experimental investigation of AC losses and effective time constant in the high current cable

The report is dedicated to the results of AC-loss measurements in the ITER (International Thermonuclear Experimental Reactor) Nb 3Sn strand and ITER design subsize cable samples. The test facility described below includes the set of superconducting magnet systems furnished with boil-off calorimetric inserts and calorimetric schemes including gas-flow meters. The calorimetric method and the magnetization loop recording method were used simultaneously for the AC-loss measurements. The coincidence between the results of both measuring techniques was confirmed. The original calorimetric scheme makes it possible to measure the released energy during one pulse of the magnet field. The results of AC loss measurements in ITER subsize conductor samples manufactured in Russia are given as an example.