Nb3Sn Cable-in-conduit Conductors Research Articles

Measurement of Tc distribution in Nb3Sn CICC

Knowledge of the actual Nb3Sn filaments’ thermal strain, εth, in a cable-in-conduit conductor (CICC) is essential to predict the CICC performance in operation starting from the strand scaling laws for the critical current density Jc(B,T,ε). To obtain a measurement of εth under relevant conditions, i.e. at low temperature and with the mechanical constraints of a long length section of CICC, the critical temperature as a function of the applied field, Tc(B), is measured by an inductive method for the CICC in situ and for the freestanding filaments used for the cable manufacture. To deduce the thermal strain in the CICC, the Tc(B) results are compared with the Tc(B,ε) curve. Starting from the susceptibility curves measured for both the CICC and the filaments, it is possible to compute the Tc distribution in the CICC using a deconvolution algorithm. The first results of Tc measured on two CICCs in the SULTAN test facility suggest a broad distribution of thermal strain, peaked at negative strain, which remains almost constant during the cyclic loading. From the knowledge of both the thermal strain distribution and the actual CICC performances, it will be possible to discriminate between reversible and irreversible degradation in Nb3Sn CICC.

Interstrand Resistance and Contact Resistance Distribution on Terminations of ITER Short Samples

The current uniformity among the strands in large cabled conductors is affected mostly by the contact resistance distribution and the interstrand resistance in the electrical terminations. While the contact resistance distribution between the strands and the copper of the termination affects the current distribution among the strands, the resistance between the strands in the termination (inter-strand resistance) has an effect on the re-distribution of the current from the most to the less loaded strands. The contact resistance distribution of an ITER conductor termination is measured in the JORDI (JOint Resistance Distribution) facility at CRPP. The sample is prepared starting from a section of Nb3Sn Cable in Conduit Conductor (CICC) and the joint is assembled following the procedure (solder dipped) used for most of the ITER qualification samples tested in the SULTAN facility. The cable is opened into 150 groups of strands (elements) and each element is connected to low temperature shunt resistors which split the current from a 10 kA power supply. The voltage drop is sensed between each element and the contact surface of the joint made equi-potential by a superconducting soldering alloy. The resistance of each channel is deduced from the measured voltage drop. The resistance distribution measured under imposed current uniformity allows deducing the current distribution in normal operation, namely under imposed voltage. The same sample is then adjusted to measure the inter-strand resistance. Two strands at the time are fed with a current of the order of 100 A, the voltage between the selected strands is sensed and the equivalent resistant deduced. The results of both measurements are compared with those obtained in previous experiments, carried out on different conductor layouts.

Qualification Measurements of the Mid-Field and Low-Field CICC for the Series-Connected Hybrid Magnet With Effects of Electromagnetic Load Cycling and Longitudinal Strain

The cable-in-conduit conductors (CICC) designed for the National High Magnetic Field Laboratory (NHMFL) and Helmholtz Zentrum Berlin (HZB) Series-Connected Hybrid magnets have been qualified. The superconducting coils for the hybrid magnets consist of three CICC configurations graded for different designed fields. Representative samples with the same Nb3Sn type, cable pattern, and CICC configuration were fabricated and the current sharing temperature (TCS) and critical current were measured with the intention to quantify the level of degradation from cyclic, transverse electromagnetic loads. In addition the intrinsic strain for the CICC was determined by measuring the TCS and IC as a function of applied longitudinal strain. Two conductor grades, for the middle and low fields regions of the coil (MF and LF), are tested at the NHMFL in a split magnet that is equipped with a system that can apply up to 250 kN force in a direction longitudinal to the samples. The samples are hydraulically connected to a cryogenic system which delivers temperature controlled supercritical helium. The reduction in TCS from electromagnetic load cycling is shown to be insignificant and measurements as a function of strain shown that the MF and LF CICC have intrinsic strains of approximately 0.65% and 0.70% respectively.

Results of Thermal Strain and Conductor Elongation Upon Heat Treatment for ${\rm Nb}_{3}{\rm Sn}$ Cable-in-Conduit Conductors

The change of length upon heat treatment of steel jacketed, Nb3Sn cable-in-conduit conductors (CICC) is driven by the difference of coefficient of thermal expansion between Nb3Sn and the other components. The cold work of the steel jacket also plays an important role. During the preparation of CICC samples for test in the SULTAN facility, the change in length has been systematically measured for 16 sections of CICC. The broad range of changes, from elongation to shrinkage, is a concern for the manufacture of the ITER toroidal field (TF) coils, where the shape/length of the conductor after heat treatment must be exactly predicted to fit into the radial plates. For a reliable assessment of the performance of Nb3Sn CICC, the axial, thermal strain in the Nb3Sn filaments is a key parameter. To gain information on the subject, the residual strain on the steel jacket is systematically measured by strain gauges on 28 ITER CICC sections tested in SULTAN. The range of results is very broad and no clear correlation is found with the conductor performance, i.e. with the thermal strain in the filaments. To gain more inside about the actual thermal strain in the Nb3Sn filaments embedded in a CICC, neutron diffraction techniques have been applied (with limited success) to deduce the thermal strain from the direct, in situ measurements of the lattice parameter.

Electro-mechanical modeling of Nb 3Sn CICC performance degradation due to strand bending and inter-filament current transfer

Performance degradation of Nb 3Sn cable-in-conduit conductors (CICCs) is a critical issue in large-scale magnet design such as the International Thermonuclear Experimental Reactor (ITER) and the series-connected hybrid (SCH) magnets currently under development at the National High Magnetic Field Laboratory (NHMFL). Not only the critical current is significantly lower than expectations but also the voltage–current characteristic is observed to have a much broader transition from a single strand to a CICC cable. The variation of conductor voltage–current characteristic as a result of cable electromagnetic, mechanical and thermal interactions is challenging to model. In this paper, we use a new numerical model, called the Florida electro-mechanical cable model (FEMCAM) benchmarked against 40 different conductor tests, to study the influence of bending strain and current non-uniformity on the critical current and n-value of Nb 3Sn strands and CICC cables. The new model combines thermal bending effects during cool-down, electromagnetic bending effects during magnet operation and current transfer in strands with filament fracture. The n-value of a strand under bending is derived from integration of filament critical current over strand cross-section for full and no current transfer. The cable n-value is obtained from the power law relation of cable electric field and critical current curve. By comparing numerical results with measurements of advanced Nb 3Sn strands and various CICC cables, we demonstrate that FEMCAM is self-consistent in modeling inter-filament current transfer. The new model predicts that I c degradation of bent strands initially follows closely full current transfer but starts deviating and falls between full and no current transfer with an increasing bending strain. The results agree with recent TARSIS measurements for less than 1% bending strain mostly interested in practice. The strand n-value degradation from FEMCAM with no filament current transfer agrees better with measurements than that from full current transfer. Finally, FEMCAM simulated cable n-values are compared with various CICC measurements. The results imply that FEMCAM is a useful tool for the design of Nb 3Sn-based CICCs and both thermal bending and electromagnetic bending play important roles in CICC performance.

AC Loss Measurements in CICC With Different Aspect Ratio

The coupling loss in a cable in conduit conductor (CICC) is strongly influenced by the cable geometry. Beside the cable pattern, twist pitch, wrapping and void fraction, also the aspect ratio has a not negligible role in the loss. In fact, it defines the area crossed by the flux variation and it determines therefore the entity of the coupling current between strands. An AC loss experiment on two Nb3Sn CICC samples (PITSAM II and PITSAM III) is carried out with a calorimetric method in the SULTAN test facility, in order to investigate the impact of the conductor aspect ratio on the loss. The two conductors, identical for cable pattern, void fraction and characteristics of both superconducting and copper strands, differ only in the aspect ratio, having PITSAM II and PITSAM III a square and a rectangular cross-section respectively. After the DC measurements scheduled in the SULTAN test-procedure (including cyclic load), each sample is tested in a sinusoidal field perpendicular both to the background field and to the conductor axis and generated by a pair of saddle shape coils. The background field during the tests is 2 T while the peak to peak amplitude of the sinusoidal signal is plusmn 0.3 T, with a frequency in the range between 0.2 and 6 Hz. The rectangular PITSAM III is tested in a sinusoidal magnetic field perpendicular first to the wide then to the narrow side of the conductor, so that the loss relative to three different aspect ratios is compared. The relation between loss and aspect ratio is also investigated with reference to the time constants tau and to the shape factor n.

The inter-strand contact resistance ofNb3Sn cable-in-conduit conductor with hydrocarbon oil

The inter-strand contact resistance(RC)of Nb3Sn cable-in-conduit conductors (CICCs) is a very important parameter which strongly correlateswith ac losses and current redistribution behavior of the CICCs. One way to obtain the desiredRC is to apply a layer of hydrocarbon oil on theNb3Sn strands beforethe CICC Nb3Sn reaction heat treatment. In this paper, we measuredRC fora Nb3Sn CICC sample fabricated with hydrocarbon oil. The measurements were performed using anapparatus designed to apply transverse load and load cycling on CICCs at 4.2 K. ResistancesRC between strands of different cabling stages were measured with transverse load up to188 kN m−1 and load cycles up to 10 000. The results show thatRC of the sample increases almost linearly with the first loading. With load cycling,RC increases rapidly at first, but becomes saturated after∼100 cycles. After10 000 load cycles, RC decreases with increasing load at a decreasing rate. These behaviors are comparable toobservations for CICC with Cr plated strands reported in the literature. Therefore from theRC pointof view, Nb3Sn CICCs with hydrocarbon oil on strands may be used as a low cost alternative to CICCswith Cr plated strands.

Performance of ITER (EU-TFPRO-2) ${\rm Nb}_{3}{\rm Sn}$ Strands Under Spatial Periodic Bending, Axial Strain and Contact Stress

The Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable in conduit conductors (CICCs) for the International Thermonuclear Experimental Reactor (ITER) show a significant degradation in their performance with increasing electromagnetic load. Knowledge of the influence of bending and contact stress on the critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands is essential for the understanding of this reduction in performance. We have measured the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands in the TARSIS facility, when subjected to spatial periodic bending using bending wavelengths from 5 to 10 mm, periodic contact stress and uni-axial strain on two strand types, manufactured by OST. These strands were used for the cables of the European TFPRO-2 sample tested in SULTAN. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">priori</i> predictions with the TEMLOP model led to the discovery that the severe Lorentz force response and degradation can be completely improved by increasing the pitch length in subsequent initial cabling stages. The TFPRO-2 /OST-II conductor, especially designed to examine this prediction, verified this significant enhancement. TEMLOP directly uses data describing the behavior of single strands under uni-axial stress and strain, periodic bending and contact loads. Here we give an overview of the TARSIS strand testing results.

Conceptual Design of Magnet System for JT-60 Super Advanced (JT-60SA)

At Japan Atomic Energy Agency (JAEA), the JT-60 is planned to be modified to a full-superconducting tokamak referred as JT-60 Super Advanced (JT-60SA) as one of the JA-EU broader approach projects. In JT-60SA, the magnet system with 1400 ton consists of 18 toroidal field (TF) coils, 4 stacks of central solenoid (CS) and 7 plasma equilibrium field (EF) coils. The TF coil with using the NbTi cable-in-conduit (CIC) conductor has the maximum magnetic field of 6.4 T in winding, the magnetomotive force of 8.2 Tm and the magnetic stored energy of 1.5 GJ which is 1.5 times larger than present superconducting fusion devices. The conductor for CS which is the Nb3Sn CIC conductor with the maximum magnetic field of 10 T was newly designed to increase the flux swing capability for the ITER plasma simulation and to accommodate the diminution of available space because of the TF coil with NbTi conductor. For CS, the faster maximum varying field of 2.88 T/s is required because of fast plasma initiation scenario of JT-60SA.

Transverse load optimization inNb3Sn CICC design; influence of cabling, void fraction and strand stiffness

We have developed a model that describes the transverse load degradation inNb3Sn CICCs, based on strand and cable properties, and that is capable of predicting how suchdegradation can be prevented.The Nb3Sn cable in conduit conductors (CICCs) for the International Thermonuclear ExperimentalReactor (ITER) show a significant degradation in their performance with increasingelectromagnetic load. Not only do the differences in the thermal contraction of thecomposite materials affect the critical current and temperature margin, but mostlyelectromagnetic forces cause significant transverse strand contact and bending strain in theNb3Sn layers.Here, we present the model for transverse electro-magnetic load optimization (TEMLOP)and report the first results of computations for the ITER type of conductors, based on themeasured properties of the internal tin strand used for the toroidal field model coil(TFMC). As input, the model uses data describing the behaviour of single strandsunder periodic bending and contact loads, measured with the TARSIS set-up,enabling a discrimination in performance reduction per specific load and strand type.The most important conclusion of the model computations is that the problem of thesevere degradation of large CICCs can be drastically and straightforwardly improvedby increasing the pitch length of subsequent cabling stages. It is the first timethat an increase of the pitches has been proposed and no experimental data areavailable yet to confirm this beneficial outcome of the TEMLOP model. Largerpitch lengths will result in a more homogeneous distribution of the stresses andstrains in the cable by significantly moderating the local peak stresses associatedwith the intermediate-length twist pitches. The twist pitch scheme of the presentconductor layout turns out to be unfortunately close to a worst-case scenario.The model also makes clear that strand bending is the dominant mechanism causingdegradation. The transverse load on strand crossings and line contacts, abbreviated ascontact load, can locally reach 90 MPa but this occurs in the low field area of the conductorand does not play a significant role in the observed critical current degradation. The modelgives an accurate description for the mechanical response of the strands to a transverseload, from layer to layer in the cable, in agreement with mechanical experiments performedon cables.It is possible to improve the ITER conductor design or the operation margin, mainly by achange in the cabling scheme. We also find that a lower cable void fraction andlarger strand stiffness add to a further improvement of the conductor performance.

Numerical Simulation of the Critical Current and n-Value in&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Strand Subjected to Bending Strain

To demonstrate the applicability of Nb3Sn Cable in Conduit Conductors (CICC's) to International Thermonuclear Experimental Reactor (ITER) coils, four Nb3Sn model coils have been constructed and tested. The experimental results showed that the measured critical current of the conductors degraded compared with what is expected from the ability of the strands. In addition, the larger is the applied electromagnetic force, the larger the magnitude of the degradation is. A degradation in n-value was also observed. One of the explanations of this degradation is supposed to be a local strand bending. This consideration has been supported by test results, in which a similar degradation in the critical current and n-value was observed when applying periodic bending strains to a single strand. However, general dependence of critical current on periodic bending strain has not been clarified in this test since the experiments were carried out at a certain magnetic field, temperature and strain. Therefore, a numerical simulation code was developed to study the general dependence of the critical current and n-value of Nb3Sn strand on periodic bending strain. A distributed constant circuit model is applied to simulate current transfer among the filaments in the strand. The simulation results show relatively good agreement with the experimental results but some modification in modeling is required for more accurate simulation