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This paper analyses the current distribution in no-insulation (NI) coils in a layer-wound configuration. The investigation aims at comparing the electrical characteristic of two coils wound from the same BSCCO tape, with or without electrical insulation between turns. Both coils are characterized by a very similar geometry, with the same number of turns and layers. Both coils, cooled in liquid nitrogen bath, are charged until the tape critical current is exceeded. To interpret the measurements and analyze the current distribution within the winding, a lumped-parameter equivalent electrical circuit is developed and solved numerically in a time-varying regime. The model results are compared with the signals acquired through voltage taps soldered at the same locations in both coils. Finally, the model is applied to investigate the impact of the transverse contact resistance and to estimate the most stressed locations of the coil during charging, in terms of power produced by Joule effect.

The measurement of transport current ac losses in high-temperature superconductor tapes with the electric method has been extensively discussed in the literature. It is well known that the configuration of the circuit used to acquire the voltage signal can affect the measured losses, potentially leading to inaccurate results. To reduce this undesired effect, the rectangular circuit arrangement, which is widely adopted, involves twisting the pair of voltage taps at a given distance from the tape middle axis. However, the explanations reported in the literature to identify the correct distance are not fully exhaustive, and an alternative interpretation is described here. Moreover, given that the conventional voltage taps arrangement creates a significant circuit area to which electromagnetic noise can be linked, an alternative configuration is proposed, aiming to reduce the linked flux. This article presents a theoretical analysis to quantify the impact of the measurement circuit configuration on the losses. To ensure clarity, the model equations involved are derived step by step. The numerical results are applied to a thorough investigation on the accuracy of the ac losses measured on a sample tape, at different frequencies and current amplitudes. Both the conventional and the alternative arrangements are studied, varying their main geometrical parameters, at the same operating conditions. The correction terms are then applied to the measurement results, highlighting the advantages and drawbacks of the different configurations.

This article presents a numerical model based on the definition of the scalar and vector potentials to calculate the ac losses generated in a high-temperature superconductor coated conductor tape by an ac transport current and by an ac external magnetic field applied with a generic orientation. The material characteristics of the different layers of a reference tape are included in the model equations, which allow analyzing the differences between a model including only the superconducting layer or all layers of the coated conductor. The model is applied to study the dependence of ac losses on the amplitude of the transport current and magnetic field (applied individually or combined) and their frequency. The numerical results are compared with analytical formulations, with a second numerical model based on the H -formulation, and with experimental results. The conditions at which the contribution of the nonsuperconducting layers to the total ac losses of the tape become relevant are analyzed. In particular, the dependence of losses on the frequency of the ac external magnetic field applied at different orientations, is investigated. The current density distribution computed on the conductor cross-section is used to explain the trend of the calculated losses.

Rutherford cables for particle accelerator magnets can be subjected to a time-varying magnetic field during the typical accelerator operating cycle, which in turn induces coupling currents flowing in the loops formed at the contacts between different strands. A proper analysis of the losses generated by these currents must be carried out for the design of the cryogenic system. A continuum model with nonuniform contact conductances between the strands was recently developed, starting from a previous continuum model based on uniform conductances between strands along the cable length. The model allows one to attain the required level of detail in the description of the interstrand currents. This model is applied here to analyze the effect of the insertion of a resistive core between the two layers of the cable, and to assess quantitatively the impact of the main core parameters, such as its width, positioning, and electrical resistance.

Quantum and radiological science and technology performed a toroidal field (TF) insert coil (TFIC) test in 2016–2017, which was newly developed in 2014 with the ITER TF conductor, in the central solenoid model coil test facility. According to the test program, the TFIC underwent more than 1000 cycles of energization to 68 kA under a 10.8 T background field, which represents the lifetime electromagnetic loads of ITER TF coils. Then, five cycles of warm-up to 290 K and cool-down to a temperature lower than 5 K were also performed. Through the test campaign, the transition of the current sharing temperature ( T cs) was measured under the above energization condition. In parallel, two SULTAN test samples fabricated from the same TF conductor as the TFIC were tested by the similar test sequence to that of the TFIC, and the T cs measurement results of the TFIC and the SULTAN samples were compared. In this paper, the test results mentioned above are reported.

The cable-in-conduit conductors (CICCs) that will be adopted for the ITER central solenoid (CS) coil of the ITER machine have been extensively characterized in the SULTAN facility in Villigen, Switzerland, by means of DC and AC tests. The AC measurements were performed superimposing a sinusoidal magnetic field, with the amplitude of 0.2-0.3 T and frequency in the range of 0.1-5 Hz, to a background constant magnetic flux density of 2 T and 9 T. This paper describes the analysis of the AC loss SULTAN tests of one CICC, identical to that used for the manufacturing of the CS Insert tested in 2015 in the CSMC facility in Naka, Japan. The numerical analysis of these experiments was performed by means of the THELMA code, by developing three models at different levels of discretization (sub-cables of different cabling stages and strands). The comparison between measured and computed losses allows one to estimate the interstrand contact conductances of the tested sample. The model provides a useful insight on the time and space distribution of the power dissipated in the conductor.

The National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL, USA, has developed a 32-T all-superconducting user magnet system combining two series-connected high-field high-temperature superconductor (HTS) nested inner coils (insert) wound with SuperPower REBCO tapes and a low-temperature superconducting (LTS) outer magnet (outsert) composed of five coils (broken in 17 electrical sections). As a part of the magnet development work, a number of quench tests were done on the HTS insert dual-coil prototype in the actual LTS outsert, mimicking the actual magnet configuration and operation. The tests on the coupled insert/outsert system were performed at 4.2 K, using a set of tailor-made quench protection heaters to be employed in the actual insert. In these tests, in the transition to the normal state, a noticeable part of the outsert stored energy could be transferred to the insert through the inductive coupling. The experimental results are analyzed here by means of a quasi-3-D COMSOL FEM model. The model, developed at the University of Bologna in collaboration with the NHMFL, enables one to calculate the coil temperature distributions, the internal voltages, and current evolution through the increase of normal zone resistances over the insert coils. A lumped parameter equivalent model of the magnet electric circuit permits including appropriately the effect of mutual inductive coupling between the insert and the outsert coils on the currents in them. Also, the model accounts for a time-varying 2-D distribution of the magnet field generated by both the insert and outsert within their coils in the event of a quench. The REBCO insert coils are described in terms of an anisotropic homogenized medium, albeit with due regard for the essentially nonlinear characteristics of the superconducting tapes and for the interturn stainless steel reinforcement and sol-gel alumina insulation. A detailed comparison between the numerical and experimental results provides detailed insights into the modes of propagation of the normal zones.

This paper describes the experimental results of a test method for determining the retained critical current after double bending at room temperature for short and straight Ag- and/or Ag alloy-sheathed Bi-2223 superconducting tapes. The critical current measurements were performed after double bending samples of a Bi-2223 tape manufactured by Sumitomo with different bending diameters set to 50, 60, and 80 mm. The results allow one to determine the critical bending diameter of the tape. An experimental technique was also developed for the measurements of the quench energy (QE) of the tapes in boiling liquid nitrogen. The impact of the bending on the QE was experimentally determined through measurements performed on the same samples previously used for the critical current measurements.

In this paper we discuss results of a one-dimensional numerical model to simulate electrical and thermal transients in the superconducting cables for the LHC machine, with specific reference to the analysis of quench due to the heat released by beam losses. Two models have been developed, one based on the analysis of the behavior of a single strand, whereas the other accounts for all the strands in the multi-strand cable. As a first step the stability margin is computed considering the single strand subjected to a variable magnetic field and heat deposit along its length. The impact of the field non-uniformity is assessed by com- parison with computations performed assuming a uniform field in the cable cross section. The results of this model are compared to those obtained with the multi-strand model. The numerical results are discussed in terms of current and heat redistribution between strands, and stability margin of the cable.

The transport performance of Nb3Sn cable-inconduit conductors (CICCs) depends on the strain distribution along the superconducting filaments determined by the combination of electromagnetic and mechanical forces applied to the strands. Experimental studies on the effect of bending strain were performed at the University of Twente by means of the Test Arrangement for Strain Influence on Strands (TARSIS) facility. The aim of this paper is to verify the agreement between a detailed electromechanical model of the wire and the experimental results obtained in TARSIS. A numerical model of two Nb3Sn internal tin strands was developed, which describes the wire through a distributed parameter nonlinear electrical circuit. The model requires a single strand to be discretized into a number of elements, connected by transverse conductances and subjected to a given strain distribution. The strain distribution maps were computed at the Ecole Centrale Paris by means of the MULTIFIL code at different experimental conditions in the TARSIS facility. The simulation results show good agreement with the experimental ones in terms of both critical current and n-value degradation.

The transverse resistivity among filaments is a crucial parameter of composite superconducting wires, since it affects the coupling losses in ac regime, the current transfer length of the wire subjected to bending strain, and the current redistribution among filaments in the case of a quench. A direct measurement of the transverse interfilament resistance as a function of temperature in several Nb3 Sn multifilamentary wires was performed by means of a 4-probe method. This paper deals with the numerical analysis of the measurements performed on a OCSI wire manufactured by Luvata, containing 82 superconducting subelements each made of 150 filaments. The measurements are analyzed by applying a 2-D FEM of the wire cross-section and a 3-D electrical circuit model of the wire sample. The results obtained have been compared with those found on two other Nb3Sn wires with different architecture and a NbTi wire taken as a reference.

Since year 2009, the joint of the toroidal field (TF) conductor samples tested in the SULTAN facility at PSI Villigen, CH, is solder-filled, so that the helium coolant can only flow axially inside the central channel. The latter is however plugged starting about 40–45 mm downstream of the joint. The helium has then to pass from the central channel into the annular cable region over such a short length that the desired homogeneity of the flow distribution among the petals in the high field region is not guaranteed a priori, since central helix and petal wrappings act as azimuthally non-uniform obstacles to the radial flow. In the paper we first present a geometrical model for estimating this interference to the radial flow, and combine it with a CFD (ANSYS-FLUENT) model of the hydraulic effect of the plug, in order to estimate the mass flow rate imbalance among the petals at the beginning of the plug. This is then used as boundary condition by the THELMA code, to parametrically assess the results of a TCS measurement.

The acceptance tests of the Cable in Conduit Conductors for the Toroidal Field coils of the ITER magnet system are being performed at the SULTAN facility in Villigen, Switzerland. The main physical quantity measured in these tests is the current sharing temperature, Tcs . In Tcs measurements, the E-T characteristic curve of the conductor is traced. This curve is utilized here to evaluate the equivalent n-index and the effective strain of the superconducting Nb3Sn cable, under the simplifying assumptions of a uniform strain, current density and temperature distribution in the cable cross section. The variations of n-index and effective strain with mechanical cycling and their sensitivity to the applied transverse load and to the warm up-cooldown process are presented for various conductor samples tested recently.

This paper presents the models of three new-generation high-performance samples of ITER full-size Nb3Sn Cable-in-Conduit conductors (CICCs). These samples were recently tested in SULTAN, at CRPP (Switzerland), and demonstrated the technical feasibility of CICCs that satisfy the ITER specifications. The improvements of these samples regard not only the cable strand and the cabling parameters, but also the manufacturing details of their SULTAN samples. A set of changes have been adopted on the SULTAN sample termination and joint, to try to achieve a more uniform current distribution among the strands and a more accurate measurement of the strand longitudinal voltages. This paper presents a comparative numerical analysis of the effects that these different joint technologies have on the DC current distribution among the strands of the SULTAN samples cable. A correlation between the current distribution in the sample and the resistance distribution at the joint is also shown and discussed.

A superconducting magnet with 200 kJ stored energy and 75 kW output/input power has been designed and manufactured. The device, which is aimed to power quality enhancement in electric grid, consists of two coaxial coils, counter-series connected in order to keep the stray field below the limit of 0.5 mT. The SC wire is a copper stabilized Nb-Ti strand. Several copper slabs are placed within the coils in order to permit heat removal deep inside the coil, thus allowing cryogen-free cooling and operation at high AC losses regime. Several tests with different ramp rate have been carried out at the liquid helium temperature. Design and manufacturing characteristics as well as test results are discussed in this paper.

In Nb3Sn strands, hundreds or thousands of fine superconducting filaments are embedded in a metallic matrix for thermal and electrical stabilization. The transverse electrical resistivity between filaments plays a fundamental role in determining the ac losses, the thermal stability, and the current transfer length of the wire. The direct measurements of the transverse electrical resistances give useful information both for stability computations and to analyze the mechanical performance of the wire. In this paper, the interfilament resistances measured with a four-probe technique on a Nb 3Sn wire produced by Europa Metalli have been interpreted through a simulation code. A 2-D finite element method model of the wire cross-section and of a 3-D electrical circuit model of the wire sample have been applied to derive qualitative and quantitative information about the transverse electrical resistance matrix. A comparison with measurements performed on a Nb 3Sn wire with a different configuration shows the relevance of the wire layout in determining the interfilament resistance between filament bundles. © 2002-2011 IEEE.