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(Recently, our group proposed a technique for the formation of crystalline joints for use in RE123-coated conductors, based on using molten bulk Y123 or Yb123 as an intermedium. The melting temperature of Yb123 is approximately 1203 K in air, which is 70 K lower than that of Y123, and so using Yb123 allows the heat treatment temperature during the joint formation process to be reduced by about 70 K. This lower temperature is likely to limit degradation of the wire. However, it is challenging to produce a highly orientated Yb123 phase, and our previous experiments indicated that crystal growth does not occur in Yb123 at the joint boundary during a short time. For these reasons, increasing the critical current at the joint requires crystal growth of an intermedium at the boundary. The present study examined the partial substitution of Y for Yb in the bulk material. YxYb1-x123 samples with x = 0, 0.1 or 0.2 were prepared and both crystal growth and the critical current at the joint boundary were assessed following heat treatment at 1203 K for 1 min under oxygen. X-ray diffraction patterns indicated that only the Y0.1Yb0.9123 phase contained highly oriented single crystals. The current-voltage characteristics of three joint samples were determined at 77 K and a current of 44.2 A was obtained from the sample with x = 0.1. This value of critical current was approximately 5 times higher than that for the x = 0 specimen (a Yb123 polycrystal), indicating a significant improvement in the critical current as a result of crystal growth in the intermedium. [ABSTRACT FROM AUTHOR]

REBCO coated conductors (CCs) are suitable for high field magnets due to their high in-field critical current density (Jc) and high tensile strength. However, in order to apply the REBCO CCs to a high field magnet, it is important not only to evaluate the characteristics of short samples but also to clarify the behavior of the REBCO coil under strong electro-magnetic forces in high magnetic fields. In this study, we investigated the tolerance to the electro-magnetic force by applying hoop stresses to a REBCO double pancake coil in high magnetic fields. The REBCO coil was wound with BHO-EuBCO CCs and impregnated with epoxy resin. In order to prevent a degradation due to thermal delamination stress, fluorine-coated polyimide tapes were wound on the REBCO CCs. When a maximum hoop stress of 608 MPa was applied to the coil, normal voltage was generated and Ic degradation was confirmed in one side of double pancake. As a result of the investigation, the degradation occurred only at lap joint part between CCs in the windings, and the cause of the degradation was thought to be stress concentration at the both edges of lap joint. Since there was no Ic degradation except for the lap joint part, it is considered that the REBCO coil can withstand larger hoop stress if we can reduce the stress concentration at the joint part. We therefore proposed a new joint structure and confirmed that it has higher tensile strength than a conventional lap joint structure. [ABSTRACT FROM AUTHOR]

The YBa2Cu3O7-x (YBCO) coated conductor (CC) is one of the most promising superconducting materials because of their high critical current (Ic) and low dependency of the Ic on the external magnetic field. For superconducting applications, a large number of joints are required to join relatively short YBCO CC tapes. In this paper, we study the electromechanical behavior of lap-jointed YBCO CC tapes fabricated by etching Cu stabilizer at its service temperatures (77 K) under self-field. The effects of etching Cu stabilizer on Ic and joint resistance (Rj) of YBCO CC tapes were discussed. The Rj was reduced and the Ic values of the joints are consistent with the parent CCs tape by etching the Cu stabilizer. Preferable electrical properties under applying axial tension are achieved and proved by the axial tension tests and finite element simulations. This fabrication technique is a unique practicable solution to reduce joint resistance for YBCO CC tapes with thicker metal stabilizing layers and improving electromechanical performance in second generation high temperature superconductor (2G HTS) devices. [ABSTRACT FROM AUTHOR]

The high-temperature superconductor (HTS) CrossConductor (HTS CroCo) is a modified stacked-tape arrangement, aiming for high current density and easy long-length fabrication. It is intended to be used as a strand for a high current cable in industrial dc applications or large high-field magnets. In order to allow for an economical operation, terminations, joints, and connector parts need to be sufficiently low-resistive to minimize the heat generation and thus the cooling cost. On the other hand, industrial applicability requires reliable and easy-to-use concepts. In this paper, three approaches to fabricate soldered joints and terminations for HTS CroCos are presented and assessed based on resistance measurements at 77 K, self-field. Termination resistances of some tens of nano-Ohms were achieved for the best termination concepts. Additionally, four types of low-resistive  HTS CroCo connectors with bending radii of less than 5 cm are introduced and assessed. For the best connector types, some tens of nano-Ohms were achieved giving rise to a heat generation of less than 1 W at operating conditions. [ABSTRACT FROM PUBLISHER]

The superconducting coils of a 40-T hybrid magnet were designed with Nb3Sn cable-in-conduit conductors wound in layers or double pancakes, which are connected in series by joints. Two kinds of electrical joints have been designed for the different terminals of the superconducting coils. The “terminal joint” is for the joints between the terminals of the superconducting coils and the ends of NbTi buslines. The “interlayer joint” is for the joints between the layers of the coils. Two prototype joint samples have been manufactured, and their dc resistance tested. The joints' manufacturing process and test results are presented in this paper. [ABSTRACT FROM PUBLISHER]

Two types of mechanical contact joint termination methods for a 2G YBCO cable have been developed. Each YBCO tape of a cable is connected to a BSCCO tape while the other ends of BSCCO tapes are bundled together and soldered inside a copper tube. Another termination method uses only YBCO tapes. It has been confirmed that both joint termination methods are very useful for 2G HTS cables. Mechanical contact joint resistances between YBCO-BSCCO tapes and YBCO-YBCO tapes have been measured under various contact pressures and contact surface conditions at 77 K. Joint resistance distribution among the tapes in the cable have been also evaluated for both termination methods. These contact joint methods can be used for a demountable joint. Joint resistance is greatly reduced with increasing applied pressure especially for YBCO-YBCO joints. However, in the case of a YBCO-BSCCO joint, the BSCCO tapes may degrade at high pressure. The contact pressure should be optimized to obtain low joint resistance. [ABSTRACT FROM PUBLISHER]

In order to assemble a magnet using second generation (2G) high temperature superconducting (HTS) tapes, one usually stacks the pancakes using the joint method. When joining the tape, it is important to keep the initial characteristics of the original tape. When two tapes are joined, appropriate pressure should be applied to each tape. The reason is that insufficient or excessive forces on the joined surface affect the inherent characteristics of each tape, such as the critical current and the n-value. The generated heat flux on the joined tapes is related to the quench occurrence when the magnet is affected by unexpected disturbance. In this study, the effect of the external disturbance on the joined tapes was examined. The experiments were carried out with a single tape and two kinds of joined tapes. The joints were exposed to various pressures. The stability and recovery characteristics of the experimental samples were analyzed and compared with each other. [ABSTRACT FROM PUBLISHER]

It is observed that measured critical current of Cable-in-Conduit-Conductor (CICC) for ITER TF coil are lower than expected. This is partly explained an imbalance of the contact resistance at the joint between double pancakes which causes an unbalanced current distribution in a cable and hence during a slow temperature increase some strands reach the critical current earlier than other. In order to estimate the contact resistances, we identify the three-dimensional positions of all strands inside the CICC, and then evaluate the contact parameters such as number and length of the strands which appear on the cable surface and have contact with a copper sleeve. It is observed that many strands do not appear on the surface of the cable, and can lead to unbalanced current distribution. We developed a numerical code to analyze all strand positions in the CICC. We evaluated the contact parameters by using the numerical code, and then compared them with those evaluated from the measured strand positions. It is found that since both results are in good agreement, the numerical code is available for evaluating the contact parameters. By using the code, we can optimize the contact parameters by varying the twist pitches of all staged subcables. The results show that all strands appear at the surface of the cable and have contact with the copper sleeve, and moreover the contact parameters have been improved. [ABSTRACT FROM PUBLISHER]

The ITER poloidal field magnet system consists of 6 pulsed coils with a diameter ranging from 8 m to 24 m and a weight of up to 400 t. The coils are made of independent modules connected to each other by joints. [ABSTRACT FROM PUBLISHER]

We experimentally and numerically evaluated the AC loss occurred in joints of the EF coil in JT60-SA. There are two kinds of the joints (pancake joint and terminal joint) in the EF coil. Firstly, the AC losses of the joints were experimentally obtained. The experimental results showed that the AC losses in the pancake and the terminal joints were smaller than the loss in the prototype joint which was the previously designed joint and we measured the loss data in two years ago. And next, we numerically analysed the AC loss of the joints. The simulated results well agreed with the experimentally measured data. And we numerically tried to decrease the loss. The AC loss could be decreased to be half of the initial loss. [ABSTRACT FROM PUBLISHER]

Combining scanning Hall-probe microscopy (SHPM) with finite element method (FEM), we have investigated three-dimensional current distribution at a silver diffusion joint of RE-123 coated conductor (CC). This research aims at the understanding of electromagnetic behaviors in a jointed CC and at the establishment of an analysis model for it. Two-dimensional distribution of sheet current density in a jointed sample was visualized by a SHPM measurement with a spatial resolution of a few hundred micrometers. Then, it was found that such a distribution included information about current transfer between the jointed pieces. Furthermore, the experimental results were successfully reconstructed by a three-dimensional FEM analysis. We believe that these will be very important findings for the establishment of jointing processes on CCs and for the designs of practical applications which need such processes. [ABSTRACT FROM AUTHOR]

The NHMFL Series Connected Hybrid (SCH) magnet will provide an energy-efficient 36 T to the DC user facility by employing a 20 kA superconducting outsert coil in series with a resistive insert. The magnet outsert consists of three concentric layer-wound sub-coils using three different grades of Nb3Sn Cable-in-Conduit Conductors (CICC). The electrical joints in the superconducting outsert require low DC resistance to minimize the refrigeration requirement at the operational 4.5 K temperatures and low AC losses to ensure good stability against ramping operation required by the users. There are four internal splice joints in the outsert, which are Nb3Sn to Nb3Sn joints with the same design configuration. There are another two terminal joints between the Nb3Sn outsert and the two NbTi buslines, which connect the outsert terminals to the two current leads. The two Nb3Sn to NbTi terminal joints are of identical configurations. All of the joints will be praying-hands configuration with an operation current of 20 kA. The R&D for the joins has been carried out at the NHMFL. The joints design and test results are discussed in this article. [ABSTRACT FROM AUTHOR]

Low-dc-resistance superconducting joints in toroidal- and poloidal-field (TF and PF, respectively) coils of the steady-state superconducting tokamak-1 (SST-1) at the Institute for Plasma Research (IPR) is under testing. The feasibility of conduction-cooled leak-tight joints made between two double pancakes in the winding pack of the TF coil is validated through experiments. The configuration of these conduction-cooled joints is comprised of a prefabricated SS304L oxygen-free high-conductivity copper leak-tight termination, into which the unconduited and soldered portion of the cable-in-conduit-conductor (CICC) is inserted. Once the cable space is inserted inside the prefabricated piece, solder filling is carried out, and the joints are realized by overlapping the mating ends and soldering them together. The supercritical helium flowing through the CICC exits prior to the termination length, and the joints are cooled by conduction. The joints are subjected to I \times B-induced and bending-induced stresses during SST-1 operational scenarios. These stresses can lead to leaks in the joint region if they exceed the material strength or the brazing/welding strength. Both the thermal and electromagnetic stresses that developed at the copper–stainless steel prefabricated brazed region are measured on the SST-1 spare TF coil. These stresses are measured using the strain gauges during the cooldown and the charging of the spare TF coil up to its operational current of 10 kA at a conventional 4.5 K and 4 bar of supercritical helium forced flow. The electromagnetic-stress behavior at the time of quench that occurred accidently during the spare TF coil test at an 8 kA transport current was also studied. The signal-conditioning electronics required for this measurement are engineered and tested at the IPR before its implementation to the spare-TF-coil test campaign. The measured thermal and electromagnetic stresses are found to be in good agreement with the simulated finite-element Ansys results. [ABSTRACT FROM AUTHOR]

Triggered by the incident in the LHC in September 2008, the thermo-electrical stability of the 10,000 soldered busbar joints in the 13 kA circuits of the LHC has been re-assessed. For this purpose the computer model QP3 has been developed. In this paper the results of some calculations are presented, and it is shown how the results have been used to improve the quench detection system and ensure safe future operation. First the layout of the 13 kA circuits, the geometry of the soldered joints and the quench detection system will be described. Special focus is put on several types of defects in the joint that have been found in the LHC. The difference between slow and fast thermal run-away will be discussed, focusing on quench propagation and reduced thermal and electrical contacts between the various parts of the joint. It is shown that an enhancement of the LHC quench detection system with a much lower threshold will greatly reduce the risk of burn-out of the joint. Finally it is discussed that protection is not feasible for fast thermal run-aways, and that a reduction of the operating current is needed for safe operation until all defective joints have been repaired. [ABSTRACT FROM AUTHOR]

Feasibility of industrial production of MgB2 cables and magnets has been established, thus leading to MRI systems realization. Apart from continuing the development in performances of both cable and magnet, a further important step consists in applying superconductive junctions to windings, to obtain a better field stability. In 2006 a technique to obtain some tens of Ampere in persistent mode operation in a joined MgB2 cable was found. Since then, short windings were repetitively built to test the progress of the performances of the junctions. Among them, a single junction, five meter long windings with a diameter of 260 millimeter were put in persistent mode (i.e. with total resistance less than 10-14 Ohm) with 300 Ampere circulating at 20 Kelvin, self-field; also windings with two junctions and about one meter long with the same diameter were put in persistent mode with 200 Ampere circulating at 20 Kelvin, self-field. [ABSTRACT FROM AUTHOR]

The YBCO coated conductor (CC) is one of the most promising superconductors for various superconducting applications. Although it has many advantages, joints of the YBCO CC are inevitable because there are technical difficulties in lengthening the YBCO CC. In the joining process, it is important to reduce the joint resistance between the YBCO CCs in order to operate in a persistent current mode. Several approaches have been proposed to eliminate the stabilizer of the YBCO CC. Among them, this paper introduces a chemical etching method that can reduce the damaging influences to the superconductor layer in the YBCO CC compared with physical or thermal methods, which may degrade the features of the superconductor. This paper discusses the characteristics of joined YBCO CCs in terms of the resistance and critical currents of the joints from the V - I curve. [ABSTRACT FROM AUTHOR]

In recent years, the YBCO coated conductor (CC) has been developed as a suitable material for power apparatuses. To fabricate a superconducting apparatus of a large capacity, the joint between the superconducting tapes are inevitable because of the need to have a long length conductor. Consequently, the joint characteristic is very important for the efficiency of the superconducting part in the apparatuses. Besides, maintaining the characteristic of the joint between the superconducting tapes even after repetitive over-current is also important for stability of power apparatuses. Therefore the joints between CC tapes were developed in differing joint length and the characteristics of the joints were compared. In this paper, we present the experimental results of several over-current experiments with the jointed CC tapes. The results of the over-current experiments and the trend of the joint characteristics are compared in point of stability of the joints. Resistance and critical current of the joints were measured from the V - I curve before and after several over-current experiments for observation of the trend of the joint characteristics. This study of repetitive over-current characteristics of the joints may be useful to the development of superconducting apparatuses in forms of magnets. [ABSTRACT FROM AUTHOR]

In superconducting coils for fusion applications, the cable-in-conduit (CIC) conductor is the most adopted solution. ENEA reported last year about a newly developed compact joint, successfully tested for Nb3Sn conductors in a configuration suitable for the EFDA Dipole which is currently in construction phase. The main advantages of this new concept are: low room occupancy, low price and easiness of manufacturing process. In the present paper, new results coming from the characterization of NbTi joint samples are presented, suggesting new applications for this innovative junction. A joint layout optimization study by means of an electrical simulation code is included as well. [ABSTRACT FROM AUTHOR]

The non-homogeneity of the contact resistance distribution in the terminations of large CICC's leads necessarily to a certain level of current imbalance among the strands in DC operation. The overloaded strands or groups of strands hit first the critical surface in the high field region and a current re-distribution process is thus driven by the initial current sharing voltage to minimize the energy of the system. This process takes place preferably at the electrical connection where the inter-strand resistance is lower. In order to study this phenomenon, a new interstrand resistance experiment is carried out at CRPP on the termination of a NbTi CICC sample. The sample was formerly prepared and tested to determine the contact resistance distribution in the JORDI test facility and it is now re-arranged for this test. Two elements at a time (strands or groups of strands) are selected and fed with a current of the order of 50 A. The voltage between them is sensed and the resistance derived. The results of this sample, together with those related to the Nb3 Sn termination previously tested, allow to evaluate the effectiveness of the current re-distribution in the joints. The comparison of the results of the two terminations allows to judge the impact of the conductor characteristics (e.g. material, coating, layout) on the interstrand resistance and therefore on the current distribution. The effect of the.current redistribution on the voltage-current and voltage-temperature characteristics in short length conductor tests is also discussed. [ABSTRACT FROM AUTHOR]

This paper presents the development status of joint method between the superconducting YBCO coated conductors (YBCO CC). In general, the YBCO-CC tape consists of an upper stabilizer, substrate, buffer, YBCO and a bottom stabilizer. It is suggested that the stabilizer between the two YBCO-CC tapes need to be removed, in order to improve the characteristics of the joints. The removing process was conducted by using heat and partial etching. The resistance and critical current of the joints were measured from the V I curve. The optical micrographs were analyzed for microstructures between the joint areas. After several times of over current test, the critical currents of the various types of joint samples were examined for applying to the high temperature superconducting magnet system. [ABSTRACT FROM AUTHOR]

Experiments conducted to determine the resistance of demountable HTS lap joints with variations in compressive stress are reported here. A joint fixture was constructed that enabled the simultaneous testing of multiple joints of different configurations. The joint fixture was installed in a loading facility, comprising a pneumatic press mounted in a vacuum bell jar and connected to a GM cryocooler. The resistance across each joint was measured by means of voltage taps over a range of compressive joint forces. The test results demonstrated that joint resistances less than 2 μΩ were possible with only light joint loading. [ABSTRACT FROM AUTHOR]

In large, cabled superconductors, a non-negligible level of current unbalance among the strands is unavoidable in dc operation due to non-homogeneous contact resistance distribution at the electrical connections. When the overloaded strands saturate their current carrying capability and start to develop current sharing voltage at the high field section, the excess current must be transferred to less loaded strands. The inter-strand current re-distribution takes likely place at the electrical connections, where the inter-strand resistance is lower. Two samples of electrical termination, made of large cable-inconduit conductors, were formerly prepared and tested for contact resistance distribution at CRPP. One of these samples is now re-arranged to measure the inter-strand resistance in different combination. A discussion of the results allows ruling out speculations about the impact of current re-distribution effects on the voltage-current and voltage-temperature characteristics in short length conductor test. [ABSTRACT FROM AUTHOR]

EFDA is presently responsible for the fabrication of a large bore 12.5 T dipole magnet, for a second Cable-In-Conduit conductor test facility to be installed in CRPP (Switzerland). This dipole magnet is wound from square and rectangular type conductors using Nb3 Sn strands. Since the magnet is built from multiple, independently cooled double pancakes, a total of twelve joints are required to interconnect them. In order to reduce the footprint of each joint ENEA, in collaboration with EFDA, developed a special joint design that allows the two conductor-ends to interpenetrate, reducing to a minimum value the increase of the magnet cross-section at the joint position. Furthermore, this design maintains the electrical performances of the conductor and gives very low electrical resistance values, on the order of some tenths of nΩ. The properties of first prototypes of this joint were successfully tested at ENEA labs. This paper describes the novel joint design, in the dipole magnet version, and discusses some of the measured results. [ABSTRACT FROM AUTHOR]

This paper deals with an analysis on the optimum lap joint method of the YBCO coated conductor (CC) wire. Effective low resistance joint is important for the development of persistent current mode system using the second generation high temperature superconducting (2G HTS) wire, YBCO CC. CC tape is stacked with copper or stainless steel stabilizer, silver layer, YBCO layer, buffer and substrate. The joint between CCs was performed with low temperature InBi solder. YBCO samples with various contact conditions were fabricated and tested. Characteristics of jointed CC tape were evaluated from the V-I curve and contact resistance was derived from experimental results. Characteristics of jointed CC were investigated by electrical analysis and optical micrograph. The BSCCO wires were jointed at different conditions, and the results were compared with the joint between the YBCO CCs. [ABSTRACT FROM AUTHOR]