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• Methods to shorten fabrication time and reduce joint resistivity were discussed for bridge-type joint of STARS conductors. • Introducing an integrated joint piece can reduce the fabrication time to be less than three hours. • Low-temperature heat treatment at 120 °C can achieve joint resistivities of 4.2–7.1 pΩm2 at 77 K and 2.1–3.6 pΩm2 at 4.2 K. • The joint performance obtained by this study can achieve ˜0.5 nΩ for a 100-kA-class STARS conductor joint. Joint-winding of high-temperature superconducting (HTS) helical coils has been proposed as a challenging option for the large helical device (LHD)-type fusion reactors, FFHR series. The coils are wound by connecting helical conductor segments using bridge-type mechanical lap joints with indium foils inserted into the joints. A previous study achieved 1.8 nΩ using a 100-kA-class STARS (Stacked Tapes Assembled in Rigid Structure) conductor joint at 100 kA, 4.2 K and 0.45 T. However, it took eighteen hours to join the conductors because each REBCO (Rare-earth Barium Copper Oxide) tape was joined individually and in series. This study introduced an integrated joint piece to join 10-kA-class STARS conductors, which possibly makes the fabrication time to be less than three hours per one conductor joint. We also applied heat treatment at 120 °C during the joining process to obtain uniform joint pressure distribution and a large true area of contact. The joint resistivity and critical current for each REBCO tape of the joint were comparable to those for a single lap joint at 77 K. The joint resistivity was also evaluated at 4.2 K, which was one-fourth to one-third of the previous result obtained during testing of the 100-kA-class STARS conductor joint. [ABSTRACT FROM AUTHOR]

Highlights • The design proposal history and R&D status of segmented HTS magnet are summarized. • Technical issues and aspects for joint design are discussed. • Low-temperature heat treatment can reduce resistance of large conductor joints. • Stress relaxation of indium does not affect the fabrication time of joints. • Mechanical joints containing indium are preferable under high electromagnetic forces. Abstract Superconducting magnets may potentially become larger and more complex for use in future demonstration and commercial fusion reactors. For such situations, the innovative design concept of remountable (or demountable) high-temperature superconducting (HTS) magnets has been proposed for both tokamak and helical reactors that uses the HTS material features of high thermal stability and low cryogenic power. In Japan, the derived concept of joint-winding of HTS coils has been proposed, in which the coils are wound by connecting short conductor segments. This paper first briefly summarizes the design proposal history and current research and development (R&D) status of the important technologies for joint design, and then reports recent R&D progress in mechanical lap joints with inserted indium foils. Based on the joint resistance as a function of temperature, the applied magnetic field and the numbers of layers and rows of HTS tapes in the conductor, the joint resistance of a 100-kA-class HTS conductor joint can be reduced to one third of its initial value by low-temperature heat treatment. Mechanical behavior of these joints indicates that stress relaxation of indium does not affect the fabrication time and that mechanical joints containing indium are preferable to soldered joints under high electromagnetic forces. [ABSTRACT FROM AUTHOR]

A mechanical lap joint of REBCO high-temperature superconducting (HTS) tapes with an indium foil inserted between joint surfaces has been proposed for segment-fabrication of HTS magnet of an advanced fusion reactor and various HTS applications. In a previous study, we successfully achieved a joint resistivity of about 3.5 pΩm2 by heat treatment during fabrication of the joint. In this study, improved heat treatment method was developed by controlling contact pressure and combining bake-out process. We evaluated joint resistance and critical current of the joint depending on the heating condition and bake-out condition. The joint resistivity after heat treatment was 2.5 pΩm2 without decreasing critical current with conditions of heating temperatures of 90–140 °C, a heating time of 30 min, a contact pressure of 100 MPa, a bake-out time of 30 min, and a bake-out temperature of 150 °C. Owing to applying the contact pressure when the indium becomes softer, the true area of contact increased and the thickness of the indium decreased greater than those in the previous method. In addition, it was shown that voids appearing by heating were removed by bake-out process from thermal desorption spectroscopy test. [ABSTRACT FROM PUBLISHER]

The remountable (mountable and demountable repeatedly) high-temperature superconducting (HTS) magnet has been proposed for huge and complex superconducting magnets in future fusion reactors to fabricate and repair easily the magnet and access inner structural components. This paper summarizes progress in R&D activities of mechanical joints of HTS conductors in terms of the electrical resistance and heat transfer performance at the joint region. The latest experimental results show the low joint resistance, 4 nΩ under 70 kA current condition using REBCO HTS conductor with mechanical lap joint system, and for the cooling system the maximum heat flux of 0.4 MW/m 2 is removed by using bronze sintered porous media with sub-cooled liquid nitrogen. These values indicate that there is large possibility to design the remountable HTS magnet for fusion reactors. [ABSTRACT FROM AUTHOR]

Mechanical joints of a high-temperature superconducting (HTS) conductor has been investigated for a “remountable” (demountable) HTS magnet. In a previous study, prediction method of joint resistance for a large-scale conductor was proposed using experimental data and current distribution analysis. However, the data were obtained by tests conducted at 77 K under self field. The joint resistance consists of contact resistance and resistance of constituted materials of HTS tapes, which depend on temperature and magnetic field. In this paper, therefore, we experimentally and numerically evaluated joint resistance characteristics with changing temperature and magnetic field using mechanical lap joint of a GdBCO tape. The experimentally evaluated joint resistance varied with the temperature and magnetic field as predicted. The joint resistance characteristic also depended on contact materials. The numerically evaluated contact resistance included in the joint resistance depending on temperature shows a tendency similar to the calculated value of the constriction resistance based on the Holm's contact theory. [ABSTRACT FROM PUBLISHER]

This paper discussed feasibility of bridge-type and multi-layer mechanical lap joint of a stacked HTS conductor. In the multi-layer mechanical lap joint, a decrease in critical current induced by transverse compressive joint force is one of the important factor to decide joint resistance. In addition, misalignment of joint section could be a problem because respective HTS tapes consisting the stacked HTS conductor could have slightly different thickness. In this study, therefore, transverse stress effect on critical current of stacked BSCCO and GdBCO conductors was experimentally investigated at first. Then, bridge-type mechanical lap joints of single layer and 2-layer HTS conductors were tested to confirm their performance and to consider the misalignment of joint section. Experimental results showed that critical current of the BSCCO conductor decreased at lower stress when the number of layers increased whereas that of the GdBCO conductor was less influenced by the stress. Joint resistance in the bridge-type mechanical lap joint was comparable to those in normal mechanical lap joint and soldered lap joint in the case of the single layer. Joint resistances in the 2-layer lap joint were two or three times larger than those in the single layer lap joint due to the misalignment. To solve this problem, compliant metal layer should be inserted into joint section. [ABSTRACT FROM PUBLISHER]

Electrical edge joint of a stacked high-temperature superconducting (HTS) conductor is explored for electrical joints for a demountable HTS toroidal field coil. Demountable coils would be very useful in small tokamak plasma-facing component-test machines, such as Vulcan. The demountable concept explored in this paper consists of forcing together the edges of stacked HTS conductor embedded in a conductive or a structural material. We carried out a testing program of the edge joint of a stacked YBCO conductor within a copper jacket. The HTS cable has a critical current of 1600 A at 77 K in self-field. The experimental results showed that joint resistance in the edge joint was higher than expected, as was not reproducible. One potential reason for the larger than expected resistance is the damage to the tape edge in preparation of the joint region due to milling/fabrication. Another potential reason was low accuracy in the fabrication of joint surface; the joint surfaces were not parallel to one another. The joint resistance will be reduced by the sophistication of the fabrication process for the joint surface. We also carried out calculations to determine the joint performance of the edge joint in an ideal condition. In the ideal condition, inserting an indium film between joint surfaces or plating copper layer on joint surfaces can prevent joint resistance from increasing due to the misalignment of the contact surfaces and would provide a compliant layer between joints. The joint resistance of the edge joint can become smaller than that of an electrical lap joint when the number of the stacked HTS tape is large. [ABSTRACT FROM PUBLISHER]

Mechanical butt joint of a high-temperature superconducting (HTS) cable with a metal jacket has been investigated for a remountable HTS magnet where parts can be mounted and demounted iteratively to allow a superconducting magnet for a fusion reactor to be assembled and repaired. In this study, an optimum structure to achieve relatively uniform contact pressure distribution on joint surface in the mechanical butt joint was suggested by structural analysis. Then, joint resistance in mechanical butt joint of a stacked BSCCO 2223 cable with copper jacket was evaluated experimentally using an experimental setup established based on the structural analysis, to confirm effect of decreasing joint resistance by the joint structure. The results showed that the structure can reduce joint resistance to 140 n\Omega for the BSCCO 2223 cable having 1 kA of critical current at liquid nitrogen temperature. [ABSTRACT FROM AUTHOR]

A concept of remountable HTS magnet has been proposed to simplify fabrication of a superconducting magnet for a fusion reactor and to reduce maintenance cost of the reactor. For realization of the remountable HTS magnet system, mechanical butt joint of a stacked BSCCO 2223 cable has been investigated experimentally with large joint stress. In this study, 10 layered BSCCO 2223 cable with copper jacket is used as a test cable, and the roughness of joint surface is controlled quantitatively by using several different particle sizes of abrasive particles. Then the relation between the joint stress and resistance is evaluated through the mechanical butt joint test of the cable under several experimental conditions. Although dependence of the joint resistance on the joint stress is observed the relationship between roughness and joint resistance is not observed clearly due to not coincidence in surface flatness of the test cables. [ABSTRACT FROM AUTHOR]

A remountable high-temperature superconducting (HTS) magnet where parts can be mounted and demounted is proposed to allow a superconducting magnet for a fusion reactor to be assembled and repaired. A conceptual study of a mechanical butt joint for a stacked HTS cable with metal jacket was carried out. In this study, contact pressure distribution at the joint surface was evaluated by structural analysis to investigate the deformation of the joint region due to the joint force and the influence of the thermal strain on joint performance. Analytical results showed that thermal strain and Young's modulus of the constituent material strongly affect the contact pressure distribution. In addition, combinations of parallel and perpendicular joint forces, and the procedure with which they are applied, have a strong impact on the realization of a remountable joint in a HTS cable. [ABSTRACT FROM AUTHOR]

The remountable HTS magnet was proposed to allow a superconducting magnet for a fusion reactor to be assembled and to allow a failed part of the magnet to be replaced or repaired. In this study, joint resistance was evaluated in a mechanical butt joint of a stacked BSCCO 2223 cable with a copper jacket. The fabricated cables could reduce joint resistance to be 400-SOD nΩ with a silver-plated layer or an indium-film applied to the joint surface. The experimental results also showed that parallel joint force is required to reduce joint resistance more without deformation of the joint surface configuration. [ABSTRACT FROM AUTHOR]

• Contact area in mechanical lap joint of REBCO tapes was evaluated with X-ray CT scan. • Graph cut image segmentation technique was chosen to improve evaluation of contact area. • Improvement of correlation between contact area and contact resistance was confirmed. • Inhomogeneous distribution of real contact area causes dispersive contact resistance. A mechanical lap joint fabricated with high-temperature superconducting (HTS) tapes is proposed for the application to joint-winding in HTS magnets for fusion reactors. The applicability of the joint has been validated, however, it could not be guaranteed because the joint resistance is unpredictable before the entire conductor is cooled and energized. Identifying the factors that affect the joint resistance is necessary to develop a method to predict this parameter at room temperature. In this report, we evaluated the correlation between the electrical resistance of contact interfaces (contact resistance) and the contact area observed using X-ray computer tomography scan (observed contact area), and discussed appropriate techniques for this prediction. A total of 40 mechanical lap joint samples were prepared. The observed contact areas were segmented from cross-sectional images of contact interfaces using a graph cut image segmentation, while the contact resistances were calculated from measured joint resistances. The correlation indicated that the prediction of contact resistance is more precise when base on the observed contact area compared to the conventional method using the nominal contact area. However, some of dispersive contact resistivity still remained due to inhomogeneous distribution of fine-structure on contact interface. [ABSTRACT FROM AUTHOR]

• Structural design of magnet system for FFHR-c1 is depicted. • Effect of coil configuration parameter on electro-magnetic force is analyzed. • Effect of curvature on a winding scheme of the helical coil is discussed. FFHR-d1A and c1 are the conceptual design of a helical fusion reactor. The positional relationship among superconducting coils, a pair of helical coils with two sets of vertical-field coils, are observed to be similar in both type of FFHR. Such a relation of coil configuration is based on the coil configuration of the Large Helical Device, which has been designed and constructed at the National Institute for Fusion Science. There is increasing demand to achieve an optimized coil configuration to anticipate improvements in plasma-confinement conditions. In this study, the structural design of FFHR based on the fundamental set of parameters of coil configuration is depicted, which satisfies the soundness of the structure. Further, the effects of the coil configuration parameters on the stress distributions are investigated. An effect of radius of curvature on a winding scheme of the helical coil is also discussed. [ABSTRACT FROM AUTHOR]

A conceptual design for a helical fusion reactor is currently being undertaken by the National Institute for Fusion Science, Japan. The coil support structure is designed from the perspective of the allowable stress of the material. A continuous helical coil winding with a low temperature superconductor and a water-cooled divertor made of tungsten and copper alloy have been considered for use in this reactor, and it is defined as the basic option. Several flexible design options have also been proposed; these options solve existing issues in the basic option and they are treated as the challenging options. They can be implemented by modifying the structural components of the basic option. The structural design modifications need to be made in order for the challenging options and the mechanical soundness were investigated. [ABSTRACT FROM AUTHOR]