High Temperature Superconducting Conductor Research Articles

Measurement and Analysis of Critical Current of 100-kA Class Simply-Stacked HTS Conductors

Based on the successful plasma experiments in the Large Helical Device (LHD), design activities of the LHD-type helical fusion reactor FFHR-d1 are progressing at National Institute for Fusion Science (NIFS). A 100 kA current capacity is required for the winding conductor under the maximum magnetic field of ~12 T. The high-temperature superconductor (HTS) is a promising option for the helical coil conductor. For the development of such a HTS conductor suitable for the helical fusion reactor, we fabricated 30 kA-class HTS conductor samples, and the excitation tests were successfully carried out. We then fabricated and tested a 100-kA class HTS conductor. The conductor sample is a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, then the critical current of the sample was measured. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the simply-stacked HTS tapes to verify the measured critical current of the samples. The critical current characteristics of a single HTS tape is evaluated by the percolation model in the precise analysis.

Investigation of Influences of Film Resistance and Magnetic Field on Contact Resistance in a Mechanical Lap Joint of GdBCO Tapes

In order to design a remountable (demountable) high temperature superconducting (HTS) magnet of a fusion reactor, it is necessary to predict joint resistance in a mechanical (electrical) joint of HTS conductors as functions of temperature and magnetic field. Holm contact theory explains that the contact resistance consists of the constriction resistance and the film resistance. The film resistance depends on the thickness of oxide layers on contact surface. In this paper, therefore, relationship between influences of the film resistance and surface roughness of the stabilizing copper layer of a gadolinium barium copper oxide tape was investigated using experimentally evaluated joint resistance. Next, dependencies of contact resistance on the temperature and magnetic field were evaluated using the experimentally obtained joint resistance. Then, a new model of contact resistance was suggested based on the results of current distribution analysis. The contact resistance based on the new model shows good agreement for the test results of a 100-kA class HTS conductor, which demonstrates the validity of the model.

Fundamental Investigation on Tensile Characteristics of a Mechanical Lap Joint of REBCO Tapes

This paper discusses tensile characteristics of a mechanical lap joint, which has been proposed and tested for joint-winding of a high-temperature superconducting (HTS) helical coil in the Large Helical Device type helical fusion reactor, FFHR-d1. Structural analysis for the helical coils in FFHR-d1 indicates that the maximum tensile strain along the HTS conductors and the maximum shear stress in HTS tapes region are 0.145% and 32 MPa, respectively under electromagnetic forces. Tensile test of the mechanical lap joint using Gadolinium Barium Copper Oxide (GdBCO) tapes with an inserted indium foil showed that the joint having 5-mm length and 5-mm width with a joint pressure of > 11 MPa had tensile strength exceeding that of the GdBCO tape itself, which also allows the tensile strain of 0.145% when used in the helical coil. To tolerate the shear stress of 32 MPa, the joint pressure of > 50 MPa was found to be needed according to the experimental results.

Development of a 66 kV-5 kArms Class HTS Power Cable Using REBCO Tapes With High Critical Current

High temperature superconducting (HTS) cables are able to achieve large power capacity and low-loss power transmission. Fujikura Ltd. worked on developing a 66 kV-5 kArms HTS power cable using high critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) REBCO tapes in the Japanese national project. One of the technical targets in this project was to reduce AC loss to less than 2 W/m at 5 kArms. REBCO tapes with 240 A/4 mm-width of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at 77 K, self field, which were fabricated by ion-beam-assisted-deposition (IBAD) and pulsed laser deposition (PLD) methods, were applied to the HTS power cable. As a result, we succeeded in developing the 20 m-long 66 kV-5 kArms HTS power cable. The measured AC loss was achieved 1.4 W/m at 77 K and 1.0 W/m at 67 K at 5 kArms. On the other hand, it is considered that the eddy current loss of the stainless-steel cryostat pipe of HTS cable become obvious in the case of the single-core cable with the large current capacity. It is suggested that the eddy current loss of the stainless-steel cryostat pipe is due to the internal magnetic flux. Therefore, the eddy current loss of the stainless-steel cryostat pipe as well as uniform current distribution of HTS conductor and shield layer should be taken into account for designing the winding direction and pitch of HTS conductor and shield layer of HTS cable. In this work, the measured eddy current loss of the stainless-steel cryostat pipe is also compared with the calculated one.

Comparison Between Different Design Topologies for Multi-Megawatt Direct Drive Wind Generators Using Improved Second Generation High Temperature Superconductors

Various design topologies were investigated to study the weights and costs for a direct drive wind turbine generator using improved second generation (2G) YBCO high temperature superconductors (HTS). Five different design topologies using combinations of air core, iron core, salient and non-salient configurations for both stator and rotor were considered for a generator with the nominal rating of 10 MW, 8 RPM and 3300 V. The main objective of the investigation was to minimize the HTS quantities and the reduction of total cost to achieve the same rating performance. The electromagnetic design was performed using transient finite element modeling including the improved HTS conductor characteristic properties. The mechanical design includes the support structures for both full load operation and sudden three-phase terminal fault conditions which can cause severe stresses.

Performance Tests of Prototype High-Field HTS Coils in Grenoble

Longitudinal and transverse quench propagation tests were performed at the Laboratoire National des Champs Magnetiques Intenses (LNCMI) high-field test facility on instrumented double pancake coils fabricated by CEA-Saclay using co-wound high-temperature superconductor (HTS) tapes. Energy deposited on an embedded heater initiated a quench and its subsequent propagations. Following the resulting thermomechanical analysis, a conductor design with stacked HTS tapes co-wound with stabilizers was conceived. A ten-turn demonstrator racetrack coil has been fabricated from the stacked HTS conductor with an aim to investigate the operation margin of an HTS coil in a background magnetic field misaligned from the coil axis as expected for the operating condition of an accelerator magnet insert. A test setup with a high current capacity up to 3 kA and angular variability that utilizes a room-temperature 376-mm-bore 10-T resistive magnet at LNCMI in Grenoble has been built. The performances and operation margin of the racetrack coil were investigated.

Fabrication of HTS Conductor Stacks for 20-kA Binary Current Leads for MagLab Series-Connected Hybrid and HFML Nijmegen Hybrid Magnets

The MagLab series-connected hybrid and the HFML Nijmegen hybrid outer superconducting coils each require a pair of binary current leads, comprised of a nitrogen vapor-cooled resistive section and a high-temperature superconducting (HTS) section, configured to minimize the heat load to the coils. The HTS sections of both pairs of leads are made with stacked and soldered Bi-2223 tapes supplied by Sumitomo. The MagLab fabricated and tested the tape stacks for both programs. The procedures and apparatus for fabricating the stacks and making the critical current measurements are described. Critical current and n-value measurement results are reported.

Numerical Calculation of Advection Heat Transfer and Fluid Flow Dynamics of $\mbox{LN}_{2}$ Flowing HTS Cable

For long-distance operation of high-temperature superconducting (HTS) cables, evaluations of heat transfer and fluid flow dynamics of liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) flowing in the HTS cable is important. However, the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow is complicated when the cable core is positioned at an eccentric position to the center of the cryostat pipe. A computer simulation analysis is effective in evaluating these characteristics with the complicated LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow. In the computer simulation analysis, design of an appropriate simulation model, which is considering the cable configuration and LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow conditions, is necessary. In this paper, at first, the accuracy of the computer simulation model was evaluated by comparison of the theoretical arithmetic analysis model for a simplified cable configuration, which is assumed that the cable core is positioned at the center. As a result, the value of computer simulation analysis showed a good agreement with that of the theoretical analysis. Then, using this computer simulation model, the effects of the eccentric arrangement on the pressure drops and the temperature rise of the flowing LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were simulated. As a result, the pressure drop of the eccentric configuration became to about two-third as compared with that of the central core configuration, and no significant temperature difference of the HTS conductor of each configuration was confirmed.

Design and development of high-temperature superconducting magnet system with joint-winding for the helical fusion reactor

An innovative winding method is developed by connecting high-temperature superconducting (HTS) conductors to enable efficient construction of a magnet system for the helical fusion reactor FFHR-d1. A large-current capacity HTS conductor, referred to as STARS, is being developed by the incorporation of several innovative ideas, such as the simple stacking of state-of-the-art yttrium barium copper oxide tapes embedded in a copper jacket, surrounded by electrical insulation inside a conductor, and an outer stainless-steel jacket cooled by helium gas. A prototype conductor sample was fabricated and reached a current of 100 kA at a bias magnetic field of 5.3 T with the temperature at 20 K. At 4.2 K, the maximum current reached was 120 kA, and a current of 100 kA was successfully sustained for 1 h. A low-resistance bridge-type mechanical lap joint was developed and a joint resistance of 2 nΩ was experimentally confirmed for the conductor sample.

Upgrade of SULTAN/EDIPO for HTS Cable Test

CRPP hosts two unique conductor test facilities SULTAN (SUpraLeiter TestANlage) and EDIPO (European DIPOle). They allow the test of high current superconductors in high magnetic fields (SULTAN 11 T, EDIPO 12.5 T). In both facilities sample currents up to 100 kA can be supplied by means of a NbTi transformer. Presently the facilities are upgraded for the test of high current high-temperature superconductor (HTS) samples. For HTS conductor testing at temperatures between 20 and 50K, the heat flux between the HTS sample under test and the NbTi transformer needs to be limited to around 10W per conductor leg by means of an HTS adapter connecting them. The second required upgrade is the supply of intermediate temperature helium (20-50 K) to the HTS test conductor. It is mandatory that the helium gas coming from the HTS conductor under test can be returned to the cryoplant as cold gas (T < 20K). To reach this goal a tube-in-tube heat exchanger has been manufactured in which 4.5K helium coming from the cryoplant is in counter flow with the warm gas leaving the HTS test conductor.

The Effect of Different Meshes on the AC Loss Calculation of HTS Conductors

The AC loss calculation is crucial for the thermal analysis of the High Temperature Superconducting (HTS) magnet. Generally, the critical state model is used to calculate the AC loss value of the HTS conductor with Finite Element Method (FEM). The mesh of the model have a large impact both on the convergence and the accuracy in calculation processes. In this paper, we investigate the effect of different meshes on the AC loss calculation of HTS conductors using a second generation tape model. Both triangular and quadrangle element with fix number of elements and predefined distribution type meshes have been used. The results illustrates that quadrangle with geometric sequence is the best choice when calculating the AC loss in HTS tape.

Irradiation Response of Next Generation High Temperature Superconductors for Fusion Energy Applications

The latest generations of rare-earth substituted and nano-doped YBa2Cu3O7-x (YBCO) high temperature superconductors (HTS) developed for applications in magnetic fields are being evaluated for potential use in fusion energy applications. The benefits include increased plasma performance and reduced system cost through more compact and cryoplant-free fusion energy systems. The response to ion irradiation of commercially produced GdBa2Cu3O7-x, (Y,Dy)Ba2Cu3O7-x, and Zr-doped (Y,Gd)Ba2Cu3O7-x samples was investigated. These state-of-the-art conductors represent different design methods for enhanced flux pinning, resulting in different responses to radiation damage. Irradiations using 5-MeV Ni and 25-MeV Au ions were used to examine cascade damage while keeping electronic energy loss levels below columnar defect thresholds. An improved radiation tolerance is found in these new generation HTS conductors. Specifically, the influences of irradiation on the superconducting critical temperatures and the electrical transport properties of the samples were much less than that observed on the earlier generation of irradiated HTS materials investigated by others.

Recent Developments in High-Temperature Superconducting Magnet Technology (Review)

The use of magnets made of high temperature superconductors (HTS) such as BSCCO and REBCO easily provide higher magnetic fields and higher operating temperatures, enabling dramatic improvements in superconducting magnet technology. The LTS magnet technology is very well summarized in text books written by M. N. Wilson (Superconducting magnets, Clarendon Press Oxford, 1983) and Y. Iwasa (Case studies in superconducting magnets, 2nd edition, Springer, 2009), covering such topics as stability, protection, ac loss and so forth. To the contrary, HTS conductors were commercialized only recently and therefore the magnet technology for HTS conductors remains undeveloped, especially so in the case of REBCO conductors. The technological problems for HTS coils thus far encountered are 1) an enormous effect of a screening current-induced magnetic field, 2) degradation in the coil performance due to excessive mechanical stresses applied along the longitudinal and transverse direction, and 3) the difficulty in protecting the magnet in the case of an abrupt thermal runaway. This paper reviews recent progress in overcoming these technological problems for HTS magnets. Both BSCCO and REBCO conductors have been used for HTS magnets in areas such as high field facilities, NMR, MRI, magnetic levitation trains and so forth. The effect of the screening current is the major problem for NMR, MRI, and accelerators, as it substantially distorts spatial field homogeneity and temporal field stability; on the other hand, degradation due to excessive stresses is substantial for high field magnets. Additionally, coil protection is a common and substantive problem among high current density HTS magnets in general. World-wide activities in developing BSCCO and REBCO magnets are overviewed in this paper.

Conceptual Design of a Toroidal Field Coil for a Fusion Power Plant Using High Temperature Superconductors

Taking a step further from the International Thermonuclear Experimental Reactor (ITER), the next step will be a demonstration fusion power plant, DEMO, i.e., a fusion power plant (FPP) prototype. As a part of European Union (EU) DEMO studies, the result of the so-called PROCESS system code has been taken as the basis to design a toroidal field coil (TFC) winding pack with high temperature superconductor (HTS) REBCO, which is a promising HTS candidate. From this, the cable space area, the winding pack current density, and the total current in one TFC has been obtained. In this paper, a conceptual design of a HTS TFC is presented, and related parameters, such as peak magnetic field at the conductor, conductor current, and coil inductance, are calculated. The results have been used to evaluate the temperature margin and the hot spot temperature in case of a quench. With the calculated results, it is shown that at 4.5 K, the actual available HTS conductor can be used to design a TFC for DEMO within the available space given by PROCESS code.

Wireless Power Transfer Using High Temperature Superconducting Pancake Coils

It is well known that the efficiency of a wireless power transfer (WPT) system depends on the resistance of the transmitting and receiving coils, especially for the case of low frequency. Since the ac loss of high temperature superconductors (HTS) is much lower than that of conventional conductors, using high temperature superconductors for WPT system is a good way to increase power transfer efficiency. Here, several kinds of WPT experiments were designed and conducted by use of HTS and conventional conductor (copper) coils, respectively. The results are presented and analyzed. It is shown that the behavior of HTS WPT system is similar to that of a conventional WPT system, but the efficiency of WPT system is much higher by use of HTS coils and HTS coil is more suitable for being as a transmitting coil than as a receiving one.

Quench Detection of HTS Current Lead Using Hall Probe

The studies on the high temperature superconducting (HTS) current lead for superconducting magnets are actively underway with rapid advancement in HTS conductor technology recently. HTS leads reduce the heat load to superconducting magnets. HTS current lead consists of several HTS tapes in parallel which are soldered to the copper blocks. The quench occurs first in the HTS tape with lowest critical current among HTS tapes during charging of the transport current. Conventionally, HTS current leads use the equipotential voltage signal to detect quench but it is difficult to estimate the quench of an HTS current lead in fault because the quench voltage level of an HTS tape in a current lead is very low. In this paper, the quench detection of an HTS current lead using a hall probe was investigated. First, the magnetic field distributions of an octagonal shaped HTS current lead before and after quench in charging of transport current were computed using Comsol. Then, this octagonal-shaped HTS current lead was fabricated and tested in order to measure its central magnetic field. Based on the results, the application feasibility of the quench detection of HTS current lead using hall probe was confirmed.

A superconducting joint for GdBa2Cu3O7−δ-coated conductors

Second-generation (2G) GdBCO-coated conductors (CCs) are promising for superconducting magnet applications because of their high critical current (Ic) density, low dependency of the Ic on the external magnetic field, good mechanical properties and reasonable cost, which offer opportunities to develop ultra-high-field magnets. However, they have not been used in high-temperature superconducting (HTS) applications with persistent current mode (PCM) operation such as nuclear magnetic resonance/magnetic resonance imaging magnets owing to unavailability of fabrication techniques for proper joining and contacts. Here we report a resistance-free joint, termed a ‘superconducting joint’, for 2G GdBCO CCs that forms a direct connection to establish a superconducting closed loop for PCM operation. The Ic value of the joined CCs is identical to that of the parent conductors in a liquid nitrogen bath (77 K). Moreover, the initially induced magnetic field of a model GdBCO coil containing a superconducting joint is maintained without decreasing, indicating the complete absence of electrical resistance. Thus, this fabrication method is a unique practical solution for lengthening the 2G HTS CCs and, more importantly, achieving PCM operation in 2G HTS magnet applications, including ultra-high-field nuclear magnetic resonance/magnetic resonance imaging magnets generating more than 1 GHz.

Ultra-high performance, high-temperature superconducting wires via cost-effective, scalable, co-evaporation process.

Long-length, high-temperature superconducting (HTS) wires capable of carrying high critical current, Ic, are required for a wide range of applications. Here, we report extremely high performance HTS wires based on 5 μm thick SmBa2Cu3O7 − δ (SmBCO) single layer films on textured metallic templates. SmBCO layer wires over 20 meters long were deposited by a cost-effective, scalable co-evaporation process using a batch-type drum in a dual chamber. All deposition parameters influencing the composition, phase, and texture of the films were optimized via a unique combinatorial method that is broadly applicable for co-evaporation of other promising complex materials containing several cations. Thick SmBCO layers deposited under optimized conditions exhibit excellent cube-on-cube epitaxy. Such excellent structural epitaxy over the entire thickness results in exceptionally high Ic performance, with average Ic over 1,000 A/cm-width for the entire 22 meter long wire and maximum Ic over 1,500 A/cm-width for a short 12 cm long tape. The Ic values reported in this work are the highest values ever reported from any lengths of cuprate-based HTS wire or conductor.

Design prospect of remountable high-temperature superconducting magnet

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, 4nΩ under 70kA current condition using REBCO HTS conductor with mechanical lap joint system, and for the cooling system the maximum heat flux of 0.4MW/m2 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.

Numerical Analysis of Heat Transfer and Fluid Characteristics of Flowing Liquid Nitrogen in HTS Cable

High-temperature superconducting (HTS) cable has heat intrusion from the termination including joule heat generation at the terminal joint and from the room temperature cable through the Cu current lead. According to the length of the HTS cable, this heat loss may become a considerable amount which cannot be ignored in the HTS cable system. In this study, referring to a high-voltage cable (HV cable) which was developed in M-PACC project, the effect of heat transfer at the interface between the terminal joint and LN2 in the terminal vessel (ho) on the temperature of the HTS cable were calculated and evaluated. The condition of flow in the terminal vessel was assumed to be natural convection, forced flow or static condition for evaluating this effect with various heat transfer condition. As a result, in the case of the natural convection, most of heats flow into the LN2 in the terminal vessel where the volumetric flow of the LN2 is large since ho becomes high. Accordingly, the temperature rise of the LN2 in the inner pipe of Cu former and the terminal vessel can be restricted. However, in the cases of the forced flow and the static condition, most of heats flow into the LN2 in the inner pipe where the volumetric flow of the LN2 is small since ho becomes small. Accordingly, the temperature rise of the LN2 in the inner pipe becomes high. This temperature rise of the LN2 in the inner pipe makes the temperature of the HTS conductor large resulting in remarkable increase of AC losses. Consequently, on the HV cable design, for restriction of the AC loss increase, it is expected that designing the HTS cable termination such as extending outer surface of the terminal joint for increasing of the heat inflow from the terminal joint to the LN2 in the vessel is effective.