High Engineering Current Density Research Articles

CORC® wires allowing bending to 20 mm radius with 97.5% retention in critical current and having an engineering current density of 530 A mm−2 at 20 T

Abstract Low-inductance, high-field insert solenoid magnets and 20 T dipole magnets for particle accelerators require flexible cables, wound from high-temperature superconductors (HTS) such as RE-Ba2Cu3O7-δ (REBCO) coated conductors, that allow bending to a 20 mm radius without significant degradation in performance. They require an operating current of at least 5 kA and a high engineering current density (Je) exceeding 500 A/mm2 at 20 T. HTS cable technologies that target such demanding magnet applications so far haven’t been able to meet the combination of these requirements.
Here we present the development of the next generation of Conductor on Round Core (CORC®) wires that improves their bending flexibility by factor of more than 2 compared to previous generation CORC® wires. CORC® wires now allow for a bending radius of 20 mm with only 2 – 3 % performance degradation. They allow bending to a radius of 15 mm with a performance retention of 83.5 %. The performance of 30-tape CORC® wires wound from 2 mm wide REBCO tapes from SuperPower Inc, SuperOx and Shanghai Superconductor Technologies was measured at magnetic fields up to 12 T. The overall performance at high magnetic fields of the next generation of CORC® wires improved by a factor of 1.5 – 1.8, depending on the REBCO tape manufacturer. CORC® wires wound from production REBCO tapes achieved a new record Je of 751 A/mm2 at a current of 8.3 kA at 12 T, and a Je of 530 A/mm2 at a current of 5.8 kA when extrapolated to 20 T. 

Study on the response of the winding direction of multi-layer CORC cable to its electromagnetic characteristics

A Conductor on Round Core (CORC) cable wound with a high-temperature superconductor is an important cable concept for high-current density applications. It is widely used in large power systems because of its advantages of good flexibility and high engineering current density. However, the complex design structure of CORC cable makes it very difficult to understand its electromagnetic properties (such as AC loss). In particular, the different winding directions of each layer in multi-layer cable have a great influence on its electromagnetic characteristics. In this paper, the H-method is used to solve the electromagnetism and mechanics equations. The influence of the winding direction of CORC cable on its electromagnetic field distribution characteristics, AC loss, and mechanical variation under the action of external magnetic field is investigated. The AC loss study of single-layer cable reveals that when the applied magnetic field is increased from 0.01 to 0.02 and 0.03 T, the AC loss peak of the cable increases by 107 and 103 orders of magnitude, respectively, indicating that the effect of low applied magnetic field on the AC loss of the cable is more significant. For multi-layer cables, cables with opposite winding directions have a greater depth of current density penetration than cables with the same winding direction. In addition, the mechanical variations of multi-layer cables with different winding orientations are explored. The results show that the Mises stress in the cable with the same winding direction is about 32% higher than that of the cable with the opposite winding direction, which indicates that the method of winding the cable in the opposite direction between adjacent layers of tape can avoid excessive mechanical stress.

Study on the electro-magneto-mechanical characteristics of CORC superconducting cable with striations

CORC superconducting cable is a compact and flexible composite superconductor. It is widely used in large power systems because of its good flexibility and high engineering current density. In these applications, reducing the AC loss generated by the superconductor is a key factor in studying the electromagnetic field of the superconducting conductor, which affects the operational stability of the entire superconducting power device. Currently, the use of striated tape-wound cables not only allows for flexible wiring at design time but also effectively reduces AC losses. This technology has been widely used in large superconducting magnet equipment. In this paper, the finite element model of single superconducting tape and CORC superconducting cable is established based on the finite element idea. The electromagnetic properties and mechanical changes are numerically calculated by H-method. The results show that both the current density and the magnetic field of a single superconducting tape show a gradual penetration from the two sides to the center. The area of maximum current density and magnetic field distribution is at the tape boundary. The distribution of current density, magnetic field and Lorentz force of three-dimensional CORC superconducting cable is similar. All of them are gradually attenuated from the maximum value at the boundary to the center position. It is worth noting that the current density, magnetic field and Lorentz force of the striated superconducting cables are smaller than the values without striations. The calculation results of the finite element model in this paper show that the striations weaken the current density, magnetic field and Lorentz force of the CORC superconducting cable by about 8.77%, 16.21% and 9.23%. Moreover, the presence of striations can effectively reduce the AC loss of superconductors.

A quench detection method for parallel co-wound HTS coils based on current redistribution

High-temperature superconducting (HTS) coated conductors (CCs) have become the preferred material for superconducting magnet applications due to their high engineering current density and high mechanical strength. However, due to the low quench zone propagation velocity of CCs, magnets wound with CCs suffer from severe quench risks. Therefore, a rapid, sensitive, and reliable quench detection method is crucial for the safe operation of such HTS magnets. In this paper, we propose a quench detection method based on current redistribution, in which two pieces of HTS CCs are soldered together at each end and insulated in the middle part, which are then parallel co-wound into a double-pancake coil. The two tightly coupled windings and low resistance joints form a very low inductance current loop, resulting in fast current redistribution between the two co-windings even at the inception of quench (with still low quench voltage). We deduced analytical solutions of the current redistribution process under different magnet operational scenarios, including constant current operation, charging and discharging, and proposed quench detection criteria. Corresponding quench tests were performed on a small scale co-wound HTS coil, and the results well verified the analytical solutions and the effectiveness of the quench detection method. Our work may be useful for lowering the risks in HTS magnet quench in various applications.

Transport AC Losses in Multiple-Layer Roebel Tapes

The Roebel tape is a novel high-temperature superconducting (HTS) wire with a multiple superconducting layer structure. Adjacent layers are patterned into meander-shaped multifilaments in opposite directions so that the magnetic fields generated by each layer can be mutually cancelled. The Roebel tape can be regarded as an HTS Roebel cable encapsulated into a tape, and hence has exclusive advantages such as high engineering current density, small critical current degradation, low AC losses, and a simple manufacturing process. This paper focuses on the transport AC losses of the Roebel tape with a multiple-layer structure. 3-D finite element method (FEM) models for the Roebel tapes are built based on the T-A formulation. Current distribution and AC losses of the Roebel tapes with up to 10 ReBCO layers are obtained. The results show that the tapes with even numbers of layers generate lower losses than those with odd numbers, which indicates the superconducting layers should always appear in pairs. Increasing layers can increase the engineering current density of the tape, but the average loss per layer also rises. Therefore, we suggest increasing the distance between the layer pairs or using double-layer Roebel tape stacks instead. The results indicate that the multilayer Roebel tapes can be a promising cable solution for industry and fusion applications because of the high current density and low AC losses.

Practical Forecasting of AC Losses in Multi-Layer 2G-HTS Cold Dielectric Conductors

With the recent progresses on the designing and manufacturing of lightweight superconducting cables with high engineering current density, the need for a reliable, fast, and accurate computational model forecasting the alternating current (AC) losses of cold-dielectric conductors is pivotal for power grid investors and operators. However, validating such models is not an easy task. This is due to the low availability of experimental data for large scale power cables, and likewise, because of the large computational burden which underlies the total number of second generational high temperature superconducting (2G-HTS) tapes in the modelling of realistic power cables. Thus, aiming to overcome these challenges, we present a detailed two-dimensional H-model capable to reproduce the experimentally measured AC-losses of multi-layer power cables made of tens of 2G-HTS tapes. Two cable designs with very high critical currents ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{1.7}~kA$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{3.2}~kA$</tex-math></inline-formula> ) have been considered. These are composed of five and six concentric layers wound over a cylindrical former consisting of 50 and 67 2G-HTS tapes, respectively. In both situations a remarkable resemblance between the simulations and experiments has been found, offering a unique view of the local electrodynamics of the wound tapes where the mechanisms of shielding, magnetization, and transport currents coexist within the hysteretic process.

High-Temperature Superconducting Undulator Prototype Coils for Compact Free-Electron Lasers

Short-period and high-field undulators are crucial for the production of coherent light up to X-rays in compact free-electron lasers (FELs). Besides, future colliders like CLIC or FCC-ee demand high-field damping wigglers to reach a low beam emittance. Both applications may benefit from the use of high-temperature superconductors (HTS), e.g. by providing high magnetic field amplitudes or higher operating temperatures. This paper presents and summarizes our HTS undulator prototype coils, designed and manufactured at CERN, made from coated REBCO tape superconductor. The coil design, described and already powered at 77K in previous works, is based on no-insulation (NI) vertical racetracks with a period length of 13 mm, assembled with iron poles. Powering tests of several HTS undulator prototype coils at 4.2 K, performed at KIT, revealed safe operations at high engineering current densities of 2.3 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with produced magnetic fields of up to 1.5 T at 3.5mm distance from the magnetic iron poles. Reasonable effective time constants of around 100 s were obtained and further powering tests in regions beyond <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> c with 3.6 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> showed the stability of our NI design and created magnetic fields in the region of 2 T.

Development of a Superconducting Wire for the Central Solenoid of a Tokamak with Reactor Technologies (TRT)

The article presents the results of the preliminary development of a superconducting wire based on the VS type design and parallel tape packages for the central solenoid of a compact tokamak with reactor technologies (TRT). One of the main problems that must be solved for successfully implementing such projects is to develop high-current high-temperature superconducting (HTS) conductors for toroidal excitation coils and central solenoid sections. The magnetic system compactness entails the need to develop a conductor with a high engineering current density up to 90 A/mm2. The operating current in the windings should be at a level of 60 kA at 15 K in a magnetic field of 15 T. The wire in the central solenoid experiences significant mechanical loads caused by Lorentz forces. In addition, in view of a significant energy stored in the magnet, there is a need to have elements in the conductor able to perform emergency evacuation of energy with an acceptable voltage across the winding and its heating that will not entail damage to its elements. The conductor structure should have enough space to accommodate stabilizing and strengthening materials, as well as cooling channels. Two design versions of the VS type conductor based on radially arranged second-generation HTS tapes and based on parallel packages are considered. The design characteristics of the proposed conductors are analyzed for various operating modes of the tokamak electromagnetic system. The results of FEM-based calculations of the magnetic field distribution in a conductor, its current carrying capacity and energy loss estimation in a varying magnetic field are presented.

Parametric Optimization of Undulator Field for Short Period NI HTS Planar Undulator

There are increasing interests in high temperature superconducting(HTS) undulator with a gradual increase of engineering current density of REBCO conductor. Additionally, no-insulation(NI) winding technique may maximize the undulator field of HTS undulators mainly benefited from its high engineering current density. In this study, we present several design options of an NI HTS undulator to investigate its anticipated performance in terms of magnetic field strength. Specifically, maximum magnetic field along the beam axis depending on the magnetic gap, magnetic period length, winding dimension, and operating temperature is investigated. Then, these results are fitted using a scaling law to easily estimate the anticipated performances of NI HTS undulator at different magnetic gaps and magnetic periods. Finally, a comparison between NI HTS undulator, NbTi based low temperature superconducting undulator, and cryogenic permanent magnet undulator is discussed.

Field Canceling Effect in Double-Layer Roebel Tapes

The Roebel tape is a novel high-temperature superconducting (HTS) cable. By incapsulating the HTS Roebel cable into a tape, it has exclusive advantages such as high engineering current density, small critical current degradation, low AC losses, and a simple manufacturing process. This paper focuses on the field cancelling effect of the Roebel tape introduced by the specially designed patterns on the buffer layers and superconducting layers. From the results obtained by the 2D and 3D models using the T-A formulation, we find this effect can make the current distributed more uniformly and can significantly reduce the AC losses. The reason and the basic principle behind this will be revealed and analysed. To sufficiently exploit it to reduce losses, the structure parameters of the Roebel tape are optimised and an optimum solution is obtained. This paper can help better understand the Roebel tape and provide references to the future design and application of this novel HTS cable.

Strain Distribution and Critical Current Characteristics of an Individual REBCO Tape and Quasi-Isotropic Superconducting Strand Under Bending Loads by the Layerwise Theory

In applications of the second generation (2G) REBCO tapes, in order to achieve high engineering current density and alleviate the anisotropy of critical current, many structures of superconducting strands/cables made of REBCO tapes were proposed. In this paper, to investigate the bending behaviors of an individual REBCO tape and the quasi-isotropic superconducting (Q-IS) strand, we firstly build a three-dimensional (3D) model by the Layerwise theory to analyze the strain distribution of an individual REBCO tape under different tension loads which gives us the stress-strain curve of the tape. Then the bending characteristics of an individual REBCO tape are investigated by experiment, calculation and simulation. In addition, critical current features of the Q-IS strand under bending loads are studied in the same way. Experimental, numerical and simulated results are in good agreement. The results indicate that the Q-IS strand possesses great electromechanical performance under bending loads and it can be applied in many fields such as power transmission, large-scale superconducting magnets and nuclear fusion.

Elaboration of a Promising Design of the HTS Conductor for the Central Solenoid of a Compact Thermonuclear Reactor TRT

The results of the preliminary development of the HTS conductor based on the VS-type design and parallel stacks for the central solenoid of the compact thermonuclear reactor TRT are presented. One of the main problems that need to be solved for the successful implementation of such projects is the creation of high-current high-temperature superconducting (HTS) conductors for Toroidal Field coils (TF) and Central Solenoid (CS) sections. The conductor must have a high engineering current density of at least 90 A/mm2. The induction of the magnetic field in the central solenoid reaches 14 T, which leads to the occurrence of large mechanical stresses due to the influence of Lorentz forces. Like many large magnets, CS has a lot of stored energy. For the safe withdrawal of stored energy from the magnet, it requires the inclusion of elements in the conductor that provide an acceptable level of electrical voltage and heating of the conductor insulation. Thus, a sufficient amount of stabilizing and reinforcing materials should be placed in the conductor. In addition, the “cable-in-conduit” type of conductor must have channels for pumping the refrigerant. Two fundamentally different versions of the conductor based on radially arranged REBCO tapes and on the basis of pre-assembled tape packages are considered. Based on the analysis of the magnetic field distribution in the conductor by finite element method, the design characteristics of the proposed conductors under various operating modes of the electromagnetic system (EMS) of the tokamak TRT was evaluated. The results of the evaluation of the current carrying capacity of the conductor and the estimation of energy losses in a changing magnetic field in comparison with known methods are also presented.

An advanced electrical heating technique for double-sided Y(Gd)BCO coated conductor: gaining high engineering current density based on MOCVD

An advanced electrical heating technique was proposed and adopted for the reel-to-reel deposition of double-sided Gd x Y1−x Ba2Cu3O7−δ (Y(Gd)BCO) films on the surface of LaMnO3/epitaxial-MgO/IBAD-MgO/Y2O3/Al2O3/Hastelloy tapes based on the metal organic chemical vapor deposition process. In this technique, heating current is introduced into alloy tape to produce heat through the electric brushes. The use of thin Hastelloy tapes is an effective method to obtain a high engineering current density. However, the reduction of the substrate thickness will directly attenuate its mechanical strength, which will lead to the deformation of tapes at high temperature based on original electric heating device. More seriously, the electrical contact between the alloy substrate and the brush will deteriorate, which could cause ignition and ablation at the edge of the tapes. Therefore, in order to improve mechanical and electrical stability, we redesigned a novel electrical heating device to deposit Y(Gd)BCO films. Furthermore, through adopting the multiple-deposition process based on the new electrical heating device, the J e of Y(Gd)BCO film can reach 900 A mm−2 (at self-field, 77 K), which has been significantly improved compared with the J e before optimization.

Magnetization loss of no-insulation coil for an electrodynamic suspension system

A high-temperature superconducting no-insulation (NI) coil, with its self-protection property, high engineering current density and unique demagnetization property, becomes a potential candidate for an electrodynamic suspension (EDS) system. Compared with the applications in high field magnets, the NI coil used in the EDS system is considered as working in a dynamic state, the magnetization loss generated in the NI coil is essential for the design of a cryogenic system. This paper presents the study on AC magnetization loss of NI coils by both numerical and experimental methods. Firstly, a 3D finite element numerical model representing the full geometry of the NI coil is built to analyse the effect of field frequency, field magnitude, as well as the radial characteristic resistance. Then, systematic discussions are conducted to figure out the working mechanism of NI coils. Finally, a calibration-free method testing platform is installed to validate the numerical model, and a modified model is proposed to represent the non-uniform radial characteristic resistance caused by stress distribution. The conclusions of this paper will be used in the future optimization of NI coils and the cryogenic design of the EDS system.

Study on Reducing Magnetization Loss in CORC Cables By Laser Cutting Technology

Conductor on round core (CORC) cables with the features of flexibility and a high engineering current density have been a potential candidate for high magnetic field magnets and accelerator magnets. In these applications, reducing the magnetization loss generated in superconductors is essential in the process of design. CORC cables fabricated by multifilament-coated conductor (CC) tapes have been proved to have a low magnetization loss. However, the degradation of the critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and additional coupling losses significantly affects the performance of CORC cables. In this article, ultraviolet laser cutting technology is used to scribe both the superconducting layer and copper stabilization layers into narrow filaments. First, the magnetization loss of a single tape with different numbers of filaments was measured by a calibration-free method. Then, based on the testing results of a single tape, two typical CORC cables are wound by 10-mm-wide and 4-mm-wide CC tapes. The magnetization loss of cables made of striated tapes was discussed by both experimental and numerical methods. It is found that the magnetization loss significantly decreases with an increasing number of filaments under a high background magnetic field. The magnetization losses decrease in CORC cables made of striated CC tape. Due to the reduced coupling loss and attenuated shielding effect, there is not much difference between the two values of the magnetization loss reduction rates in CORC cables and single tapes. The conclusions obtained in this article will be instructive for the future design of CORC cables.

New HTS Conductor Design for Large Scale Applications

We propose a new design of high-performance HTS cable and CICC conductor for large magnet application, including magnets for prospective thermonuclear installations. The design allows a large number of HTS REBCO tapes to be assembled in a relatively small cable diameter. In this case, the cable is made according to the full transposition scheme with the calculated the critical current value equal to 30-150 kA. One of the main problems that needs to be solved for the successful implementation of thermonuclear projects is the creation of high-current conductors for toroidal field coils (TF) and Central Solenoid (CS) sections. For the successful solution of this problem, development of a HTS conductor with a high engineering current density in strong magnetic fields up to 20 T is required. There should be enough space in the conductor structure, for the allocation of stabilizing and strengthening materials, and for cooling channels. Preliminary calculations show that for various electromagnetic subsystems of a thermonuclear reactor, the required engineering current density in the conductor should be about 100 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or more. The report provides an analysis of the previously proposed design options for high-current conductors based on HTS REBCO materials and proposes a new one. Original design solution allows engineering current density along the HTS cable up to several hundred amperes per square millimeter in magnetic fields 16 – 20 T to be achieved.

Characterizations of a Novel Structure of Fault-Tolerant HTS Cable

This paper presents a type of high temperature superconducting (HTS) cable, which is fabricated by winding the second generation (2G) HTS tapes on round former consisting of stranded stainless steel (SS) wires. It has the advantages of strong flexibility, high mechanical strength, and high engineering current density. With taking the resistivity of SS much larger than that of copper or aluminum, the cable has higher resistance and strength than the conventional HTS cable with former made from copper strands at condition of fault accident. After the configuration of the cable is designed in detail, the quenching resistance and fault current limiting are numerically analyzed. We fabricate a model cable and carry out the experiment of fault current. The experimental results show that the quenching resistance of this cable is much higher than the resistance of conventional underground XLPE cable and overhead conductor, which indicates that the proposed cable has a strong ability of fault current limiting and can be applicable in HTS devices with functions of fault current limiting such HTS cable, transformer, motor/generator, and current limiter.

Bending Characteristics of a Quasi-isotropic HTS Conductor with High Engineering Current Density

A high-temperature superconductor (HTS) with high engineering current density and flexibility has recently been proposed and tested. The conductor is suitable for fusion magnets because of its high carrying current capacity in high field. However, the accumulated strain during the conductor fabrication and coil winding may cause irreversible degradation of the conductor critical current. Therefore, it is important to study the bending characteristics of the conductor and the accumulated winding strain and twisting strain on the HTS tapes during the conductor fabrication process. In this paper, the finite element method is used to study the bending characteristics of the conductor, and the degradation extent of the conductor critical current under different bending radii was calculated. Bending experiment was carried out, and the conductor critical currents at 77 K in self-field under different bending radii were measured. The experimental and finite element analysis results presented in this paper are useful in application of the conductor in high-field and large-scale magnet.

Study on critical current of a flexible quasi-isotropic HTS conductor with high engineering current density

This paper presents a flexible quasi-isotropic High Temperature Superconducting (HTS) conductor with high engineering current density. The conductor consists of three components: the inner core is a twisted quasi-isotropic strand (Q-IS) with a circular cross section achieved by symmetrically stacking four sub-stacks of HTS wires and four arc aluminum fillers, the middle component consists of copper layers constructed by copper wires spirally winding around the inner core, and the outer component consists of HTS layers constructed by HTS wires spirally winding around the middle section, which is similar to the manufacturing method used for HTS cable or CORC cable. After the three-dimensional finite-element simulation model of the proposed HTS conductor is established, the dependence of the magnetic field and critical current distributions on the twisting pitch of the inner Q-IS and the winding pitch of the outer HTS cable layers are analyzed. Consequently, the model HTS conductor is designed and fabricated, and the critical current of this HTS conductor is measured in liquid nitrogen. The results show that the simulated results are in good agreement with the experimental ones. It is predicted that this type of HTS conductor has potential applications in fusion magnet and power transmission.

2G HTS wire with enhanced engineering current density attained through the deposition of HTS layer with increased thickness

2G HTS wire with high engineering current density is desired for applications where compact, high power density superconducting equipment is important. We have succeeded in enhancing engineering current density of commercial SuperOx 2G HTS wire based on GdBCO by increasing the HTS layer thickness without fast degradation of the HTS film microstructure. This was possible after improving the temperature uniformity along the HTS film deposition zone. In particular, the wire engineering current density was increased from 700-770 A/mm2 (for a 65 μm-thick wire without stabilisation) or 430-480 A/mm2 (for a 105 μm-thick stabilised wire) at the beginning of this study to almost 1200 A/mm2 (for a 67 μm-thick wire without stabilisation) or 770 A/mm2 (for a 107 μm-thick stabilised wire) at completion of this study.