Layer-wound Coil Research Articles

Electromagnetic and Thermal analysis of an Intra-layer no-insulation coil during Quench

The intra-layer no-insulation (LNI) layer-wound REBCO coil has been widely studied for its excellent high thermal stability in the research of nuclear magnetic resonance superconducting magnets. Due to the complex structure of the LNI layer-wound coil and the numerous factors that affect thermal stability, its quench characteristics are not clear. In this paper, a numerical multiphysics coupling model is established to study the transient electromagnetic and thermal behaviors of the LNI layer-wound coil during quench. The results indicate that temperature follows the diffusion law, while the currents evolve through electrical connection and electromagnetic induction. Due to the high contact resistance, the azimuthal current and axial current vary almost in units of the entire coil layer. The direction of the radial current upstream of the turn where a quench occurs is positive due to the bypassing behavior, while the radial current direction is negative downstream. The insulation material inside the LNI layer-wound coil strongly affects the quench propagation. When its thickness decreases, the equivalent radial thermal conductivity increases, thereby increasing the quench propagation velocity and current peak, and shortening the duration of the current response during quench.

Analysis of charging characteristics of a 500 MHz HTS-LTS series NMR magnet with an intra-layer no-insulation HTS layer-wound coil structure

The intra-layer no-insulation (LNI) layer-wound coil has the advantages of high spatial magnetic field homogeneity and a smaller number of joints, as well as the ability of self-protection during quench, which has great potential in making MRI or NMR superconducting magnet. In this paper, a 500 MHz NMR magnet connected in series by a high temperature superconducting (HTS) magnet and a low temperature superconducting (LTS) magnet was designed, fabricated, and tested at a low field for homogeneity, in which the HTS magnet is an LNI layer-wound coil structure. And a helical equivalent circuit model of the 500 MHz HTS-LTS NMR magnet coupled with the stacked homogenization T-A and mechanical model is established, which agrees with the experimental results. This paper provides a sufficient numerical analysis of the charging characteristics of the HTS-LTS series magnet, including magnetic field homogeneity, current distribution, loss distribution, screening current induced stress, etc. And this paper provides valuable insights into the ramping behavior of the HTS-LTS series magnet, which can be useful for practical charging experiments.

Analyses of Deformation Due to Screening-Current-Induced Force in Layer-Wound REBCO Insert Coil for 1.3-GHz LTS/HTS NMR

REBCO wires have high critical current density and thermal stability, and are being applied in various fields such as NMR, MRI, and accelerators. We are currently designing a 1.3-GHz LTS/HTS NMR magnet that operates in a persistent current (PC) mode. However, there are issues with using REBCO coils that need to be resolved. One problem unique to REBCO coils is non-uniform electromagnetic forces caused by screening currents. During the charging and discharging of the coil, screening currents are induced, resulting in a significantly non-uniform current distribution within the coil. As a result, non-uniform electromagnetic forces are generated within the coil due to these screening currents. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this study, we performed an electromagnetic stress analysis using a two-dimensional axisymmetric structural analysis that considered tension during winding, thermal stress during cooling, and electromagnetic force during charging. First, we compared the results of the numerical simulation of the layer-wound REBCO coil with the experimental results. Then, we conducted numerical simulations of an insert REBCO coil in a design example of a 1.3-GHz NMR to evaluate the stress and deformation of the REBCO coil.

Numerical analysis of the contact resistance in a no-insulation layer-wound coil with a simplified electromagnetic–mechanical model

An increase in the contact resistance of a no-insulation (NI) high-temperature superconducting coil was observed in high-field tests, which may be related to the mechanical deformation and the separation between adjacent turns in the coil. The large electromagnetic force generated in the high magnetic field can causeseparation between adjacent turns of the NI coil, which can affect the contact resistance of the magnet. An electromagnetic–mechanical model is built to study the effect of separation on the contact resistance and field delay time of an NI layer-wound coil. The numerical results show that the large electromagnetic force generated in the high field leads to the local separation between adjacent turns and the increase in contact resistance of the NI layer-wound coil. Moreover, a higher external field or target current can result in a larger area of separation, a higher contact resistance and a shorter characteristic field delay time. An overband can restrain the mechanical deformation and separation between turns of the NI coil in the high field, which suppresses the increase of turn-to-turn contact resistance.

Development of an intra-Layer No-Insulation (LNI) Winding Method for Quench Protection of a Layer-Wound REBCO Coil

The no-insulation (NI) technique is promising as a protection method for high-field high-temperature superconducting (HTS) magnet. However, it is not suitable for layer-wound HTS coils because it has extremely long magnetic field delay due to short circuits inside winding. To solve this problem, a research group, including the current authors, proposed a novel winding method called intra-layer no-insulation (LNI) method. In this paper, we summarize the series of our studies on NI layer-wound coils and LNI coils as well as discuss issues affecting high-field magnet applications.

Ramping loss and mechanical response in a no-insulation high-temperature superconducting layer-wound coil and intra-layers no-insulation coil

Ramping loss and mechanical response in a no-insulation high-temperature superconducting layer-wound coil and intra-layers no-insulation coil

Design and Characterization of the Interlayer Joint Between Low-Field Nb3Sn Conductors of a Layer Wound DEMO TF Coil

In the frame of the conceptual design studies for the Toroidal Field (TF) coils of DEMO, a solution based on a layer-wound magnet, rectangular-shaped Cable-in-Conduit conductors and W&R manufacturing approach, is being developed. The feasibility and performance of a large-size Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor operating at about 82 kA in a 13 T field has been proven in the past. Another key technology to be demonstrated for a layer-wound TF coil, is that of a joint between two different conductor grades, to be possibly manufactured in-line during winding. The proposed joint solution would provide the minimum room occupancy, with the joint embedded within the winding pack volume. A joint between two low-field conductor grades, i.e., constituted of a small number of superconducting strands and a high number of stabilization copper wires, has been designed and manufactured. The two conductor lengths are characterized by a different number of superconducting wires and different outer dimensions. A joint sample has been assembled and instrumented, in the configuration allowing testing at the SULTAN facility of the Swiss Plasma Center. Both DC and AC performance of the joint has been characterized at the DEMO TF operating conditions. The present paper reports the main manufacturing steps for the joint and its main test results. The implications on the performance and design approach of the TF coil are also discussed, based on the outcome of such tests.

Performance of Epoxy-Impregnated Intra-Layer No-Insulation (LNI) REBCO Coils at 77 K

The intra-layer no-insulation (LNI) method is effective for protecting a layer-wound REBCO coil from a quench. However, a recently reported hoop stress modification (both increase and decrease) due to screening currents is a potential problem causing conductor degradation during charging. Epoxy impregnation can suppress the hoop stress modification and in this work, we develop prototypes of epoxy-impregnated LNI-REBCO coils to be charged at 77 K. Degradation of the coil performance due to cooling down can be prevented by mechanically isolating each layer. The coil shows a reasonably short field delay and current bypassing behavior, which are major technical benefits of the LNI-REBCO coil. A structural analysis with screening current-induced electromagnetic forces qualitatively suggests that the epoxy-impregnated LNI-REBCO coil can support a compressive hoop stress which causes conductor degradation due to buckling. The tensile hoop stress, however, is still very high at the upper and lower ends of each layer, causing fracture of a superconducting layer; i.e., additional reinforcement is required.

Energizing behaviors of a no-insulation and layer-wound REBCO coil in high magnetic field

REBCO coated conductor has been regarded as one of the most viable high-temperature superconductor (HTS) options for next-generation high field magnets, mainly owing to its large in-field current carrying capacity and higher mechanical strength. To explore REBCO coil technology, a small-scale REBCO coil was designed, fabricated and tested. The REBCO coil has a winding inner diameter of 15 mm, outer diameter of 29 mm and length of 38 mm using layer-wound and no-insulation technology. The REBCO coil was tested in a 31.5 T background field, leading to a high field of 32.5 T. The no-insulation behaviors of the REBCO coil, including the charging delay and magnet constant, were investigated at 77 K and 4.2 K respectively. With the magnetic load from the high fields, we found the REBCO coil was mechanically damaged after the experiments, being two major patterns: peeling and bucking.

A novel winding method for a no-insulation layer-wound REBCO coil to provide a short magnetic field delay and self-protect characteristics

The no-insulation (NI) technique is a very effective protection method for REBCO pancake coils. However, it is not suitable for a layer-wound coil, which is the optimum configuration for the persistent operation and generation of a homogeneous magnetic field, since an NI layer-wound coil suffers from an extremely long magnetic field delay. Such a delay is caused by large short circuits across the layers inside the winding. To combat this phenomenon, a simple winding method, denoted as ‘intra-layer no-insulation (LNI)’, is proposed in the present work. The method uses insulator sheets to prevent electrical contacts between the layers and copper sheets to increase electrical contacts between the turns in each layer: such a coil has the layer-wound geometry with the NI pancake-like internal circuit. An LNI REBCO coil shows a shorter field delay time constant by three orders of magnitude than an ordinary NI REBCO layer-wound coil, and also exhibits a self-protect behavior with more rapid thermal runaway propagation compared to a NI REBCO double-pancake coil.

Numerical analysis of thermal stability and mechanical response in a no-insulation high-temperature superconducting layer-wound coil

To achieve magnetic field spatial homogeneity and reduce resistive joints, no-insulation (NI) high-temperature superconducting layer-wound coils are highly desirable for nuclear magnetic resonance and magnetic resonance imaging. A multiphysics quench model is built to study the thermal stability and mechanical behaviors in an NI layer-wound coil. The numerical results show that when an NI layer-wound coil is subjected to heat disturbance, the transport current will be redistributed owing to the low turn-to-turn contact resistance, and thus the coil can achieve post-quench recovery. Although the layer-wound coil can rapidly return to its initial temperature after a quench, the current diffusion process within the coil leads to a long characteristic field delay time, which means that the circumferential current needs more time to recover. The increase of the peak temperature for coils with high turn-to-turn contact resistivity is not significant, which indicates that the temperature rise is mainly caused by the heat disturbance. Additionally, the recovery time of the central field is dramatically shortened. Thus, it is feasible to reduce the field delay time without sacrificing high thermal stability by designing a layer-wound coil with high turn-to-turn contact resistivity. Moreover, owing to the fast temperature rise during a heat disturbance, the coil undergoes remarkable deformation. The hoop and axial stresses exhibit larger changes than the radial and shear stresses. It can be also found that the mechanical behaviors are mainly affected by the heat disturbance rather than the electromagnetic force in the self-field.

Hybrid HTS-Nb3Sn-NbTi DEMO CS coil design optimized for maximum magnetic flux generation

The present work is performed within the framework of the EUROfusion DEMO project. Previously, it was demonstrated that for a maintained magnetic flux the use of HTS conductors at highest magnetic field in a layer-wound CS coil would allow the reduction of its outer radius by around 0.5 m as compared to the DEMO reference design using only Nb3Sn conductors. Alternatively, the superior high field performance of HTS conductors can be used to maximize the magnetic flux of a CS coil with a given outer diameter, which could significantly prolong the duration of plasma burn phase and thus the overall power plant efficiency. In the present study, the maximization of the magnetic flux, generated by a layer-wound CS coil of fixed outer radius, is considered. HTS conductors are envisaged to be used at highest field, while Nb3Sn and NbTi are foreseen to be used in intermediate and low field layers, respectively. The inner radius of the CS coil is optimized with respect to the generation of a maximum flux taking into consideration the superconductor properties, the hoop stress and the axial stress. In order to provide reasonable starting values for the finite element analysis, first a simplified model with layer-dependent current densities, however, without stainless steel grading will be considered. In the final outline design of the CS coil a superconductor and stainless steel grading will be implemented.

Mechanism of notable difference in the field delay times of no-insulation layer-wound and pancake-wound REBCO coils

The characteristic magnetic field delay time for a no-insulation (NI) REBCO layer-wound coil is three orders of magnitude longer than that for a NI REBCO double-pancake coil. In a NI layer-wound coil, the circumferential current firstly flows along the periphery of the coil winding, and then it diffuses from the top and bottom turns into the middle turns of the winding, resulting in a long characteristic magnetic field delay time due to the current diffusion process. In contrast, the characteristic magnetic field delay time for a NI double-pancake coil is dominated by the circumferential current decay in individual turns. On the basis of a derived scaling law, the characteristic magnetic field delay time for a NI REBCO layer-wound coil for a 400 MHz LTS/REBCO nuclear magnetic resonance (NMR) magnet is 37 h, while that for a NI REBCO double-pancake coil is only <1 min. Thus, it is demonstrated that a double-pancake-winding is greatly preferred to a layer-winding for NMR applications from the view point of the characteristic magnetic field delay time.

A Long Charging Delay for a No-Insulation REBCO Layer-Wound Coil and its Influence on Operation with Outer LTS coils

This paper presents a long charging delay for a no-insulation (NI) REBCO layer-wound (LW) coil and an electromagnetic-induced thermal runaway, resulting in damages on the coil. A small NI REBCO LW coil with a self-inductance of 1.62 mH showed a charging delay time constant of ∼3000 s at 4.2 K. The coil was installed in the cold bore of a low-temperature superconducting (LTS) magnet, and the LTS outer coil was charged. Although the circuit of the NI REBCO LW coil was opened, the induced current inside the coil screened the magnetic field generated by the outer coils. Eventually, the NI REBCO LW coil showed a thermal runaway during charging the outer coils. After the experiment, we found burnouts of the REBCO conductor near the electrodes and buckling of the conductor for all the layers of the winding. The present results point out the technical problem and the worst case for a NI REBCO LW coil, which must be considered.

超1 GHz NMR システムの開発

The world's first Nuclear Magnetic Resonance (NMR) system with an operating frequency above 1 GHz was developed and operated at the National Institute for Materials Science (NIMS) during 2014–2015. The Nb3Sn innermost coil of an existing 920 MHz (21.6 T) NMR magnet was replaced with a Bi-2223 layer-wound coil, upgrading the magnet to 1.03 GHz (24.2 T). At 1.02 GHz (24.0 T), shimming started to achieve the homogeneous magnetic field required for NMR measurements. However, large magnetic-field inhomogeneity appeared that was capable of being compensated using the superconducting (SC) and room temperature (RT) shim coils installed. We used a powerful and fast-acting shimming method that combines ferromagnetic shim, SC shim and RT shim to compensate the inhomogeneity. This achieved effective compensation of the magnetic-field inhomogeneity, subsequently leading to an excellent NMR resolution test result of 0.7 ppb (0.7 × 10-9). This NMR resolution will enable NMR measurement of a membrane protein sample.

Combination of high hoop stress tolerance and a small screening current-induced field for an advanced Bi-2223 conductor coil at 4.2 K in an external field

An advanced Bi-2223 conductor with Ni–Cr reinforcement is a likely candidate to achieve a compact super-high field nuclear magnetic resonance (NMR) magnet capable of operation beyond 1 GHz (23.5 T). However, the conductors must show both high hoop stress tolerance, typically >300 MPa, and a small screening current-induced magnetic field, both of which are essential for a compact magnet generating a highly accurate field. These two conditions have not yet been demonstrated in a working coil. This is the first paper to systematically investigate both characteristics for a layer-wound coil made with the advanced Bi-2223 conductor operated mainly at 4.2 K in an external field of ≤17 T. The coil tolerated a hoop stress of 370 MPa, even though the conductor had a bending strain corresponding to a diameter of 120 mm. On the other hand, the coil showed a notable screening current-induced field in a low external field, which may be explained by weak-link and direct contacts between highly packed Bi-2223 filaments in the silver matrix. The field sharply decreased with increasing external field between 0–1 T. Thus, the conductor should be useful for inner coils in compact super-high field NMR magnets.

Operation of Wax-Impregnated GdBCO Layer-Wound Coil Using Cryocoolers

RE-Ba-Cu-O (REBCO) high-temperature superconducting conductors have the advantage of high critical current density in a magnetic field above 10 T as well as tolerance to high tensile stress. REBCO high-temperature superconducting coils are expected to be applied to superconducting magnets using cryocoolers. While epoxy-resin-impregnated superconducting coils are considered to be appropriate for superconducting magnets using cryocoolers because the impregnation provides sufficient thermal conductivity, the degradation of epoxy-resin-impregnated REBCO coils due to extremely low nominal cleavage strength is a serious problem. This problem is negligible for wax (paraffin)-impregnated REBCO coils. A wax-impregnated GdBCO layer-wound coil using cryocoolers was successfully operated at a current between <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex></formula> 300 and 300 A.

9 T NbTi Cryogen Free HTS Test Stand

A 9 T conduction cooled superconducting magnet with 50 mm room temperature chamber is developed and tested. The system design allows to fill the chamber with liquid nitrogen. The system is based on a commercial two-stage 4 K Gifford-McMahon cryocooler with cooling power of 1.5 W at 4.2 K. The cool down time of the magnet from room temperature to 3.5 K is 18 hours. A pair of coated conductor based current leads is employed to reduce heat leak to the magnet. The main purpose of the magnet is critical current measurements for HTS wires. Using the magnetic system the angular dependence for short sample of coated conductor (SCS4050 from Superpower) and critical current of a small layer wound coil made of the same conductor were measured at 77 K in external magnetic field up to 4 T. The n-values for short sample are also presented. The measured critical current of the HTS coil in external magnetic field is up to 1.5 times lower than calculated one.

REBCO Layer-Wound Coil Tests Under Electromagnetic Forces in an External Magnetic Field of up to 17.2 T

A nuclear magnetic resonance (NMR) system using a high-temperature superconducting (HTS) magnet and a probe with an HTS radio frequency coil is currently under development. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. RE-Ba-Cu-O-coated (REBCO-coated) conductors offer the advantages of a higher critical current density than low-temperature superconductors in magnetic fields above 10 T as well as tolerance to high tensile stress. Both of these factors are expected to lead to a reduction in the volume of the magnet. Four REBCO layer-wound test coils were fabricated in order to investigate their properties under electromagnetic forces in an external magnetic field of up to 17. 2 T. The REBCO coils with a practical inner diameter were successfully operated under electromagnetic forces of over 200 MPa in high magnetic fields.

Reduction of Magnetization in Windings Composed of HTS Tapes

Methods to reduce the magnetization in windings composed of high temperature superconductor (HTS) tapes are presented on the basis of the abnormal transverse-field effect. The toroidal arrangement as well as the coaxial arrangement of additional coils wound with non-magnetic wires such as copper wires is effective to uniformize the distribution of sheet current in the HTS tapes constituting the windings carrying a direct transport current when the non-magnetic coils are energized with an alternating transport current for a short time. By using a layer-wound coil fabricated with a commercially available coated conductor and a pair of copper coils with the coaxial arrangement, the influences of reducing the magnetization in the windings on the central magnetic fields of the HTS coil are studied in detail experimentally.