Superconducting Racetrack Coils Research Articles

Investigations on NbTi superconducting racetrack coils under pulsed-current excitations

One of the key issues in the technology of superconductors is the protection against quenches. When designing a superconductor as a magnet, a coil or even current leads, the design should be made such that the superconductor withstands all operational conditions, especially those occurring rapidly, as fast discharges or pulsed loads. A model for a superconducting racetrack coil based on NbTi winding is investigated under pulsed transport current conditions (zero external field) utilizing finite element analysis within the Simulia Opera platform. A pulse duration of a few milliseconds and a peak current exceeding 1 kA is yielded by discharging a capacitor into an RLC circuit that includes the superconductor coil as an element. A quench multi-physics analysis has been performed comprising both thermal and electromagnetic solutions. The transition to normal state and quench occurrence has agreed with the expected critical curve together with the load-line estimated for the existing coil geometry.

Electromagnetic-mechanical coupling analysis of high-temperature superconducting racetrack coil

The second-generation high-temperature superconducting (HTS) coated conductor has been recognized as one of the most promising materialsfor high field magnets due to its superior electromagnetic and mechanical performances. HTS racetrack coils wound with the coated conductors are an extensively used configuration in engineering applications, such as HTS machines and high-speed maglevs. In this paper, in order to analyze the electromagnetic and mechanical behaviors of HTS racetrack coils, a 3D coupled electromagnetic-mechanical model is used to consider the effect of coil deformation and the strain dependence of critical current. The effectiveness of the coupled model is validated by comparing the numerical results with experimental data in the literature. A numerical simulation of a 3D HTS racetrack coil subjected to an external electromagnetic field is carried out using coupled and uncoupled models. The results indicate that the structure deformation can reduce the penetration depth of the screening current, and the hoop stress and strain are mainly concentrated on the circular part of the racetrack coil. Afterwards, the influences of various parameters on the electromagnetic and mechanical responses of the HTS racetrack coil are also investigated.

Thermal Analysis of Leakage Currents of No-Insulation HTS Racetrack Coils Using a Three-Dimensional Turn-Distributed Equivalent Circuit FEA Model

In promoting novel products with superconducting magnets to transportation and science industries, the most important factor is reliability. Recently, it was reported that a no-insulation (NI) superconducting magnet could be damaged with quenches caused by a thermal runaway, significant temperature deviation, extremely high characteristic resistance, etc. To resolve this issue, numerical analysis of high-temperature superconducting (HTS) racetrack coils with NI winding technology should be conducted. The authors proposed a homogenization method to analyze operating characteristics of the NI HTS coils by simplifying the finite element analysis (FEA) model and decreasing the solution time. In order to estimate their thermal stabilities, the proposed FEA simulation is one of the efficient methods for solving the electrical and thermal factors simultaneously. In this paper, the thermal analysis of the leakage currents of NI HTS racetrack coils is presented using a three-dimensional (3D) turn-distributed equivalent circuit FEA model. The charging and discharging delay phenomena were modeled by considering the contact resistances by turns. With the equivalent circuit model of the NI HTS magnets, the 3D racetrack coils were designed by considering a single turn and stacked turns, including metal insulation layers. Their material properties according to temperature variation were applied in the transient analysis. The critical currents by turns were estimated by considering the magnetic field, its angle, and temperature with a given current simultaneously. The simulation results were compared with a fabricated NI HTS magnet and are presented in this paper.

Charging Characteristics of 2G HTS Coils With Insulation, Metal-Insulation, Non-Insulation, and Smart-Insulation Using Circuit Simulations

This study aimed to compare and analyze the output characteristics of second-generation high-temperature superconducting racetrack coils with insulation, metal-insulation (MI), non-insulation (NI), and smart-insulation (SI). First, the output characteristics of each coil were experimentally validated, and circuit equations were examined to analyze these characteristics. The output characteristics were found to be affected by the turn-to-turn resistance. Circuit analysis was used to identify the nature of dependence of output characteristics on turn-to-turn resistance in addition to internal behavior and other mechanisms. In particular, an equation relating the output characteristics of the SI coil with temperature change was derived, and each coil model was simulated using the circuit equation. Then, the simulation results were analyzed, the advantages and disadvantages of each insulation method were discussed, and the validity of the simulation was confirmed. The circuit simulation proposed in this study is expected to facilitate advanced predictions in smart insulation material candidate groups.

Ramping Loss Analysis of No-Insulation HTS Coil Under External Field Using an Improved Equivalent Circuit Model

This article is to study the ramping loss of no-insulation (NI) high temperature superconducting (HTS) racetrack coil under different external magnetic fields during the charging process. An improved equivalent circuit electromagnetic coupling model for the HTS racetrack coil is introduced with good computational speed and accuracy. The simulation results show that the turn-to-turn loss and the change of the lowest critical current position are obviously influenced by the external field that have not been mentioned from our knowledge base. During the rising time of the power supply current, there is little difference in current distribution under different external fields. But after that, steady state of charging process is deeply influenced by the external field. A larger external field will lead to a lower charging current and the external bypass phenomenon. Another result is that the relationship between direction of external field and orientation of coils will influence the lowest critical current amount and position. The relationship between turn-to-turn resistivity of co-winding materials and ramping loss is also discussed in the paper. The distributed unevenness and value of the turn-to-turn loss change with the increase of turn-to-turn resistivity.

First performance test of the iron-based superconducting racetrack coils at 10 T

Iron-based superconducting (IBS) racetrack coils were firstly fabricated by using 100-m 7-filamentary Ba1-xKxFe2As2 (Ba122) tapes at the Institute of High Energy Physics, Chinese Academy of Sciences (IHEP, CAS). The IBS tape was wound in parallel with stainless steel tape to withstand the high tensile hoop stress under high magnetic field. After the heat treatment, the coils were impregnated with epoxy resin. Then the IBS coils were tested in a low-temperature superconducting Common-Coil dipole magnet which provided a maximum background field of 10 T at 4.2 K. Most importantly, the best IBS racetrack coil quenched at 4.2 K and 10 T with an operating current of 65 A, which is still as high as 86.7% of critical current of the short sample at 10 T. The details of the fabrication process and performance test results were presented in this paper. The performance test demonstrated the IBS conductor is a promising candidate for the application of high field magnets especially for future high-energy accelerators.

Numerical Studies on Mechanical Behavior of Dry-Wound HTS Racetrack Coil

This paper presents numerical studies on the mechanical behavior of dry-wound high-temperature superconducting (HTS) racetrack coil. After calculating the Lorentz force induced from the self-field of the racetrack coil based on the 3D finite element method (FEM), the 2D mechanical stress analysis is performed based on the plane-stress assumptions to mitigate computational loads, and the mechanical behavior including displacement and stress are investigated. Based on analysis model, two following case studies are conducted: 1) analysis of mechanical behavior on various-shaped of racetrack coils having a different ratio of straight section length and semicircular section radius, and 2) analysis of stress reduction effect according to the degree of curvature when slight curvature is included in the straight section. In each case study, the factors affecting the mechanical behavior of racetrack coils are numerically analyzed and their trends are presented. The results obtained in this paper would be useful for the design of dry-wound racetrack coil, including no-insulation coil, applied in a large-scale and high-field application.

Fast iterative algorithm for calculating the critical current of second generation high temperature superconducting racetrack coils

Fast iterative algorithm for calculating the critical current of second generation high temperature superconducting racetrack coils

Local electromagnetic properties and hysteresis losses in uniformly and non-uniformly wound superconducting racetrack coils

A noteworthy physical dependence of the hysteresis losses with the axial winding misalignment of superconducting racetrack coils made with commercial second generation high temperature superconducting (2G-HTS) tapes is reported. A comprehensive study on the influence of the turn-to-turn misalignment factor on the local electromagnetic properties of individual turns is presented by considering six different coil arrangements and ten amplitudes for the applied alternating transport current, Ia, together with an experimentally determined function for the magneto-angular anisotropy properties of the critical current density, Jc(B,θ), across the superconducting tape. It has been found that for moderate to low applied currents Ia≤0.6Ic0, with Ic0 the self-field critical current of individual tapes, the resulting hysteretic losses under extreme winding deformations can lead to an increase in the energy losses of up to 25% the losses generated by a perfectly wound coil. High-level meshing considerations have been applied in order to get a realistic account of the local and global electromagnetic properties of racetrack coils, including a mapping of the flux front dynamics with well defined zones for the occurrence of magnetization currents, transport currents, and flux-free cores, which simultaneously has enabled an adequate resolution for determining the experimental conditions when turn-to-turn misalignments of the order of 20–100 μm in a 20 turns 4mm wide racetrack coil can lead not only to the increment of the AC losses but also to its reduction. In this sense, we have shown that for transport current amplitudes Ia>0.7Ic0, a slight reduction in the hysteresis losses can be achieved as a consequence of the winding displacement, which is at the same time connected with the size reduction of the flux-free core at the coil central turns. Our findings can be used as a practical benchmark to determine the relative losses for any 2G-HTS racetrack coil application, unveiling the physical fingerprints that possible coil winding misalignments could infer.

Effective 3-D FEM for large-scale high temperature superconducting racetrack coil

In various types of large-scale electrical applications, the number of coil turns in such machines is usually large. Electromagnetic simulation of large-scale superconducting coils (tens to hundreds of turns) is indispensable in the design process of superconducting electrical equipment. However, due to the large scale of the coil and the large aspect ratio of super-conducting material layer in HTS coated conductor, it is usually difficult or even unable to perform 3-D transient electromagnetic simulation. This paper introduces an effective 3-D electromagnetic simulation method for large-scale HTS coated conductor coil based on T-A formulation. The simulation and experimental results show that the 3-D model based on the T-A formulation using homogeneous strategy is more accurate than the traditional 2-D models. The memory usage is not sensitive to the number of turns and this model will be even more superior as the number of turns becomes larger.

Evaluation of a solid nitrogen impregnated MgB2 racetrack coil

To develop powerful wind turbine generators using superconducting technology, high-performance superconducting racetrack coils are essential. Herein, we report an evaluation of a multifilamentary magnesium diboride (MgB2) conductor-based racetrack coil cooled and impregnated simultaneously by solid nitrogen (SN2). The coil was wound on a copper former with 13 mm winding width, an inner diameter of 124 mm at the curvature, and 130 mm length of the straight section. An in situ processed S-glass-insulated 36-filament MgB2 wire was wound on the former in two layers with 19.5 turns, and heat treated via the wind and react method without any epoxy resin. The coil was evaluated for critical temperature and transport critical current in the SN2 environment at different temperatures up to 31.3 K in self-field. The coil was able to carry 200 A transport current at 28.8 K in self-field. During coil charging and operation, SN2 effectively acted as an impregnation material. The test results demonstrate the viability to use MgB2 racetrack coil potentially with SN2 impregnation in advanced rotating machine applications.

Design, Fabrication, and Testing of MgB2/Fe Racetrack Coils

We fabricated four superconducting racetrack coils wound by bare in situ MgB 2 /Fe mono and multifilamentary wires produced in our laboratory by using the wind-and-react method. Transport measurements in self-field were performed in a liquid helium dewar. The magnetic field flux density B = 25 mT for I = 92 A was measured to verify how the current flowed inside the coil for one of the coils by means of a cryogenic axial Hall sensor placed into the central bore region of the coil. The coil with 36 turns wound by mono MgB 2 /Fe wire of l = 24 m has a highest current-carrying capacity of more than 150 A at T = 4.2 K and self-field among the coils. The coils fabricated by multifilamentary wires have relatively low engineering critical current values of I ce = 67 A for 18 + 1 and I ce = 57 A for 4 + 5 superconducting/copper filament wires at T = 4.2 K in self-field. Our results form essential contribution toward determination of the possible use of low-cost MgB 2 /Fe wires in low-field superconductor coil applications.

Application of DIC to Extract Full Field Thermo-Mechanical Data From an HTS Coil

A digital imaging correlation (DIC) test setup was developed to accurately record the contraction of a Bi-2223 superconducting racetrack coil conduction cooled to 154 K. The coil was gripped at both ends to simultaneously measure the load and displacement induced by its own contraction. Postprocessing of the DIC data derived the displacement and strain fields from two areas of interest. The global deformation of the coil structure was influenced by how it was restrained, but the stress-strain curve in the y-direction of the HTS coil was established, and its stiffness when under thermally induced strain was found to closely match that of the epoxy resin. The coefficient of thermal expansion determined from the strain-temperature curve was in the same order of magnitude as dilatometer measurements performed upon a sample removed from a similar coil and the difference attributed to small variability in the temperatures across the coil and the mechanical restraint placed on the coil.

Demonstration of a Practical Nb$_3$ Sn Coil for an Actively Shielded Generator

This paper describes a detailed design of a 6-T Nb 3 Sn superconducting racetrack coil designed for conduction cooling. We then describe a bench test pursued as a proof of concept for one winding of an actively shielded air-core electric machine with superconducting field windings. Electromagnetic design selection is drawn from previous optimization work. The coil former design is then discussed. Numerical simulations of thermal and structural features are pursued to determine temperature distribution and strain within the winding. A coil instrumentation and experimental setup of a quasi-conduction cooled system is described. Finally, test results are presented; a maximum critical current of 480 A was reached at a peak temperature of 7.9 K, surpassing the operational current goal of 435 A. Future work and planned improvements to the test setup are discussed.

Generation and Confinement of Uniform Magnetic Field Using Second-Generation Superconducting Racetrack Coils

Advancement in a cryogen-free superconducting magnet is addressed in this paper. In the proposed laboratorysized magnet configuration, the racetrack coils are arranged as central coils to provide the majority of required magnetic field and side coils that are responsible not only to lessen the stray fields but also to improve the homogeneity of the magnetic field in the region of uniformity. The main advantage of implementing racetrack coils over solenoid coils is the ability to generate a highly uniform field for either long specimens or using several samples simultaneously. Moreover, an optimization technique with the objective of minimizing nonuniformity of magnetic field with respect to magnetic and mechanical constraints is carried out. Because of considerable numbers of designing variables, coils’ locations, and their associated currents, the problem is initially solved in 2-D Cartesian coordinates. Consequently, in order to ascertain the accuracy of the solution achieved from the 2-D model, a 3-D model based on the optimized variables is studied and the results are discussed.

Self-Inductance of a Racetrack Coil

Superconducting racetrack coils are used for magnetic levitation, magnetic resonance imaging, etc. When using racetrack coils, it is important to determine the coil characteristics (the resistance, self-inductance, mutual inductance, and distributed capacity of the coil). It is difficult to analyze the racetrack coil that has straight and semicircle parts. Therefore, the analytical solution of this coil has not been obtained yet. In this paper, we derive the analytical solution of the magnetic field of a single-layer racetrack coil based on the magnetic field of the one-turn racetrack coil. In addition, the analytical solution of the self-inductance is derived on the basis of this theory. The validity of this theory is confirmed by comparing measured self-inductance values with the calculated values.

A Cryo-Free 10 T High-Field Magnet System for a Novel Superconducting Application—Experimental Results

The experimental results of a cryo-free high-field magnet system for a novel magnetizer are presented. The magnet for the application is a superconducting racetrack coil of 0.7 m length using low AC-loss wire from Luvata. The coil selection, reaction, epoxy selection, winding trials and vacuum impregnation are reported in a previous paper. Here the experimental testing of the prototype is presented. The magnet was cooled down with a 4 K GM cryocooler to its operating temperature and ramped. The behavior during cooldown, ramping and training are presented, demonstrating the viable operation of this low-cost engineering prototype.

대형회전기기응용을 위한 GdBCO 레이스트랙형 팬케이크 코일의 ��치 발생과 전파특성에 관한 연구

The stability issue of high temperature superconducting (HTS) racetrack coils is one of the most important factors for the development of large-scale rotating machines, such as ship propulsion motors and power generators. However, The stability and normal zone propagation characteristics of HTS racetrack pancake (RP) coils are not sufficient yet. In this study, quench tests for a GdBCO racetrack pancake coil were carried out under the condition of <TEX>$LN_2$</TEX> at 77 K. Minimum quench energy and two-dimensional normal zone propagation velocities of the coil are also discussed. Normal zone propagation velocity in the coil's curved section is faster than in its straight section due to stress effects. The test results show that the protection of the straight section is of greater importance than that of the curved section when GdBCO racetrack pancake coils are applied to large-scale rotating machines.

AC Losses of Superconducting Racetrack Coil in Various Magnetic Conditions

High-temperature superconductors have created the opportunity for a step change in the technology of power applications. Racetrack superconducting coils made from YBCO coated conductors have been used in several engineering applications including SMES, rotor or stator windings of electric machines. AC loss is one of the most important factors that determine the design and performance of superconducting devices. In this paper, a numerical model is developed to calculate the AC losses in superconducting racetrack coils in different magnetic conditions. This paper first discusses the AC losses of the coils in self-field or external field only. It then goes to investigate the AC losses of the coils being exposed to AC ripple field and a DC background field. Finally, the AC losses of the coils carrying DC current and being exposed to AC field are calculated. These two scenarios correspond to using superconducting coils as the rotor field winding of an electric machine.

Quench and recovery characteristics of a racetrack double pancake coil wound withYBCO-coated conductor

The reliability of high temperature superconducting (HTS) racetrack coils is one of themost important factors for the development of large-scale rotating machines. However, it isnecessary to investigate the stability and normal zone propagation characteristics ofracetrack coils for large-scale applications such as ship propulsion motors and powergenerators. In this study, the quench/recovery characteristics of a racetrack-type, doublepancake (DP) coil, which could be applied to HTS rotating machines, were investigatedusing the voltage and temperature profiles in a cryogenic conduction cooling system. Theminimum quench heating flux and quench propagation velocity of the racetrack DP coil arealso discussed.