Superconducting Coil Research Articles

Quench initiation and normal zone propagation characteristics of smart insulated high-temperature superconducting coil

A smart insulation (SI) winding technique utilizing vanadium trioxide (V2O3) as a turn-to-turn insulation material is an effective means of achieving a suitable time constant and thermal stability in high-temperature superconducting (HTS) coils. However, the low electrical conductivity of the V2O3 makes the SI coil vulnerable to damage during quenching. To address this problem, this study explored a mixture of V2O3 and molybdenum (Mo) particles to improve the thermal stability of the SI coil. Two GdBCO coils, insulated with V2O3 and V2O3:Mo, respectively, were fabricated, and quench tests were performed at various operating currents in liquid nitrogen at 77 K. The findings revealed that the V2O3:Mo-insulated coil demonstrated improvements of 82%, 25%, and 160% in the minimum quench energy, and the normal-zone propagation velocities in the longitudinal and transverse directions, respectively, at 60% of the current-carrying capacity of the coil, which are essential performance parameters in the operation of a superconducting magnet.

Simulation for the Superconductor Linear Eddy Current Brake of the High-Speed Train

A kind of superconductive coil was designed as the linear eddy current brake for the Electric Multiple Units (EMU). 2D and 3D simulations were carried out on the designed coils. The 2D simulation results showed that the generated braking force could reach 10kN when the train speed was between 28m/s and 110m/s. The simulation results indicated that the reduced gap between the coil and the railway, the increased pole distance, and the increased coil quantity would bring more braking force. The relationship curve between speed and braking force was also investigated during the 2D simulation. The speed of 28m/s and 110m/s was applied with the 3D simulation based on the 2D results. Both the magnetic inducting intensity and the current 180A could meet the requirements of the chosen superconducting wire.

Design and Numerical Study of Magnetic Energy Storage in Toroidal Superconducting Magnets Made of YBCO and BSCCO

The superconducting magnet energy storage (SMES) has become an increasingly popular device with the development of renewable energy sources. The power fluctuations they produce in energy systems must be compensated with the help of storage devices. A toroidal SMES magnet with large capacity is a tendency for storage energy because it has great energy density and low stray field. A key component in the creation of these superconducting magnets is the material from which they are made. The present work describes a comparative numerical analysis with finite element method, of energy storage in a toroidal modular superconducting coil using two types of superconducting material with different properties bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). Regarding the design of the modular torus, it was obtained that for a 1.25 times increase of the critical current for the BSCCO superconducting material compared with YBCO, the dimensions of the BSCCO torus were reduced by 7% considering the same stored energy. Also, following a numerical parametric analysis, it resulted that, in order to maximize the amount of energy stored, the thickness of the torus modules must be as small as possible, without exceeding the critical current. Another numerical analysis showed that the energy stored is maximum when the major radius of the torus is minimum, i.e., for a torus as compact as possible.

Development of the first Tesla class iron-based superconducting coil for high field application

This study has, for the first time, combined iron-based superconductor (IBS) with high field insert coil technology to generate a higher magnetic field. Nine large-sized iron-based superconducting double pancake coils (DPCs) were fabricated, and their performance was evaluated at 4.2 K. Subsequently, seven DPCs with superior performance were assembled to form an IBS insert coil and tested at the 20 T background field. The test results indicated that the insert coil successfully generated above 1 T magnetic field under the 0 T, 10 T, and even 20 T background field, respectively. Notably, the poorest IBS DPC still had a critical current of about 84 A at the background field of 20 T, which outperformed all previously reported performance tests of IBS coils. This paper provides a detail introduction to the design and fabrication process of the IBS insert coil, presents the test results, and discusses them.

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.

Characterization of flux pump-charging of high-temperature superconducting coils using coupled numerical models

Flux pumps provide a promising solution for contactless charging of high-temperature superconducting (HTS) coils, eliminating the need for bulk current leads and reducing the heat burden for the cryogenic system. Characterizing the nonlinear effects of an HTS coil charged by a flux pump and understanding the dynamics of the charging process is essential for promoting the practical application of flux pumps. Numerical models provide a fast and cost-effective way of achieving this. In this study, we propose a methodology for coupling HTS coil and flux pump models using an electrical circuit, resulting in reduced computation costs. We validate the effectiveness of our approach against the experimental results of an HTS coil charged by a dynamo-type flux pump. Specifically, we obtain the voltage produced by the HTS dynamo using a 3D model based on the minimum electromagnetic entropy production method and apply this voltage to the load HTS coil using a formulation finite-element method model coupled via an electrical circuit. The simulated charging current shows good agreement with experimental observations, validating our modeling strategy. The results demonstrate that the flux flow state in the HTS coil is the primary factor limiting the charging performance of the HTS dynamo as the charging current approaches the coil’s critical current. Furthermore, based on the simulation, we demonstrate that, when using flux pumps, it is advisable to leave a margin between the operating current and the critical current of the coil. Overall, our approach has the potential to be applied to HTS coils charged by any device.

Design of a Conduction-Cooled Open Gradient Superconducting Magnetic Separation Magnet

Magnetic separation of wastewater is required to solve the problem of water pollution caused by a large amount of wastewater discharge in urbanization and industrialization. Superconducting magnets reduce energy consumption, increase the magnetic field strength, and improve the magnetic separation effect compared to traditional magnetic separation using permanent magnets and electromagnets. In this paper, an open gradient superconducting magnetic separation magnet conduction-cooled by a two-stage GM cryocooler was designed and developed. The open gradient superconducting magnetic separation magnet comprised four saddle-shaped coils and an optimized iron yoke. The radial magnetic flux density was 1 T at a distance of 20 mm from the surface of the coil at an operating current of 120 A, resulting in a magnetic force density of 3.5*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> N/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , with a peak field in the coil of 4.82 T. Additionally, this paper proposed a superconducting coil structure and winding method with an end positioning groove. The design and manufacture of the end former, as well as the magnet structure, was completed. Finally, this paper designed a compact cryostat with a two-stage GM cryocooler.

Dynamic resistance loss of the high temperature superconducting coil for superconducting magnetic energy storage

The Superconducting Magnetic Energy Storage (SMES) has excellent performance in energy storage capacity, response speed and service time. Although it's typically unavoidable, SMES systems often have to carry DC transport current while being subjected to the external AC magnetic fields.Under this circumstance, the dynamic resistance loss occurs in conjunction with the magnetization loss. Accordingly, the dissipative heat originating from the dynamic resistance loss will cause thermal overhead and even threaten the stability of the SMES. To ensure the safe operation of the SMES, this study examines dynamic resistance losses of high temperature superconducting (HTS) coils. In addition, this study also compares it with other losses, including magnetization loss, under various conditions such as different DC transport currents, amplitudes, frequencies, and angles of external AC magnetic fields. Furthermore, the analysis of dynamic resistance loss has also taken into account thermal coupling. This research offers valuable insights into the electromagnetic-thermal characteristics of the HTS coil and can be used as a reference for designing SMES systems.

Basic Study of a Wireless Power Transmission System Using Superconducting Coil as a Ground-Side Coil for Electric Vehicles

We proposed a large-capacity wireless power transmission (WPT) system for the electric vehicle in which a high-temperature superconducting (HTS) coil is installed only on the ground side. On the other hand, the design guideline of the 100 kW class WPT system using the HTS and the copper coils for EVs has not been sufficiently discussed. Therefore, we analyzed the electric power transmission characteristics of the 100 kW class proposed WPT system using the HTS coil and the copper coil by an electromagnetic field analysis using the finite element method (FEM) and an equivalent circuit analysis. Also, we investigated the effect of a cooling temperature of the HTS coil on the receiving power and the transmission efficiency of the proposed WPT system for EVs. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">As a result, we found that the proposed WPT systems using the high-inductance primary and secondary coils are suitable for a low-frequency operation around 1 kHz. On the other hand, the proposed WPT systems with high-frequency operation above 10 kHz are suitable for reducing the weight of the secondary coil. In addition, although the system efficiency considering the cryocooler power loss is reduced by decreasing an operating temperature of the primary HTS coil, it is possible to realize a large-capacity WPT system with the receiving power of more than 100 kW.

Overcurrent Test of High-Temperature Superconducting Coils With Liquid Hydrogen Immersion Cooling

The operating temperature of a high-temperature superconducting (HTS) coil is desired to be 15-40 K from the viewpoint of specific heat of the material and the operating current. Development of relatively small-capacity coils with gas helium cooling or refrigerator cooling in this temperature range is underway. Here, refrigerant cooling is desirable for large-capacity coils and liquid hydrogen, which has 20.7 K atmospheric pressure saturation temperature, is a candidate refrigerant. However, due to the difficulty of handling hydrogen, there have been few studies on the HTS coil cooled by liquid hydrogen. And in the HTS coil, the heat generated in the normal conducting region causes a chain of temperature rise and decrease of the critical current, which leads to an irreversible increase in coil temperature beyond the balance with the cooling conditions, and so-called thermal runaway is a problem. In this study, we report on the observation of thermal runaway phenomena in Bi2223 coils under liquid hydrogen immersion cooling. The coils were energized twice at saturated condition of 950 kPa pressure and 30 K temperature. We observed thermal runaway in both energizations and the coil tap voltage before thermal runaway was several tens of mV. From this test, it could be said that the liquid hydrogen cooling has a potential to protect HTS coils.

Compensation of NI Behaviors in Synchronous Motors With NI HTS Field Winding Using Harmonic Current Injection

Focusing on the improved operation reliability of no-insulation (NI) high-temperature superconductor (HTS) coils, various attempts are being conducted to replace conventional superconductor coils with NI HTS coils for industrial and energy conversion purposed applications. When applying NI techniques, unique behaviors of NI HTS coils due to the removal of turn-to-turn insulation should be considered with given operating conditions, because it could affect the overall characteristics of the machines. In particular, in the case of synchronous motors to which NI HTS field winding is adopted, it has been reported that periodic turn-to-turn leak currents were identified even in steady-state operation due to harmonic components, which may result in torque and loss characteristics changes. Here we report a method to offset these additional NI behaviors that may appear when NI HTS field winding is applied to synchronous motors. A brief mechanism of the proposed method was presented, and the proposed method was applied to an example model through finite element analysis. As a result, it was shown that turn-to-turn contact loss could be reduced to 2.18% without adverse effect on torque characteristics.

A Double Rotor Flux Switching Machine With HTS Field Coils for All Electric Aircraft Applications

The increasing demand for high-power density motors in electric transport industries opens a new research opportunity to develop motor topologies with less weight and high efficiency. In particular, all-electric aircraft applications require very high power density motors. This paper shows the design of a new 1 MW 20-pole/15-slot double rotor flux switching machine with high-temperature superconducting field coils and thermal management system. The proposed motor features an air-core stator. Aluminum Litz wire is used for the armature conductors, and the YBCO- high-temperature superconducting material is used for the field coils. The armature winding and field coils operate at 95K and 65K, respectively. The active part power density including rotors, armature windings and field coils obtained with the proposed design is 29.3kW/kg. The specific power density of proposed motor considering mechanical, support structure and TMS is about 18.5 kW/kg. The efficiency can be higher than 98.7% which satisfies the requirement of electric aviation.

Evaluation of Frequency Loss Induced Quench Protection Prototype at 77 K Using HTS Coils

A prototype design of the Frequency Loss Induced Quench (FLIQ) protection system for high temperature superconducting magnets was validated on a small Second-Generation High-Temperature Superconducting (2G HTS) coil. Calorimetric measurements, based on liquid nitrogen boil-off rate, were performed to quantify the heat energy introduced by FLIQ system in the coil. The technique was previously used for steady state AC loss measurements and adopted for the transient losses generated in FLIQ operation. A series of tests of the FLIQ system on 2G HTS pancake coils were performed to study the dynamics of the quench and the energies involved in the quenching process. The results suggest the suitability of the FLIQ technique for quench protection of HTS coils. A measurement protocol of extended repetitive high-frequency current pulses established in the study provided a way to estimate the energy deposited by the AC losses generated in the coil by the FLIQ system. The measurement method described is a practical way to validate the design parameters of the FLIQ system and to evaluate the sensitives of the various design parameters of the FLIQ. The paper reports an experimental technique used to study the sensitivities of various FLIQ design parameters by establishing and measuring quasi-equilibrium heat load and boil-off rate as a result of extended repetitive high-frequency current pulses.

Investigation of HTS Coil Structure Robustness Against Misalignment Between Coils in a Wireless Power Transmission System for Railway Vehicles

A wireless power transmission (WPT) system using high-temperature superconducting (HTS) coils has been investigated. However, it is difficult to precisely match the positions of the primary and secondary coils. Thus, the HTS coil structure with high misalignment tolerance is required. In this paper, we investigated the non-circular HTS coil structure with a high quality factor for the WPT system, such as a rectangular coil and a racetrack coil. Also, we evaluated the power transmission characteristics of the WPT system using the non-circular HTS coils. As a result, the quality factor of the non-circular HTS coils was maximum when the radial gap between turns was almost the same as the HTS wire width, and increased with increasing coil length in a rail longitudinal direction. Also, the receiving power and the system efficiency using the non-circular HTS coils were more robust against misalignment between coils than those using the circular HTS coils. On the other hand, the receiving power decreased with increasing the coil length of the non-circular HTS coils in the rail longitudinal direction due to thermal radiation from normal temperature. From the above, the receiving power of the WPT system with small lateral misalignments can be improved by using the rectangular coils with a short coil length in the rail longitudinal direction in parallel. Moreover, the system efficiency of the WPT system with large lateral misalignments can be improved by using the rectangular coils or the racetrack coils with a long coil length in the rail longitudinal direction.

A Study on the Critical Current Measurement of No-Insulation HTS Coils and the Effect of Screening Current-Induced Voltage

This paper presents a study on the critical current measurement of the no-insulation (NI) high-temperature superconducting (HTS) coils and the effect of the screening current-induced voltage. During the analysis of the critical current measurement data, we observed the unexpected “resistive-like” slope in the I-V curve of the NI rare-earth barium copper oxide (REBCO) double-pancake (DP) coils. Because the coil is confirmed superconducting and has no internal joints, therefore we estimate the unexpected resistive-like slope is the result of the screening current-induced voltage. The circuit analysis using the one-dimensional simple equivalent circuit could not explain the screening current-induced voltage adequately. However, using the three-dimensional advanced PEEC model, we were able to successfully reproduce the resistive-like slope.

AC Loss Reduction in HTS Coil Windings Coupled With an Iron Core

Rapid-cycling synchrotron (RCS) magnets based on a superferric design consist of high temperature superconducting (HTS) coil windings coupled with iron cores. However, the presence of iron cores significantly increases AC loss in HTS coil windings, making AC loss reduction a critical issue for applying HTS technology in RCSs. Enlarging the distance between iron cores and coil windings may reduce AC loss. In addition, magnetic materials with different saturation magnetic fields also may influence AC loss in HTS coil windings coupled with iron cores. To investigate the distance dependence, AC losses of 1DPC (double pancake coil)-, 2DPC-, 4DPC-, and 8DPC assemblies with various distances are simulated using the 3D <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> homogenization method with/without an iron core. Two magnetic materials with different saturation magnetic fields are chosen as the iron core to evaluate AC loss in HTS coil assemblies. The results show that the AC loss values in HTS coil assemblies coupled with the iron core decrease significantly with growing distance. For a given distance and current, when the iron core has a lower saturation field, the AC loss values of the 4DPC assembly are smaller than those with a higher saturation field.

Experimental Study on Quench Protection Method for HTS Coil That Uses Cu Tape Co-Wound With HTS Tape

The authors previously proposed a quench protection method for a high-temperature superconducting (HTS) coil that uses Cu tape co-wound with HTS tape (Cu-CW method). In this method, when a quench occurs in the HTS coil, part of the current in the HTS coil is quickly transferred to the co-wound Cu tape coil by shorting the Cu coil with a resistor due to the tight magnetic coupling of both coils, and the hot-spot temperature of the HTS coil is reduced due to the quick reduction of the HTS coil current. The previous work showed by a numerical simulation that the method is effective to improve the quench protection performance compared with a commonly used detect and dump method (D&D method). In this work, the effectiveness of the method was studied by an experiment using small-scale test coils wound with yttrium barium copper oxide (YBCO) coated tapes to experimentally simulate quench behaviours of HTS coils of significant scale.

Experimental and Equivalent Circuit Analysis Approach to Estimate Contact Resistances on Different Sections in a D-Shaped NI HTS Coil

With well-known self-protecting behaviors of no-insulation (NI) high temperature superconductor (HTS) coils, their performance was verified by numerous high field magnets. As one of the initial investigations of the NI HTS technique being applied to toroidal field (TF) coils, an equivalent circuit analysis is carried out on a D-shaped NI HTS coil to predict its electromagnetic behaviors regarding the contact resistance, one of the most important parameters for NI HTS coils. A charging/discharging test is performed at 77 K to obtain the experimental data for contact resistance estimation. Using the measured voltage and the pressure dependency of surface contact resistivity, a combination of contact resistance is obtained through the modified lumped circuit and the partial element equivalent circuit simulation by which the shape-dependent contact resistance can be estimated. The results are expected to provide a source for careful management in the design stage to consider the uneven contact resistance and possible uneven electromagnetic behavior among different sections of the TF coil.

Experiment and Analysis on Temperature-Dependent Electric Contact Resistivity of an NI HTS Coil

This paper reports experimental and numerical analysis results of a no-insulation (NI) high-temperature superconductor (HTS) coil in terms of temperature-dependent electric contact resistivity. A test coil was designed, constructed, and operated. The coil is divided into four sections according to the radial locations of inserted voltage taps to measure local voltages. A preliminary experiment was performed to evaluate the electromagnetic properties of the coil in liquid nitrogen. Then, the temperature-dependent contact resistance of the coil was evaluated from 10K to 80K at every 10K increment in a cooling conduction facility. To investigate the impact of the contact resistivity on the charge/discharge dynamics of the coil, we selected two arbitrary current ramp rates values, namely fast (0.5) and slow (0.01) charge/discharge. In this study, the experimental results were analyzed with equivalent circuit and finite element models. The analysis results suggest the following conclusions. First, the contact resistivity of the test coil calculated from the experiment at the coil terminals level increases by a factor 2 with respect to the temperature, ranging between 1.16 to 2.59. Second, although a constant winding tension was applied during the coil fabrication, the contact resistivity was found to increase within different sections (inner to outer radius). Third, the impact of the ramp rate on temperature dependency is negligible.

Study on the State Prediction of a Pool-Cooled Large Superconducting Coil Using Machine Learning

For the LHD subcooling system composed of pool-cooled large superconducting coils wound with NbTi superconductors, a machine learning technique was introduced to increase the reliability of the system. The machine learning model for the state prediction of the system was developed using the technique, together with the data accumulated in the LHD plasma experimental campaign. Regarding the temperature changes in the system due to coil excitation and discharging, it is possible to make predictions using the model. Especially for the usual coil current waveform in a helical coil operation, which is a trapezoidal waveform, the model achieved high prediction accuracy.