High-temperature Superconducting Coil Research Articles

AC loss measurement and simulation in a REBCO coil assembly utilising low-loss magnetic flux diverters

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

3-D Analytical Model of Racetrack HTS Coil Subject to Travelling Magnetic Fields

Magnetic levitation (maglev) train has the unique advantages of no-mechanical contact, high operating speed, and so on. All those advantages indicate the potential of maglev in the future ultra-high-speed ground transit, in which the high-temperature superconducting (HTS) linear synchronous motor (LSM) is essential because of its sample structure and excellent performance. HTS LSM is a typical application of racetrack HTS coil subject to travelling magnetic field. Motivated by an efficient method to promote the application of maglev, a 3-D analytical model combining the dynamic circuit theory and virtual displacement method was proposed for estimating the performances of racetrack HTS coil in LSM. To verify the proposed 3-D analytical model, the calculated open-circuit magnetic field and back electromotive force were compared with the numerical results of a 3-D finite element model in which the actual geometry of HTS LSM and the nonlinear conductivity of HTS tapes were taken into account. Based on the validated 3-D analytical model, the characteristics of HTS LSM was analyzed. The obtained results indicate that the validated 3-D analytical model could be a very efficient tool for the characteristic analysis and optimization of HTS LSM.

Levitation Force Improvement by Effective Magnetic Interaction in Two Magnetic Levitation Systems With HTS Coil and Bulks

In this article, we studied a magnetic levitation system which was composed of a high-temperature superconducting (HTS) coil, HTS bulks, and a ferromagnetic material (rail). We set two magnetic levitation systems in one ferromagnetic rail. We analyzed the levitation force depending on changing the distance between HTS coils of each direction of current flowing. As a result of comparing two poles that has current flowing through its coils in one direction and two poles that has current flowing through its coils in different directions, the model which had the coils in one direction was better. A reason for this result is discussed comparing with the magnetic flux density and the stress tensor in a ferromagnetic rail. In addition, the distance between coils was also important for the magnetic interaction. We show the tendency of levitation force obtained by changing the distance between coils in each direction of current flowing and explain the reason. In this article, it is found that a greater force can be obtained by using magnetic field interaction effectively. We optimized arrangement and magnetic pole combination and we succeeded to improve the levitation property.

Versatile cryogen-free cryostat for the electromagnetic characterization of superconducting radiofrequency coils

The use of high temperature superconducting (HTS) radio frequency (RF) coils in Magnetic Resonance Imaging (MRI) greatly improves the signal-to-noise ratio (SNR) in many biomedical applications and particularly in micro-MRI. However, a detailed understanding of the electrical behavior of HTS coils is important in order to optimize their performance through MR experiments. This paper presents a simple and versatile cryogen-free cryostat designed to characterize the RF properties of HTS coils prior to their use in MRI. The cryostat can be used at temperatures from 50 K to 300 K, with a control precision of approximately 3 mK at 70 K, and can measure the RF electrical power transmitted to an HTS coil over a range from 1 μW to 10 W. The quality factor and resonance frequency of the tested HTS coil are determined as a function of the temperature and the power it dissipates. This cryostat also permits the dynamic adjustment of the coil resonance frequency via temperature control. Finally, this study demonstrates that the HTS coil takes less than 12 μs to transit from the superconducting to the dissipative state, which is compatible with MRI requirements.

An Electromagnetic–Thermal Coupling Numerical Study of the Synchronous Generator with Second-Generation High-Temperature Superconducting Armatures

Generators with high-temperature superconducting armatures have an advantage in the fact that they can carry high currents. However, the AC loss of high-temperature superconducting (HTS) armatures is difficult to calculate precisely because HTS coils exist in a complex and time-varying electromagnetic environment. In addition, when the HTS coil is carrying a short circuit fault overcurrent, an electromagnetic–thermal simulation study of this process is required to ensure that the HTS coil is not damaged. In this paper, first, a fully coupled T-A formulation model is used to calculate the AC loss of HTS armatures. Then, the current and temperature distributions are simulated, considering the intrinsic characteristic of superconducting coated conductors, when the generator suffers the worst short circuit fault accidently. It is found that the turn with the lowest critical current quenches after 0.01 s, but the temperature rise cannot damage the coil if the circuit breaker can clear the fault quickly. The effects of the copper stabilizer thickness on the thermal stability of the HTS coil during the worst short circuit fault are also investigated. A thicker copper stabilizer improves the thermal stability of the HTS coil in the event of a short circuit fault, but the use of a simulation model is needed to make trade-offs between the engineering current density and the thermal stability of the HTS tapes. The work in this paper is necessary and can provide an important reference for manufacturing superconducting generators.

Influence of Coil Location and Current Angle in Permanent Magnet Wind Power Generators With High-Temperature Superconducting Armature Windings

In this article, the operating current and torque of surface-mounted permanent magnet (SPM) wind power generators with high-temperature superconducting (HTS) armature windings are analyzed. The influence of coil location and current angle on the electromagnetic performance is investigated systematically. The results show that the location of the HTS coils significantly affects the operating current and torque. Furthermore, the maximum torque current angle of SPM wind power generators with HTS armature windings can be different from 180° due to the interaction between the armature and magnet fields. A design optimization method for wind power generators with HTS armature windings is developed in this article. The optimal design is then identified with consideration of the coupling influence of coil location and current angle.

Dynamic loss and magnetization loss of HTS coated conductors, stacks, and coils for high-speed synchronous machines

Dynamic loss is an essential parameter to consider for the design of high temperature superconducting (HTS) synchronous machine windings. For aerospace electric propulsion systems, the fundamental frequency component and harmonics in electric machines can attain kHz level because of the high rotating speed. However, for HTS coated conductors (CC), the existing definition of dynamic loss only considers the HTS layer, the validity of which at high frequencies is questionable. Besides, the variation of dynamic loss and magnetization loss under skin effect due to high frequency is still unknown. Additionally, the influence of shielding effects among distinct turns on the dynamic loss of HTS stacks and coils remains unclear. In response to the above concerns, by use of the H-formulation based numerical multilayer modelling method which considers all layers of a CC, the frequency dependence of dynamic loss and magnetization loss of HTS CCs, stacks and coils over a wide range up to 20 kHz has been investigated. Results show that the existing definition of the dynamic region is no longer valid at kHz level, which shrinks rapidly with increasing frequency and magnetization loss plays a progressively important role due to skin effect. Meanwhile, the shielding effect in HTS stacks and coils can enhance the significance of dynamic loss. This paper clarifies the characteristics of dynamic loss and magnetization loss of HTS CCs, stacks, and coils over a wide frequency band, which can serve as a useful reference for accurate loss controlling of machine windings in future aerospace HTS propulsion systems.

No-Insulation High-Temperature Superconductor Winding Technique for Electrical Aircraft Propulsion

High-temperature superconductor (HTS) machine is a promising candidate for the electrical aircraft propulsion due to its great advantage in high power density. However, the HTS machine always suffers the problem of low thermal stability during quench. In this article, we apply a no-insulation (NI) coil technique on the rotor windings of HTS machines to enhance the stability and safety of the electrical aircraft. The NI HTS rotor windings experience ripple magnetic fields, which leads to induced eddy currents through turn-to-turn contacts. This induced current and accompanying losses will considerably affect the practicality of this technique. To study this issue, an equivalent circuit network model is developed, and it is validated by experiments. Then, analysis using this model shows that most of induced current flows in the outermost turns of the NI HTS coil because of skin effect, and lower turn-to-turn resistivity leads to higher transport current induced and more significant accumulation of turn-to-turn loss. A grading turn-to-turn resistivity is proposed to reduce the transport current induced and ac loss accumulation and meanwhile keep the high thermal stability of the NI HTS coil. Optimization of turn-to-turn resistivity is required when the NI HTS coil is applied in the machines' environments.

Axial and radial offset characteristics of high temperature superconducting wireless power transfer system

The usage of high-temperature superconducting (HTS) coils in magnetically-coupled resonant wireless power transfer (WPT) systems can considerably improve the transfer efficiency, and hence it attacked great attention. However, there are few studies on offset characteristics in HTS WPT systems. In order to further improve the performance of HTS WPT systems, we have studied experimentally the influences of relative displacement of the coils on the transfer efficiency and load receiving power (LRP) in a magnetically-coupled resonant WPT system. The WPT systems using copper coils and HTS coils were carefully compared. The results show that the transfer efficiency decreases monotonically with the increasing offset. However, there is an optimal position for the LRP to achieve the maximum value. Both the transfer efficiency and LRP of the HTS WPT system are higher than those of the copper WPT system under the same conditions.

Loss characteristics of superconducting pancake, solenoid and spiral coils for wireless power transfer

Wireless power transfer (WPT) is an emerging technology with widespread applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have highly nonlinear ac loss characteristics, which will be studied. This paper investigates the transport current loss and the magnetisation loss of HTS coils individually and when combined in the high frequency range relevant to WPT for EVs. A multilayer 2D axisymmetric coil model based on H-formulation is proposed and validated by experimental results as the HTS film layer is inapplicable at such frequencies. Three of the most commonly employed coil configurations, namely: double pancake, solenoid and circular spiral are examined. While spiral coils experience the highest transport current loss, solenoid coils are subject to the highest magnetisation loss due to the overall distribution of the turns. Furthermore, a transition frequency is defined for each coil when losses in the copper layer exceed the HTS losses. It is much lower for coils due to the interactions between the different turns compared to single HTS tapes. At higher frequencies, the range of magnetic field densities, causing a shift where the highest losses occur, decreases until losses in the copper stabilisers always dominate. In addition, case studies investigating the suitability of HTS-WPT are proposed.

Study on the Mechanical Properties of Ice-Impregnated HTS Coils Formed Under Confining Pressure

Epoxy resins are commonly used to impregnate the High temperature superconducting (HTS) coils for mechanical reinforcement. However, performance degradation of the epoxy impregnated HTS coils is inevitable due to the different thermal expansion coefficients between the epoxy resin and the HTS tape. The ice impregnation technique of pulsed magnets was applied to HTS coils for the first time. But cracks were easily formed in ice during cooling process and the mechanical strength of ice was rarely studied at 77 K. To solve this problem, we introduced the technique of ice impregnation under confining pressure into HTS coils. In this paper, uniaxial tension test, compression test and crack detection test of ice were conducted in liquid nitrogen. And critical current of the HTS coil was also measured under confining pressure at 77 K. The experimental results showed that ice impregnated HTS coil under confining pressure did not have performance decay in critical current and the mechanical strength was significantly improved.

Numerical Study on AC Loss of an HTS Coil Placed on Laminated Silicon Steel Sheets With Distorted AC Transport Currents

AC loss is an intractable and inevitable issue on high temperature superconducting (HTS) coils. In some HTS devices with the iron core, AC loss in HTS coils have been investigated. However, practically, partial distorted currents can flow in HTS coils, and the AC loss in this situation has rarely been conducted. Here, a normal and distorted current were respectively applied into a double-layer racetrack coil (DRC) to induce AC losses. The numerical and experimental results were obtained with and without laminated silicon steel sheets (LSSS), which showed that the AC loss of the DRC significantly increased when put on the LSSS. Furthermore, compared with the loss generated by the fundamental waveform, currents with 3rd harmonic in phase and 5th out of phase, both with 10% total harmonic distortion (THD) could increase AC loss by 9.3% and 7.8%, respectively. To reduce AC loss of the DRC on the LSSS, two methods are proposed, and it is found that increasing the 5 mm distance vertically and horizontally between the DRC and LSSS could significantly decrease the AC loss by 20.7% and 18.0%, respectively. This study could offer an indispensable reference for designing and operating HTS coils with the iron core for AC applications.

Experimental Characterization of a No-Insulation HTS Racetrack Coil Subjected to Travelling Magnetic Fields

To investigate the electromagnetic characterization of a high temperature superconducting (HTS) coil when subjected to travelling magnetic fields, a no-insulation (NI) racetrack coil was wound with YBCO coated tapes. For comparison, another HTS coil which has the same geometric and structural parameter was wound with insulated tape between the layers. The charge-discharge performance and other typical characteristics were investigated first. The terminal voltage fluctuation of the insulation (INS) coil during the critical current test proved that the NI coil has a better electrical stability. In addition, a flat stator which has traditional copper windings and a controllable three-phase inverter were introduced to provide travelling magnetic field. The HTS coils were injected with DC current and exposed to the external field generated from the stator, respectively. Different terminal voltage and current responses of the coils were measured by experiments. The INS coil burned out when the AC current in the stator rose up to 20 A @30 Hz but the NI coil kept stable during and after all the experiments even with excessive current in the coil. The terminal voltage and current results indicated that the NI coil, which is potentially going to perform as the field magnet of a HTS linear synchronous motor (LSM), has better electromagnetic stability when subjected to the external field from the stator and thus it shows better application prospects in the LSM.

Charging Characteristics of Series Connected Insulation and No-Insulation HTS Coils by Rotary HTS Flux Pump

High-temperature superconducting (HTS) coil wounds without turn-to-turn insulation (NI coils) present excellent electrical and thermal performance compared to HTS coil wounds with Kapton tape (INS coils). However, NI HTS coils are physically connected to power supplies at room temperature, resulting in increased operating costs due to complex cooling systems and constant heat loads. To overcome this issue, recent studies have investigated alternative charging methods such as flux pump, a power source without any physical contact with the HTS coil at cryogenic temperatures. Even though the NI coil is charged using a flux pump, the disadvantageously slow charge and discharge time remain. In this study, we conducted experiments to compare the charging characteristics of INS and NI coils using a rotary HTS flux pump. A superconducting circuit comprising two coils with equal inductance magnitude is connected in series. From the experiments, we compare the charging characteristics of charging time constant and magnetic field when the same current is applied to the two coils with a power supply and a rotary HTS flux pump. In the NI coil rotary HTS flux pump case, the charging time constant converged to about 6 s in the alternating magnetic field (AMF) above 20 Hz, and the value is similar to the power supply. At 60 Hz and higher AMF, the maximum magnetic field difference from the power supply converged to less than 1%, which appears to be a difference in current and voltage source.

A Design Study on No-Insulation HTS Isochronous Cyclotron Magnet for Carbon Ion Therapy

Provided that a cyclotron system requires DC magnetic field, the no-insulation (NI) high temperature superconductor (HTS) magnet may be a good candidate as it may enable the cyclotron magnet to be more compact and reliable, yet operate under the liquid-helium-free environment. Here we report on a design study on an NI HTS isochronous cyclotron magnet for an acceleration of carbon ion up to an energy level of 385 MeV/u. To obtain a target beam stability, HTS coils were designed with the so-called “hill” and “valley” structure to generate a designated magnetic field profile. After the initial design on yokes and HTS coils was completed, a final design on the cyclotron magnet design was completed and its performance was analyzed using a 3D finite element method. This paper presents the magnet design and performance analyses in details that include: (1) electromagnetic design of the HTS coils and iron yokes for isochronous magnetic field; (2) mechanical stress with the use of stainless steel co-winding to limit the peak magnetic hoop stress; (3) charging and discharging scenarios with the non-linear NI characteristics considered; and (4) first-cut post-quench analysis with an estimated peak temperature and unbalanced force within the NI HTS coils.

Performance Study on the No-Insulation HTS Coil Wound With Narrow-Stacked Wire

As a novel high-temperature superconducting (HTS) conductor structure composed of the stacked 1-mm wide REBCO tapes, Narrow-Stacked (NS) wire has been demonstrated to have a significant reduction effect of AC loss and screening current induced field (SCIF) in the insulated HTS coils. Thus, NS wire is a promising HTS conductor design for practical HTS applications. Meanwhile, compared with traditional insulated HTS coil, no-insulation (NI) HTS coil has obvious advantages in engineering current density, thermal stability and self-protecting ability, and these features benefit for HTS magnets. Although NS wire was regarded as an improvement for insulated HTS coil, the NS wire effects on NI coil are not confirmed yet due to the special characteristics of the NI coil. In this paper, NI pancake coils were wound with NS wire and conventional HTS tape respectively. To evaluate the performance of NI coil wound with NS wire, its characteristic resistance and charging delay were analyzed at the liquid nitrogen temperature of 77 K. Besides, the over-current test results show that NI NS coil also has the satisfied self-protecting feature. This paper intends to provide a possible reference for NS wire application on NI HTS magnets.

Design and Property Analysis of a Performance Evaluation System for HTS Wind Power Generators

Large-scale High Temperature Superconducting (HTS) wind generators produce strong electromagnetic force and high torque due to the high magnetic load of the machines, which is produced by HTS coils with high excitation current. Therefore, in order to properly maintain the mechanical strength of the HTS coil part, the torque and Lorentz force of the HTS wind generator must be scrutinized and reflected in the design. The authors proposed a method for assessing mechanical and electrical stability of a large-scale HTS wind generator based on a Performance Evaluation System (PES) that has the same characteristics as some of the pole-pairs of the HTS generator. The generator and the PES were analyzed using 3D finite element method. The performance of the HTS coil was evaluated by comparing the tangential, radial force and magnetic field distribution of the 10 MW HTS generator with the PES. The tangential and radial forces of the 10 MW HTS generator were 73 kN and 11 kN, respectively. The tangential, radial forces and magnetic flux density of the HTS coils in the PES were equal to one pole in the 10 MW HTS generator. Therefore, the proposed PES-based generator characteristic evaluation method can be used to estimate the mechanical stability of the generator under a high magnetic field and high torque before manufacturing the HTS wind turbine.

Loss analysis of superconducting wireless charging for electric vehicles

Wireless power transfer (WPT) is an emerging technology with widespread applications. It can be used for charging in multiple applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have an interesting ac loss characteristic which will be studied. This study investigates the transport current and magnetisation loss of HTS coils in the frequency relevant to WPT for EVs. In addition, a comparison between the full anisotropic dependency of the critical current and only considering perpendicular field components is conducted. A homogenous 2D axisymmetric coil model is used to examine three of the most employed coil configurations and their loss characteristics depending on turn numbers.

Screening Currents and Hysteresis Losses in the REBCO Insert of the 32 T All-Superconducting Magnet Using T-A Homogenous Model

The 32 T all-superconducting magnet of the National High Magnetic Field Laboratory (NHMFL) was successfully tested in December 2017 and it is expected to be soon available for users. This all-superconducting magnet, comprised of a high-temperature superconducting (HTS) insert and a low-temperature superconducting (LTS) outsert, is the first superconducting magnet reaching more than 30 T. One of the challenges facing this new magnet technology is the estimation of the screening currents, and the corresponding hysteresis losses in the two HTS coils. These coils are made of more than 20,000 turns of insulated REBCO conductor connected in series. The modelling of such system represents a significant challenge due to the huge computational load imposed by the size of the system. Up to now, only medium size magnets (made of units of thousands of turns/tapes) have been successfully modelled with methods based on the well-known H formulation of the Maxwell's equations. In the present work, a new model based on the T-A formulation and a homogeneous technique is proposed. This new approach greatly reduces the computational load and allows performing real-time simulations of large-scale HTS magnets on personal computers.

Screening Current-Induced Field and Field Drift Study in HTS coils using T-A homogenous model

The emergence of second generation (2G) high-temperature superconductor (HTS) tapes has favored the development of HTS magnets for their applications in areas such as NMR, MRI and high field magnets. The screening current-induced field and the field drift are two major problems hindering the use of HTS tapes in the mentioned areas. Both problems are caused by the screening current, then it is necessary to have a modeling strategy capable to estimate such phenomena. Thus far, the H formulation has been a widely used approach to model medium-size systems (hundreds of tapes). However, its application to large-scale systems is still impaired by excessive computation times and memory requirements. Homogenization and multi-scaling strategies have been successfully implemented to increase the computational efficiency. In this contribution, we show that using the homogenization technique with the recently developed T-A formulation allows reducing the computation time and the amount of memory up to the point that realtime simulations of slow ramping cycles of large-scale systems are possible. At the same time, the T-A homogeneous model provides enough resolution to investigate the screening current using numerical simulations.