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This paper proposes a novel approach to the modelling of HTS transformer-rectifier flux pumps that can charge HTS magnet-ic loads without the need for bulky current leads or expensive cryogenic requirements. The model was built in MATLAB/Simulink for a flux pump with 2 HTS switches, an HTS transformer with a primary inductance of 15 mH, second-ary inductance of 50 H and an HTS magnetic load with an in-ductance of 2 mH. With a 5 A pk-pk triangular current wave-form at 2 Hz the system can load the HTS load to 34.63 A in 5 s. This incremental addition of flux to the load demonstrates the advantage of flux pumps as they can charge high-field magnets without the need for large power sources. The speed of flux pumping is improved by increasing the system frequency, the magnitude of the dynamic voltage developed in the HTS switches and by increasing the primary inductance of the trans-former. This has widespread industrial applicability in fields such as the charging of high-field magnets for MRI systems.

The HTS flux pump is a promising device which can charge the HTS magnet for various high field applications. In an HTS flux pump, the superconducting tapes are the circuit connections between the charging loop and the load circuit, and using soldering joints between the superconducting layers is a general method. For a large scale HTS flux pump, the joints should endure the electrical current over kA level, and with durable mechanical structures. In our kA level HTS flux pump, the HTS-copper joint was used to connect the charging loop, the HTS bridge and the potential HTS load. Three methods of HTS-copper soldering were tested and compared for a simple HTS-copper circuit: pure indium wire pre-tinning, IN52/SN48 ribbon direct soldering, and IN52/SN48 ribbon pre-tinning. Experiments show the soldering of IN52/SN48 ribbon pre-tinning archived the lowest joint resistance among these three methods, and the individual resistances were determined. This method was used to solder the joint for the kA level HTS flux pump, and acceptably low resistance was achieved.

The High Temperature Superconducting (HTS) Conductor on Round Core (CORC) cables have been widely utilized to fabricate various superconducting applications for power transmission, fusion, and magnets with high current density. Understanding the AC loss behavior of CORC cables during the cable design process is essential for analyzing its electromagnetic performance and evaluating its future industry potential. The aim of our research is to investigate the AC loss dependence caused by the magnitude and frequency of applied external variations to the magnetic field. This paper describes the simulation models of HTS CORC cables based on the finite-element method (FEM) by using an H-formulation in a commercial software named COMSOL Multiphysics. Three-dimensional (3-D) CORC models, with three HTS tapes wound around each layer, were built and analyzed. The 3-D CORC models, built using the FEM method, were then verified with published experimental results. AC losses dissipated in the CORC cables, resulting from applied external varying magnetic field, were calculated and examined.

The HTSConductor on Round Core (CORC) Cables have been widely recognized as the proper candidate for superconducting applications such as the high-field magnets and power transmission cables. This article presents the modeling of HTS CORC cables using the FEM H -formulation model merged into the package COMSOL Multi-physics. The 2D and 3D H -formulation models of HTS CORC cables were compared and analyzed. There were two important parameters in our CORC design: (1) the gap angle between the HTS tapes in a single layer, and, (2) the degree of twist for HTS tapes (normally identified as the pitch). The 2D CORC model has much faster solving speed over the 3D model and sometimes is able to model the AC loss in a reasonable range, while, the 3D CORC model has the advantages over 2D CORC model with respect to more information in each sector with the different conditions of two parameters above, and more realistic loss calculation.

The High-temperature Superconducting (HTS) magnet is one of the most promising technologies to accommodate various high field applications. With the high magnetic field, the quench problem of HTS material should be considered with extra care, and the specific quench protection scheme should be projected. In this article, the modeling of the quench protection of an HTS magnet for the MRI system is presented. The modeling strategy was composed of two stages: Stage 1 the HTS magnet was in the normal operation circuit, and Stage 2 the HTS magnet was switched to the heat dissipation circuit. The 1st stage was based on the finite element method (FEM) using the H -formulation together with the thermal-coupled model, which simulated the current distribution of typical HTS tapes and showed the process of temperature increase when the hot source occurred. The 2nd stage was based on the analytical model to investigate the reasonable damping resistance and the hot spot temperature of the HTS magnet.

Superconducting Conductor on Round Core (CORC) cables fabricated by several high temperature superconducting (HTS) tapes are able to carry enormous electrical current density, to be widely used in a variety of superconducting applications such as power transmission, fusion, and magnetic resonance imaging (MRI). However, the AC losses induced in the superconducting tapes under alternating current or magnetic field generate heat dissipation which can reduce the electric power efficiency. Therefore, it is essential to analyze the AC loss behavior of CORC cables as well as consider methods to reduce it. This paper presents an AC loss study of CORC cables with several different superconducting layer thicknesses. The CORC cable simulation model is built in a software suite named COMSOL Multiphysics utilising the finite-element method (FEM) solved by the three-dimensional (3-D) H -formulation. By implementing FEM simulations, the cases of increasing the degree of freedom of superconducting layers have been considered, and the AC losses of a single layer CORC cable model with three tapes mounted around the core have been calculated and analyzed for different layer thicknesses. Simulation results are verified with experimental results measured in previous literature.

Conductor on round core (CORC) cables wound by high-temperature superconducting (HTS) tapes have received much attention due to the advantages in power capacity and mechanical flexibility. This paper first developed a 3D model for a CORC cable through the finite-element method (FEM). Next, the total AC loss caused by transported current only was investigated, revealing the eddy current loss in the tube former contributes to the cable loss. The numerical CORC model was validated by published measurements. Moreover, the AC loss that resulted from a simultaneous presence of transport current and magnetic field was explored. With the induced current, the magnetisation loss generated in superconductors can increase the total loss. It is concluded that the addition of a magnetic field can result in a surge in the total loss of a current-carrying cable.

The potential application of the superconducting fault current limiter (SFCL) in a voltage source convert (VSC) system has received lots of attention. On the basis of the fault current limiting characteristics, this paper evaluated the loss level of saturated iron-core FCLs (SI-SFCLs) in a VSC-based distribution grid. The models of VSC network and the SI-SFCL model were respectively developed. Then, with and without the implementation of SI-SFCLs, the fault current process under positive-to-ground and positive-to-negative short-circuit was presented. Next, the power dissipated from the superconducting coil was curved. Analysis demonstrated the power dissipation of the SI-SFCL during the short-circuit is likely to reach tens of kilowatts. This study can provide a helpful guidance for the design of SI-SFCLs.

The conductor on round core (CORC) cable wound with second-generation high-temperature superconducting (HTS) tapes is a promising cable candidate with superiority in current capacity and mechanical strength. The composing superconductors and the former are tightly assembled, resulting in a strong electro-magnetic interaction between them. Correspondingly, the AC loss is influenced by the cable structure. In this paper, a 3D finite-element model of the CORC cable is first built, and it includes the complex geometry, the angular dependence of critical current and the periodic settings. The modelling is verified by the measurements conducted for the transport loss of a two-layer CORC cable. Subsequently, the simulated results show that the primary transport loss shifts from the former to the superconductors as the current increases. Meanwhile, the loss exhibited in the outer layer is larger than that of the inner layer, which is caused by the shielding effect among layers and the former. This also leads to the current inhomogeneity in CORC cables. In contrast with the two-layer case, the simulated single-layer structure indicates stronger frequency dependence because the eddy current loss in the copper former is always dominant without the cancellation of the opposite-wound layers. The core eddy current of the single structure is denser on the outer surface. Finally, the AC transport losses among a straight HTS tape, a two-layer cable and a single-layer cable are compared. The two-layer structure is confirmed to minimise the loss, meaning an even-numbered arrangement makes better use of the cable space and superconducting materials. Having illustrated the electro-magnetic behaviour inside the CORC cable, this work is an essential reference for the structure design of CORC cables.

Magnetic Resonance Imaging (MRI) is an indispensable tool in monitoring lipid-rich tissues and the musculoskeletal system. It however requires maintaining a huge homogeneous (1 to 7 T) field. That is where superconductors come into play since they are the only feasible way to create such fields. The state-of-the-art technology utilizes low critical temperature superconductors (LTS) which require cooling down to liquid helium temperatures (4.2K). The new generation superconductors, aka the high critical temperature superconductors (HTS), can operate up to relatively elevated temperatures (technically above 100K's). In addition to that, they can withstand larger magnetic field strengths which can theoretically go up to the order of hundred Teslas in magnitude. It is evident that LTS wires of current MRI magnet technology is at their limits and new pursuits such as reaching higher fields and reducing operational/maintenance costs are in the favor of HTS tapes. There is, in fact, another advantage beyond minimization of the cryogenic costs, namely the compactness of the overall system. Utilizing certain auxiliary techniques such as flux pumping and conductive cooling of the tapes, MRI magnets can be reduced in size substantially. That, in turn, enables constructing much smaller thus mobile MRI scanners which can be brought to patients’ doorsteps. Here we present the current status of our project aiming to build an HTS MRI machine dedicated to use as a mobile head scanner.

When a superconductor is exposed to an AC magnetic field whilst carrying a constant DC transport current, a DC electrical resistance can be observed, commonly referred to as “dynamic resistance”. This dissipative effect can play a critical role in many potential high-temperature superconducting (HTS) applications, especially for the AC-field-actuated HTS switch, which is the key component in the HTS transformer-rectifier flux pump (TRFP). The characteristics of the HTS switch determine the performance and reliability of TRFP. In this paper, we report a numerical study of a new HTS switch design made of series-connected YBCO stacks. This study aims to obtain insights into the characteristics of dynamic resistance and power loss, including their relationships with the amplitude and the frequency of the AC magnetic field. The energy conversion efficiency of the HTS switch is also investigated. This work will help design a high-performance AC-field-actuated HTS switch.

High- $T_{c}$ superconducting (HTS) materials are extremely promising for high-field applications, due to their relatively high upper critical field. Flux pumps are a means ofenergizing closed superconducting magnets without direct electrical contact. Simulation is a good method to understand the physics of the flux pumping system. In this paper, we propose a simulation methodology for a transformer–rectifier HTS flux pump. A model is built, based on the electrical circuit. The simulation methodology opens up various opportunities for flux pump developers to design the whole superconducting magnet energizing system.

© 2002-2011 IEEE. The application of resistive superconducting fault current limiters (R-SFCLs) in voltage source converter (VSC)-based DC systems has been proposed, and the encapsulated high temperature coated conductors (HTS-CCs) used for R-SFCLs is getting practical. Thus, this paper respectively develops a 1-D HTS-CC model and a 2-D HTS-CC model with different thicknesses to investigate the resistance characteristics and the quench recovery time. The results show that the resistance of encapsulated HTS-CCs goes through the flux flow state before reaching the normal. Then, the encapsulation can expand the limit of fault current, and a suitable thickness is likely to reduce the total resistance of the R-FCL. Finally, due to the existence of the heat transfer between the HTS-CC surface and the liquid nitrogen bath, a thinner tape might recover faster, and the recovery time tends to rise slowly as the tolerated current increases.

Magnetic switches apply AC magnetic fields to DC current-carrying high temperature superconducting (HTS) tapes to generate DC voltages and are commonly used in the persistent current switches (PCSs) and flux pumps to charge HTS-coated conductor magnets. Normally, they are made of copper field coils and iron cores with narrow air gaps for the HTS tape to pass through. However, the perpendicular components of the self-field of the HTS tape in the air gap can be enhanced by the iron cores and cause a critical current reduction of up to 40% to the tape. If ignored, this reduction, rather than the magnets themselves, will limit the current carrying capability of the HTS magnets. To tackle this problem, we present analytical approximations to calculate the self-field distribution of a superconducting tape between iron cores. The approximate solutions are based on the method of images in electromagnetics to simplify the derivation and are then verified by the experiments and 3D finite element method models using the T–A formulation. The solutions are universal and can be applied to almost all the magnetic switches currently in use. A case study of typical magnetic switches shows that the solutions can be used to determine the critical current reduction quickly and accurately, analyse the influence of different parameters, and simplify the design process of magnetic switches. The results can significantly benefit the design and optimisation of PCSs and flux pumps for HTS magnet charging systems in the future.

This article presents a simplified mathematical model the anisotropic magnetic field dependence (AMFD) for simulating the High-temperature superconducting (HTS) Conductor on Round Core (CORC) Cables. All these simulations of HTS CORC Cable were performed based on the finite-element method (FEM) H -formulation model merged into the numerical platform COMSOL Multi-physics. The simplified model of AMFD was implemented into both the 2D and 3D H -formulation models of HTS CORC cables. Their results were studied with different conditions, such as the increasing transport current, as well as the gap angle between the superconducting tapes in a single layer. This new simplified AMFD model can give a proper approximation of the actual electromagnetic behaviours of the HTS CORC, but also establish the upper limit of the AC loss calculation, which can be fairly useful for designing future HTS CORC cables.

This paper presents the loss analysis on the saturated iron-core superconducting fault current limiter (SISFCL) for a VSC-HVDC transmission system. The numerical model of SISFCL as well as its loss calculation on superconducting parts were carried out by the finite-element method (FEM) using the H -formulation merged into the commercial package COMSOL. The SISFCL model was established for a practical ±10 kV VSC-HVDC system, and the fault current situation was simulated using the PSCAD with a SISFCL. The capability of fault current limiting was verified using the analysis of electromagnetic characteristics, and the corresponding patterns of magnetic field in the iron-core were studied. During the process of fault current limiting, the instantaneous power losses in the superconducting components were studied with the increasing DC bias current. Even in a DC grid system, results proved there were considerable amounts of losses occurred in the superconducting parts, when the SISFCL encountered the fault currents.

When a type-II superconductor, exposed to a perpendicular alternating magnetic field, carries a direct current, a biased dc voltage response can occur. The response voltage can be classified into dynamic-resistance voltage and flux flow voltage according to its triggering mechanism. Experimental results for the phase differences of the two types of voltage have been provided in the paper. Harmonic analysis on the voltage response has been carried out to differentiate the two types of the response voltages. Based on analyzing the experimental data, a useful and flexible real-time method using the ratio of the dc component to the second harmonic component of the voltage response has been proposed to serve as a monitor indicating the state of dc-carrying HTS tapes subject to an ac magnetic field. The method is promising in the application of complex superconducting systems such as flux pumps, persistent current switches, superconducting cables and dc fault current limiters.

This paper presents the power dissipation analysis on high-temperature superconducting (HTS) coated conductor tapes and coils under the action of various oscillating currents and fields. The loss calculation on HTS coil was carried out by the H -formulation model implemented in the finite-element method (FEM) software package COMSOL Multiphysics. There were four different oscillating (ac) signals in this paper: sine, triangle, sawtooth, and square. Two scenarios causing loss in HTS tapes and coils which have been analyzed: Scenario 1 ac transport current; Scenario 2 ac magnetic field. The simulation results were compared to the experiment results with the typical sinusoidal ac signal.

High-temperature superconducting (HTS) no-insulation (NI) coil is a promising design because of high current density, high thermal stability, and self-protection mechanism. Flux pumping is a contactless charging method for a superconducting magnet, which can reduce the cryogenic losses associated with current leads. In this study, we compared the charging performance of a rotating permanent magnets–based flux pump for two different types of HTS coils: an INS coil and an NI coil. Results showed that the rotating type flux pump works well with both coils. But the charging speed of NI coil is much slower, which results from the small parallel characteristic resistance of NI coil. Results also show that the output DC voltage and inherent dynamic resistance of the flux pump increase linearly with the increase of rotating speed.

High temperature superconductor (HTS) is becoming more and more attractive for the application in high field magnets, which can be utilized in the magnetic resonance imaging (MRI) and rotating machines. To charge the HTS magnets and operate them in persistent current mode, a persistent current switch (PCS) is very necessary. This paper will focus on how to build a fast persistent current switch controlled by ac magnetic field. The PCS can present a large resistance rapidly compared with conventional PCS and recover to zero-resistance in a short recovery time. Impacting factors and strategy to control the proposed PCS are discussed. An experimental system is built and tests are carried out to verify the practicality of the proposed PCS.

© 2002-2011 IEEE. This paper presents the design of an HTS magnet for an ultra compact MRI system, potentially for the rapid and early diagnosis of brain trauma. Early diagnosis and therapy stratification can reduce the risk for critically brain ill patients with the use of near patient imaging, and can aid with precision medicine. High temperature superconductors (HTS) have the ability to carry large currents in the cryogen free contrition, which can make the MRI system even smaller and lighter. The design and Genetic Algorithm (GA) optimization were based on the FEM package COMSOL Multiphysics with the LiveLink for MATLAB, together with the GA module in the MATLAB optimization toolbox. The relatively thick HTS tape, ST-12-L from the Shanghai Superconductor Technology was chosen and that made more difficulty on optimization process. Genetic Algorithm method improved the optimization performance, and the uniformity achieved 2.36 ppm in a 10 × 10 × 10 cm DSV. Two cases of the end double-pancake were compared. Further sensitivity studies were performed on the homogeneity with its relationship to the magnet length, and the thickness of HTS tape.

This report is on the design of and results from a fast-acting, high power density high temperature superconducting switch. The switch uses thin films of YBa2Cu3O7−x on a sapphire substrate switched into the off-state through application of an AC magnetic field to the superconductor, which causes any transport current to experience a dynamic resistance. Results show reliable and repeatable switching of a 10 × 30 mm section of thin film between an on-state with Ic of 100 A and an off-state of 9 mΩ, with off-state voltage exceeding 100 mV. On-state to off-state transition time is less than 1 ms and off-state to on-state transition time is around 15 ms. These results are promising for further development of a high power, fast-acting superconducting power switching device for use in applications such as flux pumps.

When a REBCO tape carrying DC transport current is exposed to a perpendicular AC magnetic field with amplitude above certain value, flux could be triggered to traversing the superconducting tape. During the process, a biased DC response voltage, called the dynamic resistance voltage will be induced. In the paper, the calculation on the time-dependent dynamic resistance voltage was carried out by the H-formulation model implemented in the finite-element method (FEM) software package COMSOL Multiphysics. Fourier analysis on the waveform of dynamic resistance voltage was utilized to figure out the main components of the response voltage. The DC component, second harmonic component and ratio of the two components can reflect how the instantaneous dynamic resistance responds to the applied AC magnetic field and the DC transport current. The findings in the paper can be used to monitor the state of DC-carrying REBCO tapes subject to an AC magnetic field.

When a type-II high T c superconductor carrying a direct current is subjected to a perpendicular AC magnetic field, a direct current voltage will appear. This phenomenon is called dynamic resistance effect. In general, the high temperature superconducting (HTS) coated conductor (CC) has two stabilizer layers. However, the impacts of two stabilizer layers on the dynamic resistance, DC electrical field, losses, and temperature rise haven't been studied yet. This paper presents the impacts of the stabilizer layers and their resistivity on the dynamic resistance effect and HTS CC tape's thermal-electromagnetic behaviors by using a temperature dependent FEM model. This work reveals that the stabilizer impacts significantly on the dynamic resistance, dc voltage, power loss, and temperature rise. It is will help design high-performance AC magnetic field-controlled PCS and switches based HTS devices.

This paper reports a newly developed high current transformer-rectifier High-Tc Superconducting (HTS) flux pump switched by dynamic resistance. A quasi-persistent current of over 1.1 kA has been achieved at 77 K using the device, which is the highest reported operating current by any HTS flux pumps to date. The size of the device is much smaller than traditional current leads and power supplies at the same current level. Parallel YBCO coated conductors are used in the transformer secondary winding as well as in the superconducting load coil to achieve high current. The output current is limited by the critical current of the load rather than the flux pump itself. Moreover, at over 1 kA current level, the device can maintain high flux injection accuracy, and the overall flux ripple is less than 0.2 mili-Weber. The work has shown the potential of using the device to operate high field HTS magnets in ultra-high quasi-persistent current mode, thus substantially reducing the inductance, size, weight, and cost of high field magnets, making them more accessible. It also indicates that the device is promising for powering HTS NMR/MRI magnets, in which the requirement for magnetic field satiability is demanding.

Multiferroic material as a photovoltaic material has gained considerable attention in recent years. Nanoparticles (NPs) La0.1Bi0.9−xSrxFeOy (LBSF, x = 0, 0.2, 0.4) with dopant Sr2+ ions were synthesized by the sol–gel method. A systematic change in the crystal structure from rhombohedral to tetragonal upon increasing Sr doping was observed. There is an obvious change in the particle size from 180 nm to 50 nm with increasing Sr substitution into LBFO. It was found that Sr doping effectively narrows the band gap from ~ 2.08 eV to ~ 1.94 eV, while it leads to an apparent enhancement in the electrical conductivity of LBSF NPs, making a transition from insulator to semiconductor. This suggests an effective way to modulate the conductivity of BiFeO3-based multiferroic materials with pure phase by co-doping with La and Sr at the A sites of BiFeO3.