Mengyuan Tian

Stacked high-temperature superconducting (HTS) coated conductors (CCs) are preferred for trapped field magnets over HTS bulk due to their improved mechanical strength and shape flexibility. In superconducting machines, the demagnetisation phenomenon in HTS CCs is crucial as it affects the machine's magnetic field loading, affecting power and torque output. The stator slotting effect, common in conventional machines, can also influence TFMs made from HTS CCs in superconducting machines, leading to magnetic flux distortion and additional losses. To gain a comprehensive understanding of these influences, numerical analyses based on the finite element method have been conducted for different variants of a superconducting machine. Findings indicate that in the machine model: (1) a slotted stator yields higher trapped field values compared to a slotless stator, and (2) closed slots prove effectively in eliminating air gap harmonics. A final design with distributed winding and closed slots is recommended for reference in HTS motor development.

High temperature superconducting (HTS) stacks of superconducting tapes provide a solution for all-superconducting electrical machines since they could work as trapped-field magnets. Most experiments with these stacks use pieces of tapes with relatively small areas as wider tapes are more difficult to acquire, therefore in this paper we investigate substitutions for these wider tapes to be implemented in superconducting electrical machines. With different stack architectures and substrate materials made by SuperOx, AMSC and THEVA, the magnetizing effect using pulse field magnetization (PFM) would be different. To explore the differences of this effect, sectioned stacks are tested in a modified synchronous machine designed by ASuMED under liquid nitrogen conditions. By changing different sectioned stacks of tape and testing them under the same condition, the characteristics of the trapped flux, magnetization and demagnetization could be affected, different stacks are simulated with COMSOL finite element models using an electromagnetic-thermal coupled model, and different stacks are tested with different charging voltages.

The Roebel tape is a novel high-temperature superconducting (HTS) wire with a multiple superconducting layer structure. Adjacent layers are patterned into meander-shaped multifilaments in opposite directions so that the magnetic fields generated by each layer can be mutually cancelled. The Roebel tape can be regarded as an HTS Roebel cable encapsulated into a tape, and hence has exclusive advantages such as high engineering current density, small critical current degradation, low AC losses, and a simple manufacturing process. This paper focuses on the transport AC losses of the Roebel tape with a multiple-layer structure. 3-D finite element method (FEM) models for the Roebel tapes are built based on the T-A formulation. Current distribution and AC losses of the Roebel tapes with up to 10 ReBCO layers are obtained. The results show that the tapes with even numbers of layers generate lower losses than those with odd numbers, which indicates the superconducting layers should always appear in pairs. Increasing layers can increase the engineering current density of the tape, but the average loss per layer also rises. Therefore, we suggest increasing the distance between the layer pairs or using double-layer Roebel tape stacks instead. The results indicate that the multilayer Roebel tapes can be a promising cable solution for industry and fusion applications because of the high current density and low AC losses.

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.

When a type II superconductor carrying a direct current is subjected to a perpendicular AC magnetic field, a DC voltage can be observed, commonly referred to as “dynamic resistance effect”. This dc voltage appears in a very short time when AC field is applied and disappears quickly when AC field is removed. These advantages make the type II superconductors to be a fast AC-field-actuated HTS switch, which plays an important role in many potential high-temperature superconducting (HTS) applications, especially for HTS flux pumps or HTS magnetic energy storage systems. However, the temperature in the superconductor varies due to this dc voltage and may result in quench for the superconductor. Therefore, it is important to investigate the thermal behavior of an AC-field-actuated HTS switch. In this paper, we build and experimentally verified a 2D electromagnetic-thermal coupled model to investigate the electromagnetic-thermal characteristics of HTS switches in different working conditions. This model can estimate the dynamic voltage rise and the temperature rise of the superconductor. This work is indicative for designing an AC-field-actuated HTS switch in terms of increasing the dc voltage, and enhancing the safety and stability.

The Roebel tape is a novel high-temperature superconducting (HTS) cable. By incapsulating the HTS Roebel cable into a tape, it has exclusive advantages such as high engineering current density, small critical current degradation, low AC losses, and a simple manufacturing process. This paper focuses on the field cancelling effect of the Roebel tape introduced by the specially designed patterns on the buffer layers and superconducting layers. From the results obtained by the 2D and 3D models using the T-A formulation, we find this effect can make the current distributed more uniformly and can significantly reduce the AC losses. The reason and the basic principle behind this will be revealed and analysed. To sufficiently exploit it to reduce losses, the structure parameters of the Roebel tape are optimised and an optimum solution is obtained. This paper can help better understand the Roebel tape and provide references to the future design and application of this novel HTS cable.

High temperature superconducting (HTS) magnets are becoming more and more attractive for many commercial applications. HTS magnets, however, need an external power supply to operate in persistent current mode. One way to charge an HTS magnet is using a conventional power supply. This, however, is not the most efficient way to charge an HTS magnet due to losses from the current leads. Flux pumping is an alternative method to charge an HTS magnet. Over the years different types of flux pumps have been proposed specific to different applications. Transformer-rectifier flux pump is one of the many types of flux pumps that have been researched widely. The design of switches plays a central role in a transformer-rectifier flux pump. In this paper, we propose a novel switch design for a half-wave transformer-rectifier flux pump. The switch proposed in this study will be used in compact and mobile HTS Magnetic Resonance Imaging (MRI) instruments.

The development of high-temperature superconducting (HTS) electrical machines results in applications of various fields, as they have higher efficiency and power density. Numerical modeling of superconducting electrical machines is usually performed by finite element method based their electrical behavior, and H -Formulation of the Maxwell's equations is one of the models that are used tremendously due to its high flexibility and simplicity. Axial field machine (AFM) is a machine topology with higher efficiency and power density, but with more complexity when modeling. This paper proposed a new 2D model for the superconducting AFM, where the superconducting coil is simulated with a 2D infinitely long model, this model also divides the electrical machine into two parts: the superconducting parts (HTS coils, bulks, etc.) and non-superconducting parts (e.g., conventional conductors, machine structure). This paper compares the results of H-A Formulation and T-A Formulation with this approach, which requires appropriate boundary conditions and continuity between the two regions, along with correct modeling of fixed and moving parts of the electrical machine, where the geometry of this model is based on a linear synchronous machine, which means the rotor is moving linearly instead of rotation.

Direct current carrying Type II superconductors present a dc electric field under external perpendicular AC magnetic field, commonly referred to as “dynamic resistance effect”. Two kinds of AC losses occur in this situation: dynamic loss due to the interaction between the DC current and moving fluxons, and magnetization loss due to shielding current caused by the external AC magnetic fields. In general, HTS coated conductors have one substrate layer, which can be non-magnetic (Hastelloy) or magnetic (NiW). In this paper, we present the impacts of the magnetic substrate and their strength of ferromagnetism on dynamic loss and magnetization loss by using a finite-element method model based on H -formulation. This work reveals that the magnetic substrate layer does influence the dynamic loss and magnetization loss of HTS coated conductors. The dynamic loss is reduced by the adjacent magnetic substrate in the low field region and is increase in the high field region. The magnetization loss is reduced significantly by the magnetic substrate. This work will help choose HTS coated conductors with different substrate layers in different applications.

The main obstacle for the application of the high-temperature superconducting (HTS) coated conductors (CCs) in AC energy applications is the relatively large AC loss. To tackle this, several forms of HTS cables for AC applications have been invented and investigated, such as the HTS Roebel cable, Cable on Round Conductor (CORC), and Twisted Stacked Tape Cable (TSTC). In this paper, the possibility of encapsulating Roebel cables into a single HTS CC is investigated, from which a novel conception of a Roebel tape is proposed. Compared with the existing HTS cables, HTS Roebel tapes are more compact, more capable at carrying large current with minimised AC losses, and easier to manufacture. A 3D finite element method (FEM) model of the Roebel tape is built to optimise the structure and verify the performance. The results show that, compared with normal ReBCO CCs, Roebel tapes with optimum patterns can reduce AC transport losses by 60% through field-cancelling and can reduce magnetisation losses by the factor of the filament number through patterning. With excellent loss reduction performance and high current capability, the Roebel tape shows great potential to become a revolutionary HTS cable for high-current power transmission and AC machine applications.