This article consists on the first finite element analysis of quasi-static test of a superconducting machine with stacks of coated conductors arranged spirally around the rotor. The behavior of the machine as a generator is also investigated. Hybrid A-V-H 2D formulation and moving mesh were used as modeling framework. The stacks are represented with the homogenization technique. Quasi-static test is simulated considering a field cooling initial state and the generator simulation, a pulse field magnetization. Current density and magnetic flux density along the stack surface as well as speed and torque results are analyzed. When the machine is not moving, major current density loops are induced in the stack, following the magnetic poles. These loops are deformed by the current density induced by speed differences between stator and rotor. The quasi-static torque does not change with position. The simulation results prove the potential this topology has of operating as a generator. The quasi-static results are in agreement with measurements made with the prototype.

Simulation of Superconducting Machine With Stacks of Coated Conductors Using Hybrid A-H Formulation

We report here a record 4.6 T trapped field generated by high temperature superconducting (HTS) persistent current loops using a HTS ring structure. By stacking 200 HTS rings into a compact magnet 90 mm in diameter, we performed a field cooling magnetisation at 25 K. The main advantage of the new magnet compared to existing trapped field HTS magnets is that the magnetic field is in the parallel direction to the ab plane of the HTS, leading to higher critical currents in the same magnetic field. Therefore, compact HTS magnets can be developed based on this principle to achieve high magnetic fields. Experimental results show that the final trapped field distribution depends on the ring geometry. We developed a new three dimensional model to simulate the magnetic field distribution within the HTS ring magnet and good agreement between experiments and simulation have been found. The temperature dependency and ramping rate dependency have been studied numerically as potential factors to influence the magnet field. The proposed HTS ring magnet will have promising applications in medical imaging devices, e.g. MRI, as well as electrical machines.

https://iopscience.iop.org/article/10.1088/1361-6668/ab794a/pdf

4.6 T generated by a high temperature superconducting ring magnet

Superconducting devices composed of the second-generation high-temperature superconducting (2 G HTS) RE-Ba-Cu-O (REBCO, RE is rare earth) tapes have extensive applications in high magnetic flux den...

Architectural configuration of a magnetic flux density amplifier composed of second-generation high-temperature superconducting (2G HTS) 8-shaped rare earth-Ba–Cu–O (REBCO) loops

The ring-shaped superconducting permanent magnet, with its great advantages in flexible sizing and trapped field, has become a potential candidate for portable medical applications. However, due to the complex geometry involved, it is difficult to predict its electromagnetic performance by traditional numerical methods. This paper presents a field-circuit coupling method to study the entire magnetization process of the ring-shaped magnet. Firstly, the principle of the numerical method is introduced and it is proved to be sufficient for a ring-shaped magnet with a large turn number. Then, the numerical model is used to discuss the relationship between pulse waveform and magnitude of trapped field. Next, the accumulation effect under multi-pulse magnetization is theoretically analyzed and proved by both experiments and simulation. Finally, based on the numerical model, a study on the decay process of ring-shaped magnets is also presented. Conclusions from this paper will be helpful for obtaining the optimization strategy of magnetization of ring-shaped magnets for practical medical applications.

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Study on the Electromagnetic Characteristics of Ring-Shaped Superconducting Permanent Magnets for Medical Applications

The so-called ring-shape superconducting trapped field magnet has been proved as a potential candidate to replace commercial permanent magnets in practical industrial applications on account of its jointless structure. In this paper, a circuit method based on Bean model, E-J power law and Faraday law has been built to investigate the magnetization mechanism of ring-shape magnet. The field dependent critical current of superconducting materials has been taken into consideration for further optimization. The numerical model was verified by both finite element method (FEM) and experiments. Compared with other methods, the circuit-based simulation method has the advantages of high efficiency, fast speed and wide applicability. On basis of this method, the paper also studied characteristics of ring-shape magnets under different working conditions to provide a guideline for the magnet design.

Numerical Study on Ring-shape Superconducting Trapped Field Magnet Based on Circuit Model

Low critical temperature superconducting coils can conduct persistent currents with almost no losses, as the technique to make joints is well developed and the resistive losses are negligible. The replacement of these superconductors by high-temperature superconductors may greatly reduce refrigeration costs. However, even though the technique for making these joints has greatly developed over the last years, there are still inherent difficulties associated to that process. Coils made with partially slit loops of high temperature superconducting tape have no joints, and therefore can conduct current in persistent mode. Since there are no terminals, its current must be induced through magnetization techniques, for instance, by the means of a transformer. The occurrence of persistent current is related to the voltage in the sample, making it interesting to study the voltage behavior for different regions of operation. In this work, a jointless 2G loop was magnetized with pulse magnetization from which the curves of voltage and current as functions of time were obtained. After several tests, it was possible to divide the superconductor behavior into three different stages of operation, depending on the current levels reached: for low current values, the material behaves as a perfect inductor; for current values near the critical current, it behaves as an RL circuit; and for higher values, resistance dependent on the temperature has to be taken into account. A direct relation between persistent current and the arise of a dissipative state was noted and the highest values of persistent current were observed when the pulse was intense enough to produce significant temperature variations in the sample.

Vagner Santos da Cruz

Papers

Study of the Voltage Behavior of Jointless Superconducting 2G Loops During Pulse Magnetization