Field decay rate is the key characteristic of superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers with an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

Large-scale complex needs high performance SMES system with 5 kA-level current carrying and kA/s current ramping rate (such as particle accelerator complex), the inductance of which should be as small as possible (<<1H) to reduce the terminal voltage and realize ideal energy evacuation. This paper introduces the world’s first application of double pancake (DP) coil with CORC cable, which can be a unit of MJ-level SMES system with a total inductance of 125 mH and 4 kA current carrying (@30 K, 4 T). The combination of low inductance and high current capacity guarantee the safety during kA/s operating condition. The CORC wound with highly anisotropic YBCO tape can realize quasi-isotropy with critical properties. The performance of the CORC with 11 layers and 3 tapes in each layer has been fully validated with excitation test at 77 K and the critical current can reach 2906 A@ 77 K, self-field. The skeleton of the DP coil is specially designed with semi-circular spiral groove and the climbing-layer area is supported by two symmetrical blocks, which can provide reasonable support to the CORC cable. The measured critical current of DP coil can realize 1750 A at 77 K (theoretical value is 1800 A). It means that the the coil winding method is feasible to avoid performance degradation during winding. The DP coil based on CORC cable is fully suitable for the SMES system which needs to realize ~4 kA current carrying and fast energy conversion. It also provides a good practice for the engineering application of CORC cable.

A high-temperature superconducting no-insulation (NI) coil, with its self-protection property, high engineering current density and unique demagnetization property, becomes a potential candidate for an electrodynamic suspension (EDS) system. Compared with the applications in high field magnets, the NI coil used in the EDS system is considered as working in a dynamic state, the magnetization loss generated in the NI coil is essential for the design of a cryogenic system. This paper presents the study on AC magnetization loss of NI coils by both numerical and experimental methods. Firstly, a 3D finite element numerical model representing the full geometry of the NI coil is built to analyse the effect of field frequency, field magnitude, as well as the radial characteristic resistance. Then, systematic discussions are conducted to figure out the working mechanism of NI coils. Finally, a calibration-free method testing platform is installed to validate the numerical model, and a modified model is proposed to represent the non-uniform radial characteristic resistance caused by stress distribution. The conclusions of this paper will be used in the future optimization of NI coils and the cryogenic design of the EDS system.

The development of high temperature superconducting (HTS) conductors is leading to the diverse structure designs of HTS cable. (RE)Ba2Cu3Ox (REBCO) tapes using spiral geometry has been a popular compact HTS cable structure, which is in the critical stage of engineering production and application. However, the winding quality of REBCO tapes is unstable for spiral HTS cables, because of the different winding methods like manual winding, device-assisted winding, or automatic winding. Although automatic winding will be the first choice for the actual applications by spiral HTS cables, the related winding quality is not monitored effectively yet. In this paper, we first discuss the possible influence of the winding quality on the critical current performance of spiral HTS cables. Then, an artificial intelligence (AI) based method is implemented to realize the detection model for the winding quality. From image data preparation to AI detection and postprocessing, the detection model provides the final results to show the winding intervals as a binary image. Through the intuitive analysis and the evaluation metrics, both error and correct winding conditions obtain acceptable detection results, and the correct one has a better performance. The identification of the winding intervals will help to determine the monitoring strategy for the spiral HTS cable fabrication.

No abstract