Li Lu

The accurate measurement of turn-to-turn resistivity (TTR) distribution in high-temperature superconducting (HTS) coils is important for optimizing performance parameters such as excitation speed and quench risk at different locations. However, this measurement remains challenging due to the non-uniformity of contact pressure. A hybrid methodology integrating the moderate equivalent circuit model with heuristic optimization algorithms (genetic algorithm and particle swarm optimization) is proposed to nondestructively detect the TTR distribution in full-scale racetrack REBCO coils for maglev applications. The systematic comparison demonstrates that Particle Swarm Optimization (PSO) is more suitable for the studied problem under the specific parameter settings, as it shows better voltage fitting accuracy and operational stability compared to Genetic Algorithm (GA). The method first estimates the global TTR range through a lumped circuit model and time constant analysis, then minimizes the deviation between measured and simulated voltage values to obtain the optimal spatial distribution of TTR. Experimental validation on three full-scale racetrack coils (C1, C2, C3) reveals characteristic TTR distributions, where the resistivity values at inner and outer turns are 2–6 times higher than those at middle turns.

Although the high-temperature superconducting (HTS) closed-loop coil operating in persistent current mode can eliminate the huge heat leakage from current leads, application of the non-insulation (NI) winding technique further improves its thermal stability and self-protection ability. Therefore, the NI closed-loop coils are preferred in certain applications, where safe operation, thermal stability, weight, and size of the magnetic are strictly demanded, e.g., electro-dynamic suspension system. It is noticeable that the magnetic field stored in the closed-loop coil exhibits a reduction when a heat disturbance occurs, because the power supply is eliminated. Unfortunately, the quench behaviors of the NI closed-loop coils have not been systematically studied. In this paper, an electromagnetic-thermal coupling model was established to study the electromagnetic-thermal behaviors during quench process. It is demonstrated that when the heat triggered quench occurs in one turn of the coil, the current at quench location flows to the turn-to-turn, which reduces the local heating. Because the traditional minimum quench energy (MQE) was not suitable as a quench characteristic criterion for the NI closed-loop coils, the concept of final remained magnetic field retention ratio (FRMFR) was proposed for the first time. The FRMFR is determined by the ratio of the remained magnetic field to the original magnetic field, when the magnetic field of the coil returns to natural decay, indicating that the coil returns to steady-state operation. Based on the simulation model, the effect of turn-to-turn resistivity on the FRMFR was clearly explained. In addition, the influences of heat quench energy, quench location, and local insulation on the quench characteristics of the coil were studied systematically.

A non-destructive method is proposed for detecting the turn-to-turn resistivity distribution (TTRD) of non-insulation (NI) coils made of REBCO tapes. In conventional designs, TTRD is often estimated to be a constant, while it is actually non-uniform. It is crucial to detect the TTRD of NI coils more accurately as it determines the behaviour of NI coils and may lead to peculiar phenomena such as local reverse currents. The proposed approach involves acquiring the temporal change of voltage distribution during an excitation/demagnetization process, which is subsequently incorporated into a system of ordinary differential equations derived from an equivalent circuit model. A genetic algorithm (GA) is then employed to fit the collected time-varying voltage data and generate the results of the ‘measured’ TTRD. The system of equations can actually be numerically solved. The solved time-varying TTRD results are averaged over the measuring period, which serve as the initial value of GA fitting, and accelerates the fitting process. Virtual measurements were performed on an artificially established mock coil, demonstrating high accuracy in reproducing the predetermined TTRD. Furthermore, an actual measurement was also conducted on a single-pancake coil, however with unknown TTRD, using eight voltage measurement points during the demagnetization process. The measured TTRD was incorporated into the equivalent circuit model to predict the temporal changes in voltage and magnetic field of the coil under additional excitation/demagnetization conditions. By comparing the predicted results with the experimental data, a high level of agreement was observed, thus confirming the potential application of the proposed method.

High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance is generated in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for the time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time that the NI magnet has been exposed to the AC field, showing several qualitatively different waveforms (e.g. some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyze/explain these experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) the azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

The second generation (2G) high temperature superconductor (HTS) has excellent current-carrying density performance in strong magnetic field. Thus, the maglev system driven by 2G HTS magnets has been studied a lot. In the design of magnets, metal-as-insulation coil retains self-protection feature of the no-insulation coil, and it performs well in charging time. On the other hand, using solid nitrogen (SN ₂ ) as a “thermal battery” may effectively reduce the volume of on-board HTS magnets. In order to design on-board magnets with fast charging, small volume and self-protection, firstly a prototype coil was manufactured. Then the experiment apparatus was set up and experiments in the persistent current mode were carried out. It was obtained by fitting that the magnetic field decay rate of the prototype coil was about 0.24% per day. This paper provides a foundation for the production of real-scale on-board magnets cooled by SN ₂ .

Heat treatments are inevitable not only in the production of rare earth barium copper oxide (REBCO) tapes, but also in their post-processing for applications, typically, in soldering and epoxy/wax impregnation during the fabrication of REBCO coils. In general, the heat treatment of REBCO tapes should be carried out at lower temperature for a shorter time, but the specific safe boundary of heat-treatment temperature and time for REBCO tapes is still unknown. Therefore, a comprehensive study on the typical electrical, mechanical, and electromechanical characteristics of REBCO tapes after heat treatments under temperature of 250 °C is necessary. This work focus on the copper-encapsulated REBCO tapes, which are more robust (while with much lower engineering current density) to be processed in application systems than the tapes without encapsulation. The critical current degradation, stress–strain characteristic, and electromechanical properties of REBCO tapes were measured after heat treatments at different temperatures in argon and oxygen atmosphere. A 2D finite element (FE) analysis model was established for detailed stress/train analyzes under tension and bending based on the analysis of residual stress/strain. The results indicate that the critical current of the copper-encapsulated REBCO tapes decreases with increasing heat-treatment temperature and dwell time, and is of no evident relation to atmosphere. In addition, increased temperature of heat treatment leads to an obvious decrease in the yield strength and critical tensile stress. This effect is mainly attributed to the degradation of mechanical properties of the encapsulated copper layer, which is demonstrated by the combination of our FE simulation and the experiments results. Interestingly, the change in the critical bending radius due to heat treatments was slight, because the bending axial strain of the REBCO layer remained almost unchanged after heating. It is also worthy to note that all the properties tested in this study were irrelevant to the external oxygen partial pressure during the heating process. As a practical conclusion for the application systems, an upper and atmosphere-irrelevant limit of processing temperature of 130 °C or 150 °C (2 h dwell time) was proposed for copper-encapsulated REBCO tapes, under which the critical current, yield strength, critical tensile stress/strain and critical bending radius of the copper-encapsulated REBCO tapes decay by <1% or 3%, respectively.

It has been previously demonstrated that currents in HTS non-insulation (NI) coils exhibit a discrepant distribution during charging and discharging. However, for NI closed-loop coils, novel phenomena that arose from the current discrepancy have not been fully studied, especially during transient processes after charging and discharging, or more strictly speaking, after the persistent current switch returns to the superconducting state. For example, in our project of a prototype maglev system, a rapid decrease in the magnetic field was observed immediately after charging, which could be easily misinterpreted as a sign of local damage. However, the subsequent stability of the magnetic field with the expected decay rate of < 1 % d⁻¹ illustrated that there was no such damage. To address this issue, in this study, a NI closed-loop coil was fabricated and simulated by an equivalent circuit model (per 46 turns as a L–R circuit element and 20 elements in total for 920 turns) coupled with a 3D finite-element-method model. Simulated results were in reasonable agreement with experimental measurements, demonstrating a significant discrepancy in azimuthal currents during charging and discharging; this is essentially due to variations in inductances across each turn of NI coils. In addition, azimuthal currents may initially flow in the direction opposite to the coil voltage in turns with lower resistivity relative to other turns. In correspondence with the observed novel phenomena, after charging or discharging, azimuthal currents undergo redistribution until converging to a stable value, which exhibits a transient process and leads to a rapid decrease or increase of magnetic field in specific positions. The following issues were also discussed: (1) the judgement of operational current could be disturbed by the discrepant current distribution, due to its impact on the spatial magnetic field, not only for NI closed-loop coils but also for open-loop ones; (2) the non-uniformity of currents in NI closed-loop coils is lower, compared to that in open-loop ones with the same joint resistance and turn-to-turn resistivity distribution; (3) a new strategy with multiple charging processes is necessary for precisely charging NI closed-loop coils to the target current.

Null-flux Electro-dynamic suspension (EDS) system promises to be one of the feasible high-speed maglev systems above 600 km/h. On account of its greater current-carrying capacity, superconducting magnet can provide super-magnetomotive force that is required for null-flux EDS system and cannot be provided by electromagnets and permanent magnets. There is already a relatively mature high-speed maglev technology with low temperature superconducting (LTS) magnets as the core, which works in the liquid helium temperature region (T≤4.2 K). 2-Generation high temperature superconducting (HTS) magnet winded by REBa2Cu3O7−δ (REBCO, RE=rare earth) tapes works above 20 K region and do not need to count on liquid helium which is rare on earth. This paper designed HTS no-insulation closed-loop coils applied for EDS system and energized with persistent current switch. The coils can work at persistent current model and has premier thermal quench self-protection. Besides, a full size double-pancake module was designed and manufactured in this paper, and it was tested in liquid nitrogen. The double-pancake module’s critical current is about 54 A and it is capable of working at persistent current model, whose average decay rate measured in 12 hours is 0.58%/day.