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In our article, we show the n value determined from measurements using the magnetic field sweep method (curves of E from B/Bc) and the current sweep method (curves of E from I/Ic). The results from these two methods allowed for a scientific n value analysis. Moreover, measurements using these two methods allow for a better understanding of the behavior of superconducting wires after the transition from the superconducting state to the normal (resistive) state. Measurements were made for the NbTi wire at 4.2 K, MgB2 at the temperature range from 4.2 to 30 K, and high-temperature superconducting tape at 77 K. The magnetic field sweep method results show that the n value has a constant value independent of an increase in the magnetic field, number of filaments, type of boron, diameter of the wire, and small amount of the dopant. Moreover, research studies point out that the n value in the magnetic field sweep method depends on the type of superconductor material, a large amount of doping at high temperatures, and the low density of pinning centers. Based on the results obtained, it can be indicated that each superconducting material will have a constant n value. Measurements carried out using the current sweep method showed that the n value decreases significantly with an increase in the magnetic field. Moreover, research studies have shown that the n value depends on the number of filaments, type of boron, dopant, wire diameter, and temperature. On this basis, it can be indicated that the n value for the current sweep method depends on the morphology and structure of the superconducting wires. In addition, based on the samples measured in this research, the current sweep method allows for higher n values than the magnetic field sweep method in low magnetic fields. However, the magnetic field sweep method allows obtaining higher n values than the current sweep method in middle and high magnetic fields. [ABSTRACT FROM AUTHOR]

• A coupled electromagnetic-mechanical model is built to calculate the coupling behaviors of superconducting coils. • Mechanical deformation impacts the motion of magnetic flux and changes the critical current of superconductors. • The roles of contact conditions in the superconducting coils are derived theoretically. • A mechanical homogenization model is proposed to simplify the model containing hundreds of contact pairs. Rare-earth based barium copper oxide (REBCO) superconducting coated conductors are promising candidates for the design of high field magnets. In a high magnetic field, these conductors have to withstand huge Lorentz force, which would threaten the safety of superconducting devices. Recently, researchers have found that the electromagnetic-mechanical coupling has a significant impact on the overall response of the magnet system. It has been demonstrated that mechanical deformation can lead to the movement of magnetic flux in superconductors and changes in the critical current of superconductors. To take into account the electromagnetic-mechanical coupling effects, REBCO coated conductors are modeled as deformable conductors based on continuum mechanics of electromagnetic solids. Then, a fully coupled electromagnetic-mechanical model is developed in this paper, which is verified with the experimental results. Furthermore, during the electromagnetic-mechanical coupling simulation, contact separation occurs in REBCO pancake coils. To characterize the global mechanical performance of the REBCO pancake coils (containing hundreds of contact conditions), the concept of flexural rigidity in elastic shell theory is employed. Then, a mechanical homogeneous model is proposed to simplify the model containing hundreds of contact pairs and enable the effective simulation of large-scale HTS systems. This framework can provide theoretical support to the mechanical research of high field magnets. [ABSTRACT FROM AUTHOR]

Our research showed that a physical phenomenon appears in MgB2 wires, which has not been reported in previous studies. We have found that the flow of transport current in the current penetration depth of normal metal areas leads to the creation of voltage in magnetic fields from 0 to Bc1. Scanning electron microscope (SEM) images showed that normal areas can be formed at the junction between the superconducting material and the diffusion barrier. SEM photos indicate that this surface is very non-homogeneous and porous. This new physical phenomenon, that is, the voltage created by these normal areas at the current penetration depth is called, in this paper, the interfacial topological effect. Further measurements showed that above Bc1, the voltage disappears and becomes unmeasurable for magnetic fields above 120 mT. This is due to the appearance of a mixed state and current flow at a deeper depth of the superconducting material. From the interfacial topological effect, transport current flows on the outside surface of the superconducting MgB2 wires not only in magnetic fields from 0 to Bc1 but also in the mixed state and disappears in magnetic fields above 120 mT. This indicates that it is related to the superconducting state. The measurements performed by using a physical property measurement system (PPMS) for the low current (100 mA) indicate that the interfacial topological effect is on the boundary of two superconducting materials: the Nb diffusion barrier and the MgB2 material. In addition, the PPMS measurement results point out that the magnetic field eliminates the interfacial topological effect. Further results indicate that the appearance of voltage (the interfacial topological effect) in a MgB2 coil does not mean that the coil is not superconducting and cannot conduct the transport current without loss. Our results indicate that test procedures for MgB2 coils should assume exceeding the magnetic field of 120 mT because in this field, the voltage can disappear to zero and the transport current flows without loss. This is a very important factor for the future production of superconducting coils made of MgB2 wires with the Nb barrier on an industrial scale. [ABSTRACT FROM AUTHOR]

Flux pumps provide a promising solution for contactless charging of high-temperature superconducting (HTS) coils, eliminating the need for bulk current leads and reducing the heat burden for the cryogenic system. Characterizing the nonlinear effects of an HTS coil charged by a flux pump and understanding the dynamics of the charging process is essential for promoting the practical application of flux pumps. Numerical models provide a fast and cost-effective way of achieving this. In this study, we propose a methodology for coupling HTS coil and flux pump models using an electrical circuit, resulting in reduced computation costs. We validate the effectiveness of our approach against the experimental results of an HTS coil charged by a dynamo-type flux pump. Specifically, we obtain the voltage produced by the HTS dynamo using a 3D model based on the minimum electromagnetic entropy production method and apply this voltage to the load HTS coil using a T − A formulation finite-element method model coupled via an electrical circuit. The simulated charging current shows good agreement with experimental observations, validating our modeling strategy. The results demonstrate that the flux flow state in the HTS coil is the primary factor limiting the charging performance of the HTS dynamo as the charging current approaches the coil's critical current. Furthermore, based on the simulation, we demonstrate that, when using flux pumps, it is advisable to leave a margin between the operating current and the critical current of the coil. Overall, our approach has the potential to be applied to HTS coils charged by any device. [ABSTRACT FROM AUTHOR]

In high-temperature superconducting magnets, slow quench propagation of YBa2Cu3O7−δ coils makes the time delay of detection signal and active protection, resulting in the local heat accumulation and further coil degradation accompanied by strong mechanical response. Based on Maxwell's equations, the heat conduction equation, and basic equations of elasticity, we build and theoretically validate a two-dimensional axisymmetric electro-thermal-mechanical model for an insulated pancake coil with real dimensions in this work. Such a model has an important advantage that the simulation domain can be restricted to the coil itself by applying appropriate boundary conditions. The operating current of the coil is ramped up to a maximum and then remains unchanged. By using the model, we perform a systematic study of the quench and stress evolution triggered by various factors in the coil. The results indicate that the quench triggered by a heater is most likely to occur at the inner and outer turns. As the temperature keeps rising, the insulation layer near the heater is most prone to mechanical failure. Whereas if the quench is triggered by a local degradation caused by defects, the electromagnetic stress is dominant in the initial period, and after the initial period, the thermal stress will be dominant. The mechanical failure probably occurs at both ends of the insulation layer close to the degradation layer. In addition, the quench in an axial stack of pancake coils containing a local degradation is also analyzed. It is found that the position where the quench occurs first is mainly determined by the position of the local degradation and the number of the stack's layers. [ABSTRACT FROM AUTHOR]

The use of a low annealing temperature during the production of coils made from superconducting materials is very important because it reduces the production costs. In this study, the morphology, transport critical-current density (Jc), irreversible magnetic field (Birr), and critical temperature (Tc) of straight wires and small 2% C-doped MgB2 coils were investigated. The coils were made using the wind-and-react (W&R) method and annealed at various temperatures from 610 °C to 650 °C for 2–12 h. Critical-current measurements were made for both the coils and straight wires at the temperatures of 4.2 K, 20 K, 25 K, and 30 K. During our research study, we determined the process window that provides the best critical parameters of the coils (annealing at a temperature of 650 °C for 6 h). Moreover, we observed that small coils made with unreacted MgB2 wire and then annealed had morphology and critical parameters similar to those of straight 2% C-doped MgB2 wires. Moreover, small-diameter bending of 20 mm and 10 mm did not lead to transverse cracks, which can cause a large reduction in Jc in the coils. This indicates that the processes of optimization of thermal treatment parameters can be carried out on straight MgB2 wires for MgB2 superconducting coils. [ABSTRACT FROM AUTHOR]

The need for energy is growing as civilization develops. Superconductors have various benefits over regular conductors, including a high current-carrying density and nearly no resistance. Therefore, the objective that scientists continue to seek is the use of superconductors to replace traditional conductors in order to satisfy the demand for energy. As a key part of the superconducting electrical system, the critical current is one of the key properties of high-temperature superconducting (HTS) coils. Scholars have created a variety of numerical simulation models to estimate the performance of HTS coils. However, the extremely nonlinear E–J power law relationship that characterizes the voltage–current relationship in superconducting materials necessitates numerous iterative refinements during the coil design stage, which is time-consuming from the perspective of computing efficiency. Consequently, it is essential to increase computational efficiency. In this study, the critical current of HTS coils was calculated using the J model (integral method with the current density J as the state solution variable), and it was proved to be an efficient research method. The accuracy of the method is verified by comparing with the H model and the experimental measurement results of the critical current of the HTS coil. Moreover, compared with the reference finite element simulation model, this model has a speed advantage of at least four times, which is a good choice, especially, for HTS circular coils with large turns. [ABSTRACT FROM AUTHOR]

Alternating current (AC) loss measurement is crucial for the theoretical evaluation and optimization in the fabrication of superconducting AC devices. Lock-in amplifier based on the lock-in phase is commonly adopted and inevitably involves with phase error. In this work, a novel approach for measuring transport AC loss by determining the minimum compensated voltage (MCV) was developed, in which the lock-in amplifier was removed. Since it just uses the voltage signal from an AC voltmeter, it is phase-error immune. Experimental results demonstrated that when using the lock-in amplifier, there existed a system error as the initial phase difference between the sampled (reference) and real current phases, which required careful compensation. In contrast, the phase error no longer needed to be considered by the MCV method, and the AC loss results were obtained much more conveniently with the relative error between the theoretical and the experimental of less than 5%. Finally, the AC loss of a coated conductor spiral wound on an epoxy bar was obtained using the presented approach, demonstrating its low cost, ease of operation, and high accuracy. [ABSTRACT FROM AUTHOR]

In this paper, mechanical responses and electro-thermal characteristics of a rare earth barium copper oxide (REBCO) high-temperature superconducting (HTS) insulated pancake coil during the quenching process are investigated through finite element modeling (FEM). Firstly, a two-dimensional axisymmetric electro–magneto–thermal–mechanical FEM model with real dimensions is developed. Based on the FEM model, a systematic study on the effects of the time taken to trigger the system dump, background magnetic field, material properties of constituent layers, and coil size on quench behaviors of an HTS-insulated pancake coil is implemented. The variations in the temperature, current, and stress–strain in the REBCO pancake coil are studied. The results indicate that an increase in the time taken to trigger the system dump can increase the peak temperature of the hot spot but has no influence on the dissipation velocity. An apparent slope change of the radial strain rate is observed when the quench occurs regardless of the background field. During quench protection, the radial stress and strain reach their maximum values and then decrease as the temperature decreases. The axial background magnetic field has a significant influence on the radial stress. Measures to reduce peak stress and strain are also discussed, which indicates that increasing the thermal conductivity of the insulation layer, copper thickness, and inner coil radius can effectively reduce the radial stress and strain. [ABSTRACT FROM AUTHOR]

Single-turn coil (STC) is a destructive pulse magnet aiming at a 100–300 T ultra-high magnetic field. A transient discharge circuit model considering the coupling of electromagnetic diffusion and conductor deformation is proposed, and the transient coil impedance characteristics are investigated. The results show that the coil resistance first decreases and then increases due to electromagnetic diffusion and temperature rise, respectively, while the coil inductance always increases because of the conductor's outward motion. By comparison, the simulation results are consistent with the experimental data, and the correctness of the model is validated. [ABSTRACT FROM AUTHOR]

The article is focused on the electromagnetic analysis of the toroidal field superconducting coils (TFCs) of the Divertor Tokamak Test facility (DTT) when electrical transients occur in the TFCs system: for example, during the operations of the fast discharge units (FDUs) and considering also, the simultaneous occurrence of a fault condition. During the FDU intervention, a transient voltage excitation lasting few milliseconds occurs at the TFC terminals and it electrically stresses the insulations of TFCs itself. To investigate the voltage distribution across, inside and between different double pancakes (DPs) of each TFC, a lumped parameters circuital model has been developed and implemented in Ansys Simplorer simulation environment. This model includes both the detailed submodel of each TFC and FDU. The transient analyses have been carried out for two different scenarios: a reference one and a failure scenario, considering three different fault resistance values and also two different values of the resistance connecting the TF case to ground. In order to verify the correct sizing of the coil insulation and the TF case-to-ground resistance value inserted in the circuit of each TFC, the voltages of each TFC (terminal-to-terminal, terminal-to-ground, across of adjacent DPs, and between other circuit points) were computed in the time domain (in the range of milliseconds) for both scenarios. An overview of calculations and simulation results is presented and discussed, pointing out a high sensitivity of fault conditions and of overvoltage values and addressing mitigation strategies. [ABSTRACT FROM AUTHOR]

A saturated iron-core type superconducting fault current limiter (SI-SFCL) can effectively restrict the magnitude of the fault current and alleviate the strain on circuit breakers in DC power systems. Design of a superconducting coil (SC), which is one of the key tasks in the SI-SFCL design, requires guaranteeing a sufficient magnetic field, ensuring optimization of the shape and size, minimizing the wire cost, and satisfying the safety and stability of operation. Generally, finite element method (FEM) is used to calculate and evaluate the operating characteristics of SCs, from which it is possible to determine their optimal design parameters. When the coil is complex and large, the simulation time may range from hours to days, and if input parameters change even slightly, the simulations have to be redone from scratch. Recent advances in deep learning represent the ability to be effective for modeling and optimizing complex problems from training data or in real-time. In this paper, we presented a combination of the FEM simulation and deep Q-network (DQN) algorithm to optimize the SC design of a lab-scale SI-SFCL for a DC power system. The detailed design process and options for the SC of SI-SFCL were proposed. In order to analyze the characteristics related to the electromagnetic properties and operational features of the SC, a 3D FEM model was developed. Then, a DQN model was constructed and integrated with the FEM simulation for training and optimizing the design parameters of the SC in real-time. The obtained results of this study have the potential to effectively optimize the design parameters of large-scale SI-SFCL development for high-voltage DC power systems. [ABSTRACT FROM AUTHOR]

Superconductivity is a remarkable state of matter exhibited by some materials, which is characterized by the vanishing of electrical resistivity at cryogenic temperatures, among other phenomena. However, the prospective economical viability of superconducting devices relies heavily on an efficient cooling system to remove the heat generated during operation. Although this would be meaningless in conventional devices, it occurs at cryogenic temperatures, impacting the efficiency and cost of the whole system. Thus, it is necessary to predict the losses of, e.g., superconducting coils in power systems. This article presents a study of ac losses in superconducting coils due to noncharacteristic current harmonics. These harmonic components are typical in electromechanical drives like the one presented in this article. A computational model of high-temperature superconducting coils and the numerical procedure used to solve the problem are described in this article. The contribution of the noncharacteristic harmonic components in these ac losses will be presented. These results, obtained by computational analysis, were validated in experimental tests that assessed the power dissipated due to triangular waveforms currents. [ABSTRACT FROM AUTHOR]

An efficient two dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors. In the approach, a thin strip approximation of the superconductor is used in which the superconducting layer is modeled as a 1-dimensional domain. The formulation is mainly based on the calculation of the current vector potential T in the superconductor layer and the calculation of the magnetic vector potential A in the whole space, which are coupled together in the model. Compared with previous T-based models, the proposed model is innovative in terms of magnetic vector potential A solving, which is achieved by using the differential method, instead of the integral method. To validate the T-A formulation model, it is used to simulate racetrack coils made of second generation high temperature superconducting (2G HTS) tape, and the results are compared with the experimentally obtained data on the AC loss. The results show that the T-A formulation is accurate and efficient in calculating 2G HTS coils, including magnetic field distribution, current density distribution, and AC loss. Finally, the proposed model is used for simulating a 2000 turn coil to demonstrate its effectiveness and efficiency in simulating large-scale 2G HTS coils. [ABSTRACT FROM AUTHOR]

Epoxy impregnated superconducting coils have better structural integrity and thermal stability. However, for REBa 2 Cu 3 O 7 − δ (REBCO, RE=Rare earth) coils, the mismatch of thermal expansion coefficients between epoxy and REBCO tapes is a serious problem. In this work, the temperature distribution, stress evolution during the cooling process, critical current distribution, and delamination sites inside REBCO coils impregnated using Stycast 2850FT have been studied. We measured the temperature distribution and the hoop strain in the penultimate turn of impregnated coils during the cooling process and analyzed the thermal stress evolution. No damage was observed for coils with the ratio between outer and inner diameter R o / R i < 1.93. The delamination behavior occurred in coils with R o / R i > --> 2.37 , where the coils even exhibited a two-stage delamination. The delamination mechanism of REBCO coils was proposed from three aspects: the mechanical analysis, the critical current degradation, and the microscopic analysis. In this work, we found that the actual delamination behavior may appear earlier than the steady-state temperature, and temperature distribution will push the radial stress peak toward the inner radius of the coil, making the inner turn more susceptible to delamination. Multiple delamination locations were accurately predicted and confirmed. The measured compressive hoop strain first increased from − 4212 μ ε to − 4684 μ ε with the increase in R o / R i and then decreased to − 3835 μ ε obviously due to delamination. This work reveals in detail the delamination mechanism in impregnated REBCO coils, which is of great significance for the development of damage-free coils. [ABSTRACT FROM AUTHOR]

The article discusses the challenges of traditional points-based grading systems in higher education and the growing popularity of alternative grading systems. The author describes their experience transitioning to a hybrid standards and specifications-based grading system over three years, focusing on Year 1 when lab sections were assessed with specifications-based grading. The lab specifications emphasized communication and process skills over correctness, with a focus on clear standards, helpful feedback, marks indicating progress, and reassessment without penalty. Student feedback on the specifications system for labs was positive, highlighting a shift towards understanding the material rather than relying on others. [Extracted from the article]

Larger fusion experiments require long beam paths for laser diagnostics, which requires mechanical stability and measures to deal with remaining alignment variations. At the same time, due to technical and organizational boundary conditions, calibrations become challenging. The current mid-sized experiments face the same issues, yet on a smaller scale, which makes them ideal testing environments for novel calibration methods, since a comparison with the established best practices is still possible. At the stellarator Wendelstein 7-X, the calibration and operation of the Thomson scattering diagnostic is hampered by beam displacements, coating of windows during operation, and access restrictions while the superconducting coils are active. New calibration techniques were developed to improve the profile quality and reduce calibration time. While positional variations of the laser beam have to be minimized, the remaining displacements can be accounted for during the absolute calibration. An in situ spectral calibration has been developed based on Rayleigh scattering, which calibrates the whole diagnostic, including observation windows. In addition, a less accurate but faster method has been developed, which utilizes stray-light of a tunable OPO to perform spectral calibration within minutes and does not require torus hall access. Finally, a workflow has been established to consider finite linewidths of the calibration source in the spectral calibration. While these methods will be used at W7-X to complement existing calibration techniques, they may also solve some of the aforementioned issues expected for even larger and nuclear experiments, where access restrictions are stringent and calibration becomes even more demanding. [ABSTRACT FROM AUTHOR]