High-temperature Superconducting System Research Articles

Modeling of contact resistivity and simplification of 3D homogenization strategy for the H formulation

The finite element method (FEM) provides a powerful support for the calculations of superconducting electromagnetic responses. It enables the analysis of large-scale high-temperature superconducting (HTS) systems by the popular H formulation. Nonetheless, modeling of contact resistivity in three-dimensional (3D) FEM is still a matter of interest. The difficulty stems from the large aspect ratio of the contact layer in numerical modeling. Nowadays, an available solution is to model the contact layer with zero thickness but requires the discontinuity conditions of the magnetic field. In this paper, the energy variational method is utilized to incorporate the contribution of contact resistivity into the H formulation. From the perspective of energy transfer, the contact resistivity is related to the energy dissipation of the radial current flowing through the contact interface. In terms of applications, this method can be employed to calculate the charging delay of no-insulation coils and the current sharing behaviors of CORC cables. One advantage of this model is that the magnetic field is continuous and hence can be easily implemented in FEM. Additionally, it requires fewer degrees of freedom and hence presents advantages in computational efficiency. Moreover, this method can be employed to simplify the 3D H homogeneous model for insulated coils. The above discussions demonstrate that the proposed model is a promising tool for the modeling of contact resistivity.

Nonlinear motion characteristics of the high-temperature superconducting levitation system with boundary crack

High-temperature superconducting levitation systems have nonlinear behaviors, such as the period-doubling bifurcation and chaotic vibration under external excitation, in connection with the nonlinear hysteresis interaction between the high-temperature superconductors (HTSC) and permanent magnet. The HTSC is a non-ideal type II superconductor in which the fracture is generated internally during manufacturing, and the material properties are brittle. The HTSC tends to crack under a strong magnetic field and electromagnetic force, and its internal defects are likely to cause structural damage with the variable Lorentz force. The fracture performance of the superconducting magnetic levitation system will affect the temperature variation of HTSC, in which the temperature is coupled with the internal electromagnetic force. In this paper, we analyze the fracture characteristics of the HTSC in nonlinear vibration with thermal effect. A superconducting magnetic levitation system model with boundary cracks is applied to study the coupling of multiple physical fields in dynamic processes. The Maxwell equation and superconducting electromagnetic constitutive equation are used to obtain the superconductor’s current density, and the temperature is calculated through the heat conduction equation. The superconducting magnetic flux flow and creep model is applied to analyze the magnetic flux motion inside the superconductors. We compare the superconducting levitation system’s electromagnetic force, temperature, and current density with four critical current densities. The numerical results show that the critical current density of superconductors significantly impacts bifurcation motion, and the temperature of the crack tip of the superconductors varies greatly during vibration, causing the superconductor to lose its superconductivity.

Levitation Force Compensation Study for HTS Maglev System Based on the Input of Electromagnetic Force

The advantages of levitation and guidance integration make the high-temperature superconducting (HTS) maglev system have great potential in the field of high-speed transportation. However, due to the influence of some factors such as the Dewar (a low temperature thermostat for superconductors) vacuum increases and magnetic field irregularity, the levitation force will decline and working height will drop. To solve the problems, this paper proposes a compensation method based on electromagnetic auxiliary device. Firstly, the thought and purpose of this method are explained in detail. Secondly, the effectiveness of levitation force compensation is verified by experiment. Then, electromagnetic simulation model is established and its correctness is verified by comparing simulation results and experimental data. Finally, dynamic simulation is conducted based on the verified model to study the vibration characteristics of Dewar system. The results show that the auxiliary device can compensate the levitation force and suppress vibration well. Meanwhile, comparing with the field cooling height (FCH) at 20 mm and 30 mm, the compensation effect of former is better than latter, but the overall maximum levitation force of latter is higher than former. This research provides an economical and effective way to achieve the levitation force compensation of HTS maglev system. Furthermore, the electromagnetic auxiliary device is an active energy input port, providing the possibility for the active control of HTS system. It is expected to provide reference for future practice in relevant application fields.

Electronic Structure of InCo2As2 and KInCo4As4: LDA + DMFT

A comparative analysis of the electronic structure obtained in the DFT/LDA and LDA + DMFT approaches of the possible isostructural analogues of iron superconductors InCo2As2 and KInCo4As4 with the electronic structure of the parent high-temperature superconductor system BaFe2As2 is carried out. It is established that in spite of the rather large value of the electron-electron correlations (local Coulomb interaction on the Co-3d shell U = 4.0 eV, the Hund exchange interaction J = 0.85 eV), in the considered systems a relatively small quasiparticle mass renormalization 1.2–1.35 at the Fermi level is observed. The correlation effects lead to the remarkable shift and compression of the spectrum below –0.8 eV. The band structure of InCo2As2 near the Fermi level is qualitatively similar to the previously studied BaCo2As2, and differs significantly from the band structure of BaFe2As2. In the KInCo4As4 system, the bands near the Fermi level resemble the band structure of BaFe2As2, and the Fermi surfaces have a similar topology. This indirectly points to the possibility of superconductivity in KInCo4As4. Also according to the results of LDA + DMFT calculations it is seen that with a rather small hole or electron doping in the KInCo4As4 system will experience topological Lifshitz transitions. We believe that the synthesis of the InCo2As2 and KInCo4As4 compounds considered in this paper is important for the study of superconductivity in this class of materials.

Coupling electromagnetic numerical models of HTS coils to electrical circuits: multi-scale and homogeneous methodologies using the T-A formulation

Numerical simulation is an effective tool for predicting the electromagnetic behavior of superconductors. Recently, a finite element method-based model coupling the T-A formulation with an electrical circuit has been proposed: the model presents the superconducting constituent as a global voltage parameter in the electrical circuit. This allows assessing the overall behavior of complex high-temperature superconductor (HTS) systems involving multiple power items, while keeping a high degree of precision on the presentation of local effects. In this work, the applicability of this model has been extended to large-scale HTS applications with hundreds or thousands of tapes by referring to two widely recognized methodologies, multi-scale and homogenization, to improve the computation efficiency. Based on the two approaches, three different models were developed and their effectiveness was assessed using the case study of a 1000 turn cylindrical HTS coil charged by a DC voltage source. The comparison of the calculated global circuit parameters, local field distributions, losses, and computation time proves that the computation efficiency can be improved with respect to a model simulating all HTS tapes, without compromising accuracy. The results indicate that the developed models can therefore be efficient tools to design and optimize large-scale HTS devices used in electrical machines and power grids. It is also found that the inductance of an HTS coil is varied according to the transport current and can be even higher than that of a normal conductor coil with the same geometry. We attribute this result to the superconductor’s non-uniform current distribution and relaxation effect during the dynamic process.

Mechanics of dielectric polyamide/ SiO 2 nanocomposite coatings for cryogenic service in superconductor systems

Mechanics of dielectric polyamide/ <scp> SiO ₂ </scp> nanocomposite coatings for cryogenic service in superconductor systems

Vibration Suppression of HTS Maglev System Based on Negative Resistance Electromagnetic Shunt Damper

The high-temperature superconducting (HTS) maglev system has tremendous possibilities for future high-speed transportation application owing to its salient advantages of self-stable levitation and zero pollution operation. However, previous study indicated that the relatively low damping of HTS system may lead the undesired large-amplitude nonlinear vibration in system. Thus, electromagnetic shunt damper (EMSD), which usually consists of electromagnets, permanent magnets and shunt impedance connected to the terminals of the coils, is introduced in the system to improve the damping of the system. The effect of EMSD is to transfer the mechanical energy to electrical energy and consume it in the form of Joule heat. For better vibration suppression performance, this paper proposes a negative resistance electromagnetic shunt damper (NR-EMSD) in HTS maglev vehicle model. The negative resistance can offset the inherent resistance of the electromagnet coils and as a consequence, the induced current of the circuit will increase as well as the induced electromagnetic force. Also, the change of total resistance of the circuit will influence the vibration reduction performance. In this paper, the effect of NR-EMSD is tested by experiment under different working conditions compared with the model without EMSD. The results are analyzed and basic conclusions are summarized. This work may provide references for further practical application in HTS system.

Calculations of the AC losses in superconducting cables and coils: Neumann boundary conditions of the T–A formulation

Numerical techniques play an important role in the design of high-temperature superconductor (HTS) systems. In the superconductivity community, the T–A formulation of Maxwell’s equations and its homogeneous technique have become popular in recent years. The T–A formulation has the capability of simulating HTS systems and high computational efficiency. However, it is still difficult for the T–A formulation to solve some special problems. For instance, the net current is not explicitly known in each HTS tape. In the present work, the contributions of the Neumann boundary condition are studied, which represent a coupling effect between the T and the A formulations. This paper firstly describes the Neumann boundary condition in detail. Then, based on the T–A formulation and its Neumann boundary condition, the non-uniform current distribution in the cables and the current decay in the closed-loop coils are respectively analyzed. This method can solve the difficulties of the T–A formulation in calculating some specific problems, and extend the application range of the T–A formulation. Furthermore, the above supplement is also applicable to the homogeneous and the three-dimensional (3D) models.

Potential Applications of MTS and HTS to MRI Imaging Systems for Particle Beam Therapy

<h3>Purpose/Objective(s)</h3> To explore the potential application of medium temperature superconductors (MTS) and high temperature superconductors (HTS) to the development of background magnets for MRI systems for use in combination with particle beam therapy (PBT) and the concept design for MRI-guided PBT. <h3>Materials/Methods</h3> In proton/heavy ion therapy, because of the sharp dose drop off owing to the Bragg peak effect, imaging guidance becomes crucial. Various imaging techniques have been considered, but MRI, with its soft tissue contrast, 3D imaging, high spatial resolution, and potential on-live monitoring feature would be particularly useful in combination with PBT. One of the key challenges is that the charged particle beam will be deflected in a transverse magnetic field. The deflection of the particle beam in the presence of the background B0 has received significant interest, and both in-line and transverse configurations have been considered. For in-line configurations, smaller deflections are seen, with some rotation, and for transverse, greater deflection, and momentum change. With the use of a pencil beam PCT approach in combination with computation, such deflections can in principle be compensated. However, if "real time" imaging is to be enabled, minimum computation and reconstruction time is desirable. For these reasons, minimizing and simplifying the deflection is desirable. This can be enabled with the use of the in-line configuration, but in this case an in-line system compacts enough to mount on a gantry may be of interest. This can be made more achievable with the use of MTS or HTS where less bulky magnets are possible, cooling is simplified, and systems are much less quench sensitive. At the same time, the minimization of fringe fields can also substantially simplify the particle beam trajectories and minimize their deflection. This may also be enabled for open configuration transverse configurations with the use of less bulky HTS and MTS magnets. For this work we performed magnetic modelling of several different configurations of MTS and HTS magnets, both in-line and transverse, using Vector fields and other modelling software. The sizes and configurations of the MTS and HTS systems as compared to potential competing LTS systems were then compared. Particle beam trajectories were then computed with GEANT software and compared. <h3>Results</h3> (1) MTS and HTS systems can allow the development of in-line MR which are sufficiently compact, open, and robust (to quench) to allow mounting on a gantry. This reduces particle beam deflection, while at the same time allowing rotation of the beam around the patient; (2) Several designs for minimized fringe fields are given, both for in-line and transverse configurations. <h3>Conclusion</h3> MTS and HTS have a significant potential for application to MRI-guided Particle Therapy based on their potential for mounting on a gantry, and enabling of more open background magnet configurations. Magnet designs with minimized fringe fields are also possible and useful.

Design and Testing of a New Cooling System using Solid Nitrogen for Pulsed Field Magnetization and Characterization of HTS Bulks

Solid Nitrogen (SN2) can provide a uniform and stable cryogenic environment for High Temperature Superconducting (HTS) systems such as bulk samples during their magnetization and/or characterization. In this paper, a SN2 cooling system consisting of a cryocooler Sumitomo CH-110 and an exchanger in a Liquid Nitrogen (LN2) bath is studied. In order to design this cooling system, an analytical model based on a nodal method coupled to a formulation of the thermal capacity was realized. The model considers the thermal parameters variation as well as the phase change of the Nitrogen. In order to compare our results, we performed a 3D simulation on COMSOL Multiphysics. The performance of the cooling system was evaluated by measurements and we estimate that 10 L of LN2 can be cooled down to 25 K in 45 hours.

Two-band conduction and nesting instabilities in superconducting Ba2CuO3+δ : First-principles study

First principles investigations of the high temperature superconducting system Ba$_2$CuO$_{3+\delta}$, recently discovered at $\delta\approx0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed `highly over-doped' superconducting phase displays stretched Cu-planar oxygen O$_{\rm P}$ distances and anomalously shortened Cu-apical O$_{\rm A}$ separations compared with other cuprates. The stoichiometric system $\delta=0$, with its strongly one-dimensional (1D) Cu-O$_{\rm P}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-O$_{\rm P}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like Sr$_2$CuO$_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $\delta$=0. We find that a `doubled bilayer' structure of alternating Cu-O$_{\rm P}$ chains and wide rung Cu$_3$O$_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-O$_{\rm P}$ chains in the superconducting phase $\delta=1/4$. Nominal formal valences of the three Cu sites are discussed. The six-fold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.

Development of a Cryogen-Free Compact 3 T Superconducting Magnet for an Electromagnetic Property Measurement System

A cryogen-free portable 3 T high-temperature superconducting magnet for an electromagnetic property measurement system has been developed to serve as a user facility at the Korea Basic Science Institute. The metallic insulation method was adopted to reduce the charging delay without sacrificing the self-protecting feature. A genetic-algorithm-aided optimized design was carried out to minimize the superconducting tape consumption while satisfying several design constraints. After the design, the compact high-temperature superconducting magnet composed of eight double-pancake coil modules was wound with high-temperature superconducting tape and stainless steel tape, and integrated with a two-stage cryo-cooler. The 3 T magnet was successfully cooled to approximately 20 K with a cryo-cooler and reached the target field of 3 T without any problems. Long-term measurements and a range of other tests were also implemented to verity the performance of the magnet. Test results demonstrated the feasibility of a cryogen-free portable high-temperature superconducting magnet system for electromagnetic property measurement experiments.

Advanced electromagnetic modeling of large-scale high-temperature superconductor systems based on H and T-A formulations

The development of the high-temperature superconductors (HTS) has allowed the emergence of diverse superconductor devices. Some of these devices, like wind power generators and high-field magnets, are classified as large-scale HTS systems, because they are made of several hundreds or thousands of turns of conductors. The electromagnetic analysis of such systems cannot be addressed by means of the available analytical models. The finite-element method has been extensively used to solve the H formulation of the Maxwell’s equations, thus far with great success. Nevertheless, its application to large scale HTS systems is still hindered by excessive computational load. The recently proposed T-A formulation has allowed building more efficient models for systems made of HTS tapes. Both formulations have been successfully applied in conjunction with the homogenization and multi-scaling methods, these advanced methods allow reducing the required computational resources. A new advanced method, called densification, is proposed here. The most important contribution of this article is the comprehensive comparison of the strategies emerged from the combined use of the two formulations and the three advanced methods.

Quantitative Comparison of LDA + DMFT and ARPES Spectral Functions

The emergence of angle-resolved photoemission spectroscopy (ARPES) made it possible to observe electronic dispersion directly as a spectral function map. On the other hand, a spectral function map can be obtained theoretically, for example, in the LDA+DMFT method. The electronic band on such a map is characterized not only by its energy position at a given $k$-point, but also by its width and intensity. To illustrate a way of quantitative comparison of theoretical spectral functions and ARPES data, spectral functions obtained by the LDA+DMFT method are chosen. It is shown that the theoretical spectral functions should take into account a number of experimental features: the photoionization cross section, the experimental energy and angular resolution, as well as the effects of the photohole lifetime arising in the process of photoemission. In this article, we present a robust procedure for taking these experimental features into account by the example of iron-based high-temperature superconductors (HTSC) systems: NaFeAs and FeSe on a SrTiO$_3$ substrate.

Influence of copper damper on bifurcation vibration in superconductive magnetic levitation system

With proper external vibrations, the dynamic behavior of a high-temperature superconducting levitation system will show interesting bifurcation and complex chaos. In order to eliminate the effects of nonlinear vibrations, a larger magnetic force generated by a high-temperature superconductor (HTSC) with strong flux pinning is necessary. However, it provides minimal inherent damping. In this article, we numerically studied the effect of different copper damper arrangements on the superconductive magnetic levitation system in nonlinear vibration by the finite element method. Furthermore, we compared the results with and without a copper damper by analyzing the displacements, temperature differences, and electromagnetic forces (HTSC and copper). The results indicate that adjusting the copper damping can increase the interference rejection capacity of the superconductive magnetic levitation system.

Chaos and period-doubling vibration in superconducting levitation systems

For high-temperature superconducting levitation systems, chaotic vibrations occurred under external disturbance, as shown in previous studies. The electromagnetic characteristic of a high-temperature superconductor (HTSC) plays a crucial role in the dynamic stability of levitated systems. In this article, we numerically investigate the HTSC-PM levitation system in nonlinear motions by the finite element method. We analyzed the displacements, temperature, electromagnetic forces, and magnetic field of the HTSC. The results show that the nonlinear vibration transitions from chaos to period-doubling vibration with the HTSC’s temperature and magnetic field change. Furthermore, we select two states from chaos and period-doubling to analyze the natural frequencies. The results show that the electromagnetic characteristics of the HTSC affect the natural frequency of the system.

Comparison of fast computing methods for ZFC High-temperature superconductors bulks

With the development of high-temperature superconducting (HTS) materials, several HTS applications are been proposed and developed. Due to the HTS non-linear and non-homogeneous electromagnetic characteristics, the optimization process of an HTS system is a complex task requiring high computational time and resources. Within this context, this paper focuses on the comparison of HTS models for a fast estimation of the magnetization level of zero-field cooled HTS bulks. Four HTS models are analyzed: Kim’s model, Bean’s model, a simplified analytical model considering J = Jc and an equivalent permeability model. As a case study, these models are used to simulate the levitation forces of a horizontal axis HTS levitation system. A 3D Finite Element model is used as a base tool for the implementation of each model. Results are compared with the experimental ones. The simplified analytical model (J = Jc) and the equivalent permeability model present accurate enough results with a very low computational time when compared with Kim’s and Bean’s models. For the 3D simulation of the levitation system, a computational time between 10 and 24 h is required for the Kim’s and Bean’s models, while only between 10 and 30 s for the simplified analytical and equivalent permeability models. Besides, the simplified analytical model (J = Jc) also provides information about the eddy-current losses inside the HTS bulks.

Analysis to the Forced Vibration of a High Temperature Superconducting System With Hysteresis

Dynamic behavior with the hysteresis of the levitation force of a high temperature magnet-superconductor system is investigated with application of a displacement harmonic excitation. The dynamic characteristics of the transient stage and the steady stage of forced vibration of the system are discussed in detail. The period-doubling and the natural frequency components are found in forced vibration. It means that the initial frozen magnetic field in superconductor plays an important role on the vibration response. The vibration amplitude vs. excitation frequency is presented. A transition excitation frequency with triple natural frequency is given to distinguish the range of vibration center. The vibration amplitude of the superconducting levitation system shows a linear increasing with the excitation amplitude, which is found to be independent on the critical current density with considerations based on the Bean's critical model. However, the critical current density greatly affects the location of the vibration center. The critical excitation amplitude vs. the field frequency to the superconductor is presented. Levitation is stable most when the sample is exposed to a low-frequency field. In this case large amplitude of the alternating field is needed to break-off of the sample.

Numerical study on natural convection of liquid nitrogen used to cool the high-temperature superconducting cable in a new combined energy transmission system

In a new high-temperature superconducting (HTS) electricity transmission system, natural convection of liquid nitrogen in the annular channel is caused by the inside wall heating and outside wall cooling. Two-dimensional numerical simulation of the natural convection is conducted using a finite volume method. The dimensionless eccentricity, ε∗, in the range of −1.0 to 1.0 is considered when the Rayleigh numbers (Ra) varied from 2 × 105 to 1 × 107. A bifurcation map of flow pattern is obtained by many numerical simulations. It is found that there are three typical flow regions in the fully developed stage, i.e. steady flow, periodic flow and chaotic flow. When the Ra is relatively low, the flow is steady and uniform. As the Ra increases, the reinforced natural convection approaches a time-dependent periodic flow and further a strongly chaotic flow. Furthermore, when the ε∗ is up to 0.6, a cluster of Rayleigh-Bénard convection cells appears on the top of the flowing channel, and enhances the heat transfer slightly. The effect of the ε∗ and Ra on the heat transfer of natural convection is analyzed quantitatively. It is demonstrated that the most suitable region for the vertical position of the HTS cable is for a dimensionless eccentricity in the range of −0.8 to −0.6 to enhance the heat transfer while guaranteeing the safety of the HTS system.

Enhanced superconducting properties of multi-wall carbon nanotubes added YBCO-123 Superconducting System

We investigated multi-wall carbon nanotubes (MWCNT’s) adding in Y1Ba2Cu3Ox high temperature superconductor system. We used solid-state method for sample preparation and 0.0 < x < 5.0 wt% MWCNT’s were added to the main matrix. Structural evolution, electrical transport and magnetic properties of material produced was investigated. According to the results obtained the superconducting onset temperature, Tconset, was not changed excessively, however, it was observed that the transport critical current density, Jctran, increased by at least 6-fold compared with the pure YBCO samples. Magnetic properties of material were also investigated by means of magnetization measurements up to 9 T together with the I–V measurements and pinning force, Fp, calculations.