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It is crucial for the successful application of high-temperature superconducting (HTS) power transformers that they are able to survive under abnormal operations, such as a short-circuit fault. Modeling, based on the design of a four-winding three-phase HTS power transformer, suggests severe boiling will occur on the surface of the winding during a short-circuit fault and will jeopardize the turn-to-turn insulation of the HTS power transformer. In this paper, a turn-to-turn insulation breakdown experiment platform involving the introduction of boiling on the surface of electrode has been built. Nomex 410 paper, one of the candidates for turn-to-turn insulation in the design of the HTS power transformer, was tested. The characteristics of dielectric strength of Nomex 410 in three different thicknesses (0.13, 0.18, and 0.25 mm) have been investigated, without boiling, as well as under different boiling situations, in an open bath of liquid nitrogen. The voltage application methods applied during both experiments test the electrical breakdown of Nomex 410 under a dynamic situation. The experiment results show that when under film boiling, compared to the test results for nucleate boiling, the dielectric strength of Nomex 410 paper decreases by approximately 38% of the value when no boiling is involved. Moreover, it is verified that the presence of an electric field accelerates the heat transfer between the liquid nitrogen and the heated electrode. [ABSTRACT FROM PUBLISHER]

High gradient magnetic separation (HGMS) for purification of wastewater from paper factory has been developed and withstand for 1 year operation. The system developed is a high gradient magnetic separation (HGMS) system with superconducting magnet which purifies wastewater continuously from a paper mill. The magnetic field generated by the superconducting magnet for HGMS is 2 T at the center of the bore, operating current 100 A, 680 mm long and 400 mm bore NbTi solenoid. The developed system which treats wastewater of 2000 t/day has been confirmed to show the expected performance. The purpose of the developed system is to separate COD constituents from the wastewater efficiently and economically in a small space. The required performances are follows, i) the set-up area is less than 6 m × 6 m, ii) COD of the treated water is less than 100 ppm and water can be recycled, and iii) a system can treat wastewater more than 2000 t/day. The design and the performance of the system are reported. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) inception characteristics of liquid nitrogen (LN2 )/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables were investigated in terms of the volume effect and the V-t characteristics. The electrical and optical measurements of PD inception characteristics showed that initial PD could be generated between PP laminated paper layers, as well as in a butt gap. Using a parameter called statistically stressed liquid volume (S SLY) based on the discharge probability in both butt gaps and LN2 filled thin layers between PP laminated papers, we could systematically analyze and evaluate the volume effect on PD inception stress (PDIE). Furthermore, experimental results revealed that n values of V-t characteristics at PD inception were as high as 80-110. On the other hand, the lower n values obtained at breakdown were interpreted by the intensified PD development in thermal bubbles generated after the PD inception. [ABSTRACT FROM AUTHOR]

This paper proposes a way to represent the anisotropic nonlinear superconducting properties using a discrete model for magnetization with the corresponding critical parameters, equivalent to the Power Law model for currents. This simplified model makes it possible to analyze magnetic systems with superconducting elements in a stationary mode, which reduces the calculation time. Using of proposed model is demonstrated by the calculation of force interactions between high-temperature superconducting (HTS) samples and permanent magnet in zero-field-cooling (ZFC) and field-cooling (FC) modes. HTS bulks in the form of a prism and stack made of HTS tape are considered. The calculation of the electromagnetic field was carried out using the method of integral equations for field sources. The data of calculations are compared with the results of experimental studies. [ABSTRACT FROM AUTHOR]

While much attention was given in the literature to improve the fault ride through (FRT) capability of a doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS) during various faults at the grid side, not much attention was given to investigate and improve the performance of the DFIG-WECS under ferroresonance conditions. This paper investigates the impact of ferroresonance on the overall performance of a DFIG-WECS. The paper also presents a new technique to improve the high voltage ride through capability of the investigated system under such condition. The proposed technique relies on incorporating a superconducting coil within the dc-link of the DFIG grid-side and rotor-side converters. Energy exchange between the coil and the system is controlled through regulating the duty cycle of the dc-chopper interfacing the coil with the dc-link. In this regard, a simple but yet effective proportional-integral (PI) controller is employed to regulate the dc-chopper duty cycle. Parameters of the PI controller are optimized using elephant herding optimization algorithm. Simulation results verify the effectiveness of the proposed controller in improving the overall performance of a DFIG-WECS during ferroresonance events. [ABSTRACT FROM AUTHOR]

This paper presents an experimental method for investigating the dependence of critical current on the applied magnetic-field intensity and the angular orientation in high-temperature superconductor (HTS) tapes and cables. Comparisons between critical-current measured and simulated data are presented as a function of field angular orientation. The obtained relation can be used as input data for numerical calculations or simulations for designing application devices that utilize HTS tapes and cables. [ABSTRACT FROM AUTHOR]

The High Luminosity LHC (HL-LHC) Project is planning to install 16 cold-masses made with Nb3Sn quadrupole magnets in the LHC Interaction Regions to significantly increase its luminosity. Half of these cold masses are fabricated at BNL, FNAL, and LBNL under the US Accelerator Research Program (AUP). Each cold mass includes two identical Nb3Sn quadrupole magnets, called MQXFA with a magnetic length of 4.2 m. Currently, the AUP project has completed the fabrication and test of the first 5 pre-series magnets, and is working on the following 16 magnets for the series production. The brittleness and strain sensitivity of the Nb3Sn superconducting material requires a careful definition of the allowable maximum stress in the windings during magnet assembly and pre-load, and a tight control of their variation within the whole coil length. Therefore, a series of assembly and pre-load specifications have been defined with the goals of minimizing the risk of conductor degradation and providing the mechanical support required to reach the nominal current during powering. In this paper we present the specifications defined for the MQXFA magnets and applied during the different assembly phases and during the pre-load process of the first 5 pre-series magnets. [ABSTRACT FROM AUTHOR]

Second-generation high-temperature superconducting (HTS) tapes strongly rely on the substrate material, in many cases Hastelloy C276 (“H-C276”). In order to ameliorate performance (and reduce the price) of superconducting tapes, replacement of H-C276 has been investigated. In this paper, the potential of the readily available austenitic stainless steels X2CrNiMo17-12-2 (“CrNiMo”) and X11CrNiMnN19-8-6 (“CrNiMnN”) as substrates is assessed. It was found that CrNiMnN is not suitable as a replacement for H-C276. Tensile stresses occurring at elevated temperatures during the HTS tape production result in plastic deformation of the CrNiMnN substrate. In the case of CrNiMo, however, plastic deformation was not observed after processing of the HTS tape. Electrical losses as a result of alternating currents beyond those of H-C276 did not occur due to the paramagnetic nature of CrNiMo. It is demonstrated that, for the very same processing parameters, HTS films deposited on electrochemically polished and buffered CrNiMo substrates exhibit similarly high critical current densities as on polished and buffered H-C276 substrates. The application of CrNiMo stainless steel reduces substrate costs considerably. More importantly, tapes processed on CrNiMo substrates withstand higher tension strains with respect to the reduction of the critical current compared to H-C276 substrates. This effect is interpreted to be a result of the matching coefficients of thermal expansion of the HTS layer and CrNiMo, resulting in lower residual tensile stress in the HTS layer after processing. [ABSTRACT FROM AUTHOR]

MgB2 in bulk form shows great promise as an inexpensive, lightweight alternative to bulk (RE)-Ba-Cu-O materials to act as trapped field magnets (TFMs), which can replace permanent magnets in applications such as desktop high-field magnet systems and rotating machines. In this paper, we investigate the thickness dependence of the trapped magnetic field in bulk MgB2 superconductors. Two bulk MgB2 samples, 20 mm in diameter and 10 mm in thickness, were fabricated using the powder-in-closed-tube (PICT) technique. The trapped field was then measured after field-cooled magnetisation for thicknesses of approximately 20 mm (both bulks stacked), 10 mm (single bulk), and then 7.5, 5, 4, 3, 2, 1.5 and 1 mm, for which the sample was machined down to the designated thickness using an automated-wet-polishing technique. A 2D axisymmetric finite-element model based on the H-formulation is used to simulate the experimental results and explain the observed thickness dependence of the trapped field. The numerical results assume a Jc(B) dependence based on the measured characteristics of small specimens taken from the bulk before and after machining. The Jc(B) measurements suggested a degradation of Jc occurred as the bulk was machined thinner and thinner: a decrease in Jc(0 T) of ∼35% was observed between the pre- and post-machining specimens. Taking this into account in the models, by assuming a linear reduction in Jc with each machining process, reproduced the experimental results with very good agreement. Consistent trapped field measurements on the top and bottom surfaces suggest this degradation occurred globally, rather than local to the machined surface. [ABSTRACT FROM AUTHOR]

In this article, to accurately analyze the magnetic properties of high-temperature superconductor (HTS), a superconductor hysteresis model is proposed. Based on the physical mechanism, the hysteresis loop of HTS can be decomposed into an antimagnetic curve and a paramagnetic loop. The calculated and measured results of silver-sheathed Bi2223 superconducting tape samples under an external magnetic field are compared to show the effectiveness and accuracy of the presented method. [ABSTRACT FROM AUTHOR]

Due to the nonlinear properties of superconductors, modeling of high-temperature superconducting (HTS) devices is difficult, especially for the ones in large scale. In this paper, we develop the multiscale model from a two-dimensional (2-D) planar geometry to a 2-D axisymmetric geometry to successfully calculate ac loss of large-scale HTS coils. By applying appropriate boundary conditions, the infinite-turn coil is built in the 2-D-axisymmetric model to produce approximated current density for an HTS coil. Besides, the multiscale model is improved in terms of coupling, so that submodels can be connected in one file and the external data processing is not needed. Validation on accuracy and efficiency is demonstrated through a double-pancake coil. Results are compared with a conventional model that considers the superconducting characteristics on each turn of the coil in actual size. This paper shows the development and improvement of multiscale model in the 2-D axisymmetric geometry is feasible. Due to its high accuracy and efficiency, the developed multiscale model can be a useful approach to analyze electromagnetic characteristics and calculate ac losses in large-scale HTS magnets. [ABSTRACT FROM AUTHOR]

On the basis of 24 T magnet, the Institute of Plasma Physics, Chinese Academy of Sciences are developing 35 T all superconducting magnet. The magnet was designed as a combination of 20 T REBCO insert coil and a 15 T 150 mm low temperature superconductor background magnet. The 20 T insert coil consists of two coils nested together. NI winding technology is applied to coil 1 to increase its current density. The 50 μm thick Hastelloy tapes will be used to co-wind with REBCO tapes of the coil 2 to further improve its over-all mechanical strength. Electromagnetic analysis shows that the maximum ratio of operating current to the critical current of coil 1 and coil 2 is 0.63 and 0.37 respectively. A more accurate model for calculating strain considering screening current is under study. The inner joint is applied to the coil 1 to avoid in-plane bending of REBCO tape, and the detailed preparation method is given in the paper. The winding and preliminary testing of all double pancakes of coil 1 have been completed, and the winding of coil 2 is in progress. This magnet is planned to be assembled and tested in 2021. [ABSTRACT FROM AUTHOR]

Second-generation high-temperature superconductors, being brittle, need to be deposited on a substrate and converted to tapes with additional reinforcement layers. These tapes are wound on a core to make superconducting wires and cables. The mechanical loads during bending, twisting, winding, etc., can degrade the superconductor. It is necessary to determine the strain-induced during these processes. Variations in induced strain with bending of superconductors have been investigated analytically and numerically for various configurations of Rare-Earth Barium Copper Oxide (REBCO) tape. The numerical simulation has been carried out using COMSOL Multiphysics software; the analytical examination of the effect has been performed applying the Flexure formula. The geometrical configuration of the superconducting tape is varied in terms of the bending radius, the thickness of constituting layers made of Hastelloy and copper. The results showed that the variation in the thickness of the substrate has a large influence on the induced strain. Electrical performance analysis is also carried out using the power law of strain dependence and the 5% critical current degradation point is noted. Results of these numerical and analytical investigations are expected to help in finding out critical parameters to avoid degradation while being in use. [ABSTRACT FROM AUTHOR]

Glass Fiber Reinforced Plastic (GFRP) is usually employed as the insulator material of superconducting cable terminal, owing to its excellent mechanical properties and insulation strength at cryogenic temperature. However, in the presence of a void defect, both the insulation life span and mechanical strength of GFRP decrease significantly at superconducting temperature conditions. In this paper, efforts have been made to investigate the degradation process of GFRP at cryogenic temperature and give insight into its degradation mechanism. By comparing PD activities at 278.7 K, 223 K and 153 K in initial phase of aging, it is found that PDs are inclined to be inhibited at lower temperature. Experiments indicate that the deposited charge accumulation and mechanical properties degradation, instead of chemical aging and PD energy, play a crucial part in dielectric degradation. At cryogenic temperatures, the local heat accumulation brought by PDs increases the material internal mechanical stress, and the accumulation of deposited charges causes instantaneous energy release. On the basis of the decrease of the mechanical properties of the material, the crack expands rapidly, which leads to the dielectric failure of the material. In conclusion, the degradation of GFRP at cryogenic temperature is the result of the combined effects of deposited charge accumulation and mechanical properties decline. [ABSTRACT FROM AUTHOR]

This paper demonstrates the feasibility of an ultra-wide band superconductor millimeter-wave triplexer. The triplexer covers the frequency range from 20 GHz–80 GHz offering three contiguous channels, each with a 20 GHz bandwidth. The triplexer is realized using a multi-layer niobium process with a physical size of 3.6 mm × 2.5 mm. The measurement results are in very good agreement with the EM simulation results. To our knowledge, this is the first ultra-wide band millimeter-wave superconductor triplexer ever reported. [ABSTRACT FROM AUTHOR]

Application method of static-field magnetization to magnetically levitated mover system with high critical temperature (Tc) superconductors is discussed. The system is composed of a levitated mover, magnetic rails, a linear induction motor (LIM) and some power supplies. In the paper, static-field magnetization is applied to superconductors in the levitated system from bottom superconductor surfaces. Then, magnetizations in superconductors are tried to control using static-field on superconductor surfaces. That is, magnetic flux density distributions are successfully forced to change to initial flux density distributions. [ABSTRACT FROM AUTHOR]

The microstructure of a bulk superconductor may fundamentally affect its field trapping and levitation performance for practical applications. Recently, a modified top-seeded melt growth using only Y2O3 and Ba2Cu3Ox precursor powder combined with a layered arrangement of graded Y2O3 content was reported to significantly refine Y2BaCuO5 particles as well as their distribution. In this paper, we have evaluated and compared the trapped field of a CPP (conventional precursor powder), an MPP (modified precursor powder), and an L-MPP (layered and compositionally graded) sample magnetized by both field cooling magnetization (FCM) and pulsed field magnetization (PFM) at 77 K. The 25 mm diameter YBa2Cu3O7-δ sample made from L-MPP shows the highest trapped field of remarkably 1.01 T and an advanced in-field performance. And the trapped fields achieved via PFM are close to (>93%) that obtained by FCM for all the three samples. However, a more dramatic flux motion has been observed for the MPP and L-MPP sample during the PFM process. The resulted trapped field distribution exhibited the effect of the homogeneity of the sample on the trapped field. [ABSTRACT FROM AUTHOR]

The Falcon Dipole (Future Accelerator post-LHC Cos-theta Optimized Nb3Sn Dipole) is a single aperture Nb3Sn cos-theta dipole short model in the framework of the Future Circular Collider (FCC) project. The Italian Institute of Nuclear Physics (INFN), in collaboration with CERN, is in charge of designing and constructing the magnet, which is a crucial step towards the construction of High Field Nb3Sn magnets suitable for a post LHC collider. This paper recalls the electromagnetic design, the field quality and performances of the Falcon Dipole. The coil ends design has been implemented in a 3D FEM to study the peak field distribution on the magnet and influence on field quality. A special focus is given to the 2D analysis to study the effect of superconductor magnetization on field quality from the injection to the final energy and then to investigate the effectiveness of compensation methods. [ABSTRACT FROM AUTHOR]

In general, when evaluating a performance of a high-temperature superconductor (HTS) no-insulation (NI) magnet, current, voltage, and center magnetic field are considered as main parameters. To achieve detailed results of the main parameters of NI HTS magnet, distributed circuits which can analyze internal physical phenomena of the magnet are required. Among distributed circuits, a circuit that divides inside of each turn into several cells in azimuthal direction is called a Partial Element Equivalent Circuit (PEEC). In this paper, we compared the main parameters of simple NI HTS magnet according to different numbers of cells in PEEC. When the number of cells per turn is different, average azimuthal current and center magnetic field of each turn coincide with each other, but voltage does not. The result suggests that PEECs with different number of cells per turn may be different circuits for each others, and the main reason for this seems to be that an accurate discussion of the characteristic resistance according to the length was not made. [ABSTRACT FROM AUTHOR]

High-temperature superconducting (HTS) maglev system, based on the magnetic flux pinning characteristic of non-ideal type II superconductors, can realize self-stable above permanent magnets. HTS maglev possesses the advantages of low-energy consumption, simple mechanical structure and environment-friendly, which make it an ideal high-speed transportation mode. When an HTS maglev vehicle crosses a ramp, the levitation height will fluctuate due to the rapid change of the track curve, which may affect the safety and stability of the system. In this paper, first, a levitation force model was obtained based on experimental data. Based on this model, a dynamic model of the HTS maglev vehicle-bridge coupled system was established. The dynamic response of the maglev train was simulated and analyzed when it operated through track grade change points under different track slopes, vertical curve radii and speeds. The influence of these parameters on the stability of the system was investigated, and the optimized design parameters for the vehicle and the track slope are proposed as well. This study provides references for the future engineering development of HTS maglev transportation. [ABSTRACT FROM AUTHOR]

The second-generation high-temperature superconducting tapes (2G-HTS) such as the REBCO coated conductors are known for their strong mechanical properties and high current carrying capacity with low ac losses compared with first-generation HTS tapes as well as low-temperature superconductors. Therefore, they seem to be good candidates to assemble coils for superconducting rotating electric machines (SuREMs). In this article, to investigate the influence of a soft magnetic composite (SMC) material on the dc and ac HTS coil performances at cryogenic temperatures, GdBCO- and YBCO-based commercial tapes from different manufactures were used to assemble ten HTS coils in double-pancake configuration. These tapes were wound using two different core structures, i.e., fixed and movable cores, having the same circular shape but different diameters as well as materials, i.e., fiberglass and SMC. Critical currents and ac transport current losses of these coils were measured using the electrical method. To evaluate the parallel and perpendicular components of the profile of magnetic flux density for each assembled HTS coil, a finite element model has been developed using Maxwell 2D. Eventually, the performance of different coils for SuREM application was discussed. [ABSTRACT FROM AUTHOR]

In this paper, we propose to study and design a coil used to magnetize, by means of a pulsed fieldmagnetization (PFM) process, an inductor of a radial flux superconducting machine with one pair of poles. Each pole contains four similar high-temperature superconducting (HTS) bulks of 30 mm diameter arranged in a square pattern. The cryostat already exists for this application and the temperature of the HTS bulks can vary from 4.2 K to their critical temperature, in a transient state. For a given primary source of energy, here a capacitor bank of 10 kJ (5mF, 2 kV) is available, the PFM process depends strongly on the value of the coil inductance used to generate pulsed field, because it defines the waveform of the current: peak value and time constant. Thus, three-dimensional modeling of the coil is required in order to be sure that its inductance and the magnetic field produced will provide a full magnetization of HTS bulks. From the practical point of view, we would like to achieve an average magnetization of each pole around 3 T. In this paper, numericalmodeling of coils with different number of turns coupled with circuit's equations is achieved. The maximum magnetic field obtained on the HTS bulks and estimated magnetization at the top center of each HTS bulk, are presented and discussed. [ABSTRACT FROM AUTHOR]

Presently, the high-speed electric machines with increased specific power are in the mainstream of research and are being developedworldwide. The high-temperature superconductor (HTS) machines allow improving the specific power even more significantly. The inductor generators are one of the perspective types of such machines. A new type of high-speed HTS electric machines is proposed in this paper. The proposed machine combines properties of generator with permanent magnets and inductor generator. In the rotor of this machine, permanent magnets are replaced by the stacks of HTS plates. In this case the HTS generator can be operated as an inductor iron-free HTS generator with combined excitation. This paper is devoted to the experiments and simulation of the proposed HTS generator. The experiment and simulation results prove the implementation of such design at three regimes: inductor, trapped-field, and combined excitation. The future development of the proposed HTS generators and perspectives of their application is discussed. [ABSTRACT FROM AUTHOR]

In previous papers, a new method to characterize HTS pellets has been presented. This method is based on the measurement of their complete penetration magnetic flux density BP . It allows determining the critical current density JC and the nvalue of the classical power law model. In this paper, a method of determination of BP of a unique HTS pellet from the measurements of the magnetic field at its top-center surface is presented. This is a simpler and cheaper method than previous methods. It uses analytical calculation based on the Biot and Savard law and numerical calculation based on power law. The maximum magnetic flux density produced by the SC pellet BZSCM is deduced from the measurement. BP is deduced from BZSCM multiplying it by a coefficient analytically calculated. The influence of JC(B) on this method is presented and analyzed. [ABSTRACT FROM AUTHOR]

In this paper we present design, construction, and preliminary results of a proof-of-concept prototype of hightemperature superconductor (HTS) shim coils operated at 77 K and energized, for the first time among all shim coils, by a flux pump, here called digital flux injector (DFI). Although the prototype shims were wound with 2-mm wide REBCO tape, and DFI with Bi2223 and REBCO tapes, the HTS Z1 and Z2 shims to be installed in the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction and operated at 4.2 K will be wound with reinforced Bi2212 wire and DFI with Nb3Sn tape. The paper concludes with two sets of Bi2212 Z1 and Z2 shims for 1.3G. [ABSTRACT FROM AUTHOR]

During the development of MQXF, the new Nb3Sn quadrupole to be used in the large hadron collider (LHC) inner triplets for the High Luminosity upgrade, three short models were tested: MQXFS1, MQXFS3, and MQXFS5. These models differ in the use of thin or thick laminations for the iron components, in the coil design, and in the superconductive strands, rod restack process (RRP) or powder in tube (PIT). In the MQXF design, the azimuthal prestress is provided at room temperature by means of the bladder-key technology, and it is further increased during the cooldown by the differential thermal contraction of the various components. Four aluminum rods provide the longitudinal prestress. Both systems allow for a flexible control of the amount of prestress applied. As a consequence, it was possible to test the models exploring different azimuthal and longitudinal prestress conditions, in an attempt to understand their impact on the magnet performances. This paper studies the mechanical behavior of these short models, also providing the strain and stresses measured by means of strain gauges installed on the aluminum shell, on the winding poles and on the rods. Finally, the paper compares the measures with the results from finite element (FE) models. [ABSTRACT FROM AUTHOR]

The Institute of Plasma Physics, Chinese Academy of Sciences was contracted by General Atomics Company to develop a superconducting feeder system containing a pair of 50-kA high-temperature superconducting (HTS) current leads for ITER central solenoid magnet test. These HTS current leads (HTSCL) are based on the previous ITER lead design experiences. However, the vertical assembly design of HTSCL which is different from ITER's horizontal assembly causes the water leakage in the insulation flange during the initial phase. Some improvements were recognized and expected to avoid the potential risk for further prototypes and series production of ITER leads. Some development experiences are shared in this paper. The HTS current leads contain BiSCCO AgAu matrix tapes tominimize the heat load to 4.5 K end. Its resistive part is cooled by 50-K helium. In the HTS part, the conduction cooling is adopted. The HTS current leads were integrated in the feeder system to perform 4.5-K full current test. The factory acceptance test was implemented to verify the highvoltage and thermodynamic performances. The maximum current of 55 kA was achieved. The test indicates that the helium mass flow consumption is 3.3 g/s per lead and the loss of flow accident time is approximate to 10 min at the nominal current with the stoppage of 50-K helium. The insulation flange passed the high-voltage test of 15 kV, while the leakage current is less than 1 mA after the improvement. More detailed test results will be shown in this paper. [ABSTRACT FROM AUTHOR]

High-Tc superconducting (HTS)-coated conductors are a promising option for the next-generation power devices. However, their thin-film geometry incurs dynamic loss when exposed to a perpendicular external ac magnetic field, which is difficult to predicate and estimate. In this paper, we propose and verify a numerical simulation model to predict the dynamic loss in HTS-thin-coated conductors by taking into account their Jc-B dependence and I-V characteristics. The model has been tested on a SuperPower YBCO-coated conductor, and we observed a linear increase of dynamic loss along the increasing field amplitude after the threshold field. Our simulation results agree closely with experimental measurements as well as an analytical model. Furthermore, the model can predict the nonlinear increase of dynamic loss at high current, while the analytical model deviates from the measurement results and still shows a linear correlation between the dynamic loss and the external magnetic field. In addition, we have used this model to simulate the distributions of magnetic field and current density when dynamic loss occurs. Results clearly show the flux traversing the coated conductor, which causes dynamic loss. These distributions have also been used to analyze the change of dynamic loss when either the transport current or the magnetic field increase individually, while the other factor remains constant. The simulation analysis on dynamic loss is done for the first time in this paper, and our results clearly demonstrate how dynamic loss changes as well as its dependence on transport current and magnetic field. [ABSTRACT FROM AUTHOR]

Given the unique electrical and magnetic properties of superconductors, high temperature superconducting (HTS) transformers will play a central role in the electricity industry in the future. Therefore, identification and analysis of this type of transformers seems necessary and indispensable. To date, many types of development plans for HTS transformers have been carried out by major power companies and important research institutes, and it is expected that this type of transformers will become one of the superconducting power equipments installed at the first stage of commercialization in the power system. However, a review of reports and papers published in this field shows that there is still much work to be done to improve the design and performance of these transformers. This article presented a review classified by subject of previous research to help orient future research into the design, analysis, and manufacturing of this type of transformers. [ABSTRACT FROM AUTHOR]

A great progress in the electrical performance of high-temperature superconductor (HTS) REBCO tapes allows for a much increased transport current capability in cables especially at high magnet fields. This is of interest to future fusion magnet applications. A variant type of twisted stacked-tape cable called quasi-round twisted stacked-tapes conductor (QisTac) is proposed and developed for the superconducting conductor of the China fusion engineering test reactor. The stack of the twisted HTS tapes is wrapped by a tight spiral made with copper wires. The stack with the copper wire is, then, soldered to form the solid round conductor, which is reliable and easy to manufacture in long lengths. A QisTac sample with four REBCO tapes was prepared and measured at 77 K and in the self-field condition. The measurement result shows the expected critical current calculated from the individual tapes considering the self-field effect. The bending characteristic of the sample was also tested under different bending radii. The critical current is reduced by roughly 5% at 0.54% bending strain. Some residual bending strain is maintained when the maximal bending force is released. This result is promising, but reproducibility testing and tests at high field and current are needed for future high field application. [ABSTRACT FROM AUTHOR]

The YBa2Cu3O7-x (YBCO) coated conductor (CC) is one of the most promising superconducting materials because of their high critical current (Ic) and low dependency of the Ic on the external magnetic field. For superconducting applications, a large number of joints are required to join relatively short YBCO CC tapes. In this paper, we study the electromechanical behavior of lap-jointed YBCO CC tapes fabricated by etching Cu stabilizer at its service temperatures (77 K) under self-field. The effects of etching Cu stabilizer on Ic and joint resistance (Rj) of YBCO CC tapes were discussed. The Rj was reduced and the Ic values of the joints are consistent with the parent CCs tape by etching the Cu stabilizer. Preferable electrical properties under applying axial tension are achieved and proved by the axial tension tests and finite element simulations. This fabrication technique is a unique practicable solution to reduce joint resistance for YBCO CC tapes with thicker metal stabilizing layers and improving electromechanical performance in second generation high temperature superconductor (2G HTS) devices. [ABSTRACT FROM AUTHOR]

Classical numerical techniques remain very computationally expensive to model the magnetization of twisted multifilamentary superconductors in three dimensions because they require solving a large system of equations. In this paper, a reduced model is proposed to approximate the ac losses of these types of wires. It is established through the projection of the physical problem in a helical Frenet basis and the use of periodic boundary conditions on a slice of the wire. The reduced model on the slice leads to systems of more reasonable size leading to smaller and faster simulations than when the whole wire is modeled. Simulation results show that the proposed method leads to computed losses similar to those obtained on a whole wire. Several simulation cases are proposed to prove the validity of the proposed model, and a comparison to some ac loss measurements is proposed to show the effectiveness of the proposed approach. The implementation relies on the H-formulation and is solved with the open-source solver, Getdp. [ABSTRACT FROM AUTHOR]

A numerical methodology is proposed to discretize a nonlinear low-frequency approximation to Maxwell’s equations using a local discontinuous Galerkin (DG) finite element method, with an upwind-like numerical flux, for modeling superconductors. In this paper, we focus on high-temperature superconductors (HTS) and the electrical resistivity is modeled using a power law. Nodal elements and the Whitney element are used. Numerical studies have been performed to verify the proposed methodology: a problem with a manufactured solution, the nonlinear magnetic front problem, and the magnetization of HTS wires. Based on the final time that can be reached for a given time-step size, the proposed strategy is compared with the $\mathbf {H}$ formulation discretized using the Galerkin finite element method with the Whitney element for the magnetic front problem. The proposed local DG strategy allows the use of a larger time-step size over a longer time interval, particularly, when we use the Whitney element. The proposed methodology can also capture sharp gradients of the current density with limited spurious oscillations. The numerical results are in agreement with Bean’s model for large values of power-law’s exponent. The proposed local DG strategy could be generalized to more complex electrical resistivity models, including multiphysics models. [ABSTRACT FROM AUTHOR]

This paper presents numerical study on magnetization characteristics of conductor on round core (CORC) cable. Numerical models introduced in this paper are based on finite-element method (FEM) with simplified structure, optimized assumptions and settings. Magnetization performance of CORC cables is represented by both full coupled model and isolated model. These numerical models are verified by experiments on CORC cables made of commercial HTS tapes. On basis of this, two topics are discussed in this paper: one is the magnetic shielding effect of superconducting layers in CORC cables; the other one is the end effect of magnetization loss in short CORC cables. Conclusions obtained in this paper will be helpful to understand the working mechanism of CORC cable and also useful in design of large-scale magnets based on CORC cables. [ABSTRACT FROM PUBLISHER]

This research paper focuses on the development of the Stirling-type pulse tube refrigerator (PTR) system. The system includes an innovative cold compression concept by introducing a superconducting linear compressor (SCLC). As one of the key goals of this research, the working gas (helium) is to be compressed directly at cryogenic temperature. The mechanical work (pressure-volume (PV) work) can be directly transmitted to the PTR system without experiencing precooling process. The unique feature of this paper is to utilize a high-temperature superconductor (HTS) coil instead of using metallic conductor coil for a linear motor. By implementing a HTS linear motor to the cold compressor, the ohmic loss can be inherently eliminated, so that the higher motor efficiency can be expected. In this research paper, the experimental validation as a proof of concept is carried out. The HTS linear motor operates at 50 Hz without creating overheat problem. The SCLC stably generates the PV work at the temperature of liquid nitrogen and it is subsequently transmitted to the compression volume of the PTR. The whole PTR system, therefore, can readily operate between 80 and 20 K with the cooling capacity of 0.26 W. [ABSTRACT FROM PUBLISHER]

This paper is about the first 25 years of the author's career. The author started working with superconducting (S/C) magnets in 1965. He has been in the field for more than 50 years. He worked on S/C magnet refrigeration and with S/C magnets from 1966 to 1971. He made the first measurement of higher multipoles generated by the circulating currents within the superconductor filaments in the first Berkeley S/C dipole magnet built in 1969. He also performed cooling experiments with super critical and two-phase helium on a small S/C magnet using a refrigerator. In the spring of 1971, the author joined the staff of the Institute for Experimental Kernphysik in Karlsruhe, Germany. He was also part of the Group European for Superconducting Super Synchrotron (GESSS) collaboration between Centre European Research Nuclear(CERN), Kernforschungzentrum Karlsruhe, Saclay, France, and the RAL, U.K. The German S/C dipole is described and his work on S/C synchrotrons is discussed. In the 1970s, he did a lot of work on quench protection of high current density S/C magnets. This paper has led to a number of insights that are shared here. [ABSTRACT FROM PUBLISHER]

The temperature behavior of the toroidal field (TF) main busbar cable-in-conduit conductors for the International Thermonuclear Experimental Reactor in 15 MA plasma current reference scenario is analyzed by Gandalf code in this paper. The current sharing temperature T\text{cs} of MBCN1 sample at SULTAN conditions and TF main busbar at 15 MA scenario is simulated by Gandalf code, and computed from the COMSOL model, and T \text{cs} of MBCN1 sample computed in this paper is close to that tested in SULTAN. The minimum quench energy of TF main busbar with heating pulse input spatial extents of 1, 3, 6, 8, and 10 mm and durations of 40, 60, 80, and 100 $\mu$s is simulated by Gandalf code in this paper. With heating pulse input region length of 1 mm and duration 40 $\mu$s, the normal length of TF main busbar is simulated by Gandalf code. [ABSTRACT FROM PUBLISHER]

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling. [ABSTRACT FROM AUTHOR]

In practical high-temperature superconducting (HTS) maglev systems, it was found that the levitation force of the onboard bulk superconductors may attenuate gradually under the inhomogeneous magnetic field. The corresponding phenomenon is that the levitation height will decay in the macroscopic and affects the long-term stability of the maglev vehicle. Thus, it is of great significance to study the dynamic levitation force of bulk superconductors under a varying external magnetic field and lay a foundation for the moderate and high-speed HTS maglev system application. In this paper, first, an experiment related to the dynamic levitation performance of YBCO bulks is carried out with a self-developed HTS maglev dynamic measurement system for the investigation of force attenuation. Then, a corresponding two-dimensional simulation model based on the finite element software of COMSOL Multiphysics 5.3 is built to research the dynamic levitation force of bulk superconductors moving above a nonuniform magnetic field. In the simulation, the electromagnetic properties are calculated using the H-formulation, and the moving mesh is employed to implement the relative movement between the bulk superconductors and permanent magnets. The experiment process is repeated in the simulation model, and the simulation results agree with the experimental results. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the effect of liquid phase (LP) source mass on crystal growth, structure, microstructure, and magnetic properties of infiltration growth processed (Gd, Dy)Ba2Cu3 $\text{O}_{{\rm{7}-\rm{x}}}$ ((Gd, Dy)BCO) bulk superconductors. The mixture of Er-123 + Ba3Cu5O8 (035) was chosen as the LP source and the ratio of 211: LP mass (wt%) was varied between 1:1 and 1:1.5. All samples were crystallized in (00l) direction; however, the supply of excess liquids for (Gd, Dy)BCO sample exhibited nonsuperconducting phases. As LP mass increased, the nonsuperconducting inclusions had refined largely due to the availability of a sufficient amount of liquids for peritectic reaction. The 211: LP mass ratio of 1:1.4 was found to be the best relation. The onset of superconducting critical temperature was not influenced by the LP mass. However, the critical current density was gradually improved with the increase in LP mass. The possibilities for further enhancement of physical properties are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel concept for realizing an RF power limiter for protecting superconductor digital receivers. A lumped-element niobium (Nb)-based filter is used as a protection circuit. It consists of lumped-element resonators formed using spiral inductors and metal–insulator–metal capacitors integrated on a multilayer Nb process. The circuit operates as a filter at low-power levels and as a reflector at high-power levels. The lumped-element filter circuit is studied in detail to explain the performance of the filter at high-power levels. It is concluded that some of the lumped-element inductors switch from being inductors when operating at low-power levels to being capacitors when operating at high-power levels. When the lumped-element inductors switch to capacitors, the filter circuit that consists of LC resonators switches to a circuit that consists of capacitors, causing the input power to be reflected back. Both the theoretical and experimental results are presented to verify this phenomenon. In addition to applications in RF power limiters, the concept can be employed to realize transmit/receive (T/R) switches in order to isolate the (T/R) circuit from the receive circuit. [ABSTRACT FROM AUTHOR]

This paper focuses on a review of the state of the art of future power grids, where new and modern technologies will be integrated into the power distribution grid, and will become the future key players for electricity generation, transmission, and distribution. The current power grids are undergoing an unprecedented transformation from the original design, changing the way how energy has been produced, delivered, and consumed over the past century. This new energy era includes the integration of renewable sources such as wind and solar, supported by the distributed or community energy storage, to power distribution grids through innovative high-frequency magnetic links and power-electronic converters. The use of emission free transportation, such as electric vehicles, and energy efficient technologies, such as superconducting generators and storage systems, are also rapidly emerging and will be integrated into the power grids in the foreseeable future. However, it is necessary to reconsider the current paradigms of system analysis and plan with a focus on how to achieve the most flexible, efficient, and reliable power grid for the future––the one that enables operation in a domain which is very different than the current one to deliver the services to consumers at an affordable cost. [ABSTRACT FROM AUTHOR]

This paper focuses on the comparison of electromagnetic performance of the superconducting permanent magnet (SCPM) generators with two different topologies. The torque capabilities of the two generators are first investigated. The peak torque is largely restricted by the material characteristics of the superconducting (SC) and the permanent magnet. The SCPM generators with iron-cored rotor and iron-cored stator topology (IRIST) is superior to the one with iron-cored rotor and air-cored stator topology in terms of torque capability. Furthermore, the flux density, line electromotive force, torque and its torque ripple, and the efficiency of the designed generators are evaluated by using numerical model. The simulation results confirm that IRIST has higher output torque and efficiency with the penalty of higher harmonics and torque ripples. [ABSTRACT FROM AUTHOR]

In this paper, an NbTi/BSCCO composite conductor made of NbTi/Cu wire and BSCCO/Ag wire cooled by cryocooler were investigated, extending the basic research on current distribution characteristics of low temperature superconductor and high temperature superconductor composite conductor and exploring its potential application. Current distribution characteristics with operating current, temperature, magnetic field, and current frequency were simulated and measured. Current-transfer characteristics between the two superconductors were also obtained. [ABSTRACT FROM AUTHOR]

With the progress of a high-temperature superconducting material production technology, the full superconducting magnets with a high magnetic field above 25 T have been realized for recent years. However, they cannot operate with persistent mode (PCM) since there is difficulty in jointing without resistance or low enough jointing resistance in HTS tapes. The bitter-like HTS magnet is a full superconducting magnet composed of ReBCO annular plates, and the power supply technology is an effective method for achieving PCM. In this paper, we choose inner and outer excitations to energize a bitter-like HTS magnet in 77 K. It was found that the HTS magnet reaches saturation faster by using the inner excitation than the outer excitation for the same bitter-like HTS magnet. The magnetic field was measured with a Hall probe at 77 K. The results illustrated that the inner excitation is much faster than the outer excitation, but the saturation field in former is lower than the latter one. [ABSTRACT FROM AUTHOR]

Surface discharge can lead to the failures of insulation materials in LN2, especially when the bubbles are generated during the quenching process. In this paper, the surface discharge properties of polypropylene (PP)/boron nitride (BN) nanocomposites were investigated in the normal and quench conditions. The results showed that the surface discharge voltage was improved in the normal condition as the BN filler content was increased from 5 wt% to 10 wt%. But it was reduced as the BN filler content was 15 wt%. As the filler content was less than 10 wt%, the free charges were captured by traps, which were with deeper trap energy level. In quench condition, the bubbles were decreased as the BN filler content increased, which were associated with the higher thermal conductivity of BN to rapidly release the joule heat. Therefore, the surface discharge voltage was increased with the BN filler content. The research has the potential to restrain the surface discharges and extent the lifetime of insulation materials in high-temperature superconducting devices. [ABSTRACT FROM AUTHOR]

The principal difficulty in large magnetic systems having an extensive flux-return yoke is to characterize the magnetic flux distribution in the yoke steel blocks. Continuous measurements of the magnetic flux density in the return yoke are not possible and the usual practice uses software modeling of the magnetic system with special 3-D computer programs. The 10 000 t flux-return yoke of the Compact Muon Solenoid (CMS) magnet encloses a 3.8 T superconducting solenoid with a 6 m diameter by 12.5 m length free bore and consists of five dodecagonal three-layered barrel wheels around the coil and four endcap disks at each end. The yoke steel blocks, up to 620 mm thick, serve as the absorber plates of the muon detection system. A magnetostatic 3-D model of the CMS magnet has been developed to describe the magnetic field outside the solenoid volume, which was measured with a field-mapping machine. To verify the magnetic flux distribution calculated in the yoke steel blocks, direct measurements of the magnetic flux density with 22 flux loops installed in the selected regions of the yoke were performed during the CMS magnet test in 2006 when four “fast” discharges of the CMS coil (190 s time constant) were triggered manually to test the magnet protection system. No fast discharge of the CMS magnet from its operational current of 18.2 kA, which corresponds to a central magnetic flux density of 3.8 T, has been performed at that time. For the first time, in this paper, we present measurements of the magnetic flux density in the steel blocks of the return yoke based on the several standard linear discharges of the CMS magnet from the operational magnet current of 18.2 kA. To provide these measurements, the voltages induced in the flux loops (with amplitudes of 20–250 mV) have been measured with six 16 bit data acquisition modules and integrated offline over time. The results of the measurements during magnet linear ramps performed with a current rate as low as 1–1.5 A/s are presented and discussed. [ABSTRACT FROM AUTHOR]

Induction coilgun can accelerate a high-temperature superconductor (HTS) projectile to higher velocity than a conventional projectile, because an HTS projectile can generate greater induced current by the premagnetized process and then obtain larger thrust in the launch process. In this paper, a numerical field model for the dynamic and electromagnetic characteristics of a single-stage induction coilgun is established, using the finite-element method. The electromagnetic force, muzzle velocity, and current density distribution of the aluminum projectile and the HTS projectile are studied, respectively. Then, the effect of magnetization on the characteristics of the HTS projectile during the launch process is considered. The result illustrates that magnetization brings a great improvement in the performance of the coilgun due to the higher induced current value in the HTS projectile. In addition, due to the limitation of the full magnetization state, the magnetization of the HTS projectile should be in a suitable range. The simulation work shows the potential of HTS projectile for future induction coilgun application. [ABSTRACT FROM AUTHOR]

This paper investigates the conditions of use and the equivalence of various constitutive laws used to model the electromagnetic behavior of high-temperature superconductors: two versions of the critical state model (CSM), the power law model, and a so-called percolation model. All these models can be used to represent the same superconducting material with some limit of accuracy. The CSM and the power law model are well known in the literature. The percolation model can be seen as a generalization of the power law model that includes also a CSM-like behavior at very low electric fields. The investigation has been carried out for three types of operating conditions: the sudden application of a dc excitation, a pure ac excitation, and combined dc and ac excitations. The equivalence between the different constitutive laws is shown to be a function of the magnitude of the electric fields and of the time scales involved. In the dc case, long time scales and very small electric fields are predominant; thus, the superconductor requires a model that is accurate at low electric fields, such as the percolation model. The losses then arise from the relaxation of the magnetic field in the sample. In the ac case, the power law and percolation models are nearly identical when considering power frequencies, so choosing the simpler power law model is fully acceptable in practice. In addition, the CSM coincidentally provides good predictions of the losses in the power frequency range. In the dc+ac case, when time scales in the range of minutes to hours are considered, it is shown that ac losses dominate over relaxation losses, and the same conclusions as for the ac case apply. [ABSTRACT FROM AUTHOR]

The superconducting magnetically levitated (Maglev) transportation system has been developed in Japan. This system is based on the side wall electrodynamic suspension (EDS). Although the EDS system has the advantage of stable levitation without active control, results of the numerical simulation show that the damping factor of the levitation system is small. Thus additional damper system has been proposed to increase damping of the system against oscillation of the bogie. The specifications of the damper coils are decided by the damping effect against the natural vertical oscillation of the bogie. The weight of the conventional damper coils is heavy. So weight reduction of the damper coils is needed. In this paper reduction of the damper coil weight is considered. Optimization of the damper coil with 50% weight reduction is studied. Running simulations with the weight reduction have been undertaken. The weight of the reduction model becomes half as that of the conventional one. This means size of the damper coils becomes small, and effect of the damper coil should decrease. But in this semi-active damper mode, it’s damping factor increases and oscillation of the bogie converges faster than that by the conventional one. [ABSTRACT FROM AUTHOR]