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ISSN: 2772-8307
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cited:8
Review of Moiré superconductivity and application of the Roeser-Huber formula
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Article Number:Article 100073
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Abstract
Corresponding email:m.koblischka@ieee.org
Moiré superconductivity represents a new class of superconducting materials since the discovery of superconductivity in magic-angle (1.1°) twisted bi-layer graphene (MATBG), forming a Moiré lattice with a much bigger crystal parameter as the original lattice constant of graphene. Hence, experimentally changing the Moiré twist angle, 0.93° 1.27, leads to a variation of the superconducting properties and enables a new way of engineering 2D superconducting materials. Details of the robust superconducting state of MATBG as function of charge carrier density, temperature and applied magnetic fields are reviewed. The influence of the top/bottom hexagonal boron nitride layer thickness on the superconducting properties of MATBG was also demonstrated in the literature. In all fabricated MATBG devices, changing of the charge carrier density leads to the appearance of insulating, metallic and even ferromagnetic states, which separate several superconducting domes in the phase diagram (longitudinal resistance, , as function of temperature T and charge carrier density, n). Further works have considered MATBG combined with WSe2-layers, twisted bi-layer WSe2, magic-angle tri-layer graphene (MATTG), and most recently, four-layer (MAT4G) and five-layer (MAT5G) stacks. The differences between the layered, cuprate high- superconductors and the Moiré superconductors are compiled together. The collected information is then used to apply the Roeser-Huber formalism to Moiré-type superconductivity to calculate the superconducting transition temperature, , using only information of the Moiré lattice and the electronic configuration. To account for the different charge carrier densities in the experimental data sets and the low charge carrier mass demands that a new parameter must be introduced to the Roeser-Huber formalism to enable the description of several superconducting domes found in the phase diagram for a given Moiré angle. Doing so, the calculated data fit well to the correlation curve defined within the Roeser-Huber formalism.
cited:30
40-meter-long REBCO tapes with critical current over 4,000 A/12 mm at 4.2 K and 13 T by advanced MOCVD
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Article Number:Article 100081
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Abstract
Corresponding email:selva@uh.edu
REBa2Cu3O7−δ (REBCO, RE = rare earth) tapes doped with 5% and 15% Zr have been scaled up to lengths more than 40 m in a pilot-scale advanced metal organic chemical vapor deposition (A-MOCVD) tool. The precursor compositions used for the long tapes were guided by a study of the influence of (Ba + dopant)/Cu content on the critical current density (Jc) of 5 and 15 mol.% Hf- and Zr-added tapes at 4.2 K and 13 T. The 40-m-long tapes exhibited a critical current (Ic) over 4,000 A/12 mm at 4.2 K and 13 T as well as over 1,400 A/12 mm at 20 K and 20 T. The critical current densities of a 40-m-long tape doped with 5% Zr at 4.2 K measured at the National High Magnetic Field Laboratory (NHMFL) were > 10 MAcm−2 and >5 MAcm−2 at 14 T and 30 T, respectively, which are over three times those of commercial REBCO tapes. The infield Jc of 5% Zr-added 40-m-long tapes was similar to those of previously-reported high-performance short samples made with 15% Zr or Hf. These results demonstrate the excellent potential of A-MOCVD for manufacturing high Ic REBCO tapes for use in ultrahigh-field magnet applications.
cited:35
Screening-current-induced magnetic fields and strains in a compact REBCO coil in self field and background field
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Article Number:Article 100082
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Abstract
Corresponding email:tmqu@mail.tsinghua.edu.cn
REBa2Cu3O7−x (REBCO) coated conductors, owing to its high tensile strength and current-carrying ability in a background field, are widely regarded a promising candidate in high-field applications. Despite the great potentials, recent studies have highlighted the challenges posed by screening currents, which are featured by a highly nonuniform current distribution in the superconducting layer. In this paper, we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil, specifically the screening-current-induced magnetic fields and strains. Experiments were carried out in the self-generated magnetic field and a background field, respectively. In the self-field condition, the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity, as the nominal center field reached 9.17 T with a maximum peak current of 350 A. In a background field of 23.15 T, the insert coil generated a center field of 4.17 T with an applied current of 170 A. Ultimately, a total center field of 32.58 T was achieved before quench. Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied. The comparative study shows that, for this coil, the electromagnetic–mechanical coupling plays a trivial role in self-field conditions up to 9 T. In contrast, with a high axial field dominated by the background field, the coupling effect has a stronger influence on the predicted current and strain distributions. Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.
cited:39
Review on the state-of-the-art and challenges in the MgB2 component manufacturing for superconducting applications
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Article Number:Article 100083
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Abstract
Since the discovery of MgB2 as a superconductor, several research groups worldwide have studied the superconducting mechanisms due to the dual gap nature of MgB2, as well as attempted to produce such a compound in wires, tapes, bulks, and thin films for a plethora of applications. While MgB2 carries the promise of replacing Niobium-based superconductors in low-field applications, less-than-desirable performance and in-operation stability has slowed down such a progress. While the properties and nature of the superconductivity of MgB2 are fairly known, the reproduction of its properties at manufacturing scales remains an unsolved problem. Therefore, this manuscript presents a systematic review on fundamental properties, phase formation, growth kinetics, and superconducting properties of MgB2-based components such as multi- and mono-core wires, bulks, and thin films. Advances, challenges, and shortcomings are utilized in consolidating research questions and directions pertaining to the manufacturing of MgB2 superconducting devices. Lastly, we evaluate the technological readiness of MgB2-based devices for applications in fusion energy systems.
cited:12
Method with reliable accuracy and fast speed for measuring operational current of HTS NI closed-loop coils in steady persistent-current-mode
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Article Number:Article 100084
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Abstract
Corresponding email:wei.wu@sjtu.edu.cn
This study proposes a method for measuring the operational current of high temperature superconducting (HTS) non-insulation (NI) closed-loop coils, which operate in the steady persistent-current-mode (PCM). HTS NI closed-loop coils are promising for many easily-quenching direct-current (DC) applications, where their performance is determined by magnetomotive forces, total number of turns, and dimensions. As the primary interface parameter in an application system, the operational current must be accurately and rapidly measured. Generally, this is achieved by dividing the measured magnetic field by the coil constant. However, even if the influence of the screening current induced field (SCIF) is not considered, existing methods for the coil constant may be disturbed by the performance and location of Hall sensors, or experience a long measuring period. Therefore, a relatively accurate and fast method is proposed in this study, which is based on adjusting the output current of the adjustable power supply and monitoring the coil voltage as an indicator. The proposed method was validated through experiments and simulations using an equivalent circuit model coupled with a finite element method (FEM) model, and its current accuracy can be equivalent to the resolution of the employed power supply. It was demonstrated that this method reduced the requirements for Hall sensor’s performance and location, and has a more reliable accuracy in contrast to the simulation method. Compared to the experimentally conventional method, the proposed method presents a significantly faster speed. The impact of the SCIF was considered and proven to be negligible for the tested pancake coils. Even for coils whose coil constant vibrates owing to the SCIF, this method can be adapted to directly measure various operational currents. Furthermore, it was demonstrated that the measurement error can be influenced by the current discrepancy among turns when the coil is not in the steady PCM, and a procedure for reducing this error was proposed.
Abstract
Corresponding email:mohammad.yazdani-asrami@glasgow.ac.uk
Power transformers are key elements for the safe and reliable delivery of electrical energy generated by renewable energy resources to consumers via transmission lines. Fault-tolerant current-limiting High Temperature Superconducting (FTCL HTS) transformers are type of superconducting transformers that tolerate fault for seconds and limit the fault current without the threat of burnout or delamination of tapes and deformation of windings. In this paper, the fault performance of a FTCL HTS transformer in a standard IEEE power system is investigated. The studied transformer is a 50 MVA 132 kV/13.8 kV transformer where both windings are made up of HTS tapes. The understudied power system consists of two microgrids with distributed generators. Part of the power in microgrids is supplied by the upstream grid which is connected to the microgrids through the HTS transformers. Two fault scenarios have been considered in this power system, in each one of these scenarios, a fault happens in one of the microgrids. Two considered fault scenarios have an approximate fault current of 18x to 23x of the rated current in the secondary windings. Results showed that insulated windings in FTCL HTS transformers could substantially reduce the peak temperature of the HTS windings, compared to bare windings. Afterwards, post-fault loading is imposed on the HTS windings, to observe their performance against the current increase after fault clearance. In this case, for the first scenario of the faults, the FTCL HTS transformer could tolerate 192% of post-fault overloading, while this number for the second fault scenario is 170%. Finally, the impact of post-fault loading on the full recovery time was discussed.
Review