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Future collider accelerators will rely on high-temperature superconductors reaching high field up to 20 T and above. Among the existing high-temperature superconducting materials, the Rare-earth Barium Copper Oxide (ReBCO) tapes arranged according to the Conductor-on-Round-Core (CORC®) concept could be a viable solution to wound accelerator magnets such as Cosine Canted Theta (CCT) magnets. Dedicated experimental characterization of the critical current to quantify the degradation due to the winding process and operating conditions should proceed in parallel to the development of numerical models capable to reproduce and, in perspective, predict the cable performance. This paper presents the development of a new multi-physics model for a CORC® wound with ReBCO tapes together with its validation. The $T-A$ formulation has been used leveraging the high aspect ratio of tapes, suitably coupled with a conduction thermal model which for the first time properly accounts for the cable convective cooling. The model developed in this work can accurately simulate the thermal, electric and magnetic behaviors and the current sharing among tapes by using a set of self-consistent boundary conditions adopted for the first time in this kind of simulations. The model is verified and benchmarked against other well-established formulations on a set of test cases. The comparison of the computed $V-I$ characteristic of the straight cable to available experimental data shows that the main physics features of the cable are well captured by the model, including performance degradation due to cable tapering at the terminations.

For the International Thermonuclear Experimental Reactor magnets, a significant fraction of the Nb3Sn conductors will be operating at a low field (B < 4 T), such as the outer turns of the central solenoid modules and toroidal field winding packs, while so far, the Ic(B,T, Ɛ) parameterization has been derived from measurements carried out at higher fields (B ≥ 7 T). It may not be accurate enough to adopt this parameterization for the extrapolation of the conductors’ operating performance in terms of critical current (Ic) and current sharing (Tcs). The critical current characteristic can be effectively extended to the lower field region by means of a relation between the magnetization and critical current. The parameterization of the derived critical current scaling law Jc(B) is processed for the field range of the magnetization measurement (0 < B ≤ 9 T) and the critical current measurements (7 ≤ B < 12 T) in order to cover the full range up to 12 T. [ABSTRACT FROM AUTHOR]

At present, HTSC-2G tape wires (hereinafter wires) are an uncontested basis for creating conductors for the magnetic system of a compact thermonuclear reactor. Since, according to calculations, the tangential mechanical stresses in the windings of its magnetic system can exceed 1000 MPa, the requirement for high electromechanical stability of the HTSC wire used comes first, which makes it possible to raise the constructive current density. In this work, comparative electromechanical test of HTSC-2G wires with different thicknesses of the superconducting layer, on a substrate made of AISI 310S stainless steel, at the temperature of liquid nitrogen and liquid helium were carried out. The factors leading to a decrease in the degradation of the critical current at high tensile stresses have been established. A new method for testing HTSC-2G wires at helium temperature and a simple method for preliminary assessment of their electromechanical properties at nitrogen temperature are proposed. [ABSTRACT FROM AUTHOR]

The performance of High Temperature Superconducting (HTS) tapes is strongly affected by the mechanical loadings they experience during manufacturing, cooldown and operation. Therefore, the development of HTS-based applications requires thorough investigations on the impact of these loadings on the critical current. In this work, a measurement procedure is applied to determine the critical current of a (Re)BCO tape at 77 K, after double bending at room temperature around a cylindrical mandrel. In this procedure, the tape is cooled down to cryogenic temperature, and the critical current is first measured on the straight sample. After warming the tape up to room temperature, a double bending around a cylindrical mandrel is applied in two opposite directions and the critical current is measured at the following cooldown. This analysis is repeated for different mandrel diameters in order to identify to study the impact of the curvature radius on the performance degradation. The application of one single bending at room temperature is compared to the results of the double bending procedure. The influence on the performance degradation of applying a known force to the tape during the double bending at room temperature is also experimentally analyzed. [ABSTRACT FROM AUTHOR]

Ion irradiation of superconductors allows both the establishment of their radiation hardness and the modification and optimization of their properties useful for applications. In this work, we investigate the effects of proton irradiation with different energies on Fe(Se,Te) thin films grown on CaF $_2$ and on buffered YSZ substrates. These systems allowed us to perform preliminary studies for the development of Fe(Se,Te) coated conductors. Critical temperature and critical current were measured for different levels of displacement damage, and with ion implantation happening at various depths inside the substrate, as evaluated by simulations with Monte Carlo codes. All measurements evidenced that these Fe(Se,Te) films are robust against proton induced damage, and that an increase of critical current can be achieved introducing pointlike defects in the superconductor. However, we find that damage induced in the substrate also plays a crucial role in modifying superconducting film properties through a variation of the strain exerted on the film. This strain degrades superconducting parameters and should be minimized. [ABSTRACT FROM AUTHOR]

We have studied the fabrication of Nb3Sn ultra-thin wire through the bronze process. The starting billet was assembled by 19 Nb rods, a bronze alloy matrix of Cu-14mass%Sn-0.3mass%Ti, Nb diffusion barrier, and the outermost Cu stabilizer. The Cu/non-Cu ratio was designed as 0.89. The outer diameter of the stating billet is 45.2 mm. After the hot extrusion with an extrusion ratio of 9.1, it was cold-drawn from 15.0 mm to 0.05 mm (50 microns) in diameter with intermediate annealing with reel-to-reel process. Eventually, we have obtained over 7,000 m in a piece length without wire-breakages. In addition, we have also demonstrated both 7 and 7/7 stranded cables using 50 microns Nb3Sn wire. The transport critical current, Ic, at 4.2 K and 4 T of both the primary 7 and the secondary 7/7 stranded cables were 14.2 A 102.0 A, respectively. A single ultra-thin wire showed 2.04 A of Ic at 4.2 K and 4 T, thus, measured Ic values of 7 and 7/7 stranded cables become almost equivalent to the calculated values from the Ic of a single wire. In addition, we investigated the bending effect on Ic of the secondary 7/7 stranded cable using the brass fixtures having a different radius. Meantime, we studied the effect of solder impregnation before bending. The Ic of 7/7 stranded cable with solder impregnation showed a dramatic degradation by bending on the radius of 11.5 mm. But the Ic of the cable without solder impregnation did not. [ABSTRACT FROM AUTHOR]

The magnetic-field anisotropy of the critical current is an extrinsic property of superconducting wires that is of greatest relevance to the design of high temperature superconducting (HTS) devices. It is also a highly useful diagnostic tool to understand the dominant flux-pinning mechanisms active in different temperature and field regimes. REBa2Cu3O7 (REBCO) coated conductors typically exhibit a large peak in critical current when the field is aligned with the REBCO a-b plane. The commercial pinning-enhanced American Superconductor Corporation (AMSC) REBCO coil wire used for this study is unusual in having a relatively small a-b plane peak at higher temperatures due to an inherently low density of stacking faults. Stacking faults can then be introduced by annealing the wire in oxygen to create the more commonly observed strong a-b plane peak. Complementary c-axis columnar defects can be added by silver ion-irradiation producing an additional peak in the critical current at 0°. The resulting complex critical current anisotropy is studied using transport critical current measurements over a temperature range from 20 K to 77.5 K and under magnetic fields up to 7 T. Through systematic studies on pristine and annealed samples, in the presence and absence of columnar defects created by silver irradiation, we investigated the flux-pinning interactions between stacking faults and columnar defects. [ABSTRACT FROM AUTHOR]

We have developed and routinely used a reel-to-reel (R2R) scanning Hall-probe microscopy (SHPM) system to characterize REBCO conductors developed at the University of Houston. By mapping the screening-current-induced field of a magnetized REBCO conductor, the SHPM system enables 2-dimensional imaging of critical current density (Jc) distribution in a contact-free, non-destructive approach. Most existing SHPM systems investigate the magnetized REBCO conductors at remnant state without external background field. However, because of the inhomogeneity in the distribution of nanoscale artificial pinning centers in advanced REBCO conductors, there is a demand for reel-to-reel in-field examination over long conductor lengths. We have been developing a new R2R SHPM, capable of measuring the Jc profile of long REBCO conductors at the temperature of 63.5–77 K and in an applied field up to 5 T. With optimal data acquisition and processing techniques, we overcame the challenge of measuring weak screening-current-field signals in a strong background field. R2R Jc mapping has been performed on long REBCO conductors with a spatial resolution as low as 0.15 mm2 and a longitudinal scanning speed comparable with our existing system. Our previous study revealed a good correlation between the REBCO conductors’ 65-K in-field Jc and their Jc over a wide span of temperatures and fields. Using this correlation, the in-field R2R SHPM will serve not only as a means of quality control, but also provide insight into high-field performance of advanced REBCO conductors at low temperatures. [ABSTRACT FROM AUTHOR]

(Recently, our group proposed a technique for the formation of crystalline joints for use in RE123-coated conductors, based on using molten bulk Y123 or Yb123 as an intermedium. The melting temperature of Yb123 is approximately 1203 K in air, which is 70 K lower than that of Y123, and so using Yb123 allows the heat treatment temperature during the joint formation process to be reduced by about 70 K. This lower temperature is likely to limit degradation of the wire. However, it is challenging to produce a highly orientated Yb123 phase, and our previous experiments indicated that crystal growth does not occur in Yb123 at the joint boundary during a short time. For these reasons, increasing the critical current at the joint requires crystal growth of an intermedium at the boundary. The present study examined the partial substitution of Y for Yb in the bulk material. YxYb1-x123 samples with x = 0, 0.1 or 0.2 were prepared and both crystal growth and the critical current at the joint boundary were assessed following heat treatment at 1203 K for 1 min under oxygen. X-ray diffraction patterns indicated that only the Y0.1Yb0.9123 phase contained highly oriented single crystals. The current-voltage characteristics of three joint samples were determined at 77 K and a current of 44.2 A was obtained from the sample with x = 0.1. This value of critical current was approximately 5 times higher than that for the x = 0 specimen (a Yb123 polycrystal), indicating a significant improvement in the critical current as a result of crystal growth in the intermedium. [ABSTRACT FROM AUTHOR]

China Fusion Engineering Experimental Reactor (CFETR) is a major scientific project independently designed and developed by China and jointly with international cooperation. Engineering design and project infrastructure construction are currently underway. The design goal of CFETR is 1.5–2.0 GW fusion power, and the central magnetic field is up to 15 Tesla, which is much larger than the design parameters of ITER under construction in Europe. In order to achieve the desired high magnetic field, the pre-research of Bi-2212 high temperature superconducting CICC magnet was carried out. The performance test of Bi-2212 cable is the most direct criterion for the successful development of the cable. New cable performance judgment method is proposed because of the long test time and high test cost of sub-size cables. Two cables are manufactured, and one cable is disassembled into single wires for performance evaluation. If the performance of single wires is not significantly attenuated in comparison with the performance of single wires that have not been twisted, we will conduct the second cable critical current test. The test results show that the method of dismantling the cable test is feasible. It is also feasible for cables of different structures. This method can be used to judge the performance of the cable in advance. This method is only suitable for detecting damage in the cable manufacturing process. [ABSTRACT FROM AUTHOR]

For the design of superconducting magnets, the magnetic field and critical current distributions are usually calculated based on finite element software such as COMSOL, ANSYS, etc. These software are necessary for accurate structure modeling and field calculation, but they occupy large memory space and comsume long simulation time. Furthermore, they are not easy to use. To achieve an easy-to-implement solution for rapid magnet field and critical current calculations, this paper investigates three types of analytical calculating methods, and builds an autonomously-operated calculating application program (APP) for the uses in various superconducting magnets. [ABSTRACT FROM AUTHOR]

The Cu-0.5 wt.% Mn alloy is selected as the inter-filament matrix in NbTi superconducting strand with low AC Loss. Three types of NbTi strand with the filament number of 10 800, 12 960 and 33 696 were prepared based on high homogeneity Nb47Ti alloy, pure Nb sheet, high purity oxygen free copper and Cu-0.5 wt.% Mn alloy. Heat treatment with 3-5 intermediate annealings has been applied to increase the Jc of the strands. When the number of annealings increases from 3 to 5, Jc raises from 2295 to 2958 A/mm2 for the 10 800 filament strand, and from ∼2500 to ∼2800 A/mm2 for the 12 960 filament and 33696 filament strands. As the filament number increases from 10800 to 33696, the ‘n’ values keep the same level, demonstrating that no “sausage like” or “nodular like” filaments exist in the final strands. When the filament diameter of the strand is reduced from 4.6 μm to 2.6 μm, the hysteresis loss decreased from 42.8 mJ/cm3 to 17.3 mJ/cm3 at ± 3 T&4.2 K. Based on this research, WST has obtained a stable mass production technology of the strands. Using heat treatment with 4 annealings, the different batches of the strands with Jc ranged from 2600 to 2800 A/mm2 have been produced with piece length level of 1-3 kilometer. [ABSTRACT FROM AUTHOR]

Based on existing quasi-isotropic strand (QIS) conductor and twisted stacked tape cable–cable in conduit conductor (TSTC-CICC), this paper proposed a novel QIS-CICC. The conductor was numerically modeled and the anisotropy of critical currents was analyzed. The simulations showed that although the critical currents of the QIS-CICC were worse than those of the TSTC-CICC for the self-field condition, the current decay of the QIS-CICC was far less than that of the TSTC-CICC under a 1 T external field, and the anisotropies of the two conductors were almost the same. [ABSTRACT FROM AUTHOR]

The performance degradation of High Temperature Superconducting (HTS) tape, which may be influenced by various heat exchange conditions due to different cooling methods, is of significant for HTS devices under repeated overcurrent. In this article, considering different heat exchange conditions, a series of repeated overcurrent experiments were carried on YBCO tapes under 50Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeated overcurrent was recorded. the resistance characteristics of YBCO tapes under different cooling conditions were analyzed. Through finite element simulation, the temperature and current distribution of YBCO tape under different heat exchange conditions were simulated. The experimental results show that the overcurrent capability of the conductor under conduction cooling is far inferior to the liquid nitrogen immersion method. [ABSTRACT FROM AUTHOR]

An intrinsic powder-in-tube technique is used to fabricate Cu-sheathed MgB2 with and without a Ti barrier. The effect of heat treatment on reaction layers between the Cu sheath, Ti barrier, magnesium and boron is evaluated by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray powder diffraction. Annealing the Cu (sheath)/Ti (barrier) prior to in situ heat treatment of the Mg + 2B reaction improves formation of a pure MgB2 core and eliminates subsequent reactions between B powder and the Ti barrier. Microstructural study of the interface between pure MgB2 and annealed Ti/Cu sheath does not show any evidence of reaction and diffusion between B and Ti which is beneficial for superconducting properties. Magnetization measurements of MgB2 superconducting wires have shown that the critical current density, Jc, of a Cu-sheathed wire combined with a Ti barrier can significantly enhance performance compared with a Cu-sheathed MgB2 wire without a Ti barrier. This enhancement is due to elimination of a MgCu2 interface layer between the copper sheath and magnesium powder for wires when a Ti barrier is involved. Pre-annealing of the Cu (sheath)/Ti (barrier) does not affect the Jc of MgB2 wires. This could make a positive effect on the Jc of the wires with finer filaments. [ABSTRACT FROM AUTHOR]

We suggested an MgB2 joint process with its own heat-treatment schedule to apply it for our 1.5-T Mg2 “finger” MRI magnet. In fabricating the MgB2 magnet, the optimal heat-treatment schedule to attain a reproducible and high critical current is different in a joint and a coil. To solve this problem, we introduced an additional heating system, which is composed of a cartridge heater and a thermocouple connected with a copper block, into a box-type furnace. Then, we carried out heat-treatments with exclusively increasing the joint-part temperature above the Mg melting point of 645 °C—the joint was actually heated up to 700 °C. We evaluated a critical current and a crystal structure of the obtained MgB2 joint. From experimental results, we found that the joint heated with the own heat-treatment schedule, which is 700 °C for 1 h + 600°C for 11 h, showed a good Ic of over 450 A at 15K under self-field. The joint resistance was estimated by the coil operation for 18 days, and it was expected to be less than 10−12 Ω. [ABSTRACT FROM AUTHOR]

Superconducting coils may suffer from electromagnetic vibration due to changing electromagnetic fields under AC conditions. In the long term, this vibration may cause performance degradation of superconducting tapes. In order to study the influence of electromagnetic vibration between superconducting tapes on their performance, the study can be started from the simplest form of electromagnetic vibration, that is, the electromagnetic vibration between two superconducting tapes. The degree of electromagnetic vibration under different AC currents were analyzed by simulation, and the variation of critical current (Ic) and the n-value of Y-Ba-Cu-O (YBCO) tapes after undergoing long-term electromagnetic vibration through experiments were measured. The results show that two YBCO tapes does not show obvious performance degradation under tens of hours the electromagnetic vibration caused by the AC current amplitude of 1.2 to 1.4 times Ic. [ABSTRACT FROM AUTHOR]

The superconducting joint between Bi2223 tapes has been developed with a method of joint by incongruent melting (JIM) in our Laboratory. The multi-junction method used to increase the contact area yielded a demonstrated increase in critical current. In previous studies, a simple boundary shape with lap-joint was chosen to connect several filaments in the tape, and this method is not suitable for connecting all about 100 filaments. Of course, more filaments are necessary to be joined with superconducting boundary, in order to increase the critical current of joint. However, due to the thin tape structure, there is difficult to use of butt joint between two cross-sections of tapes. In this study, we proposed a multi-junction method by using multiple boundaries with stepped structure. In experiments, joint samples with increasing of junction number to three from one were prepared, and the critical current for each unction number were measured at 77 K (self-magnetic field). In result, the critical current is increased with increasing of the junction number, and a highest critical current of 51.8 A was obtained in three-junction sample. in particular, the critical current at first junction (end of tapes) is highest, indicating that critical current at the end is important in total critical current of joint. Therefore, single junction at end of tapes was prepared and evaluated by using X-ray diffraction and critical current measurement. From X-ray diffraction patterns, the joint area is estimated to be about 1 cm. The critical current was 36.5 A at 77 K. [ABSTRACT FROM AUTHOR]

With excellent mechanical properties and electromagnetic properties, compact superconducting cables are considered as potential candidates for large capacity power applications and fusion magnets. In this paper, the concept of a novel compact cable, conductors in tube (CIT) cable is introduced from the aspect of design, fabrication, and performance evaluation, respectively. The principal advantages of CIT cables are that the cable can release the residual stress created from the fabrication process, and the outer former of the cable can be used as both the supporter as well as the container of refrigerant liquid. Firstly, the topological structure and the detailed fabrication process of a CIT cable are presented. Then, a typical CIT cable is manufactured. The feasibility of this new structure is verified by its critical current test. Moreover, bending characteristics of CIT cables are evaluated and compared with Conductors on Round Core (CORC) cables from the aspect of both simulation and experiments. The superior bending performance enables it to be potential in the future compact magnet application. [ABSTRACT FROM AUTHOR]

Advanced metal organic chemical vapor deposition technique (A-MOCVD) was used to grow 4-5 μm thick REBa2Cu3O7−δ (REBCO, RE = rare earth) films, doped with 5% and 15% Hf. Thick REBCO films doped with 5% Hf exhibit self-field critical current density (Jc) over 1.8 MA/cm2 at 77 K and 3 MA/cm2 at 65 K, 1.5 T. Increasing the Hf content to 15% resulted in Jc above 9 MA/cm2 at 20 K, 9 T. The effect of (Ba+Hf)/Cu content in Hf-added (5 and 15 mol.%) REBCO films on the critical current density at 77 K, 0 T, c-axis lattice parameter and out-of- plane texture has been studied. In addition to growth of thick film REBCO tapes with superior performance, the A-MOCVD reactor has been used to quadruple the precursor-to-film conversion efficiency above 45%. This achievement has a first order impact in lowering the cost of REBCO tapes made by A-MOCVD. [ABSTRACT FROM AUTHOR]

The Japanese national project started on standardization of the test method of the DC critical current of superconducting cables. A round robin test of critical current measurement is carried out to survey underlying problems for the standardization. The sample cables are made of Bi-2223 tapes and the inner core conductor has two layers and the outer shield conductor is composed of a single layer. The diameter of the shield layer is about 40 mm and the length of the cable is 2.00 m. The critical current was determined by the electric field criterion of 1 μV/cm. The n value is also measured for the inner core and the outer shield conductor. Three institutes participated in the test for two sample cables. The obtained experimental relative standard deviation for the critical current at 77.30 K is in the range of 0.2 to 1.3%, and is sufficiently small. Hence, it can be said that the present method that is widely used is suitable as a standard test method. On the other hand, the experimental relative standard deviation of the value is in the range of 2.8 to 18.6%, which is relatively large. This is caused by the measurements done at different temperatures. [ABSTRACT FROM AUTHOR]

To reduce the time and risk of sample damage during critical current characterization of long Coated Conductors (CC), a pulse current method is proposed previously. The dynamic current distribution in the tape during pulse current ramping is studied in this work for better understanding of the behavior of the CCs under measurement, based on a current-driven version of Brandt's method. in addition, a coil with 2000 turn was used in the experiment to improve the signal-to-noise ratio. The ramping rate is compared with the voltage criterion in the standard four-point measurements. Especially, a scaling behavior is found for simple but precise description of the effect of the current ramping rate on the measured critical current, utilizing the concept of an effect electric field, which can be used to calculate the critical current with commonly used criterion of 1 μV/cm, with no free parameter. Effects of copper protection layers are also considered for fast pulses. The numerical results are also compared with those of experiments. [ABSTRACT FROM AUTHOR]

Jelly-roll (JR) processed wires using metal thin foils as starting materials have excellent cold workability and drawability. We have successfully fabricated Nb/Al composite mono-core wires having 50 μm in outer diameter and over 400 meters in length. Furthermore, some of the wires were additionally drawn to 30 μm in outer diameter. It is known that the critical current density (Jc) of Nb3Al synthesized by low-temperature diffusion reactions increases with decreasing the diffusion distance between Nb and Al. In this study, we have confirmed that Jc would increase with decreasing the wire diameter, and the maximum Jc has been obtained on Nb3Al wires with 30 μm in diameter. In addition, a 7-wire round cable has also been demonstrated using 80 μm diameter wire, with an Ic 7 times the Ic of a single wire. A transport current test was performed for the cable sample simulating W&R and R&W processes using a 50 mm diameter G10 holder. There were no differences between the Ic and the n-values in the W&R and R&W processes. Moreover, as a feasibility study, a 3rd bundle Rutherford cable has been successfully fabricated using 3,108 pure copper wires 50 μm in diameter. [ABSTRACT FROM AUTHOR]

As compute demands and their large energy costs continue to rise in the datacenter, it is imperative to identify low energy compute solutions. One approach is to investigate device schemes based on collective mode phenomena arising at low temperatures where power dissipation and switching voltage can be extremely low. Additionally, such devices could also have applications in quantum computing where such a low power logic/memory scheme could possibly be integrated onto the qubit plane, which is typically at ∼10 mK temperature. Josephson Junction Field Effect Transistor (JJFET) is a promising candidate for low power operation with zero voltage drop across its drain-source terminals while operating in the superconducting regime. JJFET logic gate is typically realized as a n-type JJFET connected in a common-source configuration with a current source load. This results in high noise margin for logic-1 input but not for both logic-1 and 0. In this paper, we propose an overdamped region, n-type JJFET based digital logic using a cascaded common-source configuration-based design yielding high noise margin for both the logic inputs. DC noise margin sensitivity analysis is performed for the bias current and threshold voltage modulation. The noise margin can be further improved (approaches ideal inverter case) by cascading multiple common source stages yielding higher inverter gain. [ABSTRACT FROM AUTHOR]

Electrical machines with high-temperature superconducting (HTS) armature windings can benefit from high power density and efficiency. Furthermore, by using permanent magnets (PM) on the rotor, the cryogenic system can be significantly simplified compared with the fully superconducting (SC) machines or the machines with SC field windings. Compared with the conventional electrical machines, the performance of machines with HTS armature windings can be more sensitive to the design parameters due to the interaction between the operating current and the flux density. In this article, the influence of stator geometry on the machine with HTS armature windings and PM rotor is investigated. By considering the interaction between the operating current and the flux density, an iteration procedure for calculating the operating current of the HTS armature windings is proposed. A 10-MW surface-mounted PM wind power generator with HTS armature windings is studied by the finite element analysis. It is found that machines with different coil angle have different behaviors when changing the tooth width and the dimensions of tooth tips, which is different from conventional electrical machines. A new design and optimization method is then developed. Finally, the optimal design is identified with global optimization. [ABSTRACT FROM AUTHOR]

We propose and investigate the new scheme of a two-section vircator with a photonic crystal (PC) as a section of modulation and interaction with a virtual cathode (VC). The model demonstrates the high stability of the generation frequency for such type of devices and the electronic efficiency of the order of 12.8%. We achieve this result by implementing a feedback mechanism that stabilizes the frequency and increases the generation efficiency. We show that the primary generation mechanism in the proposed scheme is based on the preliminary modulation of a beam passing through the PC and forming a VC. The model is also characterized by distributed feedback inside the PC, which ensures the effective interaction of a VC with the field in the PC due to spatial synchronism. This regime arises when the beam current lies between the critical currents for each of the sections. The advantages of the proposed scheme include operating in a wide range of values of the beam energy and current, relative design simplicity, and scalability. [ABSTRACT FROM AUTHOR]

In this article, the operating current and torque of surface-mounted permanent magnet (SPM) wind power generators with high-temperature superconducting (HTS) armature windings are analyzed. The influence of coil location and current angle on the electromagnetic performance is investigated systematically. The results show that the location of the HTS coils significantly affects the operating current and torque. Furthermore, the maximum torque current angle of SPM wind power generators with HTS armature windings can be different from 180° due to the interaction between the armature and magnet fields. A design optimization method for wind power generators with HTS armature windings is developed in this article. The optimal design is then identified with consideration of the coupling influence of coil location and current angle. [ABSTRACT FROM AUTHOR]

Temperature-dependent current transfer properties of copper (Cu)- and iron (Fe)-sheathed magnesium diboride (MgB2) superconducting wires are studied by the estimation of current transfer length (CTL) and the electric field for various cross-sectional area combinations of the superconducting core, barrier, and outer metallic sheath. The result indicates that the CTL values increase with increasing operational temperatures. The least CTL values are obtained for Fe-sheathed wires (MgB2)/V/Fe) for all the studied cross-sectional area combinations in the wide temperature range (10–25 K) as compared to Cu-sheathed wires (MgB2/V/Cu). The change in CTL values with varying operational temperature is small for both types of studied superconducting wires. The estimation of heat flux density generated on the metallic sheath suggests that the electrical dissipation is lower for Fe-sheathed superconducting wires. Hence, the lower CTL values suggest that the Fe-sheathed MgB2 wire with vanadium barrier is operational with the lowest electrical dissipation. [ABSTRACT FROM AUTHOR]

Superconducting cables or fault current limiters may encounter short-circuit currents during operation. When experiencing multiple overcurrent cycles, the critical current degradation of yttrium barium copper oxide (YBCO) tapes may occur. The performance degradation of YBCO tapes under overcurrent is a gradual process. It may take dozens or hundreds of overcurrent or overheating pulses. To get a better understanding of this process, we explored the performance change of YBCO tapes under repeated overcurrent. A life characteristics test platform was established. The electromagnetic performance degradation of YBCO tape under 50 Hz sinusoidal ac operating conditions was investigated, taking the effect of the overcurrent amplitude and duration into account. Meanwhile, the relationship between the number of overcurrent cycles and the $I_{c}$ degradation is proposed. The reason for this performance change is analyzed by comparing the 2-D current density distribution of the YBCO film before and after experiencing overcurrent. In addition, the U–I property and resistance variations of these tapes are presented. It is inferred that the cracking of the superconducting layer is the main cause of $I_{c}$ degradation. The experimental results provide meaningful references for the performance degradation of the YBCO tapes and related devices. [ABSTRACT FROM AUTHOR]

Divertor Tokamak Test facility (DTT) is going to be built in Italy, and is expected to be operated along with ITER to explore the potential solution for the power exhaust to be eventually implemented in DEMO. 77 tons of Nb3Sn strand in two types are required by DTT. For type one, the weight is 55 tons. The dominant performance requirements are Ic >285 A (4.2 K, 12 T) and hysteresis loss <1000 mJ/cm3 (4.2 K, ± 3 T). For type two, the weight is 22 tons. The dominant performance requirements are Ic >260 A (4.2 K, 12 T) and hysteresis loss <400 mJ/cm3 (4.2 K, ± 3 T). These performance requirements are quite challenging in the respect of performance stability in Nb3Sn strand production. WST has successfully developed three types of Nb3Sn strand to meet these requirements especially for DTT. For DTT type one, considering the ultra-high level of Ic, Nb and Sn volume was increased based on the structure of WST Nb3Sn strand for ITER. Ic >300 A (4.2 K, 12 T) and hysteresis loss <700 mJ/cm3 (4.2 K, ± 3 T) have been obtained from both 164-filament structure and larger-filament structure. For DTT type two, Cu split structure was selected to guarantee low hysteresis level. For the structure with 160 filaments and one Cu split, Ic >260 A (4.2 K, 12 T) and hysteresis loss <400 mJ/cm3 (4.2 K, ± 3 T) have been achieved. Finally, all the performance of DTT Nb3Sn strand has been accomplished by WST. [ABSTRACT FROM AUTHOR]

The properties of superconductors, such as a critical current and an n-value, have a significant effect on their loss characteristics. This effect is essential for the design of superconducting devices and their loss reduction and, therefore, needs to be thoroughly understood. The dependence of the ac loss in high-temperature superconductors (HTSs) on these properties has been well studied. However, it is still unknown how the dynamic loss is affected. The aim of this article is to address this unsolved problem and provide comprehensive analyses through both numerical simulation and experimental measurements. In this article, HTS coated conductors with a wide range of critical currents and n-values have been extensively studied. Their dynamic losses and resistance under various conditions have been modeled and compared to available experiment measurements. The results clearly show the dependence of the dynamic loss and resistance on the critical current and n-value. The cases with the high current load under strong external magnetic field have been studied as well. All these results show a rapid change of the dynamic loss around certain magnetic fields, which we defined to be the “corner field,” Bcor. This article clearly shows the dependence of the dynamic loss and resistance on the critical current and n-value, which can be used as a reference for material selection and design optimization. [ABSTRACT FROM AUTHOR]

High-temperature superconducting (HTS) composite conductors with high carry-current abilities become a preferable choice for superconducting devices to meet the requirements of the power system. In actual operation conditions of superconducting power devices, it is always accompanied by variational current ramp rates, which may cause the ac loss in the conductors. Therefore, this work carries out research on the ac losses of a stacked conductor and a twisted stacked-tape cable (TSTC) with different current ramp rates by changing their structural parameters. Critical current measurements at multiple rates of HTS tapes are initially investigated. Furthermore, the influence of the number of stacked tapes and the twist pitch on the critical current of the conductors is studied. Based on the experimental results, the ac losses of the samples are calculated in COMSOL multiphysics by using a T–A method. This work provides variation trend references of the critical current in HTS tapes and conductors under several rates. Moreover, the ac losses of a stacked-tape conductor and a TSTC with different structural characteristics have been presented in detail. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model for evaluating the inductance and critical current characteristics in solenoid-type superconducting magnetic energy storage (SMES) magnets. The inductance submodel covers the estimations of self and mutual inductances among the single-pancake coils, double-pancake coils, and solenoidal magnets. The critical current submodel reveals the transient critical current characteristics such as the minimum critical current and initial quenching position inside one specific magnet. As a case study, numerical calculation and experimental verification of a small-scale ReBCO magnet are investigated. Being verified by both the simulations and experiments, the proposed SMES model can be expected to evaluate the energy storage property prior to practical magnet fabrication. [ABSTRACT FROM AUTHOR]

A key factor concerning the superconducting application of HTS tapes is the volume of current they can carry. The resistivity of the HTS tape is theoretically zero if lower than the critical temperature, but it cannot carry an infinitely large current. Therefore, the critical current of different HTS tapes must be evaluated to determine their maximum current carrying capacity. First, this study proposed an experimental structure based on controllable dc current ramp rate (R-value) and the critical current of three kinds of tapes were measured, respectively. Finally, this research provides new insight into the understanding of HTS tapes with the magnetic substrates and the nonmagnetic substrates. [ABSTRACT FROM AUTHOR]

High-temperature superconducting pulsed power transformer (HTSPPT) is an important component of superconducting pulsed power supply. Six superconducting coils and four normal conducting coils have been designed and manufactured in our previous work. The six superconducting coils and four normal conducting coils can be assembled into a large HTSPPT to obtain high energy storage. Five arrangement modes are proposed and analyzed in this paper. The self-inductance, coupling coefficient, magnetic field distribution and critical current of the five modes are obtained by finite-element simulation and calculation. The results show that Modes 1, 2, and 4 are feasible. Especially, Mode 1 has the highest coupling coefficient, the largest critical current, the highest energy storage capacity, the lowest capacitor voltage, and the shortest current rise time. As a result, Mode 1 is the best assembly sequence for the HTSPPT. The alternate arrangement of all primary and secondary coils can obtain the highest coupling coefficient and energy storage capacity. [ABSTRACT FROM AUTHOR]

Angular dependence of second generation (2G) high-temperature superconducting (HTS) tape is a crucial parameter for its industrial applications. Known as soldered-stacked-square (3S) HTS wire, a novel structure of HTS wire has been proposed in our previous research. In this paper, angular dependence of 1 mm HTS tape, 4 mm tape, 1 mm 3S HTS wire, and 1 mm 3S HTS twisted wire were measured in liquid nitrogen bath, respectively. It can be found that the 3S HTS wire would not enhance the critical current density under external magnetic field, while an isotropic angular dependence is obtained when 3S HTS wire is twisted. The test results obtained in this paper provides useful information for the future applications of 3S HTS wire. [ABSTRACT FROM AUTHOR]

A high-temperature superconducting (HTS) conductor with quasi-isotropic and high engineering current density is proposed. The HTS conductor consists of inner quasi-isotropic strand and outer power cable. The inner quasi-isotropic strand, equivalent to a former, is a twisted quasi-isotropic conductor made from four components stacked by the second generation (2G) HTS wire with symmetrical arrangement. The outer conductor is made by winding the 2G wire on the inner conductor that is equivalent a power cable. Simply speaking, the HTS conductor is a power cable with replacing the copper former by HTS quasi-isotropic strand so that its engineering critical current is high. The HTS conductor has quasi-isotropic critical current density, since it is a combination of a quasi-isotropic strand and an isotropic power cable. The engineering current density is improved by changing twisting pitch of inner quasi-isotropic conductor and pitch of outer power cable, the conductor is also of benefit to mechanical flexibility and quasi-isotropic critical current density. Additionally, the critical current density of inner quasi-isotropic strand is higher than the one in self-field by considering the effect of longitudinal magnetic fled taking into account. [ABSTRACT FROM AUTHOR]

Narrowing high-temperature superconducting (HTS) tapes can effectively enhance the ability of antielectromagnetic interference. In addition, different structures of HTS wires are necessary for HTS applications. Thereby, a kind of soldered-stacked-square (3S) HTS Wires has been proposed. This paper presents a process of fabricating several 3S HTS wires. A normal second-generation (2G) HTS tape having 4-mm width was narrowed into four 1-mm-width narrow HTS tapes. The main propose of this paper is to study different effects of mechanical force on the critical current of 3S HTS wires. Therefore, we measured the critical current variation of these wires from the torsion, bending, and tension effects points of view, respectively. Compared with their initial critical current values, if the reduction of the influenced critical current is smaller than 5%, it can be considered that these 3S HTS wires are workable. Based on the results of the experiment, the limitations of mechanical force for these wire applications are: minimal twisting pitch of 64 mm, minimal bending diameter of 54 mm, and maximum tensile force of 374 N. [ABSTRACT FROM AUTHOR]

Flux-pinning mechanisms of Al2O3-doped YBa2Cu3O7- y (Y-123) have been investigated by electrical and magnetic measurements. Using the Ambegaokar and Halperin model applied to magnetoresistivity ρ(T, H), we calculated the critical current density in the grain boundaries at zero temperature. The field dependence of the critical density ${J_c}(B)$ was analyzed within the collective pinning model. A crossover field Bsb from the single vortex to the vortex bundle-pinning was observed. It has been found that there is competition between $\delta l$ and $\delta {T_c}$ pinning in the nanosize Al2O3-doped Y-123 (Y-123:Al) samples. The $\delta {T_c}$ and $\delta l$ pinning mechanisms play an important role, each of them in its own temperature region; $\delta l$ and $\delta {T_c}$ are dominant mechanisms at low and high temperatures, respectively. It is noted that $\delta l$ and $\delta {T_c}$ mechanisms coexist at 50 K for the Y-123:Al sample. We have also plotted the new B–T phase diagram of Y-123:Al samples. [ABSTRACT FROM AUTHOR]

A method to calculate second-generation (2G) high temperature superconducting (HTS) coils’ critical current is presented. Critical current and n value of HTS tapes measured under different magnet field are used for the calculation of the coil. The distribution of HTS coils’ magnetic field intensity, current density, and electric field intensity is calculated with COMSOL 2-D based on H formulation. A partial differential equation (PDE) model and the magnetic field formulation (mfh) model of HTS tape are constructed. The modeling method and key points are also introduced and discussed. AC loss of the HTS tape is calculated with the above-mentioned two models, respectively. The calculation results of the two modeling approaches are coincident. The cross sections of the straight segment and bending segment of a HTS racetrack double-pancake coil are modeled, respectively, with a 2-D and 2-D-axisymmetric model in COMSOL, and the critical current is calculated. E–I properties of each turn of the coil and the critical current are calculated. HTS coils with seven pairs of voltage lead are wound. Voltages at different parts of the coils and the critical current are measured. The measurement results of the magnetic field, electric field, and critical current agree with the calculation results. The method presented in this paper can also be referred to when designing an HTS machine with HTS tapes or coils. [ABSTRACT FROM AUTHOR]

We report on the experimental evaluation of dispersion effects on superconducting quantum interference device (SQUID) arrays made with high-temperature superconductor technology. The dispersion of Josephson junction parameters not only results in different operational temperature ranges but also in a flux shift of the voltage versus flux response of the SQUID. Another contribution to the phase shift originates from unbalanced inductances of each branch of the SQUID loop. Measurements show that both contributions can be discriminated. Optimal operation of SQUID arrays is obtained in principle only when these self-induced fluxes are negligible. Degradation of the response is more important when the involved currents are larger, i.e., at lower temperature. We compensated these phase shifts for an array of a few SQUIDs connected in series by direct current injection in one of the branches of each SQUID, thereby recovering the characteristic “antipeak” response. This demonstrates that in the lower temperature range of SQUID operation, phase shifts are mainly responsible for the degradation of the SQUID array response. [ABSTRACT FROM AUTHOR]

Axions and axion-like particles appear in well-motivated extensions of the standard model of particle physics and may be the solution to the long-standing puzzle of the dark matter in our Universe. Several new experiments are foreseen in the next decade searching them in a wide range of the parameter space. In the mass region from few to several tens of microelectronvolt, detector sensitivity will be limited by the standard quantum limit of linear amplifiers and a new class of single microwave-photon detector will be needed. We have developed a single photon counter based on the voltage switching of an underdamped Josephson junction that is coupled to a coplanar waveguide. By measuring the switching voltage, we can register single photons at 14 GHz with the rate less than 1 photon per 3000 s. [ABSTRACT FROM AUTHOR]

We consider conditions allowing attending the highly linear voltage response of a bi-SQUID, taking into consideration the inductances of both loops and their shared inductance. The existence of the small latter one allows improving the law of the flux to phase transformation and essential enlarging margins of the high-linearity response. The reported data represent the design map for creation practical high-performance bi-SQUID-devices, taking into account a restricted fidelity in both design and fabrication. [ABSTRACT FROM AUTHOR]

In this paper, a field coil is optimized for a kW-class high-temperature superconducting (HTS) generator prototype to decrease tape consumption and improve the use efficiency of the HTS tapes by increasing the operating current. First, the structure of the field coil is preliminarily designed based on a novel graphic method. Then, the field coil is optimized by using a genetic algorithm to decrease the tape consumption. Finally, the field coils are fixed in the finite element method generator prototype to study and compare the electromagnetic characteristics. The current distribution is calculated based on the Jc-B-θ characteristics, and the minimum current to determine the operating current is also derived. After the optimization, the electromagnetic characteristics are improved and the operating current is increased. [ABSTRACT FROM AUTHOR]

Superconducting coils limit current is usually measured on unloaded short samples of strands extracted from the same cable used to wound the coil. However, the critical current density of Nb3Sn strands is strongly dependent on the applied strain. This dependence is well reproduced by the so-called exponential scaling laws on strands subject to uniaxial loading. Unfortunately, no standard procedure exists to apply these laws under the complex stress condition experienced by the strands inside Rutherford cables for particle accelerator magnets. As a consequence, an empirical limit from magnet tests was set in the past: 150-200 MPa of maximum equivalent stress within the coil, considered as a block of uniform material. It is not clear how this number relates with the actual stress/strain state in the strands and if it is possible in magnet applications to infer the coil degradation from single strands tests. In this context, this paper presents a model to estimate the critical current degradation of superconducting coils as a function of the applied strain. The model results were compared against the critical current degradation measured on impregnated cable stacks subject to transverse pressure. [ABSTRACT FROM AUTHOR]

VAMAS TWA 16 promoted an international round robin test (RRT) on critical current (Ic ) measurement of RE-Ba-Cu-O (REBCO; RE=rare earth) superconductors to establish the transport Ic measurement method in liquid nitrogen bath without an external magnetic field. Ten laboratories from five countries participated in the RRT and tested commercially available REBCO superconductors. Ic and measurement conditions were reported. In parallel, intrinsic Ic nonuniformity of the superconductors were evaluated by reel-to-reel scanning Hall-probe microscopy for 10-m conductors. Statistical analyses of Ic provided uncertainty of the measurement method. Combined standard uncertainty was analyzed considering the intrinsic nonuniformity of Ic . The analysis indicated that the intrinsic nonuniformity has the largest contribution to the uncertainty of Ic. [ABSTRACT FROM AUTHOR]

Recent development in the superconducting REBCO joints is an encouraging prospect for realizing an HTS magnet in persistent current mode operation. In our ongoing project, we are developing a cryocooled 1 T REBCO magnet system with joints to demonstrate the joint performance in an operating condition expected, for example, for an MRI. Preliminarily, the small test coils have been fabricated to verify the winding method. Additionally, we have designed and manufactured the new leads structures for feeding the conductor from the winding to the joints. [ABSTRACT FROM AUTHOR]

We measured the critical tensile strains at room temperature and the critical bending strains of in situ round MgB2 wires, and both critical strain values agreed well with each other. The characteristic I-V curve was seen at the Ic measurement of the critical strained wire of which the filament was thought to be partly damaged. [ABSTRACT FROM AUTHOR]

The effect of Nd3.6Ba2.4Cu1.8O5-δ [Nd(422)] dopant on the microstructure and superconducting properties of Nd0.9 Ba2.1Cu3O7-δ [Nd(123)] superconductormelt-processed in air has been studied using various techniques. It has been confirmed that the addition of Nd(422) phase prior to melt-processing significantly enhances the superconducting prosperities of Nd(123) superconductor when melt-processed in air atmosphere. Microstructure analysis using an electron probe micro-analyzer revealed the presence of Nd(422) inclusions in the 123 matrix and these inclusions are better dispersed and more refined in the doped sample. Compared to the undoped sample, the Nd(422)-doped sample showed significant improvement of current density at lowfields with largely depressed fishtail peak effect at high temperatures. These results are ascribed to the effect of Nd(422) phase, which yet unoptimized to serve as efficient pinning centers and also allow the introduction of sufficient field induced pinning centers in the sample. [ABSTRACT FROM AUTHOR]

Superconductor/insulator/superconductor trilayer structure can be used to increase the superconducting properties in self-field of thick REBa2Cu3O7-δ (RE = rare metal) films by inhibiting the thickness effect, as well as improving in-field properties through the interlayer, which acts as a flux pinning interface. The conventional trilayer structure was always prepared symmetrically, namely, the bottom superconducting layer has the same thickness with the top superconducting layer. In this paper, Y0.5Gd0.5Ba2Cu3O7-δ/SrTiO3 /Y0.5Gd0.5Ba2Cu3O7-δ (YGBCO/STO/Y-GBCO) trilayer films were prepared asymmetrically by pulsed laser deposition on CeO2 buffered IBAD-MgO tapes, which means the bottom superconducting layer has a different thickness than the top superconducting layer. YGBCO/ STO/YGBCO trilayer structure had the same total YGBCO layer thickness of 720 nm and STO interlayer thickness of 80 nm, but a bottom YGBCO superconducting layer of different thickness. Microstructure, surface morphology, and self-field and in-field superconducting properties were systematically investigated. It was found that thicker bottom-layer had almost no internal residual stress, better crystalline quality, and surface morphology, which contributed to better superconducting properties of the YGBCO/STO/YGBCO trilayer sample both under self-field and magnetic field. The phenomenon was explained by the growth model. These results can provide useful information for fabricating trilayer structure with high in-field performance. [ABSTRACT FROM AUTHOR]