Table of contents

33^rd International Symposium on Superconductivity (ISS2020)

It is our pleasure to report that the Proceedings of the 33^rd International Symposium on Superconductivity (ISS2020) held at AIST Tsukuba, Japan with virtual meeting rooms, December 1–3, 2020, are now published in Journal of Physics: Conference Series (JPCS). ISS2020 was co-organized by the National Institute of Advanced Industrial Science and Technology (AIST) and the Victoria University of Wellington (VUW). At first, ISS2020 was planned to be held in Wellington, New Zeeland. However, owing to the COVID-19 pandemic, the venue was changed to Tsukuba, Japan, and the symposium was organized to have both real and virtual meeting rooms. Participants from abroad joined online through the virtual meeting rooms.

Total of 462 scientists, engineers, students, corporate executives and so on (182 from abroad) registered for ISS2020, and fruitful discussions were made to promote superconductor sciences and technologies. Total of 206 papers were presented, which included 5 plenary lectures, 59 invited talks, 142 contributed oral presentations. The papers published in JPCS were categorized into relevant topics in the following four fields; (a) Physics and Chemistry, (b) Wires and Bulk, (c) Electronic Devices, (d) Large Scale System Applications.

To ensure the high publication standard mandated by JPCS, every paper was peer reviewed by a reviewer with expertise before it was accepted for publication. As editors of the Proceedings, we would like to express our sincere appreciation to all the reviewers involved in the evaluation of the papers for their invaluable contribution.

List of Organization of ISS2020, Committees of ISS2020, International Advisory Committee are available in the pdf

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All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

• Type of peer review: Single-blind

• Conference submission management system: via ISS2020 Homepage

• Number of submissions received: 41

• Number of submissions sent for review: 41

• Number of submissions accepted: 38

• Acceptance Rate (Number of Submissions Accepted / Number of Submissions Received X 100): 93

• Average number of reviews per paper: 2.1

• Total number of reviewers involved: 42

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• Contact person for queries:

Name : Hirofumi Yamasaki

Affiliation: Secretariat of International Symposium on Superconductivity

Email : h.yamasaki@iss-secretariat.org

We study three-dimensional structures of vortices in a type II superconductor under a magnetic field by solving the Ginzburg-Landau equations. First, we show under a perpendicular magnetic field, two vortices are almost parallel but bend toward the center of superconductor due to the Meissner current. Second, we show under a tilted magnetic field, two vortices enter the superconductor from edge to edge and they are parallel to the magnetic field.

A reversible-irreversible transition (RIT) is studied using a periodically-driven vortex system in an amorphous film with random pinning that causes local shear, as a function of shear amplitude d. The relaxation time to reach the steady state exhibits a power-law divergence at a threshold value d_c with critical exponents in agreement with the values predicted for an absorbing phase transition in the two-dimensional (2D) directed-percolation (DP) universality class. In our previous work, the experiment was conducted at relatively high frequency f, giving rise to a large mean vortex velocity v. Here we use lower f to study the effects of reduced v and increased dynamic pinning on the RIT. The results show that the critical behavior of RIT stays essentially unchanged, while we find a trend for d_c to increase with decreasing v. We will propose a possible model to qualitatively explain this unexpected result.

Effects of three kinds of columnar defects in NbSe₂ single crystals introduced by 320 MeV Au irradiation with a dose equivalent matching field B_Φ up to 4 T were studied. Through the magnetization measurement using a SQUID magnetometer, two pronounced peak effects in the external field dependence of critical current density (J_c) were observed in crystals introduced with splayed and tilted columnar defects. In crystals with splayed columnar defects, J_c at the peak field reaches the maximum for splay angle θ_CD = ±10°, which is a little different from that found in YBa₂Cu₃O₇ and iron-based superconductors, where optimal θ_CD is equal to ±5°.

In this paper we report the results of the first-principles calculations for (Ca_1-xSr_x)Pd₃P for various crystal structures observed in experiment. The calculated electron specific heat coefficient shows a good agreement with the experimental values. We found the centrosymmetric phase has a higher density of state at the Fermi level than that of the non-centrosymmetric phase. Possible parity mixing in the non-centrosymmetric phase is estimated as small in this system.

The combined scanning tunneling microscopy/spectroscopy (STM/STS) and the break-junction tunneling spectroscopy (BJTS) measurements of the three-layered Pb_xBi_2-xSr₂Ca₂Cu₃O_10+y cuprate superconductors were carried out. The averaged dI/dV spectrum obtained by the STS shows the gap Δ_STS ≈ 45 meV, while that of BJTS shows Δ_BJ ≈ 35 meV. In case of the BJTS measurements, we also referred the zero bias (ZB) peak as being due to the maximum Josephson current I_Jc. The product of I_Jc and normal resistance R_N (I_JcR_N) was obtained I_JcR_N ≈ 3.5 mV at T = 11 K, which is one order lower than that of the BCS-based Ambegaokar-Baratoff theory with Δ_BJ =35 meV taken into account. With increasing the temperature, the I_JcR_N was reduced and vanished just below the T_c (≈ 106 K), indicating the bulk superconductivity. Simultaneously, from the temperature dependence of the gap features from 80 K to 120 K, the gap Δ was shown to persist across the T_c, indicating that the I_JcR_N product is a significant parameter for understanding the cuprate superconductivity.

Effect of co-substitution of Ca and Sr in the Y-124 phase of Y_1-xCa_xBa_2-ySr_yCu₄O₈ has been investigated. Polycrystalline samples of Y_1-xCa_xBa_2-ySr_yCu₄O₈ with 0 ≤ x(Ca) ≤ 0.2 and 0 ≤ y(Sr) ≤ 0.2 have been prepared by the molten KOH method. The lattice parameters decrease with increasing y(Sr), indicating that the smaller Sr ions are systematically substituted for the larger Ba ions. The value of T_c increases from 83 K to ∼ 90 K through the Sr-substitution for x(Ca) = 0, while T_c ∼ 90 K are almost independent of y(Sr) for x(Ca) = 0.1 and 0.2.

Polycrystalline samples of the Bi-2212 phase of Bi_2−xPb_xSr₂CaCu₂O_8−zF_z with x(Pb) = 0.6 and 1.0 have been prepared. All the samples are almost of the single phase with the Bi-2212 structure for x(Pb) = 0.6. The value of T_c ∼ 82 K for z(F) = 0 increases up to 86–88 K for z(F) = 0.2–0.6. This may be due to the decrease in the amount of vacancy at the O-site by the introduction of F⁻. In addition, we have successfully synthesized the partially Ba-substituted Bi-2212 phase of Bi_1.4Pb_0.6Sr_2−yBa_yCaCu₂O_7.8F_0.2 with 0 ≤ y(Ba) ≤ 0.5. T_c increases up to 98 K for y(Ba) = 0.5. The enhancement of T_c through the Ba-substitution may be due to the increase in the distance between Cu and the apical oxygen.

We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As_1-x'P_x')O exhibits the re-emergent antiferromagnetic (AFM) order at x' ~ 0.6 (AFM2) separated from the parent AFM order at x' =0 (AFM1). Systematic ³¹P-NMR study on [Sr₄Sc₂O₆]Fe₂(As_1−xP_x)₂ (SrSc42622), which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As_1−x'/P_x')O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around x ~ 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer. In order to elucidate the further universality and diversity, we have carried out ⁷⁷Se-NMR measurement on Li_x(NH₃)_yFe_2−δSe₂ (T_c = 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (T) dependences of Knight shift and nuclear relaxation rate (1/T₁T). We discuss the universality and diversity of the relationship between the T_c and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.

We measured the complex conductivity, σ, of the FeSe_1−xTe_x (x = 0 – 0.5) films below T_c which show a drastic increase of the superconducting transition temperature, T_c, when the nematic order disappears. Since the magnetic penetration depth, λ (> 400 nm) of Fe(Se, Te) is longer than the typical thickness of the film (∼100 nm), we combined the coplanar-waveguide-resonator- and cavity-perturbation techniques to evaluate both the real and imaginary parts of σ. Films with the nematic order showed a qualitatively different behavior of the quasiparticle scattering time compared with those without the nematic order, suggesting that the nematic order influences the superconducting gap structure. On the other hand, the proportionality between the superfluid density, n_s/m* (∝ λ⁻²), and T_c was observed irrespective of the presence or absence of the nematic order. This result indicates that the amount of the superfluid has a stronger impact on T_c of Fe(Se, Te) than the presence or absence of the nematic order itself.

We report the fabrication and the transport measurements of FeTe_1-xSe_x (x=0.4, 1) nanobridges along the c axis, toward the appearance of Josephson effects in single-crystal devices. Cross sectional areas of both FeTe_0.6Se_0.4 and FeSe nanobridges were systematically reduced to 0.06 μm² by using a new method based on the focused ion beam (FIB) techniques. The critical current I_c measured by the current-voltage characteristics is roughly two orders of magnitude smaller than that for the conventional microbridges with larger cross sections, while the I_cR_n product, where R_n is the normal-state resistance along the c axis, is still 3-4 times larger than the theoretical value for the appearance of Josephson effects. We argue the importance of the development of single-crystal Josephson devices and the comparison between FeTe_0.6Se_0.4 and FeSe nanobridges.

We have synthesized Fe_1+xTe_0.6Se_0.4 large bulk single crystals by the melting method with a homogenization heat treatment. The relationship between the crystal structures and the superconducting properties was investigated. Critical temperature T_c onset reached the highest value of 14 K for the crystal with x = - 0.005. The maximum critical current densities J_c in the a-b plane at 4.2 K achieved 1.53 and 0.83×10⁵ A/cm² at 0 T and 5 T for the crystal with x = 0, respectively. The crystal lattice and structural parameters were refined by Rietveld analysis. The anion height was estimated by using the obtained parameters. The anion height dependence of T_c and J_c were investigated. The T_c decreased as the anion height increased, and it showed the same tendency as other Fe-based superconductors. On the other hand J_c increased initially and then decreased with the increase in the anion height. We believe that it comes from the trade-off between the increase in flux pinning centres of excess Fe sites and the deterioration of superconductivity.

Enhancement and non-monotonic field dependence of critical current density (J_c), called anomalous peak effect, were observed in Ba_1−xK_xFe₂As₂ with splayed columnar defects. In principle, anisotropy of in-plane J_c should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of J_c observed in 2.6 GeV U irradiated Ba_0.6K_0.4Fe₂As₂ with splay angles of ±15° and ±20°. Results show that J_c component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B_Φ/3 more.

Novel nematic states as well as remarkable superconducting properties were reported in iron-based superconductors. We synthesized high-quality single crystals of (Ba_1-xRb_x)Fe₂As₂ close to the optimal composition of x ≈ 0.4, and have characterized normal state and superconducting properties for pristine and proton irradiated samples. The highest T_c was found to be ∼36.6 K for pristine samples. A maximum critical current density of 12 MA/cm² was achieved in a proton irradiated sample, which is comparable to that in irradiated (Ba_0.6K_0.4) Fe₂As₂. Twin boundaries were observed optically in both non-doped and under-doped samples. We also attempted local magnetic field measurements induced by transport current in a sample with twin boundaries for future exploration of nematic states in different regions of doping.

Superconductors can trap large magnetic fields. However, magnetic fields perpendicular to the trapped field reduces the trapped field effective, which is known as crossed-field demagnetization. Crossed-field demagnetization becomes a problem for the application of stacked magnet. We investigated the crossed-field demagnetization of a EuBa₂Cu₃O₇ coated conductor with BaHfO₃ nonorods which has large critical current density. When an AC transverse field is applied after the magnetization of the coated conductor, the decay of trapped field was significantly accelerated. However, for the AC field smaller than the full penetration field parallel to the ab-plane, the decay becomes slow enough. All these results suggests that we can apply the stacked magnet for rotating machines if the alternating transverse magnetic field is reduced below the penetration field.

In this work, the phenomena of electromigration in samples of HTSC tapes of the 2nd generation was investigated. Samples of REBa₂Cu₃O_7-x (REBCO, where RE is a rare earth element) were used in the form of a copper-plated tape. A superconducting bridge was preliminarily formed on the surface of the tape by the method of chemical etching (to decrease the total value of critical current). The sample was exposed by a direct current of I = 0.9I_c. The duration of exposure was up to 350 hours. The value of the current density in the area of the bridge was J = 1.38×10¹⁰A/m². The measurements were carried out at liquid nitrogen temperature (77 K). The influence of the electric current flow on the value of the sample critical current is controlled at the selected time intervals. The experiment showed high stability of the tape under the specified conditions.

In high temperature superconducting (HTS) applications, joining two HTS wires is considered as an essential process for extending the available length. Many soldering and diffusion joints have been designed in recent years, but these joints mainly focused on joining two HTS virgin tapes with 4mm width and 0.15mm thickness. As for the recently developed soldered-stacked (SS) HTS wire, it does not have applicable methods of joining. In this paper, we proposed a novel technique to join two SS wires by soldering and stacking, and measured the joint resistance to demonstrate its feasibility of engineering. First, we join separate layers in SS wires using spot-welding machine. Second, we coat these layers in the solder bath. This method allows making joints and manufacturing SS wires simultaneously. This work will introduce the specification of this jointing method for SS wire, and the related experimental results will provide a reference for the SS wire joint fabrication in the future.

In this study we prepared Y_xYb_1-xBa₂Cu₃O_7-δ (x = 0.1, 0.2, 0.3, 0.4, 0.5) powders and bulks to use as intermedium for superconducting joint between REBCO-coated conductors. Superconducting joints are essential in the development of high-field REBCO persistent magnets. All the powders and bulks were sintered at 850 °C for 10 hours in air. Bulks were prepared by pressing at 70 MPa. Calcination of the bulks was performed under the same conditions as the powders. Two joint samples between GdBa₂Cu₃O_7-δ-coated conductors were prepared by using the bulk samples. For x=0.1, a superconducting joint was successfully obtained.

We aimed to improve the trapped magnetic flux density obtained with a waveform control pulsed field magnetization (WCPM) by using a negative feedback control of the magnetic flux density on the growth sector boundary of a GdBaCuO superconducting bulk sample. The WCPM method with negative feedback control has previously shown that it could help increase the trapped magnetic flux density, compared to conventional passive pulsed flux magnetization, if the magnetic field penetrates the bulk centre substantially using a flux jump. The flux jump sometimes greatly changes the magnetic and thermal state of the bulk, which limits the maximum trapped magnetic field. The active control method of the applied magnetic field helps to overcome this limit if the control conditions are appropriate. While searching for the ideal control conditions of the WCPM, the magnetization characteristics have been investigated. We found that the flux jump, which assists the flux in penetrating the centre of the bulk, can be "slowed down" thanks to the negative-feedback WCPM. This operation helped to decrease the heat generated by the moving flux inside the bulk and reduced the temperature rise, which contributed to the increase of the trapped magnetic flux density.

MgB₂ superconducting bulk materials are characterized as simple and uniform metallic compounds, and capable of trapping field of non-distorted conical shapes. Although pulsed-field magnetization technique (PFM) is expected to be a cheap and an easy way to activate them, the heat generation due to the magnetic flux motion causes serious degradation of captured fields. The authors precisely estimated the flux trapping property of the bulk samples, found that the flux-shielding effect closely attributed to the sample dimensions. The magnetic field capturing of Ti-5.0wt% sample reached the highest value of 0.76 T. The applied field which reached the centre of the sample surface shifted from 1.0 T to 1.2 T with increasing sample thickness from 3.67 mm to 5.80 mm. This means that the shielding effect was enhanced with increasing the sample thickness. Moreover, Ti-addition affected the frequency of flux jump happenings. The occurrence of flux jumps was suppressed in 5.0wt%Ti-added sample. This means that the heat capacity of the compounds was promoted by Ti addition.

We report the fabrication of 10 m-class (Ba, Na)Fe₂As₂ long round wires and coils using these round wires. Round wires are fabricated by powder-in-tube (PIT) method and hot-isostatic-press (HIP) technique. To increase I_c keeping the diameter of the wire constant, the cross section area of the superconducting core is enlarged by changing the dimensions of metal sheaths. At 4.2 K under a high field of 100 kOe, J_c of a short segment of HIP wire reaches 42 kAcm^-2, which is comparable to the highest J_c of IBS round wires of 44 kAcm^-2. Furthermore, I_c of the HIP wire at 4.2 K under 10 kOe and 100 kOe reaches 95 A and 54 A, respectively, which is about twice the value of the previous report. We also fabricated a small demonstration superconducting coil using 12.5 m long PIT wires by HIP method. I_c of the whole coil is ∼60 A under the self-field, and magnetic field at the center of the coil reaches 2.6 kOe. The coil generated the magnetic field of 43 Oe/A, which is almost consistent with the calculated field from the coil dimensions.

We have developed a system using high-temperature radio frequency superconducting quantum interference device (RF-SQUID) for detecting metallic contaminants in the liquid component of a lithium-ion battery. Although we have executed detection experiments using a simulated system without liquid in the past^[1], we have developed a new system to inspect real liquid components. Small cylindrical metallic contaminant samples were fabricated using a gallium-focused ion beam to evaluate the detection performance. Tap water containing the metallic contaminant sample was poured into the tube using a pump, and the magnetic signal of the contaminant matter was detected using the RF-SQUID. Among the tested small metallic contaminant samples, the volume of a minimum detectable metallic contaminant was evaluated to be 2 × 10⁴ μm³, which corresponded to that of a spherical sample with a diameter of 33 μm and a sensitivity of a signal-to-noise ratio of more than three. Moreover, the dependence of the detected signal strength on the volume of the metallic contaminant samples is discussed here.

The operation of the RF SQUID is restricted by the condition that the inductance parameter β_L must be in the range of 1−3. However, since both ends of the Josephson junction (JJ) of RF-SQUID are shorted, it is difficult to non-destructively estimate the critical current (I_C). Thus, we proposed a technique for the non-destructive measurement of the I_C of a high-temperature superconducting (HTS) RF-SQUID ring by evaluating the behaviour of the flux in superconducting thin films using a SQUID magnetometer. A superconducting ring sample with JJ was placed below the HTS SQUID magnetometer and cooled down to 77 K. The change in the SQUID output was monitored on application of the magnetic field. When increasing the field, the waveforms indicated that the screening current of the ring sample exceeded the I_C of the JJ, and the JJ became a normal-conducting state. As a result, we estimated the I_C of the JJ of this sample as 134 μA using the values of mutual inductance and the coupling coefficient α between the coil and the sample.

The current-biased kinetic-inductance detector (CB-KID) is a solid-state superconducting neutron detector with high spatial and temporal resolutions, and multi-hit tolerance. We demonstrate high temperature operation of CB-KID at 7.9 K with the delay-line method. High temperature operation reduces imaging pixel size by suppressing signal propagation velocity. High spatial neutron transmission image for a mixed metal alloy consisting of heavy elements Sm and Sn is successfully constructed. We also examine the capability of element discrimination imaging based on the resonance dip analysis.

The design of plug-in wire system is improved for 200-pixel superconducting tunnel junction (STJ) array of X-ray detector on a helium three cryostat. The electrical connections between the STJ array to the room temperature electronics consists of plural wire modules, which have connectors at the both ends. A flexible coaxial cable with electrical conductors of stainless steel (SUS-FFC) is manufactured and attached to make the electrical connection between room temperature to the 3 K stage. After the installation of SUS-FFC, temperature of the cold stage is 302 mK, and the holding time is more than 115 hours. Compared with the previous setup, the temperature of 3He stage decreased 5 mK, and the holding time increased 30 hours.

An RSFQ flexible-precision multiplier is proposed. The circuit can perform multiplication with specified bit-width within a predefined bit range. The calculation bit-width can be changed in every operation. When the bit-width of a calculation decreases, the latency in cycles is reduced. The proposed circuit calculates the multiplication result with bit-level processing to save the circuit area. The circuit carries out multiplication by counting pulses on a signal line. An RSFQ flexible-precision matrix multiplication circuit based on the proposed multiplier is also proposed. Its internal multipliers share many component circuits and it is implemented in a compact area.

We examine digital behavior of faults caused by a change of the order of the pulse arrivals in Rapid Single-Flux-Quantum (RSFQ) logic circuits. Based on the timing fault model, we present a flow of automatic test pattern generation for testing digital RSFQ chips. As the test pattern generation repeatedly executes fault simulation, the scalability depends on computational cost of the simulation. We propose an efficient fault simulation method based on two ideas. First, we share computation for the fault-free circuit and faulty circuits as much as possible. Secondly, we exploit the pipelined behavior of RSFQ logic circuits to suppress computation for unnecessary signals. We have implemented our simulation method and obtained evaluation results to show that our method is effective for large circuits.

With recent advances in InductEx and the TetraHenry numerical engine, it is now possible to analyze the coupling of trapped flux in moats to superconductor circuit structures. Here we show the analysis of moat position in the layout of an adiabatic quantum-flux-parametron (AQFP) buffer cell on the coupling to critical inductors such as the load inductor. This inductor is often carefully laid out to balance current flow and cancel out stray coupling between clock and dc current excitation lines. We investigate the effect of trapped flux that couples asymmetrically to the circuit—either due to the moat position and shape or due to only one of a symmetrical duo of moats being filled with a trapped fluxon—on circuit operating margin and performance. We conclude with suggestions on moat placement to reduce the chances of circuit failure when fluxons are trapped in the moats.

We have studied the manipulation and the separation of substances using the horizontal magneto-Archimedes effect. By ingenerating a high magnetic field gradient in the horizontal direction, it is possible to move the target substance to a designated position in the horizontal direction. We attempted to separate several kinds of valuable metals by applying the horizontal magnetic force to the metals falling vertically into a paramagnetic medium. In order to enhance the magnetic force, we set up the ferromagnetic core array in magnetic fields. According to the trajectory analysis for the metal particles and the magnetic separation experiments, we succeeded in dynamically separating the metal mixture dropped to the vertical direction due to the strong horizontal magnetic force in relatively low magnetic fields. It is expected that this method creates a new device for the separation of substances.

High-temperature superconductivity (HTS) has the potential to be a useful technology for space applications, allowing for high current densities and magnetic field generation in compact devices. However, HTS requires cryogenic temperatures and it is not well understood how this can best be achieved in a space environment. Using a modelling approach, the expected heat load on a hypothetical 3U CubeSat with an HTS coil during a sun-synchronous low Earth orbit was predicted. The direction and magnitude of solar, albedo and infrared incident radiation toward the satellite was calculated for each orbital position of a circular 732 km, Ω = 0° longitude of ascending node orbit. Using a finite element approach, the surface radiosity and temperature of the CubeSat was predicted and validated. Finally, the instantaneous heat load on the HTS magnet, which generates a 1T magnetic field, was calculated as a function of orbital position. This study provides technical information about the characteristics of the refrigeration device required to maintain cryogenic temperatures for an HTS coil on a space faring vessel, such as a portable cryocooler. Selection and design of a satellite cooling system must be optimised according to the calculated heat load and available solar power.

High-temperature superconducting (HTS) magnetic levitation (maglev) systems have been studied by various research groups regarding both experimental and modelling point of view. However, there exists a trade-off between levitation and guidance forces acting on the vehicle, especially in the case of high-speed curve negotiation. To overcome this trade-off, we proposed a multi-surface permanent magnet guideway (PMG) design, for the small-scale maglev vehicle, in which polarization of the permanent magnets (PM) changing with track segments. The HTS-PM interaction model was constructed by utilizing H-formulation implemented in COMSOL Multiphysics^®. The hysteretic levitation and guiding force expressions used in the dynamic simulation have been obtained by a polynomial fit to force-displacement curves obtained by the finite element model built in COMSOL Multiphysics® environment. Also, the damping effect derived from free-fall dynamic tests is incorporated into the model to construct a more realistic simulation model. The effectiveness of the proposed track design has been validated through comparisons with Halbach-derived PMG. Finally, it can be thought that the proposed PMG design is a good candidate for the high-speed operation of a maglev system when the increased levitation and guiding stiffness values are considered.

The electrodynamic suspension system (EDS) is one of the magnetic levitation system and has advantage of stable levitation without the gap control. However, the damping factor of the EDS system is not enough. Once the oscillation of the bogie occurs by the external disturbance, it does not converge immediately. The additional damper system is needed, so the damper coil system is installed. By the semi-active damper system, the levitation stability is improved. This paper discusses about superconducting Maglev system as application of the EDS system. Superconducting Maglev has been the magnetic levitation system developed by Central Japan Railway Company and Railway Technical Research Institute. The operation speed of the train is v = 120[m/sec]. At low speed range such as v = 60-80[m/sec], the damping factor is smaller than that of the normal operation v = 120[m/sec], and it is difficult to keep the stable levitation. In this paper, running simulation when the bogie passes large displacement at low speed is undertaken, and the improved switching method for the semi-active damper system at low speed and large displacement is discussed. From the results, to increase the period that the damper coil works. The damping factor becomes larger, and the bogie keeps the levitation.

This paper proposes the design of the magnetic circuit to improve stability of the magnetic bearing using HTS. From the rotational characteristic, it is shown that if the flux of the rotor is concentrated to the outer part of the HTS, the rotor will be able to rotate stability. In order to achieve stable rotation, the yoke equipped ring type rotor is proposed. The ring type permanent magnet equipped the yoke is used of this rotor. The change in the flux when the gap between the yoke and the permanent magnet, the yoke thickness and the width are changed is considered. As each value increases, the flux amount increases. As the gap between the yoke and the permanent magnet increases, the flux distribution is more concentrated to the outer part of the HTS. Considering the overall diameter, the boundary where the increasing rate decreases is adopted as the optimum value. In the yoke thickness and the width, the minimum value where the magnetic saturation is not observed is adopted as the optimum value. As a result, the magnetic circuit that the flux is concentrated to the outer part of the HTS is designed. By concentrating the flux to the outer part of the HTS, the stable rotation is given.

Electric propulsion is seen as a potential solution for reducing greenhouse gas emissions from the aircraft industry. However, electrical machines must achieve high power to mass ratios (PtM) to meet aviation requirements. Superconducting technologies are a promising option for creating compact and efficient machines. Indeed, superconductors make it possible to generate large magnetic fields while reducing the need for ferromagnetic materials. In previous works, a 50 kW partially superconducting flux modulation machine has been realised. The flux modulation machine is an unconventional topology where the inductor is composed by a large static superconducting coil and rotating superconducting bulks acting as magnetic field shields. This topology allows controlling the inductor excitation while being brushless. In this paper, we design a 500 kW flux modulation machine considering the results of the 50 kW prototype and the constraints due to the structure change of scale. The presented machine aims to reach a power-to-mass ratio of 10 kW/kg.

In many high-temperature superconducting (HTS) rotating machines of radial flux type, the magnetic field from one side surface of the field pole is employed facing the armature winding. However, both side surfaces of the field pole can be effectively utilised to face the armature windings. Thus, we have applied the double armature structure in which both inner and outer armature rotors sandwiched the field pole stator in the HTS synchronous generator. We introduce the first step design and the electromagnetic study of the double armature synchronous machine for tidal current power generation. The double armatures with copper winding were adopted with a field pole composed of the HTS bulks. The power and the torque density were calculated with a low speed that depends on a 500 kW contra-rotating turbine design. In this paper, we comparatively studied three electromagnetic designs in which (1) a permanent magnet was used for the field pole, (2) the HTS bulk was employed as the field pole and (3) the field pole HTS bulk was arranged at an angle in the x-y plane, reducing torque ripple due to torque consistency and tilting of the HTS bulk plate. A generator model consistent with the input mechanical torque of the turbine was obtained from the analysis of compatibility with the 500 kW class tidal current power generation.

Rotating machine applications with superconductors attract much interest in the industry. For maintaining the operating temperature of high-temperature superconductor (HTS) field poles, a sophisticated cooling system is required. A thermosyphon (TS) is one of the ways for this purpose. The cryo-mechanical system composition is relatively simple, lightweight, and based on natural convection therefore not using any mechanical pump. It can supply the refrigerant from the static condenser part to the rotating evaporator part by using a successful rotary joint. The operating temperature of the TS cooling system depends on the refrigerant. Considering the current HTS materials, the suitable refrigerant is neon, hydrogen. Nitrogen which provides a higher operating temperature than neon and hydrogen is employed for future development. In this work, we focused on and investigated the heat transfer capacity of TS using nitrogen as the refrigerant. The heat transfer performance with rotating was also studied. As a result of the experiment, the TS cooling system could maintain the evaporator temperature by rotating. Moreover, it could transfer the 90 W heat load with rotating.

There is a recent surge in activity to develop high power electric (or hybrid electric) aircraft. Part of this development effort is the creation of lightweight and small volume high-performance motors and airborne power transmission cables. As part of the power transmission of a distributed propulsion aircraft will be T-terminals to extract power to individual motors from a "main" power cable. In this research, a standard pressed plate high purity Cu T-terminal, with cylindrical high-temperature superconducting cables (main cable current of 20 kA, branch cable current of 2.5 kA), were investigated using Multiphysics simulations. Then, a more geometrically optimized high purity Al-Cu composite T-terminal was simulated under similar conditions. Discussed are the influence of T-junction geometry, operating temperature (30 to 50 K), contact resistance, and magnetoresistance on joule losses of terminals with different masses. It is shown the Al-Cu terminal can greatly reduce joule losses/mass of the T-terminal while also having an intrinsic clamping force from thermal expansion of the Al shell of the composite structure.

In this study, we designed a power cable based on the longitudinal magnetic field (LMF) effect using an iterative approximation method. An evaluation of the cable shows that it is more efficient than conventional-design power cables. A cable with four inner layers and four shield layers and a maximum winding angle of 30° had a current capacity of more than 10 kA. Based on calculation results, we fabricated a 2-m-long LMF power cable and a conventional-design power cable. Experimental results show that the proposed power cable can carry more current (8.8% more current per tape and 18% more current overall) than a conventional-design power cable due to the LMF effect. These results well agree with the theoretical calculation results.

Too high loss levels can severely limit the efficiency and the safe operation of several high-temperature superconductor (HTS) AC applications. A reliable estimation of AC losses, which includes the observed field-dependence of the superconductor's critical current density on the magnetic field, is therefore paramount. In this contribution, we use numerical simulations to evaluate the AC losses of HTS coated conductors in a variety of working scenarios: individual wires carrying AC transport current and/or subjected to AC magnetic fields, and wire assemblies like stacks and arrays carrying AC transport current. Numerical results are compared to the corresponding analytical models and to experimental results. This work presents some general guidelines regarding the extent to which the dependence of the critical current density J_c on the magnetic flux density B modifies the AC loss characteristics with respect to a constant-J_c model based on the superconductor's self-field critical current.

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All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

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• Number of submissions received: 41

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• Number of submissions accepted: 38

• Acceptance Rate (Number of Submissions Accepted / Number of Submissions Received X 100): 93

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Name : Hirofumi Yamasaki

Affiliation: Secretariat of International Symposium on Superconductivity

Email : h.yamasaki@iss-secretariat.org