The no-insulation (NI) winding method is widely known for its ability to improve the stability of high-temperature superconducting (HTS) coils by allowing the current to bypass the local hot spot during a quench. However, one major drawback of this method is the occurrence of charge and discharge delays due to current leakage between winding turns. To address this limitation, an impregnation technique has recently been proposed, in which the contact resistance is controlled through the use of an electrically-conductive epoxy composite. This approach has shown the potential to reduce the charge/discharge delays while maintaining the inherent quench protection of NI HTS coils. Following this study, this paper aims to evaluate the proposed impregnation technique in a racetrack HTS coil, which is widely used in applications such as superconducting motors and generators. This study involves the fabrication and performance testing of racetrack HTS coils impregnated with an electrically-conductive epoxy composite, focusing on key aspects such as charge/discharge behavior and over-current characteristics under varying electrically conductive powder. Taking the NI coil value of 20.6 mu Omegacm(2) as a reference, the fabricated coils exhibit surface contact resistances ranging from 8.77 to 8055.59 mu Omegacm(2). The ability to control such a wide range of surface contact resistance while maintaining the self-protecting characteristics enables future applications to HTS electrical machines and other industrial systems.

The conceptual design, prototype fabrication, and experimental evaluation of a pipe-integrated cryogenic pump motor using a bulk YBCO superconductor in the rotor were conducted. An induction-type topology was adopted, utilizing a commercial copper-wound stator. The cylindrical bulk YBCO, mounted as the rotor, was designed under the assumption of liquid nitrogen flowing through a pipeline, allowing for direct cooling of the rotor. A locked-rotor experiment was performed on the fabricated prototype in a liquid nitrogen bath, and the equivalent critical current density was calculated through the finite element method (FEM) analysis based on the measured voltage and current data. Using the fitted parameters, the torque and output power characteristics of the motor were calculated, and the motor had approximately 50 W of output power under the given operating conditions.

This article presents a partial-depth impregnation method using electrically conductive epoxy composites in high-temperature superconductor (HTS) coils. In a previous study, we proposed a wet winding technique using electrically-conductive epoxy composites to control the contact resistance of no-insulation (NI) HTS coils. In that method, the epoxy composites were applied across the entire contact surface between winding turns, allowing the contact resistance to be controlled by adjusting the mixing ratio of electrically-conductive powder. However, this approach led to significant current degradation due to delamination caused by thermal contraction mismatch between the epoxy composites and the HTS tape. The partial-depth impregnation method addresses this issue by allowing the epoxy composites to penetrate only partway between winding turns, thereby minimizing critical current degradation. In this study, HTS coils were dry-wound with 4.1 mm-wide HTS tape and insulated with 3 mm-wide polyimide tape on each turn. By applying the electrically-conductive epoxy composites to the edge of the HTS coil, the epoxy composite penetrated to a depth of 1.1 mm. This configuration allows the current to bypass through the edge of the insulated coil, providing self-protection characteristics. Additionally, the contact resistance can be controlled by adjusting the mixing ratio of the electrically-conductive powders. The feasibility of the proposed impregnation technique was demonstrated through over-current and sudden-discharge tests on the partial-depth impregnated coils with different mixing ratios of electrically conductive powders.

This study focuses on developing an electric propulsion platform that utilizes liquid hydrogen (LH2) as both fuel and coolant. The cooling system aims to cryogenically cool the rotor of a high-temperature superconductor (HTS) motor with LH2 while supplying the discharged hydrogen to the fuel cell. By omitting an additional cryocooler for the HTS motor, this approach enhances efficiency and overall performance. The research includes the thermohydraulic and structural design of the HTS magnet rotor, incorporating LH2 and a triple tubemagnetic fluid seal. A prototype cooling system was fabricated, and its performance evaluated using liquid nitrogen (LN2) as an alternative refrigerant to ensure safety. LN2 verified the flow and boiling heat transfer characteristics of the HTS rotor cooling system, with the flow rate controlled by the pressure build-up (PBU) method in the LN2 vessel, relying on ambient heat load.

This paper examines the electrical characteristics of no-insulation (NI) high-temperature superconducting (HTS) coils impregnated with an electrically conductive epoxy. The NI winding technique refers to a method of winding HTS coils without insulating materials between the winding turns and is regarded as the most promising method to protect the HTS coils from the quench phenomenon. Thus far, many analytical and experimental studies focusing on the electrical characteristics of NI HTS coils have been conducted. From these studies, the quench tolerance levels of NI HTS coils were verified, and the possibility of using NI HTS coils in high-field-magnet applications was confirmed. In addition, studies have recently attempted to apply NI HTS coils to electrical rotating machines. The use of NI HTS coils as field coils in electrical rotating machines is expected to enhance their thermal/electrical stability, but there may also be technical issues related to charging/discharging delays and mechanical reliability. In this paper, we propose a NI HTS coil impregnated with an electrically-conductive epoxy to resolve these issues. In the proposed method, a commercially available epoxy resin is mixed with electrically conductive particles, after which HTS coils are wet-wound using the electrically conductive epoxy. In addition, it was confirmed in charging/discharging tests and in a quench test that HTS coils manufactured by the proposed method show reduced charging/discharging delay times in conjunction with quench-tolerant characteristics. The results here demonstrate the potential of the proposed electrically conductive epoxy impregnated NI HTS coil for electrical rotating machine applications.

Implementation of the persistent current mode (PCM) provides an effective solution for mitigating thermal load on superconducting magnets used in maglev trains and addressing external power supply issues. However, the HTS magnet is affected by harmonics by the armature coil during running operation. Notably, the dynamic resistance by the external time-varying magnetic field under the DC transport current generates additional AC loss. This must be considered during the design of a quasi-persistent current switch (quasi-PCS) as it causes additional field attenuation and thermal loss. This paper describes the characteristics of field attenuation of HTS magnet of lab-scale with the quasi-PCS due to the dynamic resistance for DC transport currents condition under various external AC field by experiments and numerical analysis.

This study investigated the thermal and electrical characteristics of a high-temperature superconducting motor intended for electric vehicle traction. Particular attention was paid to mitigating the temperature increase induced by eddy currents in the aluminum bobbins and to increasing the operating time while maintaining the critical current margin. Adjustments were made to incorporate multiple axial segments composed of anodized aluminum, thereby establishing segmented eddy current loops. Models ranging from two to 40 segments were investigated at the maximum power of 300 kW for the motor. It was observed that the implementation of a 40-segment bobbin reduced the eddy current losses by approximately 78%. A marked decline in the temperature rise was realized, considerably extending the operational time of the motor. The operational time increased from an initial 7 seconds at maximum power to 221 seconds, attributed to the reduced eddy current losses and the resulting decrease in temperature rise.

In this study, we designed gradient coils for a 6 T high-temperature superconducting (HTS) MRI magnet and analyzed their effects on the main magnet which was fabricated using rare-earth barium copper oxide (REBCO) conductor. First, we designed a total of 6 coils, which composed the X, Y, and Z gradient and shield coils. To ensure the performance of gradient system, we designed the gradient coil that satisfies the essential requirements such as linearity and uniformity. Second, we analyzed the effects of the gradient coil operations on the main magnet. Although the gradient shield coils prevented the main HTS magnet from experiencing magnetic field changes, the unshielded magnetic field leakage could generate screening currents in the REBCO conductor which could affect the performance of the main magnet. We simulated and analyzed the electromagnetic behaviors of the HTS REBCO magnet affected by the high-frequency varying field caused by the gradient coil operation.

Rare-earth barium copper oxide (REBCO) racetrack coils can be used for superconducting rotating machines to achieve higher power density than conventional ones. Unlike solenoid coils, winding turns in a straight section of a racetrack coil can easily move outward generating stress concentrations at the intersection of a straight section and a curved section by winding tension and strong Lorentz force. Abrupt change in curvature at intersections of straight section and half circle even create singularities in terms of mechanical stress. Clothoid curves, whose length and curvature are proportional, are often used to create smooth changes in curvature, making all higher order derivatives of curvature continuous. This study investigates the relaxation of stress concentration by applying Clothoid curve to a REBCO racetrack coil for 500 kW class superconducting motors. Non-linear anisotropic equivalent mechanical properties of the stacked REBCO tape are used for precise and efficient analysis. The modification of the coil shape will be applied to the detailed design of the 500 kW superconducting motor field coil.

This study introduces a cooling topology for a rotating field coil that integrates the supply of liquid hydrogen (LH 2 ) fuel with the cooling flow path in a High Temperature Superconductor (HTS) motor. The distinguishing feature of this system is the use of cryogenic LH 2 as both refrigerant supplied to the HTS rotor and fuel supplied to a fuel cell. The HTS rotor is fitted with a simple reservoir-shaped heat exchanger and includes both thermal and mechanical design considerations. To supply LH2 to the high-speed rotating HTS rotor, a cryogenic coupling structure is implemented using a coaxial triple pipe that includes both vacuum insulation layer and Ferro fluid seal. This research describes design process to minimize external heat load and ensure cooling down to 30 K during initial cooling and operation. In addition, thermal analysis is conducted through a commercial FEM analysis in consideration of the rotor components. Based on the design process and results, the fabrication and performance evaluation of the HTS rotor will be conducted.

Recently, structural analysis related to stress and strain has become increasingly important in the design of high-field, high-temperature superconducting (HTS) magnets using Rare-Earth Barium Copper Oxide (REBCO) tapes. Superconducting magnets can be subjected to compressive loads due to various factors such as winding tension, Lorentz force, and thermal contraction. Deformation in superconducting magnets is typically predicted based on the material properties of REBCO tapes. However, the deformation behavior in the stacking direction under compressive stress conditions, influenced by uncertain contact conditions, continues to be an area of investigation. In this study, we measure and analyze the stress-strain characteristics of stacked REBCO tape under compression in the stacking direction to obtain a mechanical property. The experiments are conducted at room temperature using specimens with varying numbers of stacked layers, during which we measure displacement and deformation while gradually increasing the compressive stress up to 100 MPa. Additionally, consecutive three loading-unloading experiments are conducted to analyze the changes in stress and strain characteristics due to local plastic deformation. Furthermore, we employ a 3D laser microscope to measure the change of surface roughness. The stress-strain characteristics obtained from the experiments are expected to contribute to improving the accuracy of various high-magnetic field superconducting magnet designs.

For the first time, we demonstrated experimentally 4F 2 single-gated IGZO-VCT, monolithically stacked on top of core/peripheral transistors without wafer bonding process for sub-10nm DRAM. Sufficiently low leakage current (I OFF ) of <1 fA/cell, subthreshold swing (SS) of 164 mV/dec and V T of -1.73 V at 85°C is obtained with advanced processes. In order to achieve higher on-current (I ON ) and positive V T , the impacts of fabrication processes including thickness, deposition condition and post deposition treatments of IGZO channel, and its top/ bottom interfaces are investigated utilizing top-gated planar devices. By optimizing processes for gate dielectric interface, planar devices of 70 nm gate length show excellent on-off ratio of 13 order-of-magnitude at 85°C with improved N/PBTI lifetime; extremely low I OFF of 2e-18 A/um, I ON of 25 uA/um at V GS -V T = 1.0 V, and SS of 90 mV/dec with positive V T of 0.19 V. This result implies that 4F 2 single-gated IGZO-channel VCT can be an excellent candidate to scale down a unit cell volume for high DRAM capacity, high bandwidth and low power consumption.

Due to the high upper critical field and transition temperature, REBCO coated conductor show great promise for conduction-cooled magnet applications. The normal zone of high temperature conductor (HTS) spreads slowly, and the conversion of a weak spot into a hot spot is localized and quite fast. Hence, in-field critical current weak spot evaluation under the nonuniform temperature gradient is important for conduction-cooled magnet stability estimation. Cooling path design needs to take the weak spot into account. In this study, a temperature-field-angle dependent critical current estimation neural network was first trained based on a public database of HTS critical current. The equivalent thermal conductivity was applied for the interface contact between the turns. Thermal contact conditions were given to the interface between the coil and bobbin considering actual production situations, such as thermal grease. Combing the non-uniform magnetic field and thermal distribution, the critical current distribution was evaluated to track the weak part. The results of this study will be employed in the design analysis of high magnetic field conduction cooling magnets in the future.

In promoting novel products with superconducting magnets to transportation and science industries, the most important factor is reliability. Recently, it was reported that a no-insulation (NI) superconducting magnet could be damaged with quenches caused by a thermal runaway, significant temperature deviation, extremely high characteristic resistance, etc. To resolve this issue, numerical analysis of high-temperature superconducting (HTS) racetrack coils with NI winding technology should be conducted. The authors proposed a homogenization method to analyze operating characteristics of the NI HTS coils by simplifying the finite element analysis (FEA) model and decreasing the solution time. In order to estimate their thermal stabilities, the proposed FEA simulation is one of the efficient methods for solving the electrical and thermal factors simultaneously. In this paper, the thermal analysis of the leakage currents of NI HTS racetrack coils is presented using a three-dimensional (3D) turn-distributed equivalent circuit FEA model. The charging and discharging delay phenomena were modeled by considering the contact resistances by turns. With the equivalent circuit model of the NI HTS magnets, the 3D racetrack coils were designed by considering a single turn and stacked turns, including metal insulation layers. Their material properties according to temperature variation were applied in the transient analysis. The critical currents by turns were estimated by considering the magnetic field, its angle, and temperature with a given current simultaneously. The simulation results were compared with a fabricated NI HTS magnet and are presented in this paper.

This paper presents a dynamic characteristic analysis of an electrical rotating machine that applies no-insulation (NI) high-temperature superconductor (HTS) field coils. The HTS electrical rotating machine applying HTS field coils has high power density compared to conventional electrical rotating machines that utilize normal conductor field coils, meaning that it has great potential for next-generation mobility applications such as electric ships and aircraft. Recently, several studies have reported the possibility of solving the quench protection problem of the HTS field coil, which is one of the most pressing issues in HTS electrical rotating machines, and the key technology is the no-insulation (NI) winding technique. The NI winding technique enhances the electrical and thermal stability of the HTS field coil because the over-current can be automatically bypassed through the turn-to-turn contacts. Although there have been many studies of the electrical and thermal stability of NI HTS field coils, there are still issues related to NI HTS field coil applications to electrical rotating machines. In particular, there are few studies of the behavior of electrical rotating machines with a NI HTS field coil under dynamic conditions, such as active short-circuit and speed drop conditions. In this study, we designed an electrical rotating machine that utilizes NI HTS field coils and analyzed the dynamic characteristics of the designed machine. The results demonstrate there is a possibility of larger torque fluctuations and armature current fluctuations in the NI HTS machine than in the insulated HTS machine.

The no-insulation (NI) winding method has been widely used in the fabrication of superconducting coils owing to its excellent thermal stability and mechanical stiffness. In the NI coil, there is a charging delay and heat loss due to leakage current. Therefore, the metal insulated (MI) winding method was proposed to reduce the charging delay and the heat loss due to the leakage current and the increased contact resistance by the metal tape. However, it is difficult to quantify the contact resistance between the turns of the coil during the design stage. To resolve this problem, a new winding method, called soldered metal insulation (SMI), was proposed by the authors, and the electrical properties were evaluated in a bath of liquid nitrogen. As a follow-up to the previous research, experimental investigations were conducted in a conduction cooling test apparatus to investigate the electrical and thermal characteristics of the SMI coil below 77 K. The electric contact resistances were evaluated through sudden discharge experiments. Then, the thermal contact resistances were measured using a heater installed on the outer turn of the coil. It is believed that the SMI winding technique can be applied to fabricate rare-earth barium copper oxide (REBCO) coils with predictable electrical and thermal contact resistances.

Recently, development of a cryogenic fluids storage tank for storing or transporting liquid hydrogen is actively in progress. In cryogenic fluids storage tanks, hydrogen evaporates due to the extreme temperature difference inside and outside the tank. As the mass of the cryogenic fluids changes with continuous vaporization, the fluids level also changes. Therefore, there is need for a method of accurately measuring the level change in the storage tank. In the case of general cryogenic fluids, it is difficult to accurately measure the level because the dielectric constant is very low. As a method of measuring cryogenic fluids level with low dielectric constant, it can be used an Millimeter wave (MM wave) FMCW radar sensor. However, the signal sensitivity is very weak and the level accuracy is poor. In this paper, the signal sensitivity is improved by designing the horn lens antenna of the existing 80 GHz FMCW radar sensor. Horn lens antenna is fabricated by FDM/SLA type 3D printer according to horn and lens characteristics. The horn is used to increase the signal gain and the lens improves the signal straightness. This makes it possible to measure the level of cryogenic fluids with a low dielectric constant.

With the innovative development of bio, pharmaceutical, and semiconductor technologies, it is essential to demand a nextgeneration transfer system that minimizes dust and vibrations generated during the manufacturing process. In order to develop dustfree and non-contact transfer systems, the high temperature superconductor (HTS) bulks have been applied as a magnet for levitation. However, sintered HTS bulk magnets are limited in their applications due to their relatively low critical current density (J(c)) of several kA/cm(2) and low mechanical properties as a ceramic material. In addition, during cooling to cryogenic temperatures repeatedly, cracks and damage may occur by thermal shock. On the other hand, the bulk magnets made by stacked HTS tapes have various advantages, such as relatively high mechanical properties by alternate stacking of the metal and ceramic layer, high magnetic levitation performance by using coated conductors with high Jc of several MA/cm(2), consistent superconducting properties, miniaturization, light-weight, etc. In this study, we tried to fabricate HTS tapes stacked bulk magnets with 60 mm x 60 mm area and various numbers of HTS tape stacked layers for magnetic levitation. In order to examine the levitation forces of bulk magnets stacked with HTS tapes from 1 to 16 layers, specialized force measurement apparatus was made and adapted to measure the levitation force. By increasing the number of HTS tapes stacked layers, the levitation force of bulk magnet become larger. 16 HTS tapes stacked bulk magnets show promising levitation force of about 23.5 N, 6.538 kPa at 10 mm of levitated distance from NdFeB permanent magnet.

The authors have proposed a performance evaluation system (PES) for large-scale high temperature superconducting (HTS) wind power generators, which produces strong electromagnetic force and high torque due to the high excitation current and the high magnetic load of the machine, based on the computer simulation results. The PES can physically test the structural stability of HTS coils and armatures against high torque and the effect of the magnetic field before mounting the HTS coil on the generator. This paper deals with the design, fabrication, and performance testing of a full-scale HTS coil to be installed in the PES, a performance analysis system for a 10 MW HTS wind power generator. The HTS coil with metal-insulation consists of four racetrack-type single coils with 310 turns per layer. The manufactured HTS coil was tested at 77 K with and without an iron-core. As a result, the critical currents of the tested HTS coils with and without the iron-core were 90 A and 106 A, respectively. Compared to the simulation results, it was found that the magnitude of the magnetic field was the same. These results will be effectively used to study and fabricate high magnetic field coils for HTS applications, as well as the PES being fabricated.

Researches on a magnetic levitation (maglev) train traveling in a vacuum tube have been actively conducted as a next generation ultra-high speed vehicle. For strong propulsion and levitation, a REBCO magnet is considered a feasible option. Due to the limited facility space of the maglev train, it has been proposed to eliminate the cumbersome cryogenic cooling system and power supply while the train is running. Applying a thermal battery instead of the on-board cooling system can suppress the temperature rise of the REBCO magnet while the maglev train is running. In this study, a solid nitrogen (SN 2 ) is proposed as the thermal battery since it has a large sensible heat and an additional latent heat by α-β solid phase transition at 35.6 K. This can delay the temperature increase during the operating time. In addition, a semi-persistent current switch (semi-PCS) is applied with detachable current leads to operate the magnet without the power supply while the train is running. The detachable current leads can eliminate heat loss through the current leads. This paper describes the experimental results on the thermal and electromagnetic performance of REBCO magnet with the thermal battery after disconnecting the cooling system and the power supply.