Jae Young Jang

No-Insulation (NI) and NI-like winding techniques have become essential for quench protection in high-temperature superconducting (HTS) coils. However, the turn-to-turn resistance in these techniques arises from uncontrolled mechanical contact, making current decay behavior difficult to control. One of the major challenges is asymmetric quench propagation, which can cause mechanical imbalance in the magnet system. In this study, we propose a design approach using Partial Electrical Connectors (PECs) to guide current bypass paths and achieve near-symmetric current decay. This concept was demonstrated through experiments on sample double pancake coils equipped with PECs, where heater-triggered quenches resulted in almost symmetric field decay across the coil stack. These results indicate that PEC-based designs can effectively mitigate quench asymmetry while providing a tunable means of controlling current redistribution in HTS magnets. Although building a perfect quench protection circuit remains challenging, the PEC approach represents a practical step toward optimized quench dynamics and improved stability in future high-field magnet systems.

In this paper, we present a method for determining the appropriate charging current waveform to mitigate the screening-current-induced field (SCF) in an NMR class HTS magnet. It has been observed that screening currents degrade the magnetic field quality of NMR class HTS magnets, and the extent of this degradation varies depending on the charging current waveform. We developed a simulation program to predict the effects of screening currents when applying specific charging current waveforms to given HTS magnets. Using the program, we were able to determine which current waveforms are most suitable for a given HTS magnet. To validate the performance of the program, we applied the same current waveforms to the actual NMR class HTS magnet and measured the central magnetic field. We compared the experimental results with the simulations and confirmed that they were nearly identical. This technique is expected to allow us to determine the most suitable charging current waveform for reducing screening currents before conducting time-consuming and resource-intensive charging experiments.

This study discusses the application of Partial Electrical Connector (PEC) in various patterns on high-temperature superconducting (HTS) coils. The PEC is a technology that partially attaches metal foil to the coil surface to form a direct current path between the turns, primarily aimed at protecting against sudden discharge accidents in no-insulation (NI) coils. A small sample of NI coil was fabricated with 60 turns, and a 10 $\mu $m thick PEC was installed to connect the turns. The PEC was applied to areas that covered 1/8, 1/4, and 1/2 of the coil surface to conduct charge tests and sudden discharge experiments. The experimental results confirmed the possibility of controlling the contact resistance by adjusting the amount of PEC. This research aims to optimize the protective performance of the developed contact resistance control technology according to magnet specifications, making it a useful technique for the protection of large magnet systems in the future.

This article addresses the design, manufacturing, and testing of a contactless current charging system for high-temperature superconductor (HTS) coils. Traditional methods of charging HTS coils typically involve power-driven techniques, which result in a substantial thermal load due to thermal conduction. To mitigate this issue, we developed a system using an inductive coupling technique. The design of this system focuses on optimizing the magnetic coupling coefficient to ensure efficient power transfer to the HTS coils. Essential components, such as a magnetic coupler, a resonance circuit, and a rectifier, were designed and manufactured to match the operating environment of the HTS coil. A series of practical current charging tests were conducted to evaluate the feasibility of the system. The results from these tests confirmed the successful contactless charging of HTS coils, indicating the system's potential applicability in various HTS magnet applications.

A Partial Electrical Connector (PEC) offers a promising self-protection mechanism for high-temperature superconducting (HTS) coils, providing an alternative to no-insulation (NI) coils. Unlike NI coils, PEC-applied coils have limited current bypass paths, potentially limiting their self-protection capabilities. This study investigates how the location of hot-spots, created by heaters positioned at different sections within the coil, affects the performance of PEC. By analyzing voltage and center field responses, we found that the proximity of the heater to the PEC significantly influences the efficiency of current bypass. Specifically, closer hot-spots result in more effective current redirection through the PEC. These findings provide valuable insights for optimizing PEC patterns, potentially enhancing protection mechanisms for larger HTS coils by improving current bypass capabilities based on hot-spot location.

This paper presents research on the dynamic characteristics of electrical rotating machines using no-insulation (NI) high-temperature superconductor (HTS) field coils, particularly focusing on changes resulting from the application of dampers. Previous studies have indicated the potential for increased torque and armature current fluctuations in NI HTS machines compared to insulated HTS machines. NI winding enhances the electrical and thermal stability of HTS field coils, as overcurrent can be automatically bypassed through turn-to-turn contacts. However, the use of NI HTS field coils may compromise the operational stability of electric rotating machines under dynamic conditions. To address this issue, various studies are being conducted to increase the contact resistance between winding turns of the NI HTS coil. However, this could deteriorate the electrical and thermal stability of the HTS coil. Therefore, this study investigates the potential of improving the operational stability of electric rotating machines under dynamic conditions, like active short circuits, by applying dampers to the NI HTS machines.

Given that REBCO wire is typically manufactured as a tape, it is possible for REBCO magnets to exhibit a relatively larger screening current-induced field (SCF). It is well known that the SCF in an REBCO magnet can be effectively reduced by the so-called current sweep method. However, it requires an overshooting in the magnet current, often close to the magnet's critical current, to effectively suppress the SCF, which may not be favored in terms of operation reliability. In this study, we experimentally validated an alternative approach which mitigates the SCF using thermal cycling together with the current sweep reversal. Well known is the fact that the large load factor, defined as the ratio of operating current to critical current of an REBCO magnet, deteriorates the SCF impact on field uniformity. The key idea is to increase the magnet temperature to increase the effective load factor of the magnet close to its quench limit momentarily, minimize the SCF, and then return back to the nominal operating condition. The combined approach of the thermal cycling and the current sweep may be effective when attempting to eliminate the SCF without ramping an REBCO magnet to a current that may be dangerously high, especially when the magnet is operated at a high operating current. This paper presents the test results of the combined approach in the conduction-cooled REBCO magnet, and the results demonstrate the potential of the approach for more safe and effective elimination of the SCF.

The feasibility of the wireless current charging technique for high-temperature superconductor (HTS) magnets is verified through previous studies. Because the magnitude of the charged current strongly depends on the circuit parameters of the wireless current charging system, this study proposes an optimal design method that increases the efficiency of the system by employing a genetic algorithm. An equivalent circuit model to improve the computing speed is proposed and the overall design process reflecting the equivalent circuit model is introduced. The optimal design for a 3T class target HTS magnet is derived using the proposed method, and a circuit analysis using Simulink is carried out to check the design results. Additional circuit simulations of randomly selected cases for a comparison of the optimal design result are also carried out to verify the performance capabilities of the proposed method.

We report room-temperature (RT) shimming results and a long-term operational protocol for a Korea Basic Science Institute (KBSI) 400 MHz high-temperature superconducting (HTS) nuclear magnetic resonance (NMR) magnet composed of 48 all-REBCO double-pancake (DP) coils cooled by a cryo-cooler to be operated under 20 K. The spatial magnetic field homogeneity of the 400 MHz HTS NMR magnet improved from 0.95 ppm to 0.2 ppm due to the use of a genetic-algorithm-based RT shimming method, of which the feasibility had been verified beforehand through an earlier study of a low-temperature superconducting (LTS) NMR magnet. We also confirmed that the field homogeneity of the HTS NMR magnet varied slightly over time due to the screening-current-induced field (SCF) and the temporal instability of the magnet power supply during long-term operation. Because fluctuations in the field quality affect the performance of the HTS NMR magnet, we established a long-term operation protocol for HTS magnets when used in NMR or magnetic resonance imaging (MRI) applications. Through the proposed operational protocol, the homogeneity of the HTS NMR magnet was maintained within the target range for 24 hours. The experimental results here demonstrate the potential of the proposed RT shimming method and long-term operation protocol for HTS NMR/MRI applications.