A cryogen-free portable 3 T high-temperature superconducting magnet for an electromagnetic property measurement system has been developed to serve as a user facility at the Korea Basic Science Institute. The metallic insulation method was adopted to reduce the charging delay without sacrificing the self-protecting feature. A genetic-algorithm-aided optimized design was carried out to minimize the superconducting tape consumption while satisfying several design constraints. After the design, the compact high-temperature superconducting magnet composed of eight double-pancake coil modules was wound with high-temperature superconducting tape and stainless steel tape, and integrated with a two-stage cryo-cooler. The 3 T magnet was successfully cooled to approximately 20 K with a cryo-cooler and reached the target field of 3 T without any problems. Long-term measurements and a range of other tests were also implemented to verity the performance of the magnet. Test results demonstrated the feasibility of a cryogen-free portable high-temperature superconducting magnet system for electromagnetic property measurement experiments.

In this paper, a new method is presented for sensitive quench detection in high-temperature superconductor (HTS) rotating machinery. The normal zone propagation velocity of an HTS is about 1000 times slower than that of a low-temperature superconductor. Therefore, the propagation of normal zone resistance, which occurs when the HTS transits from the superconducting state to the normal state, is also slower. Thus, it is difficult to detect the abnormal signals by voltage measurement using voltage taps. Moreover, the monitoring signal includes noise generated by interaction between the HTS rotating machinery and the industrial environment. Therefore, when quenching occurs in the HTS rotating machinery, a thermal runaway occurs in the hot spot. Furthermore, the magnetic energy stored in the HTS coil can damage the machinery. For these reasons, a new method is proposed for sensitive quench detection that reduces the noise generated from the power supply and from the HTS rotating machinery, using both an RLC resonance circuit and fast Fourier transform analysis.

The high-temperature superconducting magnets with high magnetic field can store a relatively large amount of energy because of their large inductance and operating current. The magnetic energy stored in the magnet is converted into heat energy when a quench occurs, and the heat generated by the quench resistance and magnet current is sufficient to damage the magnet. Therefore, the large amount of energy stored in a superconducting magnet must be removed quickly to protect the magnet. In this paper, a new quench protection method using a capacitor and dual-switches is proposed and compared to a conventional energy extraction system that uses only a dump resistor. The simulation and test results show that this new protection system is a feasible method for reducing the temperature increase by rapidly reducing the magnet current when a quench occurs.

In recent years, a new quench protection system using capacitor and switches has been announced to rapidly extract energy from high temperature superconducting (HTS) magnets. When a quench occurs, the quench protection system activates four metal oxide-semiconductor field-effect transistor switches in sequence, and the energy stored in the magnet is extracted by a capacitor to an external resistor. To verify the feasibility of the proposed dual-capacitor switching (DCS) quench protection system, DCS system was tested using a small-scale coil with a 3.3 mH self-inductance. The experimental results show that the energy extraction of the magnet is faster than the quench protection system using a conventional dump resistor. However, as the system is sensitive to the capacitance, inductance, and four resistances, it is necessary to analyze and optimize the design parameters for an application to various magnet systems. In this study, a modified DCS system is introduced to remove the constraints of capacitance, and the effect of all the design parameters on DCS protection performance is analyzed. The optimal design of modified DCS protection system for the best performance is determined. The performance of the system is also evaluated by applying a design that is optimized for the scale-up HTS coil with a 30 mH self-inductance.

A superconducting fault current limiting cable (SFCLC) with a hollow former is composed of various elements with different electrical and thermal properties, including liquid nitrogen inside the former. This type of cable can be used to mitigate the impact of short-circuit faults. This paper introduces a computational model that combines thermal and electrical behavior to investigate the thermal and electrical characteristics of SFCLCs according to the composition of a hollow former. To verify the computational model, a superconducting power cable with a hollow former was fabricated and a fault current test was performed. Good agreement was obtained between the calculation results obtained from the proposed computational model and the fault current test results. In addition, using the proposed computational model, the relationship between increases in temperature and limitations on current was analyzed for each configuration of the former according to the length of the cable and the duration of the fault. The characteristic analysis provides information on the hollow former composition that most effectively limits the fault current, depending on the length of the cable and the duration of the fault. The results can be used to determine the material and dimension of the hollow former for SFCLCs design.

A HTS (High temperature superconductor) current lead, which provides an electrical connection between a power supply and a superconducting magnet, needs to be designed to minimize the heat leakage transferred to the cold end to save the cooling power of the superconducting magnet system. Since the heat leakage of the HTS current lead can be reduced not only through the material properties of the metal lead but also through structural analysis, we will introduce study results of the shape design method of a metal lead. We performed a numerical analysis based on a segment algorithm using the general solution of temperature distribution and heat leakage to mathematically express the design method. The results of the simulation revealed that the heat leakage at the cold end of the vapor cooled HTS current lead is determined by conduction, Joule heating and vapor cooling, thus demonstrating that liquid helium consumption can be reduced by adjusting the perimeter and hydraulic diameter of the metal lead. Based on these results, we performed a numerical analysis of the temperature distribution and heat leakage on a metal lead with an axially varying cross-sectional area. After the design, the metal lead was manufactured with the proposed method and applied to a 5 T class superconducting magnet system to evaluate the performance of the system. We compared the evaluation results to the simulation results to verify the feasibility of the proposed numerical analysis method. The study confirmed that the proposed metal lead design method can be applied to the superconducting magnet system to save cooling power.