Earticle

This study explores the enhancement of magnetic field homogeneity in Nuclear Magnetic Resonance (NMR) magnets through optimized ferromagnetic shimming design. Traditional shimming techniques, which involve attaching ferromagnetic materials to the magnet bore, often lack manufacturability considerations, limiting their effectiveness in real-world applications. To address this, we employ a topology optimization (TO) approach using the Solid Isotropic Material with Penalization (SIMP) scheme for design parametrization. The proposed optimization framework includes volume and perimeter constraints to improve the practical manufacturability of the shim. Numerical analysis demonstrates that the TO-based design method achieves superior magnetic field homogeneity, achieving 0.45 ppm with integer thickness shims, compared to conventional designs. This approach also effectively reduces manufacturing complexity by minimizing design sensitivity to the thickness and placement of individual shimming elements. The proposed design framework is broadly applicable to superconducting magnets in NMR and MRI systems, where high magnetic field homogeneity is essential. This study presents a significant advancement in ferromagnetic shimming technology, offering a viable solution for enhancing the performance and manufacturability of high-precision magnetic field devices.

High-temperature superconductors (HTS), owing to their high critical current and critical magnetic field, are utilized in various superconducting applications such as superconducting power cables and high-field magnets. However, REBCO (Rare Earth Barium Copper Oxide) tapes face significant challenges in long-length fabrication compared to low-temperature superconducting wires. Consequently, joining REBCO tapes is essential for their application in superconducting devices. The most widely used method for joining REBCO tapes is mechanical pressure soldering, which inevitably introduces contact resistance at the joint interface. Since superconducting devices operate in cryogenic environments where maintaining minimal losses is critical, minimizing joint resistance is essential. In this study, we evaluated the effect of heating temperature on joint resistance in the mechanical pressure soldering process by selecting four types of solders (Pb37Sn63, In52Sn48, Sn42Bi57Ag1, and In66.3Bi33.7). Lap joint samples were prepared by increasing the heating temperature in 10°C increments above the melting points of each solder, and the surface resistance of each sample was measured. The results were analyzed to identify the appropriate heating temperature.

The Small Punch Test (SPT) was developed to evaluate the softening and embrittlement of materials such as power plants and nuclear fusion reactors by taking samples in the field. Specimens used in the SPT are very thin and small disk-shaped compared to specimens for general tensile test, and thus have economic advantages in terms of miniaturization and repeatability of the test. The cryogenic SPT can also be miniaturized and has a significantly lower heat capacity than conventional universal test machines. This leads to reduced cooling and warm-up times. In this study, the cryogenic SPT was developed by modifying the existing room temperature SPT to be cooled by liquid nitrogen using a super bellows and a thermal insulation structure. Since the cryogenic SPT was first developed, basic experiments were conducted to verify the effectiveness of it. For the validation, aluminum alloy 6061- T6 specimens were tested for mechanical properties at room and cryogenic temperature. The results of the corrected tensile properties from the SPT experiment results were compared with known room temperature and cryogenic properties. Based on the correction results, the effectiveness of the cryogenic SPT test was confirmed, and the surface fracture characteristics of the material were analyzed using a 3d image scanner. In the future, we plan to conduct property evaluation according to the development of various alloy materials.

Accurate measurement of contact resistance in no-insulation (NI) high-temperature superconducting (HTS) coils is crucial for their performance evaluation. The sudden discharge test is a widely used method for measuring the contact resistance of NI HTS coils. However, there are several issues regarding the data acquisition system (DAQ) during the test. In particular, the DAQ must operate at a sampling rate exceeding 500 S/s. An infinite impulse response (IIR) filter can introduce a phase delay into the filtered signal compared to the original data. For instance, applying a 4-Hz low-pass Butterworth filter to a high-frequency signal may lead to measurement errors due to this delay. Moreover, remnant field induced by the screening current after the current transportation introduces uncertainty of conducting sudden discharge tests. This paper proposes the qualified settings of DAQ system configuration for sudden discharge test and suggests an alternative method for measuring the contact resistance of NI HTS coils.

As global environmental regulations intensify, the electrification of the mobility industry is accelerating, thereby increasing the demand for high-power electric propulsion systems. Among the various candidates for application, superconducting motors have attracted considerable attention owing to their potential for delivering exceptionally high-power density. In this study, we fabricated two types of GdBa2Cu3O7-δ (GdBCO) superconducting magnets and evaluated their trapped field performance using a 14T superconducting magnetization system. A single grain bulk magnet, grown by the top seeded melt growth (TSMG) method, exhibited a trapped field of 1.93T at 20K. The stacked tapes, fabricated by commercial GdBCO tapes, showed a maximum trapped field of 1.25T at 20K. While the single grain bulk magnet has an advantage in trapped field performance, the stacked tapes offer benefits in terms of simple manufacturing processes and shape flexibility. These results suggest that simultaneous improvement of trapped field performance and motor design optimization could establish superconducting motors as a core technology for nextgeneration electric propulsion systems.

We investigated the flux pinning properties of GdBa2Cu3O7-x (GdBCO) superconducting films grown on a LaFeO3 (LFO) buffer layer forming a bilayer structure. LFO films with different thicknesses and GdBCO thin films were all prepared by using a pulsed laser deposition technique. The magnetization measurements revealed that the critical current density of the GdBCO bilayer structure with the LFO buffer layer increased in low magnetic field regions compared to pure GdBCO films. Notably, the variation in critical current density and the position of the maximum pinning force density did not exhibit a simple dependence on the LFO buffer layer thickness. Dew-Hughes’ model fitting confirmed that normal surface pinning is the primary pinning mechanism for both the thinnest and the thickest LFO films. However, for intermediate LFO buffer layer thickness, additional pinning mechanisms beyond normal surface pinning were identified, likely due to the defects from misoriented crystallites, as indicated by the close correlation between the fitting exponent and the grain size of the bilayer structure.

Since the fiber reinforced polymeric (FRP) composites are considered in next generation of space transportation systems, reliable thermal expansion properties should be well provided for structural design of composite materials. To obtain accurate mechanical behaviors at a cryogenic temperature, precise strain measurement and calibration must be provided. In this work, apparent strains (or thermal output) of temperature self-compensated strain gages were deliberately investigated for epoxy, CTBN modified epoxy and carbon fabric composite system from room temperature to liquid nitrogen temperature. Also, fourth-order thermal output curves were presented for the further calibration. The results showed that the thermal output is heavily dependent on test materials and a large amount of apparent strains were observed for the polymer resins.

A linear compressor generates pulsating pressure and oscillating flow in a cryocooler such as Stirling cryocooler and pulse tube refrigerator. It is driven by AC power source and designed to operate at resonance of piston motion. The driving voltage level is determined by electric parameters of resistance, inductance and thrust constant of linear motor. From voltage equation on linear motor, the power factor of driving power is inherently less than 1. The phase difference between voltage and current of supplied power can be zero using capacitor and this can minimize a supply voltage level. Especially, the linear compressor of kW class requires high voltage and thus can cause a difficulty in selecting power supply unit due to limitation of voltage level. The capacitor in driving electric circuit is useful to settle this problem. In this study, the electric circuit of linear compressor is analytically investigated with assumption of mechanical resonance. The electric parameters of commercial linear motor are used in the analysis. The effects of capacitor on driving voltage level and power factor are investigated. From analytic results, it is shown that the voltage level can be mimized with using capacitor in driving electric circuit.

Cryogenic vessels are special equipment that requires periodic evaluation of their thermal insulation performance. At the current standard, the test is considered as the loss product or heat leakage of cryogenic vessel, which takes over 72 h to evaluate; consequently, a large amount of working medium is discharged to the environment in the process. However, hydrogen is flammable and explosive, and the discharged gas may be dangerous. If liquid hydrogen is replaced with liquid nitrogen before testing, the operation then becomes complicated, and the loss product or heat leakage cannot respond to the thermal insulation performance of cryogenic vessels for liquid hydrogen. Therefore, a novel method is proposed to evaluate the heat leakage of cryogenic vessels for liquid hydrogen in self-pressurization. In contrast to the current testing methods, the method proposed in this study does not require discharge or exchange of working medium in all test processes. The proposed method is based on one-dimensional heat transfer analysis of cryogenic vessels, which is verified by experiment. When this method is used to predict the heat leakage, the comparison with the experimental data of the standard method shows that the maximum error of heat leakage is less than 5.0%.

High-entropy alloys (HEAs) consist of five or more principal elements with atomic fraction between 5 % and 35 %, randomly distributed throughout the lattice points. Recently, a simple body-centered cubic structure HEA superconductor, Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6, was discovered and classified as a type-II superconductor with a critical temperature of approximately 8 K. Ta-Nb-Hf-Zr-Ti HEA superconductors exhibit a large critical current density of 1 MA cm-2 and demonstrate robust superconductivity against irradiation-induced disorder. We have synthesized HEA superconductors based on the composition Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 with various Ce doping concentrations (0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 4 wt%, 8 wt%), investigating the effects of Ce addition on the crystal structure and superconductivity. X-ray diffraction and energy dispersive x-ray spectroscopy results indicate the complex Ce addition effects on the crystals. Notably, while the superconducting transition temperature remains largely unaffected by the presence of magnetic impurities, it is found to be closely related to the lattice constant, which changes with varying Ce doping concentrations.