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Structural disorder within grains formed by carbon doping is widely known to enhance the in-field transport critical current performance of MgB2 wire. The structural formation usually requires high-temperature sintering above 600 °C due to low reactivity of the dopant. In this study, we selected a carbohydrate (malic acid) that has a low-decomposition temperature as the carbon dopant and performed low-temperature sintering (below 600 °C) to introduce greater disorder within the MgB2 grains. We revealed that the doping and low-temperature sintering process cause severe disorder, especially along the out-of-plane (c-axis) direction. The disordered formation, in addition to the in-plane disorder combined with well-connected grains, eventually raised the in-field transport performance.

We report an ultra-low resistance superconducting joint using unreacted multifilament MgB2 wires produced by tailoring the powder compaction pressure within the joint with heat treatment conditions. The joint demonstrated an ultra-low resistance of 5.48 × 10−15 Ω and critical current (Ic) of 91.3 A at 20 K in self-field. The microstructural and composition studies of the joint revealed cracks and a high amount of MgO, respectively. These two features reduced the Ic of the joint to some extent; nevertheless, the joint resistance was not affected by it. Our tailored joining process will play a pivotal role in the superconducting joints development.

Nanocomposites have been actively studied for understanding the superior electrical characteristics and for utilizing in the electric devices of HVDC systems. Nanocomposites in electrical devices are not used unilaterally, but rather as the hetero-dielectric composites, which form an interface with other insulators. In this study, the hetero-dielectric composites comprised of nanocomposites and transformer oil were analyzed using the migration–migration model incorporating the finite element method. Until now, hetero-dielectric composites have been usually studied by using the Migration–Ohmic model, where the specific space charge behavior cannot be analyzed. Although interface conditions are significant for the transport and local accumulation of charge, the BCT models, including conditions for interface with other insulators, have not been reported. To investigate the space charge behavior of the hetero-dielectric composites, charge carriers of positive, negative ions and electrons are considered in the liquid region and electrons and holes in the nanocomposites. These hetero-dielectric composites are applied to the needle-bar electrode system to analyze the surface discharge phenomena at the interface. The BCT model was applied to the nanocomposites, and the drift dominated charge continuity model was applied to the transformer oil. These numerical results were compared with those from the previous research works validated with the experimental results. We analyzed the propagation of the streamer with an equivalent parallel current circuit and adopted the effective mobility of the electrons according to the field strength to improve the applicability of the numerical analysis model.

As an insulator, dielectric composites should be guaranteed the specific level of insulation performance to maintain the electric device. It is essential to understand the physical phenomena at the interface of the dielectric composites, where the electric field and charge distribution are concentrated. We, therefore, focused on the surface discharge propagated on the interface forming with dielectric composites, composed of nanocomposites as a solid region and oil as a liquid region. Nanocomposites have a broad range of conductivities depending on the content of nanoparticles. Thus, the conductivities of nanocomposites were analyzed from 0 to 10−3, 10−6, 10−7, 10−8, and 10−9 S/m with the experimentally verified numerical model. Our numerical model was expanded incorporating with the migration-ohmic model to link the physical phenomena between liquid and solid. With extending the simple model that handles only one charge carrier, three charge carriers were employed to analyze the temporal discharge dynamics in the liquid. Using the fully coupled finite element method, the patterns of the streamer were investigated with different conductivities. The higher the conductivity, the slower the propagation speed of the streamer. Conductivity changes the amount of the surface charge accumulated on the surface. This surface charge affects the speed of the streamer. Furthermore, the equivalent circuit model was implemented to explain the relations between the propagation speed and conductivity of solid.

A cryogen recondensing cooling system was designed for a 7 T superconducting solenoid magnet in an electron beam ion source (EBIS) system. In an EBIS system, it is difficult to access the apparatus due to strong X-ray emission when in full operation. The proposed cooling system was adapted for liquid-helium recondensation. An EBIS is a typical ion source that is feasible for highly charged ion production. Thus, the EBIS system can meet various requirements of end applications, and the development of such a system is strongly required. Estimations of the thermal and structural stability levels are very important during the design of a cryostat; therefore, these values were calculated by means of a numerical analysis. In the thermal analysis, the heat loads were calculated according to the temperature gradient after calculating the temperature distribution. The cooling margins were estimated at the first and second stages of the cryocooler, respectively, with respect to the calculated heat load. It was also estimated that the thermal stress due to the temperature gradient. Finally, it was confirmed that the proposed cryostat design was thermally and structurally stable and suitable for application to a superconducting magnet for an EBIS system.

The propagation of surface discharge due to interfacial polarization was numerically analyzed at the oil-nanocomposite interface using fully coupled finite-element analysis incorporating the relative permittivity from experiments. To improve the insulation ability, a new nanodielectric insulating material has been proposed in which a pressboard is coated with epoxy resin mixed with silica nanoparticles; this nanocomposite material can enhance the breakdown voltage in power systems with a certain level of silica nanoparticles. To specify the electric breakdown performance of this nanocomposite material, we measured the bulk relative permittivity of epoxy resin containing different percentages of silica nanoparticles on the pressboard. Surface discharge, or creepage discharge, tends to propagate along the solid–liquid interface and then leads to flashover. The mechanism of surface discharge, therefore, is a critical issue for understanding the dielectric breakdown strength in solid–liquid interface problems. To quantitatively analyze and explain the characteristics of surface discharge, here, the fully coupled finite-element analysis technique has been applied and tested with various relative permittivity values of nanocomposite materials. This phenomenon has been simulated using the fully coupled governing equations using Poisson’s equation for electric field and charge continuity equations, including surface charge accumulation for charge transport. After verification of our numerical setup in a conventional oil-pressboard system, a needle-bar electrode system was proposed and applied to the analysis of surface discharge propagation for the new nanocomposite materials with bulk dielectric permittivity. The propagation speed at the oil-nanocomposite interface was compared with different percentages of nanosilica. Finally, the physical mechanism of surface discharge due to the interfacial polarization was analyzed with the space, bounded, and surface charge densities at the oil-nanocomposite interface based on the numerical results.

A method has been developed for demonstrating a flow nanogenerator by using a mixture of magnetic nanofluid (MNF) and bubbles in a fluid circulating system, and notable phenomena related to the power generation properties of the nanogenerator have been explored. MNF is widely used in various areas because of its interesting magnetic properties under an external magnetic field. The objective of the proposed technique is to obtain the induced electromotive force (EMF) based on Faraday’s law due to the flow of MNF in a closed-circulating system. To maximize the induced EMF, magnetic nanoparticles (MNPs) should pass through the induction coil with a perpendicular magnetization direction in accordance with Faraday’s law. To control the magnetization direction of the MNPs, a permanent magnet was employed to produce an external magnetic field that considers the Brownian and Neel motions. To obtain a continuously induced voltage, a circulation system was implemented ensuring the flow of the MNF in the closed cycle. Further, power generation properties were investigated considering electric, magnetic, and fluidic effects. To analyze this complicated physics, a multiphysics analysis was used to calculate the flow pattern of the MNF according to its magnetic properties, and the acquired results were compared with those obtained from the experiment. From these experiments, we investigated the generation properties of the nanogenerator considering the flowrate of the MNF as well as the presence or absence of bubbles within the MNF. Our experimental tests demonstrated that the continuous power generation mode was successfully achieved with a mixture of MNF and bubbles.

Superconducting joints are essential for persistent-mode operation in a superconducting magnet system to produce an ultra-stable magnetic field. Herein, we report rationally designed niobium-titanium (Nb-Ti) superconducting joints and their evaluation results in detail. For practical applications, superconducting joints were fabricated by using a solder matrix replacement method with two types of lead-bismuth (Pb-Bi) solder, including Pb42Bi58 as a new composition. All the joints attained a critical current of >200 A below 1.43 T at 4.2 K. Our optimized superconducting joining method was tested in a closed-loop coil, obtaining a total circuit resistance of 3.25 × 10−14 Ω at 4.2 K in self-field. Finally, persistent-mode operation was demonstrated in an Nb-Ti solenoid coil with a persistent-current switch. This work will pave the way to developing high-performance Nb-Ti superconducting joints for practical applications.

Use of the systematic process of design optimization was proposed using the sequential approximation technique for the 10 MeV cyclotron magnet, which is a positron emission tomography cyclotron for producing radioactive isotopes. To design cyclotrons with the conventional routine approach, too much time and an iterative process are required, because even a very small change in the magnet shape is directly linked to success or failure, as verified using the beam orbit and energy level. To resolve this sensitive and tricky problem, designers need a systematic numerical approach for developing a new design. Until now, however, related research articles have not been seen in the literature. To build a systematic optimization process for the cyclotron magnet, we adopted the Latin hypercube sampling method, one of the design of experiments, and generated sampling data. On the basis of these data, an approximation model was modeled using the Kriging technique. To find the optimal shape, an iterative searching technique was implemented, which incorporates the approximation model, restricted evolution strategy, and boundary shifting technique. The initial magnet model was obtained from the conventional routine procedure for a cyclotron. To verify our proposed model, the final magnet model was tested with a beam simulation for checking the trajectory of $^{18}\text{F}$ at a level of 10 MeV at the extraction point.

Predicting the electrical breakdown of polymers is critical for certifying the endurance and lifetime of high voltage power equipment. Since various factors contribute nonlinearly to the breakdown phenomena of polymer insulators, it is difficult to assess the impact of each factor independently. In this study, we numerically analyzed the breakdown phenomenon because of the ramp rate of the DC voltage applied to a polymer insulator, low-density polyethylene (LDPE), using the finite element method (FEM). To predict the breakdown initiation, we analyzed the relaxation time of the conduction current through the insulator as a significant indicator. The bipolar charge transport (BCT) model was used to analyze the charge behavior within the LDPE, and the breakdown voltage was predicted by incorporating the molecular displacement model. This analysis was conducted for a wide range of ramp rates from 10 to 1500 V/s. The current density was calculated using two different methods, namely the energy and average methods, and the results were compared with each other. The results of the numerical model were further verified by comparing with those from experiments reported in the literature.

Positive and negative discharge mechanisms in atmospheric air have been numerically analyzed using a fully coupled finite element analysis. Until now, many research works have examined discharge analyses, but the discharge mechanism has not yet been explained in detail numerically. In a positive discharge, the electric field intensity at the streamer head can be enhanced owing to the superposition of a Laplacian field and a field from the positive space charge density. With this increased electric field intensity and attraction force between the negative electrode and positive space charge density, the positive streamer can move straight from the positive tip electrode to the negative plane electrode. In a negative discharge, the polarity of the electrode is reversed, and the positive space charge covers the negative electrode. Excess negative ions move toward the positive electrode, and a Trichel pulse is formed in the discharge current. To analyze this space charge propagation, we employed charge continuity equations for carriers such as electrons and positive and negative ions. These equations include ionization, attachment, and recombination effects, and Poisson's equation for electric fields. A secondary emission effect is included as a boundary condition on the cathode surface. Tip-plane electrodes were implemented in 2D axial symmetric model with a gap radius of 1 mm. To verify our numerical setup, the numerical results were compared with experimental data found in the previous literature.