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This paper reports calculation and measurement of magnetic fields from a 3 T 66-mm room-temperature bore no-insulation high-temperature superconductor nuclear magnetic resonance (NMR) magnet with screening current and manufacturing uncertainty considered. The magnet reached its full field, 3 T under a conduction-cooling environment at 17 K. The measured magnetic fields along the magnet axis showed a substantial discrepancy to the originally designed ones. In simulation, measured dimensions of the 66 total single pancake coils of the magnet were used in consideration of the manufacturing errors of each pancake coil. And, the two-dimensional edge-element method with the domain homogenization technique was adopted to simulate screening currents within pancake coils and calculate the consequent screening current induced fields. The normalized magnetic fields between simulation and measurement showed good agreement that implies a potential to calculate “actual” magnetic fields of an REBCO NMR magnet with our approach.

This paper represents a numerical calculation method that enables highly-accurate simulations on temperature analysis of superconducting magnets considering the heat flow between the magnet and liquid helium during a quench. A three-dimensional (3D) superconducting magnet space was divided into many cells and the finite-difference method (FDM) was adopted to calculate the superconducting magnet temperatures governed by the heat transfer and joule heating of the each cell during a quench. To enhance the accuracy of the temperature calculations during a quench, the heat flow between the superconducting magnet surface and liquid helium, which lowers the magnet temperatures, was considered in this work. The electrical equation coupled with the governing thermal equation was also applied to calculate the change of the decay of the magnet current related to the joule heating. The proposed FDM method for temperatures calculation of a superconducting magnet during a quench process achieved results that were in good agreement with those obtained from an experiment.

In this study, we introduce a highly accurate NMR magnetometer, which is capable of measuring the magnetic field for inhomogeneous HTS magnets. The magnetometer is based on the pulsed and Fourier-transformed NMR and 1H NMR signal analyses. The magnetometer system was fabricated according to these concepts and magnetic field measurements on a 3 T HTS magnet were carried out using various mapping paths. The test results demonstrate that the magnetometer can measure magnetic fields accurately in inhomogeneous conditions as high as approximately 864 ppm with a spherical volume of 10 mm in diameter nearly centered at the magnet center. The stronger and hence shorter RF pulse resulting from the high-power amplification enables measurements of magnetic fields in more inhomogeneous conditions. Because the permissible field homogeneity range is very wide, the magnetometer can be used for mapping fields at the preshim stage of the inhomogeneous magnets. To also verify the high-resolution performance of the magnetometer, the results are compared with measurement data obtained with a Hall magnetometer ordinarily used in such an inhomogeneity range. We expect that the proposed magnetometer will enable the precise measurements of the magnetic fields over a wide homogeneity range, and thereby bring about improvement in the shimming process of the inhomogeneous magnets.

This paper investigates an optimal design methodology for multiwidth (MW) high-temperature superconductor (HTS) magnets, where pancake coils wound with the narrowest tapes were placed at and near the magnet center and those with progressively wider tapes toward the top and bottom of the magnet. For a given design, target of field strength and winding bore, input parameters include tape width, number of grading tapes, and number of pancakes coils, while the main objective is to minimize the magnet volume, i.e., essentially the stored energy and thus the cost. A magnet with a single-width tape is also designed as a control sample for comparison. A case study on a design of 3-T 240-mm HTS magnet was carried out. In that case, MW magnets save 40% wire consumption of single-width magnets. The results are expected to be beneficial to determine the practical level of HTS tape grading and estimate a volume of an MW magnet.

A 3-T 100-mm winding bore conduction-cooled REBCO magnet has been designed, constructed, and tested. It consists of a stack of double pancake coils wound with metallic cladding (MC) REBCO tapes, for which 1–2 $\mu {\text{m}}$ stainless steel was clad in a hermetic way around the tapes. This approach, first proposed by the SuNAM Company Ltd. in 2015, is expected to substantially reduce the charging time delay of a no-insulation magnet, which in turn will be effective in the manipulation of screening currents in such a way as to improve the field homogeneity of magnets, particularly for nuclear magnetic resonance or MRI. This paper presents test results for our approach on the current sweep cycling, which is used to reduce the screening current-induced fields in a newly constructed 3-T 100-mm MC all-REBCO magnet

This paper presents a high-resolution magnetic field mapping system in development that is capable of collecting spatial magnetic field data for NMR magnets. An NMR probe was designed and built with a resonant frequency of 5.73 MHz. The measured Q-factor of the NMR probe is ∼191 with a half-power bandwidth in the range of 5.72–5.75 MHz. An RF continuous-wave technique with magnetic field modulation was utilized to detect the power dispersion of water molecules. The zero-crossing frequency of the NMR dispersion signal corresponds to the magnetic field at the center of the water sample. An embedded system was developed to sweep the frequency and record the reflected RF power simultaneously. A numerically controlled digital oscillator is able to provide a precise frequency step as small as 0.02 Hz, which is equivalent to 4.7 e-7 mT for hydrogen atoms. An RF preamplifier was built to supply up to 4 W of RF power to a bidirectional coupler. The coupler supplies RF power to the NMR probe and channels reflect the RF power back to the detection circuit, which detects the reflected RF power from the NMR probe during the frequency sweep. The homogeneity of an NMR magnet can be determined by magnetic field data.

Flux diverters made of ferromagnetic materials can be used in high-temperature superconducting (HTS) magnets to change the magnetic field distribution and increase the critical current of the magnet. In this paper, the notch-shaped flux diverter increases the critical current of the HTS magnet by detouring the magnetic flux more effectively than the flat surface-shaped (simple) flux diverter. Numerical analyses are carried out to calculate the critical current using the simple-shaped and the notch-shaped flux diverters. Then, experiments are conducted to verify the results of the numerical analyses. As a result, the HTS coil with a notch-shaped flux diverter shows the increased critical current.

This paper presents a passive shimming design approach for a magic-angle-spinning (MAS) NMR magnet. In order to achieve a 1.5-T magic-angle field in NMR samples, we created two independent orthogonal magnetic vector fields by two separate coils: the dipole and solenoid. These two coils create a combined 1.5-T magnetic field vector directed at the magic angle (54.74° from the spinning axis). Additionally, the stringent magnetic field homogeneity requirement of the MAS magnet is the same as that of a solenoidal NMR magnet. The challenge for the magic-angle passive shimming design is to correct both the dipole and solenoid magnetic field spherical harmonics with one set of iron pieces, the so-called ferromagnetic shimming. Furthermore, the magnetization of the iron pieces is produced by both the dipole and solenoid coils. In our design approach, a matrix of 2 mm by 5 mm iron pieces with different thicknesses was attached to a thin-walled tube, 90-mm diameter and 40-mm high. Two sets of spherical harmonic coefficients were calculated for both the dipole and solenoid coil windings. By using the multiple-objective linear programming optimization technique and coordinate transformations, we have designed a passive shimming set that can theoretically reduce 22 lower-order spherical harmonics and improve the homogeneity of our MAS NMR magnet.

In this paper, we present the fabrication and experimental analysis of a 6.6 kV/100 A class superconducting fault current controller (SFCC) as a novel prototype fault current limiter. Recently, the integration of distributed generations into a grid has been issued due to increased power demand and their lower environmental impacts. The addition of distributed generators can increase the level of the fault current in the grid. To resolve the issue of increased fault current level, we propose a SFCC consisting of a superconducting coil, full-bridge thyristor rectifier, and control unit. The proposed SFCC can be described as a combination of a thyristor type solid-state breaker and a superconducting DC reactor coil. The inductance of the superconducting coil, which is about 10.4 mH, reduces the fault current amplitude. After the half cycle, through the control of the thyristors' phase angle, SFCC can control the level of the fault current to a proper level. In this paper, we present the design and fabrication of the superconducting coil, a rectifier, and a thyristor control unit. Based on the results of short circuit tests, which simulated the power grid, we expect that SFCC and its behavior can be helpful in protecting power grids from unpredictable fault currents.

A semipersistent current-mode operation with a high-temperature superconducting (HTS) coil is very useful for some applications, including maglev. An HTS coil for semipersistent mode operation includes at least one residual joint to make a closed-loop coil. A low current decaying rate is achieved by reducing the residual joint resistance. This paper suggests a winding method with a turn-to-turn soldering process. The turn-to-turn soldering process provides a wide flow path for the charged current in the HTS coil during the semipersistent mode operation. Therefore, the total equivalent residual resistance is reduced. However, this reduced residual resistance also prevents the current from participating in the charging process. Therefore, a new current charging method for the turn-to-turn soldered HTS coil needs to be tested. In this paper, a small-scale sample HTS coil with turn-to-turn soldering method is fabricated for testing. A simple charging-discharging experiment is performed on the sample coil. The time constant of the sample coil is calculated from the experimental results. A fast current charging sequence for the turn-to-turn soldered coil is also tested and analyzed from the perspective of power and energy consumption during the current charging operation.

In order to obtain the target field homogeneity of a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) magnet, commonly used are active shim coils, i.e., axial and radial, that typically locate radially outside of the magnet. In a high-temperature superconducting (HTS) magnet, however, we reported a “strong” hysteresis in the field-to-current ratio of an external (placed radially outside the HTS magnet) active shim coil by the screening-current-induced fields (SCFs). This nonlinear behavior of an external active shim due to the SCF is one of the major technical challenges to achieve the target field homogeneity of an HTS NMR or MRI magnet. In this paper, we constructed and operated active shim coils, i.e., two axial ( Z1 and Z2) and two radial (X and Y) , that were installed inside of an HTS magnet to investigate effectiveness of internal shim coils. The HTS magnet, wound with GdBCO coated conductors, consists of ten double pancakes. With the active shim coils, a customized field mapping and shimming system was constructed, which consists of a 3-D field mapper, a field analysis software package, and a set of power supplies to control the shim coils. A pre-shim field error of 953 ppm at a 10-mm-diameter cylinder was improved to 464 ppm after iterative automated shimming using the internal shim coils. The results demonstrate that the internal shim coils are effective to eliminate the SCF-oriented field errors, both radial and axial, which was practically impossible for the external shim coils.

This paper presents quench analysis of a superconducting magnet system for 28 GHz electron cyclotron resonance (ECR) ion source. The magnet system consists of a hexapole coil and four solenoid coils located outside of the hexapole one. All coils were wound with NbTi wire and impregnated by epoxy. To analyze the characteristic of superconducting coil when the quench occurs, a numerical code was developed. The analysis procedures are as follows. First, normal zone propagation (NZP) velocity which is as a function of magnetic field was calculated. Second, a fraction of the winding volume was obtained by transient analysis, considering longitudinal and transverse NZP velocities. Third, a generated resistance and temperature rising over time were simulated. Lastly, current trace of the coil was calculated. The current trace calculated by simulation well agrees with the test result. Also the result of hot-spot temperature is reasonable. Since simulated hot-spot temperature and experimental result are 60.32 K and 63 K when the operating current is 169 A. The normal zone resistances are also identical for 1.13 s which is the convergence time of simulation. The final resistances are about 10.2 Ω and 11.23 Ω. Therefore it is expected that the analysis code can be used to estimate the characteristic of superconducting magnet when the quench occurs.

Sulfonated hollow microporous organic polymers (S-HMOP) were prepared by template synthesis and post synthetic approach. The S-HMOP showed excellent non-covalent fixation ability towards various cationic dyes through ionic interaction. Among various dye systems, Zn-porphyrin loaded S-HMOP showed promising activity and stability in the decomposition of 4-chlorophenol under visible light irradiation.

This paper reports the effects of thermal energy on reducing the overshoot of the current sweep cycle method to reduce the screening current-induced field (SCF) in a 2G high temperature superconducting (HTS) coil. A disadvantage of the current sweep cycle method is the necessity for large overshoot in the coil current. For a 2G HTS coil, excessive overshooting of the coil current is undesirable (Yanagisawa et al 2012 AIP Conf. Proc. 1434 1373–8). In an effort to circumvent this overshooting problem, the thermal energy effect was investigated in combination with the current sweep cycle method based on experiments in this study. The experimental results show that greater SCF reduction in the HTS coil was obtained upon increasing thermal energy by heater current.

This paper presents a warm bore ferromagnetic shimming design for a high resolution NMR magnet based on spherical harmonic coefficient reduction techniques. The passive ferromagnetic shimming along with the active shimming is a critically important step to improve magnetic field homogeneity for an NMR Magnet. Here, the technique is applied to an NMR magnet already designed and built at the MIT's Francis Bitter Magnet Lab. Based on the actual magnetic field measurement data, a total of twenty-two low order spherical harmonic coefficients is derived. Another set of spherical harmonic coefficients was calculated for iron pieces attached to a 54 mm diameter and 72 mm high tube. To improve the homogeneity of the magnet, a multiple objective linear programming method was applied to minimize unwanted spherical harmonic coefficients. A ferromagnetic shimming set with seventy-four iron pieces was presented. Analytical comparisons are made for the expected magnetic field after Ferromagnetic shimming. The theoretically reconstructed magnetic field plot after ferromagnetic shimming has shown that the magnetic field homogeneity was significantly improved.