The discovery of high Tc superconductors has led to the development of novel applications of bulk materials for practical industries, as well to the application of the wiring for metallic low-temperature superconductors. Uniform REBCO bulk materials have been successfully manufactured with the size reaching more than 60 mm in diameter, and these cylindrical superconducting bulk magnets have created applications for compact cryogen-free NMR magnets. In the latest research, a newly designed magnet composed of eight-stacked bulk magnets has been developed. Furthermore, a GdBCO tape was wound around the copper pipe, and this was inserted into the bore of the magnet. The NMR spectrum resolution of 1 Hz at the full width at half maximum was observed on this magnet. This proved that the magnetic field homogeneity required for the high-resolution NMR has been accomplished using a cylindrical superconducting bulk magnet. In the near future, the mechanical strength of the bulk superconductor itself will be improved, leading to the common use of 400 MHz (9.4 T) NMR magnets.
Compact NMR magnets comprised of stacked HTS bulk annuli trapped using a field cooling (FC) method and cooled by liquid nitrogen have been developed. The strength and homogeneity of the magnetic field required for the NMR relaxometry device are 1.5 T and 150 ppm/cm3, respectively, and these values are much lower than a conventional NMR device. The target magnetic-field strength of over 1.5 T at liquid nitrogen temperature can easily be obtained using stacked HTS bulk annuli. However, it is hard to obtain a target field homogeneity above 150 ppm/cm3 using a conventional superconducting magnet (SCM) as the magnetizing magnet. As a result, experimental and analytical studies have been conducted to improve the spatialfield homogeneity of HTS bulk magnets developed for NMR relaxometry. A method for re-applying the magnetic field, a field compensation method using several iron rings and hybrid model magnet comprised of HTS bulk and HTS coil, has been proposed and its effectiveness was confirmed. Finally, asymmetric problems of magnetic-field uniformity occurred owing to the different Jc–B characteristics of the HTS bulk annuli, and the degraded HTS bulk annuli were analyzed using the three-dimensional (3-D) finite element method (FEM).
We have assessed various superconducting materials for NMR application, and designed and fabricated a compact, lightweight, mobile permanent high-Tc superconducting magnet system using RE annular bulk. Using this bulk magnet system, we successfully magnetized several melt-processed bulk samples at 77 K. Additionally, we experimentally produced a compact NMR magnet system using RE annular bulks and tapes. Using simple superconducting magnets for magnetizing bulk materials can open new technological windows in various industrial areas.
A new type of superconducting bulk magnet for compact nuclear magnetic resonance (NMR) devices with high magnetic-field homogeneity has been developed by inserting an HTS film cylinder into a bulk superconductor bore. Annular 60 mmφ Eu-Ba-Cu-O bulk superconductors with a larger inner diameter (ID) of 36 mm were sandwiched between bulk superconductors with a smaller ID of 28 mm, and the total height of the bulk superconductor set was made to be 120 mm. The inner height of central wide bore space was optimized by magnetic-field simulation so that the influence of the bulk superconductor's paramagnetic moment on applied field homogeneity was minimized during the magnetization process. An HTS film cylinder, in which Gd-Ba-Cu-O tapes were wound helically in three layers around a copper cylinder, was inserted into the bulk bore in order to compensate for the inhomogeneous field trapped by the bulk superconductor. The superconducting bulk magnet composed of the above bulk superconductor set and the film cylinder were cooled by a GM pulse tube refrigerator and magnetized at 4.747 T using the field cooling (FC) method and a conventional superconducting coil magnet adjusted to below 0.5 ppm in magnetic-field homogeneity. The NMR measurement was conducted for an H2O sample with a diameter of 6.9 mm and a length of 10 mm by setting the sample in the center of the 20 mm ID room-temperature bore of the bulk magnet. The magnetic- field homogeneity derived from the full width at half maximum (FWHM) of the 1H spectrum of H2O was 0.45 ppm. We confirmed that the HTS film inner cylinder was effective in maintaining the homogeneity of the magnetic field applied in the magnetization process, and as a result, a magnetic field with a homogeneity of less than 1 ppm can be generated in the bore of the bulk magnet without using shim coils.
The effect of inserting high-Jc HTS thin cylinders into a 200 MHz (4.7 T) nuclear magnetic resonance (NMR) superconducting bulk magnet using a field-cooled magnetization (FCM) process and three-dimensional (3D) numerical simulation was studied. High-Jc thin cylinders of various lengths and shapes were inserted in various positions. A recent experiment in which a cylinder wrapped with a high-Jc material was inserted in a bulk cylinder was confirmed to be effective in reducing the inhomogeneity of the trapped field in the hollow space. We also modeled an actual cylinder with several kinds of slits. As a result of the defect in the NMR cylinder bulk magnet, the inhomogeneity of the trapped field can be drastically improved by inserting a HTS thin cylinder that has a higher Jc than a bulk material the same length as the bulk magnet. However, the level of improvement in the trapped field homogeneity changes depending on the spatial relationship between the defect and the HTS thin cylinder, and on the length and shape of the HTS thin cylinder with and without slits. The superconducting current flows in the HTS thin cylinder to compensate for the inhomogeneous trapped field caused by the defect. The optimum length and shape of the high-Jc HTS thin cylinder is discussed in terms of realizing a practical compact NMR bulk magnet.
Various experimental attempts have been made to obtain a uniform magnetic field in the space between face-to-face HTS bulk magnets that could possibly be utilized as NMR magnets. In general, the magnetic fields emitted from the magnetic pole surfaces containing HTS bulk magnets are characterized as non-uniform field distributions. Since the NMR magnets require highly uniform magnetic-field spaces, it has been assumed to be difficult to form uniform magnetic-field spaces between magnetic poles placed face-to-face. The authors modified the shapes of the magnetic-field distribution from convex to concave by attaching ferromagnetic iron plates to the pole surfaces. The magnets were then set face-to-face with various gaps of 30-70 mm, and the experimental data on magnetic-field uniformity was precisely measured in the space. In order to detect the NMR signals, the target performance for uniformity was set as 1,500 ppm throughout the 4-mm span on the x-axis, which is equivalent to performance in the past when the world's first detection of NMR signals was observed in the bore of hollow-type HTS bulk magnets. When we combined the concave and convex field distributions to compensate the uneven field distributions, the data of the best uniformity reached 358 ppm and 493 ppm in the 30 mm and 50 mm gaps, respectively, which exceeded the target value for the purpose of detecting the NMR signals within the space. Furthermore, it was shown that the field distributions change from concave to convex shape without any change at 1.1 T in the range from 7 to 11 mm in the 30-mm gap, indicating that the distributions are uniform. This suggests the possibility that the uniform magnetic-field space between the HTS bulk magnets set face-to-face expands.
The authors have studied the co-winding coil method (CW method) using the co-wound coil electrically insulated from the HTS coil. In this method, the quench is detected by the voltage difference between the coil of the HTS tape (HTS coil) and the coil of the normal conductor (CW coil). The voltage induced in the CW coil caused by the change of the magnetic field is almost the same as that in the HTS coil because the coils are magnetically coupled close to each other. Therefore, it is expected that the induced voltage will be canceled with high accuracy and that the resistive voltage in the HTS coil will be detected with greater sensitivity compared to the bridge balance method, which is used commonly. In this study, quench detection applying the CW method is demonstrated using an experimental double-pancake coil. A tape with the copper layer deposited on the polymer substrate was used as the insulated conductor wire to form the CW coil. An additional pancake coil was used to expose the experimental double-pancake coil to the external magnetic field asymmetrically. It was shown that the CW method can detect the resistive voltage with greater sensitivity even when the HTS coil was exposed to the changing asymmetric external magnetic field.