Search

This study explores the possibility of achieving magnetic field homogeneity comparable to that of nuclear magnetic resonance (NMR) magnets in a low temperature superconducting (LTS) magnet originally designed for physical property measurements. The LTS magnet used in the experiment has a significant advantage due to its large spatial margin for improving the magnetic field homogeneity of the NMR magnet, as it has a room-temperature (RT) bore diameter of 102 mm. However, certain low-order gradients pose a serious problem due to the shape of the magnet. Additionally, the absence of other shimming devices, typically present in NMR magnets, makes it difficult to improve magnetic field homogeneity. This study was conducted to determine whether it is possible to overcome these limitations and upgrade the field homogeneity of the magnet solely using multiple ferromagnetic shimming technology. This research addresses the enhancement of magnetic field homogeneity by effortlessly applying ferromagnetic shimming to the non-NMR LTS magnet, which incorporates both shimming to minimize specific gradients and traditional lattice structure shimming.

This study explores the influence of the Partial-Electrical-Connector (PEC) on High-Temperature Superconducting (HTS) coils. The PEC method emerges as a promising alternative to the conventional No-Insulation (NI) technique, establishing a direct current path between turns through partially soldered metal foils, such as copper, on the coil surface. This innovative approach achieves comparable performance to NI without necessitating a complete path between turns, offering advantages even in insulated or paraffin-impregnated coils. For the investigation, an insulated HTS coil and two NI HTS coils with paraffin impregnation are prepared. These coils undergo testing under overcurrent conditions, and their performance is compared with PEC-applied samples. The results demonstrate that coils with PEC application exhibit definitive current bypass characteristics. This finding highlights the potential of PEC to effectively create current bypass paths in both insulated and paraffin-impregnated coils.

We report room-temperature (RT) shimming results and a long-term operational protocol for a Korea Basic Science Institute (KBSI) 400 MHz high-temperature superconducting (HTS) nuclear magnetic resonance (NMR) magnet composed of 48 all-REBCO double-pancake (DP) coils cooled by a cryo-cooler to be operated under 20 K. The spatial magnetic field homogeneity of the 400 MHz HTS NMR magnet improved from 0.95 ppm to 0.2 ppm due to the use of a genetic-algorithm-based RT shimming method, of which the feasibility had been verified beforehand through an earlier study of a low-temperature superconducting (LTS) NMR magnet. We also confirmed that the field homogeneity of the HTS NMR magnet varied slightly over time due to the screening-current-induced field (SCF) and the temporal instability of the magnet power supply during long-term operation. Because fluctuations in the field quality affect the performance of the HTS NMR magnet, we established a long-term operation protocol for HTS magnets when used in NMR or magnetic resonance imaging (MRI) applications. Through the proposed operational protocol, the homogeneity of the HTS NMR magnet was maintained within the target range for 24 hours. The experimental results here demonstrate the potential of the proposed RT shimming method and long-term operation protocol for HTS NMR/MRI applications.

Given that REBCO wire is typically manufactured as a tape, it is possible for REBCO magnets to exhibit a relatively larger screening current-induced field (SCF). It is well known that the SCF in an REBCO magnet can be effectively reduced by the so-called current sweep method. However, it requires an overshooting in the magnet current, often close to the magnet's critical current, to effectively suppress the SCF, which may not be favored in terms of operation reliability. In this study, we experimentally validated an alternative approach which mitigates the SCF using thermal cycling together with the current sweep reversal. Well known is the fact that the large load factor, defined as the ratio of operating current to critical current of an REBCO magnet, deteriorates the SCF impact on field uniformity. The key idea is to increase the magnet temperature to increase the effective load factor of the magnet close to its quench limit momentarily, minimize the SCF, and then return back to the nominal operating condition. The combined approach of the thermal cycling and the current sweep may be effective when attempting to eliminate the SCF without ramping an REBCO magnet to a current that may be dangerously high, especially when the magnet is operated at a high operating current. This paper presents the test results of the combined approach in the conduction-cooled REBCO magnet, and the results demonstrate the potential of the approach for more safe and effective elimination of the SCF.

The critical current of a High-Temperature Superconductor (HTS) tape varies with temperature, external magnetic field magnitude, and field angle. However, there is no single standard curve for the dependency due to the complex nature of the HTS tape. Therefore, we need many direct measurements of the critical current of different samples. In this research, we introduce a cryostat design for the critical current measurement system in which cooling is based on the gaseous helium to reduce total helium consumption for a single measurement. The cryostat is integrated into a conduction-cooled, 102 mm room-temperature bore, 15 T center field magnet. An approximately 5 cm long HTS tape sample will be installed near the center of the magnetic field. We will use a GM-cryocooler with helium gas to cool down the HTS tape sample, and also use a heater to control the sample temperature. A pair of HTS-copper hybrid current leads will feed the HTS sample. In this paper, we will present the operation process of the system, a brief heat input calculation, and future plans.

A Partial Electrical Connector (PEC) offers a promising self-protection mechanism for high-temperature superconducting (HTS) coils, providing an alternative to no-insulation (NI) coils. Unlike NI coils, PEC-applied coils have limited current bypass paths, potentially limiting their self-protection capabilities. This study investigates how the location of hot-spots, created by heaters positioned at different sections within the coil, affects the performance of PEC. By analyzing voltage and center field responses, we found that the proximity of the heater to the PEC significantly influences the efficiency of current bypass. Specifically, closer hot-spots result in more effective current redirection through the PEC. These findings provide valuable insights for optimizing PEC patterns, potentially enhancing protection mechanisms for larger HTS coils by improving current bypass capabilities based on hot-spot location.

We present a recently developed shimming method that uses a system of room-temperature (RT) orthogonal shimming coils to improve the spatial field homogeneity of high-resolution nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) superconducting magnets. Conventional RT shimming methods require recursive routines of field mapping and RT shim coil current adjustments, which lead to a less effective and time-consuming process. In this paper, we introduce a new RT shimming method that utilizes a process of gradual and automatic search to find an optimal operating point with a genetic algorithm (GA), a type of optimization technique. The proposed algorithm is capable of determining the optimal set of currents for a system of shimming coils to create the best spatial field homogeneity by minimizing the NMR spectral linewidth. We used a commercial 600-MHz NMR magnet with a full NMR spectral linewidth at half maximum (FWHM) of 2.2 ppm, as determined by an NMR test sample contained in a cylindrical glass tube with a diameter of 5 mm to demonstrate that the shimming method can improve the FWHM to 0.15 ppm. The results demonstrate the strong potential of the proposed RT shimming methodology for the auto-shimming process of NMR/MRI applications.

We have batch-fabricated cantilevers with ∼100 nm diameter nickel nanorod tips and force sensitivities of a few attonewtons at 4.2 K. The magnetic nanorods were engineered to overhang the leading edge of the cantilever, and consequently the cantilevers experience what we believe is the lowest surface noise ever achieved in a scanned probe experiment. Cantilever magnetometry indicated that the tips were well magnetized, with a ≤ 20 nm dead layer; the composition of the dead layer was studied by electron microscopy and electron energy loss spectroscopy. In what we believe is the first demonstration of scanned probe detection of electron-spin resonance from a batch-fabricated tip, the cantilevers were used to observe electron-spin resonance from nitroxide spin labels in a film viaforce-gradient-induced shifts in cantilever resonance frequency. The magnetic field dependence of the magnetic resonance signal suggests a nonuniform tip magnetization at an applied field near 0.6 T.