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In order to achieve a persistent current mode operation with high-temperature superconducting (HTS) coils, we have been trying to improve the concept of a jointless HTS coil. The coil has a perfect closed loop without any joints in a coil, and we have proven that a persistent current can be induced in the coil by several charge up methods such as PCS, flux pump, or field cooling with a background magnet. In this paper, based on the previous experiment, which showed partially successful results [10], we designed and fabricated a small solenoid type jointless HTS magnet with a 100-m-long ReBCO wide conductor and tested it in a liquid nitrogen bath. After the persistent current mode operation of 15 h, we tried to measure temporal and spatial homogeneity by a field mapper we developed under the DSV of 10 mm .

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.

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 presents the characteristics analysis of the flux-switching synchronous generator (FSG) applying no-insulation (NI)-type high-temperature superconducting (HTS) field coils. The HTS FSG can be cost effective, since the field coils and armature windings are both installed in the stator, while the rotor is only made of iron core with tooth. Along with low cost, since HTS field coils in the FSGs are located in the stationary part of the generator, FSG can be a solution for the design complexities of the excitation and cooling systems for HTS field coils. In addition, several studies on HTS coils without turn-to-turn insulation (NI) have been reported for HTS field coils for applying in electrical rotating machines. The NI-type winding technique enhances the stability of the HTS field coil since the overcurrent is automatically bypassed through the contacts between winding turns. Nevertheless, there is still an issue about whether the NI-type HTS field coil can be applied to the electrical rotating machines. In this study, we fabricated and tested an NI-type HTS field coil. In addition, a 600-kW HTS FSG was designed using a fundamental design process and the characteristics analysis of the generator was performed based on the test results of the fabricated NI-type field coil. The results show that the HTS FSG having NI-type HTS field coils is able to operate normally under normal operating conditions. However, the field current charging problem issue has been revealed through this study. Therefore, further research will need to improve the problem.

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

Due to the self-protecting feature, no-insulation (NI) high-temperature superconductor magnets have been regarded as a reliable option to generate high fields, yet their intrinsic charging delay still remains a major drawback. To apply the NI technique for actual high-field user magnets, however, postquench transient behavior of such magnets need to be fully understood, particularly the electromagnetic interaction among magnetically coupled subcoils. Recent publications have shown successful simulations of the transient behavior of a single NI coil or a multicoil magnet using distributed network models. Even though these approaches are very accurate, they often require substantial computation time, especially when multiple iterations are required during design or analysis of an NI magnet having a large number of coils. This paper presents a simple circuit approach that may be effective for quench simulation of multicoil NI magnets. Each NI subcoil in a magnet is lumped into a single inductor with a resistor in series and a resistor in parallel. This approach has allowed us to simulate the whole magnet system within reasonable time without compromising the understanding of mutual interactions of all of the subcoils after quench, namely change in parameters such as voltage, current, and temperature with respect to time and coil to coil normal zone propagation in an electromagnetic manner. We verified the proposed approach by analyzing the quench process in a previously built 7 T, 78 mm all-REBCO NI magnet (by MIT), and by doing the first ever comparison between the simulated results with the data measured from actual experiment.

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using an electrically conductive epoxy resin. Recently, several studies of HTS coils without turn-to-turn insulation have been reported for field coils used in rotating machines such as motors and generators. The NI winding technique enhances the thermal stability of the HTS coil without requiring complicated protection techniques because the quench current is automatically bypassed through the turn-to-turn contacts within the HTS coil. Nevertheless, there is still a question as to whether the NI technique can be applied to rotating machines. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. For HTS field coils intended for use in rotating machines, epoxy impregnation is generally necessary to protect the HTS field coil from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability [1] , [2] . However, the NI HTS coil cannot be fabricated by wet winding using epoxy resin because epoxy resins such as Stycast 2850 FT and CTD 521 are electrically insulating materials. This study examines the electrical stability of an NI HTS coil impregnated with an epoxy resin containing electrically conductive particles. The results are likely to present useful data for the application of electrically conductive epoxy impregnated NI HTS coils to rotating machines.

We present a design of a 400-MHz/60-mm all-REBCO nuclear magnetic resonance (NMR) magnet (H400) that consists of a stack of 56 double-pancake (DP) coils. With the multiwidth no-insulation technique incorporated, DP coils were wound with REBCO tapes of five different widths, i.e., 4.1, 5.1, 6.1, 7.1, and 8.1 mm; DP coils placed at and near the magnet midplane were wound with the narrowest (4.1 mm wide) REBCO tapes, whereas those with progressively wider tapes were placed toward the top and bottom of the magnet, where the “perpendicular field B ⊥ ” is at its peak within the magnet. The magnet was designed to be operated under a conduction cooling environment at 20 K. Once successfully completed, the magnet will be installed as an NMR user facility in the Korea Basic Science Institute. Basic magnet performances and major technical challenges were discussed.

This paper presents experimental and theoretical studies of the no-insulation (NI) winding method of second-generation high-temperature superconducting (HTS) wire. We compared two single pancake coils wound by two different HTS wires. One pancake coil is made of a normal HTS wire with an electroplated copper stabilizer. The other pancake coil is made of the same HTS wire with the only difference in an additional outermost layer made of stainless steel. We employed an equivalent circuit model to evaluate our experimental results. We tested both coils by the same simple operating procedure consisting of two steps: first, ramping up of current from zero to holding current (IH) and second, keeping the IH at minimum 500 s. We also tested the stability of the coil wound by an HTS wire with an additional layer of stainless steel by applying a current exceeding a critical current of the coil. We observed a charging time of the metal-cladding HTS coil reduced to a quarter of copper-electroplated HTS coil.

A nuclear magnetic resonance (NMR) magnet composed of a high-temperature superconductor (HTS) is expected to contribute to a smaller size and a higher magnetic field (>1 GHz) of the NMR magnet due to its high critical current density under high magnetic field and better mechanical properties. The high operating temperature of HTS NMR magnets also makes conduction cooling by a cryocooler available. The conduction cooling system combined with a cryocooler is compact and user-friendly but has critical issues about the design of conduction cooling passages and the mechanical vibration. The small-scale HTS magnet with the conduction cooling system is fabricated and tested to check the issues and establish the design basis as a preliminary step for the development of a 400-MHz HTS NMR magnet. Thermal analysis for heat loads and thermal resistances at main joints is performed from the temperature distribution of the cooling system. The vibration levels at the crycooler and the vacuum chamber are discussed in comparison to the vibration criterion of NMR magnets. In addition, the design approach to the conduction cooling system for the HTS NMR magnet is discussed.

A high-temperature superconducting (HTS) DC cable system has attracted a great deal of interest from the view point of low loss, dense structure and large capacity compared to HTS AC cable system. It has a HTS cable and a termination. A HTS DC cable system consists of a conductor, cooling system and electrical insulation. To realize the HTS DC cable system, it is important to study not only high current capacity and low loss of conductor but also optimum electrical insulation at cryogenic temperature. The electrical insulation technology of HTS DC cable and termination must be solved for the long life, reliability and compact of cable. In this paper, we will discuss mainly on the electrical insulation characteristics and the insulation design of 220 kV class HTS DC cable. Voltage-time (V-t) characteristics of laminated polypropylene paper (PPLP) in LN2 were studied. Furthermore, the surface flashover characteristics of glass fiber reinforced plastic (GFRP) for termination insulators under DC and lightning impulse voltage were studied.