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Degradation of a REBCO coil under external magnetic fields is one of the major technical problems in the field of HTS magnet technology. A possible cause of such degradation is an inhomogeneous hoop stress distribution, or hoop stress modification (both increase and decrease), induced by the screening current. In this work, we investigate such a hoop stress modification by a small coil experiment with a strain measurement and a numerical simulation. An experimental result shows a very high stress increase factor of >4.1, defined by the maximum circumferential stress over BzJR stress, and the simulated result is in qualitative agreements. The strain (stress) shows a hysteresis effect corresponding to the screening current behavior. A large hoop stress modification causes not only a hoop stress increase, but also buckling of the conductor, which induces delamination and micro-clacks of the superconducting layer. We also show the stress modification can be reduced by bonding turns with epoxy.

Recently, we proposed a method of joint by incongruent melting (JIM) for making a superconducting connection between two multi-filamentary Bi2223 tapes. At high temperatures, the Bi2223 phase separates into solid Bi2212 and a liquid phase by incongruent melting. In the JIM method, the Bi2223 filaments in a tape are first melted to Bi2212 and liquid. Then, Bi2212 is connected by the liquid to form a superconducting current pathway. In the present experiment, a joint between multi-filamentary Bi2223 tapes was prepared, resulting in a superconducting joint with a critical current of 8 A at 77 K. We then investigated the microstructure of the superconducting joint interface using X-ray diffraction and SEM/EDS, to clarify the superconducting joint formation. In examining samples prepared using the JIM method, Bi2212 and Sr-Ca-Cu-O system compounds were found at the joint interface, which had a high crystalline orientation.

Superconducting magnet technology changed dramatically with the discovery of high temperature superconductors (HTS) in 1986, an event which drove the development of much higher field magnets. However, this technology paradigm shift has been delayed by as much as a decade in the case of NMR magnets. In this paper, we will provide a historical perspective to the reasons for this delay and assess the future prospects for high- and ultrahigh-field NMR magnets resulting from current trends in the development of HTS magnet technology.

International audience; Occasionally in proton high-resolution magic angle spinning nuclear magnetic resonance spectroscopy (1H HR-MAS NMR), the standard field shimming across the region-of-interest with the active shims is not sufficient in the presence of substantial magnetic susceptibility gradients. This can be ascribed to the presence of large air pocket within the sample, the proximity between sample and probehead (especially with micro-sized probes), or data acquisition at high magnetic fields. Herein, the study demonstrates a simple approach to enhance the capacity of the shim fields—by supplementing a specific passive ferro-shim—for unifying the field distributions across the sample. Qualitative field numerical analyses were carried out to illustrate the effectiveness of the applied passive shims together with the active shims.

A superconducting joint between two Gd123-coated conductors was successfully formed using crystal growth in the bulk (CJMB). An intermediate Yb123 layer was used at the junction, and was melted to form a joint by heat treatment at a temperature below the melting point of the RE123 (such as Gd123) in the coated conductor. This liquid-phase bonding results in high tensile strength, which has exceeded 100 MPa in previous studies. Nevertheless, the joint principle has not been sufficiently clarified. In this study, we performed microstructural analysis of the joint using X-ray diffraction and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy to form a clearer understanding of the joint formation mechanism in order to achieve a high critical current in the junction.

The design and development for a compact 1 GHz (23.5 T)-class NMR magnet are presented. The magnet uses inner coils wound with Ni alloy-reinforced Bi-2223 conductors in combination with LTS outer coils. The series-connected coils are operated in a power supply-driven mode. The total magnet size is very compact, and is comparable to that of a currently commercialized 600-MHz (14.1 T) NMR magnet. This owes to a high current density operation of the Bi-2223 inner coils, which contribute as much as 53% of the total magnetic field. This paper investigated the following properties of the Bi-2223 inner coils: First, joint resistance under external magnetic fields; second, tensile stress tolerances of the Bi-2223 conductor under bending condition; third, high compressive stress on Bi-2223 coils; fourth, the effect of screening current-induced magnetic field on the field homogeneity and stability; and fifth, the maximum permissible Joule heating to avoid a thermal runaway. Two Bi-2223 inner coils for the magnet were built and tested in liquid nitrogen, which provided important knowledge for the magnet excitation.

For application to ultrahigh-field nuclear magnetic resonance spectroscopy (e.g., 30 T), we have started to develop a REBa2Cu3O $_{7-\delta}$ (RE123, RE: rare earth) multi-core coated conductor in which the ceramic layers (RE123 and buffer layers) are electrically separated to create multiple filaments. This method is called electrical separation by inner splitting, and the wire is called a split wire. The multi-core structure is fabricated using electrical separation by a phase stress, which utilizes the difference in toughness between ceramics and metal, such as partial V-bending by stress along the longitudinal direction of the coated conductor using a commercially available single-core RE123 coated conductor. In addition, about ten narrow cores (width: 5–15 μm) can be formed by one bending. These cores are called subcores. The wire is composed of main cores and subcores. In this study, a 4-mm-wide multifilamentary RE123 split wire with 16 main cores and 150 subcores was fabricated and evaluated. The manufacturing method, microstructure, and critical current properties under an external magnetic field and tension are presented.

High-field high temperature superconducting (HTS) coils, in particular REBCO coils, tend to show electromagnetic force-induced degradation and premature thermal runaways. Operation of high-field HTS coils is practicable with resistive voltage if cooling power permits. In this light, understanding coil voltage and the corresponding Joule heating power that may give rise to a thermal runaway is essential for stable magnet operation. In this paper, we report investigations of continuous heating criteria for safe operation, Q safe , and the dependence of Q safe on applied field, operating conductor current density, heating length, cooling condition, and number of coil layers using a numerical simulation based on results of model coil experiment. In addition, Q safe values for actual HTS coils were estimated; the results showed that the simulation can predict Q safe values of actual coils.

This paper overviews the combined effects of winding, cool-down, and screening current-induced stresses in REBCO coils. First, a simulation method to model the circumferential stress modification effect due to the screening-current is overviewed. The simulation includes coil winding, cooling down, and coil charge up to the operating current. Second, we will compare the numerical simulation results with the experimental results. The numerical simulations for a dry coil and an epoxy impregnated coil agree well with the experimental results. Third, the enhanced circumferential stress did not degrade the performance of a dry winding REBCO coil, but the improved increased compressive stress buckled the coil structure. Finally, it is demonstrated that epoxy impregnation has beneficial effects in reducing the stress modification effect. However, the circumferential stress is enormously enhanced at the coil ends, sometimes resulting in degradation of the coil performance.

Recently, we have proposed the concept of a crystalline joint by melted bulk (CJMB) for making a superconducting connection between GdBa2Cu3O7− δ (Gd123)-coated conductors (wire) using a wire-bulk-wire structure. In a model experiment, a joint between a Gd123-coated conductor and large piece of Y123 bulk (wire-bulk) as one-half of a joint was demonstrated, resulting in a superconducting joint with a critical current of 10 A at 77 K, and a high tensile strength greater than 100 MPa. In this study, a joint between coated conductors in a wire-bulk-wire structure was constructed using a Yb123 sheet fabricated from bulk material. As a result of using the CJMB method, we successfully fabricated a superconducting joint over the short time of less than 1 day (including joint formation and oxygen annealing). A persistent current coil having a superconducting joint was fabricated using the CJMB method. Its resistance was estimated from the current decay rate of the coil, to have a low value not exceeding 10−10 Ω after one day following the external power cutoff.

Recently, degradation of a high-field REBCO coil due to strong electromagnetic forces, has been identified. This issue is related to a conductor movement, forming a kink in the conductor body, and hence epoxy impregnation should be effective to prevent it. The purpose of this paper is to examine the effect of epoxy impregnation on the electromagnetic force-induced degradation of a REBCO coil. We made an epoxy impregnated solenoid coil and charged it at 4.2 K in an external field of 11 T. A notable characteristic behavior, which is different from that of a dry or paraffin impregnated coil, was observed in the coil’s performance. The coil did not show any normal voltage below 408 A, at 65% on the coil load line, but it showed a sudden voltage jump at 408 A, resulted from a sudden fracture of the REBCO conductor. The outward bending, combined with a strong circumferential stress, caused the REBCO layer to fracture. Although epoxy impregnation is effective to suppress a conductor movement inside the winding, avoiding self-supported sites at a coil edge is required to eliminate degradation of the thin and flexible REBCO conductor.

This paper describes the first persistent-mode medium magnetic field (400 MHz; 9.39 T) nuclear magnetic resonance (NMR) magnet which uses superconducting joints between high-temperature superconductors (HTSs). As the ultimate goal, we aim to develop a high-resolution 1.3 GHz (30.5 T) NMR magnet operated in the persistent-mode. The magnet requires superconducting joints between HTSs and those between an HTS and a low-temperature superconductor (LTS). Towards this goal, we have been developing persistent-mode HTS inner coils to be operated in a 400 MHz (9.39 T) NMR magnet and here we present the first prototype inner coil wound with a single piece (RE = rare earth)Ba2Cu3O7−x (REBCO) conductor. The coil and a REBCO persistent current switch are connected with intermediate grown superconducting joints with high critical currents in external magnetic fields. To evaluate the performance of the joints in an ultimately stable and homogeneous magnetic field, the coil is operated in the persistent-mode, generating 0.1 T, in a 9.3 T background magnetic field of a persistent-mode LTS outer coil. The magnetic field drift over two years of the 400 MHz LTS/REBCO NMR magnet is as small as ∼1 ppm, giving high-resolution NMR spectra. The magnetic field drift rate over the second year was 0.03 × 10−3 ppm h−1, which is more than three orders of magnitude smaller than that required for an NMR magnet, demonstrating that the superconducting joints function satisfactorily in a high-resolution NMR system. The corresponding joint resistance is inferred to be −14 Ω.

The tensile stress tolerance of a Bi-2223 conductor has been doubled by the use of a mechanically strong Ni-alloy reinforcement material and a pretension technique. It is possible that, however, in the case of overheating due to a thermal runaway in a coil, release of bending strain in the mechanically strong reinforcement tape buckles the conductor itself. The present paper experimentally investigates the basic mechanism of such a buckling phenomenon. If the bending diameter of the coil is small, typically ≤50 mm, the release of the bending strain of the reinforcements instantaneously buckles the conductor itself when the solder between the reinforcements and the Bi-2223/Ag composite is melted. This causes fractures of the Bi-2223 filaments and a permanent degradation of the coil performance. The buckling is prevented by applying a radial compressive stress to the conductor; a smaller diameter coil winding requires a higher amount of compressive stress.

Solid-state NMR observations of low-gamma half-integer quadrupolar nuclei, 35Cl and 37Cl, were demonstrated using a 24 T hybrid magnet (1H resonance frequency of 1.02 GHz) comprised of the high-temperature (HTS) and low-temperature (LTS) superconductors, and compared with results using a 14.1 T standard NMR magnet. While at 24 T the linewidth is 1.7 times narrower than that at 14.1 T, the gain in the sensitivity is 7.0 times because of enhanced polarization, reduced linewidth, and the use of larger rotor. A simple theoretical model was used to rationalize the sensitivity enhancements. The ratio of 35Cl and 37Cl quadrupolar couplings agrees well with the ratio of quadrupolar moments, and no isotope-dependent chemical shift has been observed. In addition, the 3QMAS spectrum of 35Cl is shown to demonstrate the high sensitivity rendered by the 24 T spectrometer.

Magnets using low temperature superconducting conductors require liquid helium cooling, yet can only generate a magnetic field Super-High Field NMR Initiative in Japan. 2) Development of ultra-low resistance joints between superconducting dc feeder cables, enabling to connect adjacent railway substations that are 2–5 km apart. This paper first overviews the MIRAI Project. Second, it will describe initial results from investigations on superconducting joints for a persistent mode NMR magnet. Preliminary designs of the 1.3 GHz NMR magnet will also be discussed.

This paper presents experimental results on a quench of an intra-layer no-insulation (LNI) (RE: rare earth)Ba2Cu3O7−δ (REBCO) coil in a 31.4 T central magnetic field and simulated results on the quench. We have been designing a persistent-mode 1.3 GHz (30.5 T) nuclear magnetic resonance (NMR) magnet with a layer-wound REBCO inner coil. Protection of the REBCO coil from quench is a significant issue and the coil employs the LNI method to obtain self-protecting characteristics. We conducted high-field generation and quench experiments on an LNI-REBCO coil connected to an insulated Bi2Sr2Ca2Cu3O x (Bi-2223) coil under a background magnetic field of 17.2 T as a model of the 1.3 GHz NMR magnet. The coils successfully generated a central magnetic field of 31.4 T. Although the LNI-REBCO coil quenched at 31.4 T, this quench did not cause any degradation to the coil. A numerical simulation showed the current distribution during the quench was non-uniform and changed rapidly over time due to current bypassing through copper sheets between layers, resulting in faster quench propagation than in an insulated REBCO coil. During the quench propagation, the peak temperature (T peak) and the peak hoop stress BzJR (σθ, peak) were calculated to be 330 K and 718 MPa, respectively. These are below critical values that cause degradation. The simulation also showed that the high electrical contact resistivity (ρ ct) of 10 000 µΩ cm2, between REBCO conductors and copper sheets in the LNI-REBCO coil winding, played an important role in protection. When ρ ct was as low as 70 µΩ cm2, the quench propagation became too fast and large additional currents were induced, resulting in an extremely high σθ, peak of 1398 MPa, while the T peak was as low as 75 K. In short, the high ρ ct in the present coil caused a high T peak, but succeeded in suppressing σθ, peak and protecting the coil from the quench.

This paper presents the results of a high field generation test for layer-wound HTS coils in a background magnetic field. The test revealed new types of degradation for a REBCO coil caused by an electromagnetic force. A series-connected REBCO coil and a Bi-2223 coil were charged at 4.2 K in a background magnetic field of 17.2 T to generate a central magnetic field of 28.2 T (nuclear-magnetic-resonace frequency of 1.2 GHz). However, a premature normal voltage appeared on the REBCO coil, and the charging was stopped at 25 T. Unwinding the REBCO coil after the experiment found: 1) delamination, due to a cleavage and peeling, of the conductor in a soldered joint inside the winding; and 2) axial movement and edgewise bend deformation of the REBCO conductor at the outermost layer. The keyword of the present degradation is stress concentration, which is also the origin of the more widely known thermal stress-induced degradation.

Using a Bi-2223 innermost coil, the world's first NMR magnet with a frequency beyond 1 GHz has been developed and operated at the National Institute for Materials Science during 2014-2015. The existing 920 MHz (21.6 T) NMR magnet was successfully upgraded to a 1030 MHz (24.2 T) magnet by replacing the Nb3Sn innermost coil with a Bi-2223 coil. After charging the magnet to 1020 MHz (24.0 T), a shimming operation was started to obtain the homogeneous magnetic field required for NMR measurements. However, a large magnetic field inhomogeneity appeared, which could not be compensated using conventional shimming methods, i.e., superconducting and room temperature shim coils. Therefore, a new ferromagnetic shimming technology was applied, which achieved powerful and fast-acting field compensation and performed comparably to active shimming. This enabled effective compensation of the magnetic field inhomogeneity, leading to a subsequently excellent NMR resolution test result of 0.7 ppb. This NMR resolution enables NMR measurements for a membrane protein sample.

We succeeded in upgrading the 920-MHz nuclear magnetic resonance (NMR) superconducting magnet (21.6 T) to 1020 MHz (24.0 T) by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10 (Bi-2223) coil. The 920-MHz NMR spectrometer had been installed in the National Institute for Materials Science, Tsukuba, Japan, in 2001. It has been operated in the persistent mode for six years. The upgrading project started in 2006. A Bi-2223 coil was developed as the innermost coil instead of the Nb3Sn one. The newly installed Bi-2223 innermost coil is connected to Nb3Sn and NbTi coils in series. The upgraded NMR magnet was seriously damaged by the Great East Japan Earthquake in March 2011. After more than two years of restoration and additional improvements of current leads and the power supply system, the magnet was cooled down to below 1.8 K in August 2014. The magnet successfully generated 24.0 T, corresponding to 1020 MHz, in October 2014. To achieve the required homogeneity and stability of the magnetic field, not only superconducting and room-temperature shim coils but also ferromagnetic shims were used. The 1020-MHz superconducting NMR magnet has been operated in a power-supply-driven mode for six months.

This study reports a first successful demonstration of a single channel proton 3D and 2D high-throughput ultrafast magic angle spinning (MAS) solid-state NMR techniques in an ultra-high magnetic field (1020 MHz) NMR spectrometer comprised of HTS/LTS magnet. High spectral resolution is well demonstrated.

We recently suggested a joint method by using incongruent melting for high-temperature superconducting (HTS) wires, such as joint between RE123-coated conductors and between Bi2223 multi-filamentary tapes. It is important to obtain a suitable mechanical performance in joint to develop a persistent current HTS magnet, and this joint method with the melting is considered to have a higher tensile strength than the solid diffusion. In this study, the tensile strength of such a joint at critical current under tension was investigated toward application in a HTS magnet, such as Bi2223-NMR. We prepared two types of lap joint samples between RE123-coated conductors and between Bi2223 multi-filamentary tapes, with wrapping Ni sheets on junction (joined part in joint sample). The critical current of the samples was a similar value about 15 A at 77 K. The Ic of the samples showed a dependence on a small tension of junction below 40 MPa. With the reinforcement effect of the Ni sheet, the tensile stress tolerance of both joints that Ic of joint is reversible was higher than 90 MPa.

The screening current-induced magnetic field (screening field) for REBCO tape coils is a critical problem for superconducting coil systems which require accurate magnetic fields, such as NMR, MRI and accelerators. The magnitude of the screening field depends on the width of a REBCO layer in a tape. Therefore, a fundamental method to reduce the screening field is to use multi-filamentary REBCO tapes. Experimental and simulated results demonstrate that the use of multi-filamentary REBCO tapes reduced the screening field and the effect is pronounced for medium to high load factor operation, due to full penetration of the magnetic field and the transport current effect. The multi-filamentary tapes also reduce the temporal drift of the magnetic field after the coil charge. If we use multi-filamentary REBCO tapes with a filament width of 0.6~0.9 mm, the screening field is reduced to the same level as Bi-2223 coils.

High-temperature superconductors (HTS) are the key technology to achieve super-high magnetic field nuclear magnetic resonance (NMR) spectrometers with an operating frequency far beyond 1 GHz (23.5 T). (RE)Ba 2 Cu 3 O 7− x (REBCO, RE: rare earth) conductors have an advantage over Bi 2 Sr 2 Ca 2 Cu 3 O 10− x (Bi-2223) and Bi 2 Sr 2 CaCu 2 O 8− x (Bi-2212) conductors in that they have very high tensile strengths and tolerate strong electromagnetic hoop stress, thereby having the potential to act as an ultra-compact super-high field NMR magnet. As a first step, we developed the world’s first NMR magnet comprising an inner REBCO coil and outer low-temperature superconducting (LTS) coils. The magnet was successfully charged without degradation and mainly operated at 400 MHz (9.39 T). Technical problems for the NMR magnet due to screening current in the REBCO coil were clarified and solved as follows: (i) A remarkable temporal drift of the central magnetic field was suppressed by a current sweep reversal method utilizing ∼10% of the peak current. (ii) A Z2 field error harmonic of the main coil cannot be compensated by an outer correction coil and therefore an additional ferromagnetic shim was used. (iii) Large tesseral harmonics emerged that could not be corrected by cryoshim coils. Due to those harmonics, the resolution and sensitivity of NMR spectra are ten-fold lower than those for a conventional LTS NMR magnet. As a result, a HSQC spectrum could be achieved for a protein sample, while a NOESY spectrum could not be obtained. An ultra-compact 1.2 GHz NMR magnet could be realized if we effectively take advantage of REBCO conductors, although this will require further research to suppress the effect of the screening current.

In coils wound with high-temperature superconducting (HTS) tapes, large screening currents are induced in the tape by the radial component of the magnetic field. Furthermore, when a transport current is supplied to the HTS tape under a magnetic field, the total current (which consists of the transport and screening currents) flows inhomogeneously. The magnetic field generated by the screening currents affects the magnetic field distribution in cyclotron applications. Investigating the effects of the screening currents on the reduction, hysteresis, and drift of the magnetic field in the HTS coil is therefore necessary. In our previous studies, we have developed a novel simulation code for a three-dimensional electromagnetic field with the aim of clarifying these effects. In the present study, we measured the reduction and hysteresis of the magnetic field generated by the screening currents for a small model of HTS pancake coil. Additionally, we numerically investigated the influence of the screening current in the HTS coil on the spatial and temporal behavior of the magnetic field using our developed simulation code, which considers the nonlinear voltage-current characteristics.

The use of REBCO coils is a promising approach to achieve super-high field NMR magnets. As the first step in this direction, we have started to develop an LTS/REBCO NMR magnet operated at 400-600 MHz (9.4-14.1 T). In this paper, based on numerical simulations, we describe a study of the thermal runaway of the REBCO innermost coil, which is one of the major problems of this type of magnet. Based on the simulation results, if an LTS coils quench occurs, the current in the REBCO coil continues to increase due to the electromagnetic induction and leads to a natural thermal runaway with a very high current density. The thermal runaway is limited to only the top and bottom ends of the REBCO coil, as the radial magnetic field is highest there and the quench propagation velocity is quite low. The current density for this type of thermal runaway is much higher than that of the operating current, resulting in the catastrophic overheating of the coil. This is the worst case scenario which must be prevented for the LTS/REBCO NMR magnet. An optimal size dump resistor for the REBCO coil can prevent the natural thermal runaway by suppressing the current increase due to electromagnetic induction.

The use of magnets made of high temperature superconductors (HTS) such as BSCCO and REBCO easily provide higher magnetic fields and higher operating temperatures, enabling dramatic improvements in superconducting magnet technology. The LTS magnet technology is very well summarized in text books written by M. N. Wilson (Superconducting magnets, Clarendon Press Oxford, 1983) and Y. Iwasa (Case studies in superconducting magnets, 2nd edition, Springer, 2009), covering such topics as stability, protection, ac loss and so forth. To the contrary, HTS conductors were commercialized only recently and therefore the magnet technology for HTS conductors remains undeveloped, especially so in the case of REBCO conductors. The technological problems for HTS coils thus far encountered are 1) an enormous effect of a screening current-induced magnetic field, 2) degradation in the coil performance due to excessive mechanical stresses applied along the longitudinal and transverse direction, and 3) the difficulty in protecting the magnet in the case of an abrupt thermal runaway. This paper reviews recent progress in overcoming these technological problems for HTS magnets. Both BSCCO and REBCO conductors have been used for HTS magnets in areas such as high field facilities, NMR, MRI, magnetic levitation trains and so forth. The effect of the screening current is the major problem for NMR, MRI, and accelerators, as it substantially distorts spatial field homogeneity and temporal field stability; on the other hand, degradation due to excessive stresses is substantial for high field magnets. Additionally, coil protection is a common and substantive problem among high current density HTS magnets in general. World-wide activities in developing BSCCO and REBCO magnets are overviewed in this paper.

Development for superconducting wires of materials such as Nb3Al and the high-temperature superconductors (HTS such as REBCO, Bi2223, and Bi2212) has been carried out for high-field magnet applications. It is known that these types of wire exhibit very different characteristics and performance for different applications. The development of Nb3Al wire for high-field accelerator magnet has resulted in remarkable achievements in critical current using a Rapid Heating and Quenching (RHQ) method by High Energy Accelerator Research Organization (KEK) and National Institute for Materials Science (NIMS). As one example of a characteristic of Nb3Al, the strain sensitivity of the critical current in the RHQ-Nb3Al wire is better than that of Nb3Sn wire. A strain study is needed to further the development of a high-filed magnet; therefore, we have carried out experimental studies using the neutron diffractometer at J-PARC Takumi. Researchers have recently achieved the highest critical current density for REBCO wires in a high-field above 15 T. For this reason, REBCO wire has been considered for high-field magnet NMR applications in Riken. But several obstacles remain, including coil degradation, shielding current and thermal runaway. In this paper, R&D on recent advances for applications will be presented.

This paper clarifies the basic mechanism of self-healing from natural thermal runaway for uninsulated REBCO pancake coils. Based on the numerical simulation and experimental results, it is demonstrated that current flow patterns for an uninsulated REBCO pancake coil sequentially changes with the overcurrent value. Due to natural thermal runaway above the coil critical current, the current flow changes from “multi-turn-coil” mode to “single-turn-coil” mode, reducing the conductor current density and Joule heating, and thus the thermal runaway shrinks and the coil is self-healed; i.e. the REBCO coil is self-protected. For higher overcurrents, however, transverse currents across both electrodes become dominant, here called “terminal-to-terminal current” mode. It is dangerous for REBCO coils as the winding volume connecting both electrodes is extremely overheated, damaging the conductor winding.

Solid-state NMR observations of low-gamma half-integer quadrupolar nuclei

The no-insulation (NI) technique is a very effective protection method for REBCO pancake coils. However, it is not suitable for a layer-wound coil, which is the optimum configuration for the persistent operation and generation of a homogeneous magnetic field, since an NI layer-wound coil suffers from an extremely long magnetic field delay. Such a delay is caused by large short circuits across the layers inside the winding. To combat this phenomenon, a simple winding method, denoted as 'intra-layer no-insulation (LNI)', is proposed in the present work. The method uses insulator sheets to prevent electrical contacts between the layers and copper sheets to increase electrical contacts between the turns in each layer: such a coil has the layer-wound geometry with the NI pancake-like internal circuit. An LNI REBCO coil shows a shorter field delay time constant by three orders of magnitude than an ordinary NI REBCO layer-wound coil, and also exhibits a self-protect behavior with more rapid thermal runaway propagation compared to a NI REBCO double-pancake coil.

Recently, we proposed a method of forming superconducting joints between high-temperature superconducting wires based on the incongruent melting of an intermedium, called crystalline joint by melted bulk. Using this technique with GdBa2Cu3O7−δ -coated conductors, a superconducting joint with a critical current of 7 A at 77 K was obtained. In the present study, a joint between multi-filamentary Bi2Sr2Ca2Cu3O10+δ (Bi2223) tapes was prepared without an intermedium, employing incongruent melting of Bi2223 filaments in the tapes. A superconducting joint was successfully fabricated, with critical currents of 12.2 A and 177 A at 77 K and 4.2 K, respectively. A Bi2223 coil terminated with the present joint method was operated in persistent current mode and showed an ultra-low field decay rate with a characteristic resistance of 10−12 Ω at 77 and 4.2 K.

One apparent shortcoming of REBCO conductor coils is an extremely low fraction of conductor in the coil winding, as the REBCO conductor is as thin as 100–150 μm, comparable to the insulation thickness. If we wish to achieve high overall current densities for REBCO coils, the insulation thickness on REBCO conductor must be substantially reduced. Therefore, we have developed a 4 μm thick ultra-thin polyimide insulation coating by electro-deposition, which is 10-fold-thinner than conventional insulations on REBCO conductors. The fraction of REBCO conductor in the coil winding manufactured by this novel conductor becomes as high as 90%, which is comparable to that for a low temperature superconducting coil winding manufactured from Formvar-coated NbTi conductor. Thus, one important shortcoming of REBCO coils is satisfactorily overcome by using ultra-thin polyimide coated conductors, making possible a dramatic increase in the overall current density of such coils.

Conventional low-temperature superconducting (LTS) NMR magnets cannot exceed 23.5 T, as the critical current density decreases steeply beyond 23 T. The high-temperature superconducting (HTS) magnet provides a high current density even at the field above 23 T, enabling 1H NMR at a frequency above 1 GHz. Thus, super-high-field NMR magnets are possible if we employ LTS outer coils and HTS inner coils; that is, an LTS/HTS NMR magnet. We firstly conducted basic experiments and made a precise comparison between two 500 MHz-class LTS/HTS NMR spectrometers operated in driven mode; one with a Bi-2223 innermost coil and another with an REBCO innermost coil. We then evaluated the effect of screening current induced in the HTS coil, the NMR sensitivity and NMR signal resolution, and the quality of protein NMR spectra. Based on these results, we constructed the world's first beyond 1 GHz LTS/Bi-2223 NMR spectrometer; the magnet was successfully charged to 1.02 GHz (24.0 T). It was demonstrated that the quality of the 2-D solid-state NMR spectrum of a membrane protein was considerably better than that obtained by a conventional 700 MHz NMR magnet. Finally, a perspective on the super-high-field NMR spectrometers (1H at 1.2 GHz), is discussed. Keywords: super high field NMR magnet; high temperature superconductor (HTS); driven mode operation; internal/external lock; screening current induced magnetic field; temporal magnetic field drift; magnetic shielding due to screening current; NMR magnet operated beyond 1 GHz; membrane protein; quadrupolar nuclei

Degradation of the epoxy impregnated YBCO coil performance is due to tensile radial stress concentration on the outer edge of the conductor during cool down. This stress acts as a cleavage stress and opens the conductor edge, fracturing the YBCO layer. The fracture propagates to another edge of the conductor, resulting in degradation of the coil performance. Degradation of the epoxy impregnated YBCO coil is eliminated, if we use a polyimide-electrodeposited YBCO-coated conductor: tensile radial stress concentration on the outer edge of the conductor is reduced due to plastic deformation of the ductile polyimide. Polyimide electrodeposition onto the YBCO-coated conductor is reliable, uniform, easy to apply, and can be extended to larger YBCO magnets, removing the risk of coil degradation.

Cleavage strength for an YBCO-coated conductor at 77 K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5 MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor; cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils.

NMR magnets using high-Tc superconductors (HTS) are currently by several organizations designed to generate high fields exceeding 25 T. In such designs, the HTS are used for the inner coils, and the outer coils consist of NbTi and Nb3Sn wires. A ReBCO wire with a Hastelloy substrate has excellent critical current performance over wide range of magnetic fields and tolerates high tensile stress up to 700 MPa. We have previously shown conceptual designs in which the main coils are assumed to use ReBCO wires. The designs suggest the potential to realize much more compact and lightweight magnets than conventional magnets with low-Tc superconductors (LTS) and HTS. On the other hand, the problem of screening current induced by the perpendicular field remains to be solved. In this study, we evaluate numerically the amount of the magnetic field generated by the screening current in ReBCO coils. Compensation coils for improving field homogeneity were found to reduce the shielding current by several percentage points. The improved coil arrangement is reported considering the shielding current effect.

Screening current induced in a YBCO-coated conductor coil causes two major problems; (i) reduction in the central magnetic field and (ii) temporal magnetic field drift due to flux creep. They constitute disadvantages for YBCO coil applications such as NMR, MRI, accelerator and high field magnets. The second problem is effectively suppressed by current sweep reversal, while the first remains unsolved. The present paper demonstrates that the screening current-induced magnetic field (screening field) is dominated by (a) the YBCO coil shape, (b) the YBCO-coated conductor width, (c) the coil inner diameter and (d) the ratio of operating current to the coil critical current. The dependence on these quantities is systematically investigated by numerical simulations. We conclude that coils with a smaller width of YBCO-coated conductor, a larger inner diameter and a higher ratio of operating current to the coil critical current generate a smaller central screening field ratio.

Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8 A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10 MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem.