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High magnetic fields play a critical role in the development of modern science and technology, breeding many significant scientific discoveries and boosting the generation of new technologies. In the last few years, China has undertaken a great deal of work on the application of ultra-high-field (UHF) superconducting magnet technology, such as for the Synergetic Extreme Condition User Facility in Beijing, the UHF nuclear magnetic resonance/magnetic resonance imaging, nuclear fusion energy, particle accelerator, and so on. This paper reports the research status of UHF superconducting magnets in China from different perspectives, including design options, technical features, experimental progress, opportunities, and challenges.

Rare-earth-based barium copper oxide (REBCO) coated conductors are promising candidates for the development of ultra-high-field (UHF) magnets, due to its high in-field performance, engineering current density, tensile strength and commercial availability. However, technological challenges pertaining to the large screening currents still remain. The major issues caused by the screening currents in REBCO conductors in UHF applications involve two aspects: the screening current induced magnetic field (SCF), and the screening current induced stress (SCS). In the past decades, extensive research has been devoted to the SCF, offering a variety of possible remedies. With latest advances in the construction and testing of UHF magnets, new observations of the SCF involving REBCO coils were reported. The SCS was identified in recent years and has raised growing concerns. The excessive and highly concentrated Lorentz force, rooted in the high magnetic field and the screening currents, poses threats to the mechanical strength of REBCO coated conductors. The aim of this paper is to review recent research efforts in understanding and tackling the screening current related technological issues. For the SCF, we focus on the latest observations in high-field experiments and its various mitigation methods. For the SCS, we present recent studies including experimental characterizations, numerical modelling and possible countermeasures. It is still an open question to precisely predict the SCS in large-scale HTS magnets. How to minimize the influence of SCF and SCS is one of the key technical challenges for the design of future UHF magnets.

The nonuniform superconducting current distribution in REBCO coated conductor, including a varying-field-induced screening current, is responsible for a significant magnetization effect that not only degrades the field quality of REBCO magnets, but also introduces risks of overstressing conductor. This paper presents our experimental and simulation studies on the screening current effect on an 800-MHz (18.8-T) REBCO insert (H800) that together with a 500-MHz LTS NMR magnet (L500) constitutes the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G). To develop our simulation model, which subsequently validated by a good agreement between simulation and experiment, we chose H800, Coil 1 of the 3-coil assembly operated alone and the entire H800, for the sources of experimental data, specifically their remnant fields after current discharge and diminished axial fields during operation. Armed with this valid model, we examined in detail the negative effects of screening current on H800, an important 1.3G component. Our simulation indicates that the screening current, nonuniformly distributed in the REBCO conductor, not only deteriorates H800 field, both strength and homogeneity, thus that of 1.3G, but also may overstress the REBCO conductor.

A high-temperature superconducting coated conductor can be practically applied in electric equipment due to its favorable mechanical properties and critical current (I c) performance. However, the coated conductor can easily delaminate because of its poor stress tolerance along the thickness direction. It would be interesting to investigate whether the I c of the delaminated YBa2Cu3O7−δ (YBCO) layer can be preserved. In this study, coated conductor samples manufactured through the metal organic deposition route were delaminated by liquid nitrogen immersion. Delaminated samples, including the YBCO layer and silver stabilizer, were obtained. Delamination occurred inside the YBCO layer and near the YBCO–CeO2 interface, as suggested by the results of scanning electron microscopy (SEM) and x-ray diffraction. A scanning Hall probe system was employed to measure the I c distribution of the original sample and the delaminated sample. It was found that approximately 50% of the I c can be preserved after delamination, which was verified by I c measurements using the four-probe method. Dense and crack-free morphologies of the delaminated surfaces were observed by SEM, which accounts for the I c survival of the delaminated YBCO layer. The potential application of the delaminated sample in superconducting joints was discussed based on the oxygen diffusion estimation.

Ferromagnetic material was electroplated onto the surface of the Bi2223/Ag multi-filamentary tape and as a result changed the Ic(B) characteristic of the tape correspondingly. A numerical simulation was used to investigate the influence of ferromagnetic shielding (FS) on the performance of the tape, in particular the Ic behaviour under perpendicular external fields. Using finite element analysis, we are able to understand how FS alters the flux distribution within the superconductor region under any arbitrary shielding structure. The shielding width, thickness and nonlinear property of the ferromagnetic material were taken into account for the simulation. Finally, optimized shielding parameters in association with different operating fields were suggested and these values could be considered for the next run of experimental work.

The phase evolution of lead oxide compounds in (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) precursor powder during heat treatment was studied. The BSCCO powder oxygen release and absorption processes occurring during heat treatment were investigated by using an oxygen analyser. The experimental results show that the oxygen release and absorption processes are related to evolution of the lead oxide compounds in the BSCCO powder. Oxygen absorption may result from the segregation of Pb from the (Bi,Pb)-2212 phase as well as the formation of the Pb3Sr2.5Bi0.5Ca2CuOy (3321) and Ca2PbO4 phases. Oxygen release could be related to decomposition of the 3321 and Ca2PbO4 phases as well as incorporation of Pb into the (Bi,Pb)-2212 phase. The formation and decomposition of the 3321 and Ca2PbO4 phases, which depend on temperature and oxygen partial pressure, have also been identified.

This paper presents construction details and test results of a persistent-mode 0.5-T MgB2 magnet developed at the Francis Bitter Magnet Lab, MIT. The magnet, of 276-mm inner diameter and 290-mm outer diameter, consisted of a stack of 8 solenoidal coils with a total height of 460 mm. Each coil was wound with monofilament MgB2 wire, equipped with a persistent-current switch and terminated with a superconducting joint, forming an individual superconducting loop. Resistive solder joints connected the 8 coils in series. The magnet, after being integrated into a testing system, immersed in solid nitrogen, was operated in a temperature range of 10–13 K. A two-stage cryocooler was deployed to cool a radiation shield and the cold mass that included mainly ~60 kg of solid nitrogen and the magnet. The solid nitrogen was capable of providing a uniform and stable cryogenic environment to the magnet. The magnet sustained a 0.47-T magnetic field at its center persistently in a range of 10–13 K. The current in each coil was inversely calculated from the measured field profile to determine the performance of each coil in persistent-mode operation. Persistent-current switches were successfully operated in solid nitrogen for ramping the magnet. They were also designed to absorb magnetic energy in a protection mechanism; its effectiveness was evaluated in an induced quench.

Most present demonstrations of high-temperature superconducting (HTS) synchronous motors/ generators are partially superconducting, only installing HTS coils on the rotor as excitation windings. The possible applicability of HTS armature windings is an interesting research topic because these windings can certainly increase the power density attributed to a potentially high armature loading capacity. In this study, we analysed the thermal behaviours of a developed 2.5 kW–300 rpm synchronous generator prototype that consists of an HTS stator with Bi-2223–Ag armature windings on an iron core and a permanent magnet (PM) rotor. The entire HTS stator, including the iron core, is cooled with liquid nitrogen through conduction cooling. The rated frequency is set at 10 Hz to reduce AC loss. The properties of the HTS windings and the iron core are characterized, and the temperatures in the HTS stator under different operation conditions are measured. The estimated iron loss is 11.5 W under operation in 10 Hz at liquid nitrogen temperature. Conduction cooling through the silicon iron core is sufficient to cool the iron core and to compensate for the temperature increment caused by iron loss. The stable running capacity is limited to 1.6 kW when the armature current is 12.6 A (effective values) due to the increasing temperature in the slots as a result of the AC loss in the HTS coils. The thermal contact between the HTS coils and the cooling media should be improved in the future to take away the heat generated by AC loss.

High temperature superconducting (HTS) armature windings have the potential for increasing the electric loading of a synchronous generator due to their high current transport capacity, which could increase the power density of an HTS rotating machine. In this work, a novel synchronous generator prototype with an HTS stator and permanent magnet rotor has been developed. It has a basic structure of four poles and six slots. The armature winding was constructed from six double-pancake race-track coils with 44 turns each. It was designed to deliver 2.5 kW at 300 rpm. A concentrated winding configuration was proposed, to prevent interference at the ends of adjacent HTS coils. The HTS stator was pressure mounted into a hollow Dewar cooled with liquid nitrogen. The whole stator could be cooled down to around 82 K by conduction cooling. In the preliminary testing, the machine worked properly and could deliver 1.8 kW power when the armature current was 14.4 A. Ic for the HTS coils was found to be suppressed due to the influence of the temperature and the leakage field.

In the reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. However, further lowering the F/Ba ratio is important according to the researches of YBCO thick film. On the other hand, the F/Ba ratio is necessary to be at least 2 for the full conversion of the Ba precursor to BaF_2 to avoid the formation of BaCO_3, which is detrimental to the superconducting performance. In this study, a novel solution with the F/Ba mole ratio of 2 was developed, in which the fluorine content was only about 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transformed-infrared spectra(ATR-FT-IR) revealed that BaCO_3 was remarkably suppressed in the as-pyrolyzed film and eliminated at 700 Celsius degree. Thus YBCO films with a critical current density (J_c) over 5 MA cm^{-2} (77 K, 0 T, 200 nm thickness) could be obtained on LAO single crystal substrates. In-situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all of the F atoms might remain in the film as fluorides. X-ray diffraction (XRD) \theta/2\theta-scan showed that BaF_2, but neither YF_3 nor CuF_2, was detected in the films quenched at 400 - 800 Celsius degree. The formation priority of BaF_2 over YF_3 and CuF_2 was interpreted by the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of the precursor solution for MOD-YBCO fabrication and open a gate to study the fluorine content in the precursor solution continuously and systematically.

During the fabrication of YBa2Cu3O7?? (YBCO) thin films via the metal organic deposition (MOD) method, the water content of the precursor solution should be reduced to prevent precipitation and other detriment. In most of the reported MOD-YBCO studies, water was used as a solvent, and vacuum distillation was utilized to reduce the water content of the precursor solution. In this study, we propose a water-free MOD method using propionic anhydride and methanol as the solvents. The crystal water of raw precursor salts was removed by heating, and the water content in the final precursor solution was as low as 0.2?wt%. Using this solution with low fluorine content and the optimized maximum temperature of pyrolysis, the final YBCO thin films about 250?nm thick had the critical current density (Jc) of 3.8?MA?cm?2 (77?K, self-field). The water content of the precursor solution was further reduced to less than 10?2?wt% by vacuum distillation. According to the Jc performance, thermal analysis, x-ray diffraction investigations, and morphology observations, no significant improvement was found for the final YBCO thin films fabricated by the distilled solution, indicating that vacuum distillation is not necessary in the water-free MOD method.

Trifluoroacetate metal–organic deposition (TFA-MOD) is a promising technique to fabricate YBa2Cu3O7−δ (YBCO) superconducting films. However, its slow pyrolysis process, which usually takes more than 10 h, constitutes a barrier for industrial production. In this study, polyethylene glycol (PEG) was utilized to reduce the stress generation inside the coated films when the strong pyrolysis reactions happen. With the addition of 30 wt% PEG2000 to the precursor solution, a smooth film surface could be obtained through a rapid pyrolysis process of 15 min. After the optimizations of the crystallization and oxygenation processes, mass percentage and molecular weight of PEG additive, YBCO thin films with Jc of about 4.5 MA cm−2 (77 K, self-field) could be routinely fabricated using (20–30) wt% PEG(1000–2000) additive with a total treatment time of about 2 h including the 15 min pyrolysis process time. The effects of PEG additive were discussed using one of the mechanisms of buckling formation. The reduction of compressive stress by PEG additive was suggested to be the reason for preventing buckling.

A two-dimensional testbed that aims to demonstrate the feasibility of magnetically driving several spacecrafts was successfully constructed. The testbed has two vehicles, each of which consists of two Bi2223 coils placed in an orthogonal way. The magnetic force between the two vehicles with a 1 m separation distance is less than 1 N, which is big enough to attract, repel and rotate two 50 kg vehicles in a 1.2 m × 2 m platform. The hardware for the testbed, which includes the coils, a cryostat and an air pressured system, is introduced. A parallel power supply circuit was specifically designed for the high temperature superconductor (HTS) coil operating in space, for the purpose of reducing the Joule heat and providing redundancy. Two key questions relating to the HTS coil used in the magnetic driving are discussed. First, the magnetic force between the two coils, a preliminary index directing the coil parameters, is concisely expressed as an arithmetical equation. The accuracy of the equation is checked by a more accurate model based on the Biot–Savart law and a FEA calculation. Second, the interaction between the orthogonal coils with respect to the Ic(B) characteristic of the Bi2223 tape is discussed. A minimum safe distance between the two coils is defined and numerically calculated. Two coils whose separation distance is larger than the minimum safe distance can be regarded as having no magnetic interaction.

In this work, unsealed short Bi-2223/Ag tapes were treated at the high pressure of 100 bar and at different temperatures. It was found that the tapes which had undergone a pre-treatment at high temperature at normal pressure before they were subjected to high pressure processing (HPP) could be compressed by pressure. The thicknesses of these tapes could be reduced from about 0.230 to about 0.220 mm, and most of the pores and cracks in the cores disappeared. These results might provide a feasible method for manufacturing Bi-2223/Ag tapes by HPP on a large scale.