Jintao Hu

Compact and mechanically reinforced REBCO insert magnets provide greater flexibility for high-field magnet design and construction. Meanwhile, conduction-cooled REBCO magnets have attracted increasing interest because they can operate below 20 K with lower cooling costs than conventional liquid-helium-cooled systems. An extreme no-insulation conduction-cooled REBCO magnet with a 40 mm cold bore was constructed and preliminarily tested as a prototype for a 40 T high-field magnet project. The prototype was wound using 4 mm-wide, 50 μ m-thick REBCO tapes with 1 μ m-thick copper stabilizer layers on both sides. The ultra-thin REBCO tapes enabled a compact winding structure, while the surface-shunting layer provided additional turn-to-turn current bypass capability. Stycast epoxy was applied to the solder-shunted surfaces of the double-pancake (DP) coils to enhance the thermal conduction between the REBCO coils and the cooling plate. The edge-bonded epoxy also mechanically reinforced the REBCO turns against electromagnetic stresses. The prototype was cooled below 10 K within 14 h and charged to 105 A within 2 h, at which point a localized fault-mode quench occurred in one DP coil. The magnet steadily dissipated the stored energy with the assistance of an external shunt circuit and was subsequently re-operated up to 120 A, generating a center field of 9.3 T after bypassing the damaged coil. Dedicated sudden-discharge tests at 100 A further demonstrated the effectiveness of the external shunt, reducing the peak temperature rise from 33 K to 13 K during power-supply shutdown events.

This paper introduces the design and experimental demonstration of a novel insulated two-in-hand REBCO magnet for high-field applications. The insulation between turns ensures that the current flows the designed path, resulting in improved predictability during quench events with properly designed protection circuits. Conventional insulated magnets typically use Kapton tapes between turns or wrapped around the conductor, which is functional but limited by its thickness, softness, and lack of mechanical robustness. Our magnet design incorporates key innovations: the application of Pyralux insulation—a very thin Kapton layer laminated between two outer copper sheets without adhesive bonding between conductor turns—to improved mechanical properties and compactness, a four-tapes co-winding technique, which includes two REBCO tapes in parallel to enhance defect tolerance, a copper tape for protection and a Pyralux insulation tape, and the integration of intermediate rings within each single pancake coil to effectively manage mechanical stresses in large-diameter pancakes. The copper laminates in Pyralux provides a robust way to introduce a much thinner Kapton layer between conductor turns than would otherwise be possible. These innovations collectively demonstrate significant advancements in insulated magnet technology for high-field applications.

High-field magnets are being developed for various applications in fundamental-physics experiments, materials sciences, and nuclear magnetic resonance (NMR). Here we report our recent work focusing on enabling technologies for a 40 T all-superconducting solenoid magnet with a 20-mm warm bore, intended for potential high-energy physics experiments. Adopting commercial REBCO conductors, we propose an Extreme-no-insulation (NI) insert magnet conceptual design with over-banding reinforcement to make it compact and enhance its mechanical integrity. The charging sequence of the nested-coil 40 T system was optimized to reduce the peak hoop strain as much as possible. A double-pancake extreme-NI trial coil was wound by REBCO bare tape and tested in a liquid-nitrogen cryogenic environment. The trial coil showed degradation after several cooling cycles and unexpected poor turn-to-turn electrical contact due to the slitting-edge feature of the REBCO bare tape. After attaching an additional shunt on the upper pancake, it showed better thermal self-protecting ability than the lower pancake during multiple quench events. This work provided a strain-management concept for the extreme-NI insert magnet inside high-field nested-coil systems, and experimental references for protecting REBCO magnets from quench.

In this paper, we present the design and test results of a low-AC-loss Nb3Sn model coil developed to validate key enabling technologies for a fast-switching-field magnetic resonance imaging (MRI) magnet concept that can change the magnetic field very quickly in time, within 1 second, between significantly different field strengths: a high field (3 T) for relaxometry and prepolarization and a low field (0.5 T) for spectroscopy and imaging. While conventional MRI magnets require a static magnetic field, we expect that our proposed superconducting magnet with rapidly changing fields can provide opportunities for novel contrast mechanisms, which include level-crossing between spin-1/2 and quadrupolar nuclei, accelerated spin-lattice relaxation, and adiabatic demagnetization/ remagnetization, by permitting differential relaxometry enabled by a large field strength difference, and ratiometric molecular/ superthermal imaging. We have developed and demonstrated an innovative magnet design that uses a very low-AC-loss Nb3Sn coil and a novel cooling technology featuring highly heat-conductive thermal links between the coil and solid nitrogen surrounding the coil. These thermal links in solid nitrogen are anchored at one end to the cryocooler cold head. This design enables rapid switching between two magnetic fields in the superconducting magnet without inducing quench. The paper provides details on the construction, test results, and an analysis of the maximum temperature rise in the coil of the small-scale fast-switching-field magnet system.

Thanks to their compact size, reduced energy losses, and enhanced power density, superconducting motors utilising high-temperature superconducting (HTS) materials are particularly appealing for propulsion systems such as those used in electric aircraft. Upon magnetisation, stacked HTS coated conductors are transformed into trapped field stacks (TFSs), which represent a key variant of trapped field magnets. TFSs offer a promising alternative to traditional permanent magnets, providing rotor fields for motor operation, particularly in synchronous machines. To date, there has been a lack of comprehensive investigation into the interaction between HTS TFSs and cryocooled iron cores, as well as the losses generated in iron cores during pulsed field magnetisation (PFM). Furthermore, the optimal magnetisation strategy for achieving the maximum trapped field in TFSs through PFM for motor applications remains an unresolved issue. To address these challenges, a 2D electromagnetic-thermal coupled numerical model was built using the finite element method and validated against experimental data. This model was employed to analyse a superconducting machine featuring TFSs as rotor magnets and cryocooled iron cores. This study introduces two key novelties. The primary novelty lies in identifying the optimal combination of pulsed current amplitudes and TFS layer numbers required to achieve maximum magnetisation fields. The secondary novelty involves evaluating the overall losses in both superconducting and non-superconducting components of the machine during PFM. This includes heat generation in various layers of the TFS and losses in the motor’s active materials, such as copper windings and iron cores. Additionally, two practical motor configurations were proposed, and two calculation methods using linear interpolation of iron losses and steel grades were introduced to estimate the iron losses for the studied iron material, M270-35 A. This pioneering study is expected to serve as a valuable reference for loss analyses and structural design considerations in developing superconducting machines.

This paper experimentally investigates the characteristics of the linear traveling-wave flux pump by varying the rise-time to fall-time duty ratio, DC offset, and amplitude of triangular magnetization waveforms continuously over time and observing the generated current in the high-temperature superconductor (HTS) magnet. From the experimental results, the asymmetric triangle waveform is observed to have a certain rise-time duty cycle between 15% and 17% that maximizes the generated current and a threshold magnitude of 0.625 A below which no flux pumping is achieved. The results also confirm the ability of the magnitude and duty ratio of the asymmetric triangle waveform to alter the magnitude and direction of the generated current, respectively. Using these properties, a closed-loop control system is implemented in which the magnetization wave amplitude is the scaled difference between a demand current and the actual HTS current, and the duty ratio of the waveform is 1:9 or 9:1 based on the sign of the difference.

High temperature superconducting (HTS) stacks of superconducting tapes provide a solution for all-superconducting electrical machines since they could work as trapped-field magnets. Most experiments with these stacks use pieces of tapes with relatively small areas as wider tapes are more difficult to acquire, therefore in this paper we investigate substitutions for these wider tapes to be implemented in superconducting electrical machines. With different stack architectures and substrate materials made by different manufacturers with different widths, the magnetizing effect using pulse field magnetization (PFM) would be different. To explore the differences of this effect, different stacks are tested in a modified synchronous machine designed by ASuMED under liquid nitrogen conditions. By changing the stacks of the tape and testing them under the same condition, the characteristics of the trapped flux, magnetization and demagnetization are affected, the difference is then compared to a COMSOL finite element model using an electromagnetic-thermal coupled model to investigate this effect. This paper shows the experimental setup for a PFM for HTS stacked coated conductor (CC) within an electrical machine, different stack architectures are tested and simulated, and the magnetic characteristics are compared and discussed, where the simulation and experiment agree in a certain degree, and the sectioned HTS CC stacks can proven to be useful in an electrical machine.

The high-temperature superconducting (HTS) Roebel tape is a latest developed concept of high-current and low-loss HTS cables for high-power AC applications. In this work, samples of Roebel tapes with single superconducting layer are manufactured, tested, modelled, and analysed. In the experiments, the critical current degradation and the transport AC losses are measured. In the simulation, with the 3D finite element method (FEM) models based on the T-A formulation, the transport AC losses are calculated and compared with the experimental data, and the current distribution is analysed in detail. This model is then used to simulate long HTS Roebel tapes which could be used in practice and calculate the transport AC losses. The losses are found over 30% lower than that of the normal HTS ReBCO tapes, which verifies the loss-reduction performance of the HTS Roebel tapes. This work provides both experimental and simulation proof of the advantage of the HTS Roebel tape as a high-current and low-loss HTS cable. It could potentially become the key to the compact fusion magnets which require high power density and suppressed losses.

Stacked high-temperature superconducting (HTS) coated conductors (CCs) are preferred for trapped field magnets over HTS bulk due to their improved mechanical strength and shape flexibility. In superconducting machines, the demagnetisation phenomenon in HTS CCs is crucial as it affects the machine's magnetic field loading, affecting power and torque output. The stator slotting effect, common in conventional machines, can also influence TFMs made from HTS CCs in superconducting machines, leading to magnetic flux distortion and additional losses. To gain a comprehensive understanding of these influences, numerical analyses based on the finite element method have been conducted for different variants of a superconducting machine. Findings indicate that in the machine model: (1) a slotted stator yields higher trapped field values compared to a slotless stator, and (2) closed slots prove effectively in eliminating air gap harmonics. A final design with distributed winding and closed slots is recommended for reference in HTS motor development.

The use of High-temperature superconducting (HTS) flux pumps has emerged as a promising approach for energizing HTS magnets. Compared to conventional power supplies, HTS flux pumps enable the wireless injection of a substantial dc current into a closed HTS magnet, allowing the magnets to operate in a persistent current mode. There are a lot of advantages to energize the HTS magnets in this manner. First of all, it avoids sophisticated power supplies and heavy current leads. Secondly, excessive cryogenic losses caused by thick warm-to-cold current leads can be reduced. Additional benefits include cost-effectiveness, reduced energy consumption, and a compact form factor. The most important parameter to evaluate the performance of the flux pump is its dc output voltage, which determines the charging capability. In this paper, we study both instantaneous and time-averaged dc output voltage for two most popular flux pumps – transformer-rectifier flux pump (TRFP) and high-Tc superconducting dynamo by a finite-element method model. Moreover, their working principles, structures and operation strategies are discussed. This work reveals characteristics and limitations of two flux pumps and is indicative for designing a high performance HTS flux pump for the specific application.