Low-temperature Superconductors Research Articles

Strain analysis and preliminary test of an all-superconducting high-field magnet generating 32.4 T direct-current magnetic field

Abstract A higher available magnetic field can give more flexibility for significant research and constructions. Inserting feasible high-temperature-superconducting (HTS) magnet inside low-temperature-superconducting (LTS) external is becoming an efficient method to build ultra-high-field (UHF) magnets. However, HTS insert magnets exhibited concentration of magnetic strain during the energization, significantly owing to the screening-current effect. This phenomenon affect the structural integrity of HTS magnets, posing a difficulty for UHF magnets to reach a center field larger than 30 T. In this paper, we report the recent progress of the 35 T all-superconducting UHF magnet project. The full-cycle development of this homemade UHF magnet was a strong collaboration with multiple domestic institutions in China. We used commercial REBa2Cu3O7-x (RE = rare earth element, REBCO) conductors to wind the 20 T HTS insert. Specifically, we optimized the electromagnetic design of this HTS insert with consideration of screening-current induced strain. We applied the no-insulation and metal-as-insulation winding techniques simultaneously to balance the time constant of the two nested REBCO coils. During the excitation test, the HTS insert maintained a 256 A operation current as designed for nearly 18 hours. With the energization of LTS external magnet, the entire facility reached a center field larger than 20 T for 16 hours, 30 T for 6.35 hours and 32 T for 2 hours. It finally reached a 32.4 T center field, at which point the LTS external quenched, inducing the HTS insert to quench as well. After quench, the HTS magnet can maintained 229 A operation current during the postmortem standalone test. This 32.4 T result exhibits effectiveness of considering the screening-current induced strain to enhance the structural integrity of REBCO magnets, and verifies the high reliability of REBCO magnets by generating a long-duration high field environment without any quench accident originating from the REBCO magnet.

Review of the research status of practical superconducting materials and their current carrying performance

Abstract Superconducting materials hold great potential in high field magnetic applications compared to traditional conductive materials. At present, practical superconducting materials include low-temperature superconductors such as NbTi and Nb3Sn, hightemperature superconductors such as Bi-2212, Bi-2223, YBCO, iron-based superconductors and MgB2. The development of low-temperature superconducting wires started earlier and has now entered the stage of industrialized production, showing obvious advantages in mechanical properties and cost under low temperature and middle-low magnetic field. However, due to the insufficient intrinsic superconducting performance, low-temperature superconductors are unable to exhibit excellent performance at high temperature or high fields. Further improvement of supercurrent carrying performance mainly depends on the enhancement of pinning ability. Hightemperature superconductors have greater advantages in high temperature and high field, but many of them are still in the stage of further performance improvement. Many high-temperature superconductors are limited by the deficiency in their polycrystalline structure, and further optimization of intergranular connectivity is required. In addition, it is also necessary to further enhance their pinning ability. The numerous successful application instances of high-temperature superconducting wires and tapes also prove their tremendous potential in electric power applications.

Calculation of AC loss using 2D homogenization method for HTS synchronous condenser rotor and validation

Compared with low-temperature superconductors, second-generation high-temperature superconducting materials offer higher current densities and temperature margins, which provide a boost to the development of superconducting condenser towards large capacity and light weight. In this paper, a 3D-to-2D dimensionality reduction and simplification method is proposed to address the problem of massive computation of 3D finite element model during the optimization of superconducting condenser rotor performance parameters. This method splits the pole based on the geometric characteristics of the rotor pole, establishes a 2D simplified model of each part of the pole after splitting, and uses the simplified model to calculate the key physical quantities such as the magnetic field, AC loss, temperature change, etc., during the excitation process of the superconducting condenser rotor. This method effectively reduces the computational complexity of the model and improves the efficiency of the calculation and iterative optimization of the magnet pole parameters. In order to verify the correctness of the electromagnetic calculation results of the simplified model, the rotor magnet model was assembled to carry out experiments in liquid nitrogen bath, and the experimental results were in good agreement with the calculation results.

High Current Measurement of Commercial REBCO Tapes in Liquid Helium: Experimental Challenges and Solutions

Recent advances in high-temperature superconductors (HTS) have made them extremely attractive for low-temperature, high-magnetic-field-power applications such as in fusion technology, where the advantages over traditional low-temperature superconductors (LTS) allow for the design of fusion reactors operating in different and more convenient regimes. However, the performance enhancement exhibited by novel conductors poses several challenges for the measurement of their superconducting properties. The high critical currents coupled with the relatively low thermal stability of the conductors and their mechanical fragility render this task a challenge, as the angular anisotropies complicate the experimental setup. In this work, we describe the development of our novel high-current measurement facility, focusing on the solutions introduced regarding critical aspects such as the superconducting leads and the sample holder design. We show how simple but effectively designed solutions can be adopted to combat the complexity of the measurement. The results reported in this work guide the development of a measurement system able to withstand high critical currents (I > 1500 A) at high magnetic fields (µ0H > 12 T) by evaluating the angular response of 4 mm wide short samples (L ~ 7.5 cm) in a robust and reproducible manner.

Elastic-plastic conductor damage evaluation at over 0.4% strain using a high-stress REBCO coil

Recent reports on screening current stress simulations of high-field REBCO magnets frequently present peak stresses over 1 GPa. However, this result is probably an unrealistic artifact of purely elastic calculations, considering the macroscopic yield and fracture stresses of approximately 900 MPa and less than 1.1 GPa for Hastelloy substrate-coated conductors. Here, we evaluate elastic-plastic conductor damage at over 0.4% strain using a high-stress REBCO coil exposed to a high field to explore this elastic-plastic regime. The coil was located off-center in a low-temperature superconductor magnet so as to induce a significant screening current in the enhanced radial field. Voltage taps, a Hall sensor, and two strain gauges were used for the instrumentation. We obtained strains exceeding 0.4% near the outward edge during the coil current charge from 350 A to 390 A, where the coil was exposed to external axial and radial magnetic fields of 13 T and 0.5 T. Post mortem results showed wavy plastic deformation, electrical damage, and REBCO defects. An elastic-plastic simulation reproduced the measured strains and predicted that ∼1 GPa stress is sufficient to induce ∼0.9% strain, thus validating our initial concerns with purely elastic models. This paper provides our experimental and simulation results.

Regulation strategy and analysis of the ultra-low temperature heat and mass transfer capacity of 3He-4He mixture in the mixing chamber of dilution refrigerator

As an important method for achieving millikelvin temperatures, dilution refrigerators (DRs) have been widely utilized in the fields of low-temperature superconductivity, low-temperature measurement, and space exploration. The mixing chamber (MC) uses a 3He-4He mixture as the refrigerant and serves as the cold end of the DR. However, the current research lacks a detailed study of the effect of the structure of porous media on the MC cooling capacity. In this study, the heat and mass transfer model in the MC is established using the finite element method. The model is based on the improved two-fluid model to evaluate its cooling capacity and propose regulation strategies. The reliability of the model is verified through experiments. When the load power stabilizes at 10 μW, the steady-state temperature is 59.6 mK, and the Kapitza thermal resistance significantly affects the cooling process of the MC. Three strategies for regulating Kapitza thermal resistance are provided. First, the optimal porosity of the sintering cylinder for the porous medium is ε = 0.5. Second, the extreme point of the sintering cylinder is at a height of h = 1.75 cm, and the cooling capacity is at its minimum at this location. Third, the steady-state temperature initially increases and then decreases with an increase in the cylinder radius. This study is of great significance as it reveals the influence of a porous medium heat exchanger on the heat and mass transfer process of mixtures at ultra-low temperatures, guiding the design of DR.

Оптимизация ВТСП кабелей постоянного и переменного тока с учетом эффекта продольного магнитного поля

It is known that in magnetic fields parallel to the transport current, an increase of the critical current is observed in most low-temperature and high-temperature superconductors — the effect of a longitudinal magnetic field (LMFE). LMFE has been theoretically predicted and experimentally confirmed in many studies. This article presents various methods for optimizing power coaxial cables based on high-temperature superconductors (HTS), considering LMFE. LMFE provides a higher current-carrying capacity of HTS DC cables and higher stability for HTS power AC cables, including lower AC losses. The results of optimization of HTS cables for both DC and AC applications with consideration of LMFE are presented and discussed. Two methods for calculating the magnetic field in coaxial HTS cables are presented: analytical and using the finite element method. The results of AC loss calculations and measurements for HTS cables are also presented.

Cryogen-free 400-MHz nuclear magnetic resonance spectrometer as a versatile tool for pharmaceutical process analytical technology.

The discovery of new ceramic materials containing Ba-La-Cu oxides in 1986 that exhibited superconducting properties at high temperatures in the range of 35 K or higher, recognized with the Nobel Prize in Physics in 1987, opened a new world of opportunities for nuclear magnetic resonance (NMRs) and magnetic resonance imaging (MRIs) to move away from liquid cryogens. This discovery expands the application of high temperature superconducting (HTS) materials to fields beyond the chemical and medical industries, including electrical power grids, energy, and aerospace. The prototype 400-MHz cryofree HTS NMR spectrometer installed at Amgen's chemistry laboratory has been vital for a variety of applications such as structure analysis, reaction monitoring, and CASE-3D studies with RDCs. The spectrometer has been integrated with Amgen's chemistry and analytical workflows, providing pipeline project support in tandem with other Kinetic Analysis Platform technologies. The 400-MHz cryofree HTS NMR spectrometer, as the name implies, does not require liquid cryogens refills and has smaller footprint that facilitates installation into a chemistry laboratory fume hood, sharing the hood with a process chemistry reactor. Our evaluation of its performance for structural analysis with CASE-3D protocol and for reaction monitoring of Amgen's pipeline chemistry was successful. We envision that the HTS magnets would become part of the standard NMR and MRI spectrometers in the future. We believe that while the technology is being developed, there is room for all magnet options, including HTS, low temperature superconducting (LTS) magnets, and low field benchtop NMRs with permanent magnets, where utilization will be dependent on application type and costs.

Significant enhancement of Jc and flux pinning mechanism of MgB2 doped with PEG-polymers

Compared with traditional low-temperature superconductors, the advantage of MgB2 lies in its critical temperature close to 40 K, which provides potential application at 20 K. In order to further improve its superconducting current carrying capacity at 20 K, a series of PEG polymer (including PEG2000, PEG4000, and PEG6000) doped MgB2 with doping level of 15 wt% were prepared by combining hot-pressing sintering with chemical solution route. The best result is achieved in PEG4000 doped sample, with a Jc at 20 K and 2 T reaching 51,000 A/cm2, increased by 42.8 % in comparison with the undoped sample. Study on the mechanism of the improvement reveals that organic carbon doping enhances the irreversibility field and consequently the flux pinning force. In response to the deviation of MgB2 pinning characteristics from Kramer theory, an irreversibility field increment (IFI) model was proposed which provides a satisfactory explanation for the experimental results. Our results demonstrate that the combination of hot-pressing sintering and organic carbon doping is an effective means to enhance the superconducting performance of MgB2 at 20 K for practical applications.

Low-Temperature Chiral Crystal Structure and Superconductivity in (Pt0.2Ir0.8)3Zr5.

We report the observation of superconductivity in (Pt0.2Ir0.8)3Zr5 with a chiral space group (P6122) at low temperatures. The bulk nature of the superconductivity at a transition temperature of 2.2 K was confirmed using specific heat measurements. We revealed that (Pt0.2Ir0.8)3Zr5 obeys the weak-coupling Bardeen-Cooper-Schrieffer model, and the dominant mechanism in the upper critical field is the orbital pair-breaking limit rather than the Pauli-Clogston limit. This indicates that the antisymmetric spin-orbit coupling caused by the chiral crystal structure does not significantly affect the superconductivity of (Pt0.2Ir0.8)3Zr5.

Prospects of an alternative superconductor technology for fusion reactors

New concepts for Tokamak fusion reactors are enabled by ReBCO high temperature superconductors, either to achieve toroidal magnetic fields in the range of 20 T, and/or by operating temperatures above 4.2 K, e.g., 20 K. The application of ReBCO tapes is challenging because common techniques for the manufacturing and quench protection of magnets, developed for classical multifilamentary superconductors, such as NbTi and Nb3Sn, cannot be applied directly. Less risky would be the use of a ternary molybdenum chalcogenide (TMC) superconductor, which was under development before the discovery of high temperature superconductors. Although a low temperature superconductor, the upper critical field is extremely high resulting in a comparable field dependence of the critical current to ReBCO. Because of the improved superconductor fraction of a multifilamentary TMC conductor, the expected engineering current density can be one order of magnitude higher. In addition, a TMC conductor has the potential for cost efficiency and a performance index around 1 $/kAm at 20 T seems be possible.

Analysis of charging characteristics of a 500 MHz HTS-LTS series NMR magnet with an intra-layer no-insulation HTS layer-wound coil structure

The intra-layer no-insulation (LNI) layer-wound coil has the advantages of high spatial magnetic field homogeneity and a smaller number of joints, as well as the ability of self-protection during quench, which has great potential in making MRI or NMR superconducting magnet. In this paper, a 500 MHz NMR magnet connected in series by a high temperature superconducting (HTS) magnet and a low temperature superconducting (LTS) magnet was designed, fabricated, and tested at a low field for homogeneity, in which the HTS magnet is an LNI layer-wound coil structure. And a helical equivalent circuit model of the 500 MHz HTS-LTS NMR magnet coupled with the stacked homogenization T-A and mechanical model is established, which agrees with the experimental results. This paper provides a sufficient numerical analysis of the charging characteristics of the HTS-LTS series magnet, including magnetic field homogeneity, current distribution, loss distribution, screening current induced stress, etc. And this paper provides valuable insights into the ramping behavior of the HTS-LTS series magnet, which can be useful for practical charging experiments.

Effect of charging sequence of background coil and insert coil on screening current in high-field non-insulated hybrid superconducting magnets

The hybrid superconducting magnet with low-temperature superconductor (LTS) for background coil and high-temperature superconductor (HTS) for insert coil is a dependable approach towards achieving high magnetic fields. However, challenges related to screening currents considerably limit the realization of higher field strengths. Due to the historical dependence of screening currents in superconductors on the magnetization path, the shunt currents in non-insulated (NI) superconducting coils can alter the effective magnetization path. Consequently, the charging sequence of LTS coils and NI HTS coils can influence the behavior of screening currents. We establish an electromagnetic and mechanical model for a 32 T NI hybrid superconducting magnet and conduct a comprehensive comparative analysis of screening current induced stress (SCIS), screening current induced field (SCIF), and loss under three different charging sequences. The results indicate that when the NI insert coils are charged first, the hoop stress is slightly lower compared to the other two charging sequences, but the instantaneous power and total losses are the highest. When both the insert and background coils are charged simultaneously, the hoop stress is the highest, yet the instantaneous power and total losses are the lowest, accompanied by a more uniform magnetic field ascent. These findings provide valuable insights for the design and selection of charging sequences of ultra-high-field NI hybrid superconducting magnets.

First-order quantum phase transition in the hybrid metal-Mott insulator transition metal dichalcogenide 4Hb-TaS2.

Coupling together distinct correlated and topologically nontrivial electronic phases of matter can potentially induce novel electronic orders and phase transitions among them. Transition metal dichalcogenide compounds serve as a bedrock for exploration of such hybrid systems. They host a variety of exotic electronic phases, and their Van der Waals nature enables to admix them, either by exfoliation and stacking or by stoichiometric growth, and thereby induce novel correlated complexes. Here, we investigate the compound 4Hb-TaS2 that interleaves the Mott-insulating state of 1T-TaS2 and the putative spin liquid it hosts together with the metallic state of 2H-TaS2 and the low-temperature superconducting phase it harbors using scanning tunneling spectroscopy. We reveal a thermodynamic phase diagram that hosts a first-order quantum phase transition between a correlated Kondo-like cluster state and a depleted flat band state. We demonstrate that this intrinsic transition can be induced by an electric field and temperature as well as by manipulation of the interlayer coupling with the probe tip, hence allowing to reversibly toggle between the Kondo-like cluster and the depleted flat band states. The phase transition is manifested by a discontinuous change of the complete electronic spectrum accompanied by hysteresis and low-frequency noise. We find that the shape of the transition line in the phase diagram is determined by the local compressibility and the entropy of the two electronic states. Our findings set such heterogeneous structures as an exciting platform for systematic investigation and manipulation of Mott-metal transitions and strongly correlated phases and quantum phase transitions therein.

Thoughts on the influence of Alex Mueller on high magnetic field technology

I briefly summarize some of the impacts that Alex Mueller's drive to discover superconductivity in low carrier density oxides has had on high magnetic field technology, especially on the desire to use the cuprate superconductors to generate magnetic fields now almost twice those possible with the Nb-based low temperature superconductors. There were some important lessons for applications of any new exotic temperature superconductor that had to be learned before the Bednorz and Mueller discovery could really impact applications. Above all is the lesson that a high transition temperature is no guarantee of applications unless conductors with high current density can be manufactured. Recent reports and hopes for an ambient pressure room temperature superconductor remind us that the hopes for pervasive superconducting technologies germinated with the discovery of superconductivity in LaBaCuO and went truly ballistic following the discovery of YBa2Cu3O7-δ (YBCO) with a 92 K transition temperature. Now that magnets made out of rare earth alloys of YBCO (REBCO) have entered routine service in NMR labs and prototype magnets made with REBCO appear ready to enable compact fusion reactors, we can finally say that Bednorz and Mueller's magnificent scientific achievement is having real world applications impact too.

Development of the first non-planar REBCO stellarator coil using VIPER cable

The benefits of operating fusion devices, such as tokamaks and stellarators, at high fields make high-temperature superconducting magnets necessary to realize a compact fusion power system. Superconducting stellarators, such as W7-X, have used standard low-temperature superconductor technology niobium-titanium. ARPA-E has recently funded a two-year project led by the startup Type One Energy and involving the Fusion Technology Institute at the University of Wisconsin-Madison, the Plasma Science to design and fabricate the first non-planar high-temperature superconductor (HTS) rare-earth barium copper oxide (REBCO) coil for a high-field stellarator based on the SPARC tokamak’s VIPER cable concept. The design consists of a 1.5-turn non-planar REBCO coil supported by a pair of 3D printed stainless steel radial plates. The ultimate goals of the project are to determine if commercial REBCO tapes and additive manufacturing can be used to fabricate high field () non-planar coils with tight bending radii () and with a degradation of the critical current smaller than 20% with respect to the expected performance. In this work we present numerical analysis for non-planar coils (critical current, magnetic field map, Lorentz forces and quench aspects) at the operating conditions of 77 K and 20 K and the fabrication and testing in liquid nitrogen (77 K) of the first two non-planar demonstrators for stellarators based on a VIPER cable. The first demonstrator is a short NOn-planar VIPER cabLe (cable demonstrator at which we will refer to as NOVEL) equipped with 100 HTS REBCO tapes and with a critical current of 5700 A at 77 K (self-field); the second is a MultIple turns (1.5-turns) NOn-plANar coil (coil demonstrator at which we will refer to as MINOAN) equipped with 30 HTS tapes and with a critical current of 2100 A at 77 K (self-field). Both demonstrators were tested at 77 K (liquid nitrogen bath) and the results showed that—even after being bent into non-planar shapes with bend radii —the degradation of the critical current was within 15%, meeting the expected goals of the project.

Conceptual Design of a 20 T Hybrid Cos-Theta Dipole Superconducting Magnet for Future High-Energy Particle Accelerators

High energy physics research will need more and more powerful circular accelerators in the next decades. It is therefore desirable to have dipole magnets able to produce the largest possible magnetic field, in order to keep the machine diameter within a reasonable size. A 20 T dipole is considered a desired achievement since it would allow the construction of an 80 km machine, able to circulate 100 TeV proton beams. In order to reach 20 T, a hybrid Low-Temperature Superconductor (LTS) - High-Temperature Superconductor (HTS) magnet is needed, since LTS technology is presently limited to ∼16 T for accelerator magnet applications. In this paper, we present the design of a 6 layers 20 T hybrid dipole magnet using Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn (LTS) and Bi2212 (HTS). We show that it is possible to achieve this magnetic field with accelerator field quality, with sufficient margin on a realistic conductor, keeping the stresses within safe limit, avoiding conductor degradation.

High Temperature Superconductor Detector Magnets for Future Particle Physics Experiments

Particle physics experiments make use of magnetic fields up to 4T to bend electrically charged particles such that their charge and momentum can be determined. The particle energy measurement requires a low amount of material, or ma-terial that is highly transparent to particles inside the calorimeter volume. The conflict between the small volume of space reserved for a magnet and having a field of several teslas inside the detector is often resolved by using superconducting magnets. Up to now, large particle physics detector magnets have been constructed with low temperature superconductors, but there are clear benefits from using high temperature superconductors in future particle physics detector designs, such as allowing for an elevated operating temperature and the reduced amount of superconductor needed. In addition to the HTS material itself, additional material is needed to support the Lorentz forces, and to temporarily carry the current in case of a quench since these magnets are always one-of-a-kind and they need to operate reliably and without damage in case of a failure scenario. The stabilizer has to be a low-density material for high particle transparency, such as aluminium. Since the density of the superconductor is a factor of 4 higher than the density of aluminium, a reduction of superconducting material also means an improvement of the particle transparency: the density of a material is directly related to its particle transparency. This paper presents a conceptual design for high temperature superconducting detector magnets and a study of the type of aluminium stabilizer used.

20 T Hybrid Nb3Sn-HTS Block-Coil Accelerator Dipole With Stress-Management

In the framework of studies for high energy particle colliders, design concepts for high field dipoles are being explored. In particular, relatively compact 20 T magnets can be achieved in a hybrid configuration, combining a High Temperature Superconductor (HTS) and a Low Temperature Superconductor (LTS). Preliminary concepts have been previously proposed using Bi2212 for the HTS and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn for the LTS. One of the main difficulties of 20 T magnets is the management of the very high stresses developing during operation. The design concepts rely on a rectangular block-coil layout, which offers the advantage of aligning the conductors with the main magnetic field, therefore submitting the conductors to a perpendicular electromagnetic force for a better control of the stresses. In addition, the layout allows a specific stress management, with adequate horizontal and vertical plates to intercept the stresses. The paper presents the improvements provided to the initial concept. In terms of magnetic design, the field quality has been improved. In terms of mechanical design, the stress management has been optimized to provide a compact coil with a reduced peak stress on the HTS. Concepts for flared-end coils with joints in the coil-ends are finally presented.

Conceptual Design of 20 T Hybrid Accelerator Dipole Magnets

Hybrid magnets are currently under consideration as an economically viable option towards 20 T dipole magnets for next generation of particle accelerators. In these magnets, High Temperature Superconducting (HTS) materials are used in the high field part of the coil with so-called insert coils, and Low Temperature Superconductors (LTS) like Nb3Sn and Nb-Ti superconductors are used in the lower field region with so-called outsert coils. The attractiveness of the hybrid option lays on the fact that, on the one hand, the 20 T field level is beyond the Nb3Sn practical limits of 15-16 T for accelerator magnets and can be achieved only via HTS materials; on the other hand, the high cost of HTS superconductors compared to LTS superconductors makes it advantageous exploring a hybrid approach, where the HTS portion of the coil is minimized. We present in this paper an overview of different design options aimed at generating 20 T field in a 50 mm clear aperture. The coil layouts investigated include the Cos-theta design (CT), with its variations to reduce the conductor peak stress, namely the Canted Cos-theta design (CCT) and the Stress Management Cos-theta design (SMCT), and, in addition, the Block-type design (BL) including a form of stress management and the Common-Coil design (CC). Results from a magnetic and mechanical analysis are discussed, with particular focus on the comparison between the different options regarding quantity of superconducting material, field quality, conductor peak stress, and quench protection.