Superconducting Magnet Research Articles

Paramagnetic Meissner Effect in the Magnetic Superconductor DyRh3.8Ru0.2B4

The paramagnetic Meissner effect, previously observed in yttrium and bismuth cuprates, was discovered in DyRh3.8Ru0.2B4, a single-phase superconductor possessing its own intrinsic magnetic subsystem. Measurement of the magnetic moment in DyRh3.8Ru0.2B4 was carried out in the FC (field-cooling) and ZFC (zero-field cooling) modes: cooling in or without a magnetic field, followed by heating in the magnetic field. Appearance of a positive magnetic moment was established in the study of complex rhodium boride in the FC mode in the fields of ~20 Oe (below the first critical value). The appearance of a paramagnetic Meissner effect in DyRh3.8Ru0.2B4 at fields B < Bc1 is related to the peculiarities of the magnetic subsystem behavior when the sample is cooled in the FC or ZFC modes at a temperature below the phase transition point from the paramagnetic to the ferrimagnetic state. Correlation of this anomaly with the intrinsic magnetic subsystem of the compound formed by a cluster crystal structure of the LuRu4B4 type is discussed. The influence of the intrinsic magnetic subsystem on the increased stability of the superconducting state is established. The presence of the intrinsic magnetic subsystem distinguishes the studied single-phase rhodium boride from other type II superconductors, which do not have a magnetic subsystem.

Numerical study on the model of HTS bearing by coupling the magnetic scalar potential and magnetic field intensity

HTS magnetic bearing is one of the important applications of HTS suspension system. It is a relatively complicated problem to describe the relative motion of superconductor and permanent magnet in the numerical calculation of HTS suspension system. For this problem, firstly, this paper analyzes the characteristics and limitations of the existing methods, and then proposes a new solution: using the COMSOL Multiphysics® to solve the magnetic scalar potential and magnetic field intensity for the permanent magnetic domain and the superconducting domain respectively, and then coupled them. The domain of solving the magnetic scalar potential can use discontinuous meshes to ensure the continuity of the physical field, thereby the problem of the relative motion can be solved effectively, and the complex structure can also be modeled easily and calculated quickly. Finally, this paper selects two different forms of motion of the HTS magnetic bearing for calculation, and compares them with the experimental results reported publicly. The results show that within the acceptable error range in engineering, the modeling and calculation in this paper can describe the motion process of the system quantitatively, so it can be used to design, optimize and study the characteristics of HTS magnetic bearing.

The performance of first CORC cable solenoid insert for development of CFETR high-field magnet

A ReBCO coated conductor maintains high current-carrying capability under a high magnetic field and superior mechanical performance. As a promising candidate material for high-field high-temperature superconductor magnets, it has been designed for application in the China Fusion Engineering Test Reactor CS coil, with a maximum magnetic field requirement higher than 17 T. In practical application of the ReBCO tapes, the characteristic Ic response with respect to electromagnetic (EM) and thermal stress is important for safe operation of the magnet. In this paper, a nine-turn solenoid magnet was wound from 4 m of CORC cable and tested at 4.2 K in a background magnetic field of up to 19 T. The aim is to check the stability of the current-carrying properties of the ReBCO cable under combined thermal and EM loads. The solenoid coil results in a combined peak magnetic field on the conductor of 19.4 T at a critical current of 888 A. Furthermore, no performance degradation was observed after 20 cycles of operation at 800 A (90% Ic) under a background field of 19 T and 10 cycles of warm-up–cool-down between 77 K and room temperature. This demonstrates the stable performance of ReBCO conductors for high-field magnet application with EM and thermal cycles.

Improvement on the series control strategy for ITER AC/DC converters

Abstract The Coil Power Supply System is one of the most critical systems in the ITER machine to provide controlled DC power to the superconductive magnets by large power AC/DC converters for the control of plasma initiation, shape and position. The AC/DC converter system leads to serious harmonic variation and risk of the low frequency oscillation, which has become a major concern. In addition, the deficiency of the voltage response for AC/DC converters could downgrade the performance of the plasma operation. This paper describes the configuration and series control strategy of the ITER AC/DC converter. Then, according to the theoretical analysis of the time-varying and broad-spectrum distributed ITER load harmonics, an improved hybrid control strategy is proposed based on the existing sequential control strategy. The hybrid control strategy can restrain harmonic current and improve voltage response performance by redistributing the voltage of each series connected unit. Finally, the representative simulation confirms the effectiveness of the control strategy.

Odd-frequency superconductivity in dilute magnetic superconductors

We show that dilute magnetic impurities in a conventional superconductor give origin to an odd-frequency component of superconductivity, manifesting itself in Yu-Shiba-Rusinov bands forming within the bulk superconducting gap. Our results are obtained in a general model solved within the dynamical mean field theory. By exploiting a disorder analysis and the limit to a single impurity, we are able to provide general expressions that can be used to extract explicitly the odd-frequency superconducting function from scanning tunneling measurements.

Suppression of superconducting parameters by correlated quasi-two-dimensional magnetic fluctuations

We consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside superconducting state and the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. An example of such material is RbEuFe$_{4}$As$_{4}$. We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is sensitive to the relation between the magnetic correlation length, $\xi_{h}$, and superconducting coherence length $\xi_{s}$ defining the 'scattering' ($\xi_{h}<\xi_{s}$) and 'smooth' ($\xi_{h}>\xi_{s}$) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations. Since the magnetic-scattering probability varies at the energy scale comparable with the gap, the quasiclassical approximation is not applicable in the crossover region and microscopic treatment is required. We find that the corrections to both the gap and superfluid density grow proportionally to $\xi_{h}$ until it remains much smaller than $\xi_{s}$. When $\xi_{h}$ exceeds $\xi_{s}$, both parameters have much weaker dependence on $\xi_{h}$. Moreover, the gap correction may decrease with increasing of $\xi_{h}$ in the vicinity of the magnetic transition. We also find that the crossover is unexpectedly broad: the standard scattering approximation becomes sufficient only when $\xi_{h}$ is substantially smaller than $\xi_{s}$.

Effects of proton irradiation on the magnetic superconductor EuFe2(As1 − xPx)2

We report on the effects of 3.5 MeV proton irradiation on single crystals of the magnetic superconductor EuFe2(As1 − xPx)2, investigated at microwave frequencies by a coplanar waveguide resonator technique. We studied the relative strength of the two collective phenomena by analyzing the dependence of their onset temperatures on different perturbations: magnetic fields and structural disorder. Results suggest that superconductivity and magnetism in this material are two competing orders: as the former is suppressed by irradiation or by excess P doping, the latter reinforces and manifests itself at higher temperatures. The ability to modulate superconductivity and magnetism in EuFe2(As1 − xPx)2 by disorder and in-plane magnetic field, respectively, offers the chance to tailor, even on a local scale, their relative weights. This could be useful for the aim of engineering this material for specific applications.

Design of control algorithm for accelerator magnet power supply with large time constant load

High Energy Photon Source-Test Facility (HEPS-TF) is a pre-research project for the construction of high energy synchrotron radiation source in the 12th five-year plan period. The purpose is to research the key technology and develop the key equipment of high energy synchrotron radiation source. Superconducting 3W1 magnet is the first self-developed superconducting wiggler magnet in China, and it is also one of the key research topics of HEPS. The author has completed a new digital closed-loop control algorithm for the superconducting 3W1 magnet with large load time constant and the nonlinear characteristics of inductance increasing with current, namely three-branch structure algorithm. In the face of the rapid development of high energy accelerator technology, the application of intelligent technology has become an inevitable development trend in the field of accelerator magnet power supply technology. Although the digital control of accelerator magnet power supply has been widely used, it is the first time to apply the new closed-loop control algorithm to realize fast adjustment and precision tracking in accelerator superconducting magnet power supply in China. According to the nonlinear characteristics of inductance and output current of superconducting magnet, a new digital closed-loop control algorithm for the load of superconducting magnet power supply with large time constant is proposed. This algorithm is quite different from the traditional algorithm and can attain the independent tracking and adjustment of the control target. Finally, by testing the ripple, error and stability of superconducting 3W1 magnet power supply, the correctness, practicability and reliability of power supply system as well as the digital control algorithm are verified. The results provide a new idea for the control of accelerator magnet power supply.

Thermal-Hydraulic Analysis on Quench Behavior of Indium-Tin Soldered REBCO Composite Conductor

The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

Superconductivity in Quantum Complex Matter: the Superstripes Landscape

While in XX century the theory of superconductivity has focused on a homogeneous metal with a rigid lattice which can be reduced to a single effective conduction band in the dirty limit. Today in the XXI century, the physics of superconductivity is focusing on complexity of quantum matter where novel quantum functionalities with lattice inhomogeneity at nanoscale (between 1 nm and 100 nm) and at mesoscopic scale (in the range 100-10000 nm), where the electronic structure need to be described by multiple Fermi surfaces and multiple gaps in the superconducting phase in the clean limit. The present issue of Journal of superconductivity and Novel magnetism collects papers presented at the International Conference Superstripes 2019 which was held on June 23-29, 2019, in Ischia Island, Italy. The series of Stripes conference started on Dec 8 1996 and it has contributed to scientific advances in the new physics of superconductivity in quantum complex systems in these last 23 years where polarons, strain, multigap superconductivity, exchange interaction between condensates, granular superconductivity and percolation play a key role. The articles collected in this issue cover hot topics of the new quantum physics of complex matter including the mechanism of high temperature superconductivity, complex magnetic orders and orbital physics in strongly correlated materials which have been growing rapidly in these last two years opening new perspectives also in mesoscopic quantum engineering.

2D conductive metal-organic frameworks for electronics and spintronics

Two-dimensional (2D) materials showcase great potentials in both fundamental research and technology development, thanks to their unique chemical and physical properties that are usually not available in corresponding bulk counterparts. As an emerging class of 2D materials, 2D conductive metal-organic frameworks (2D c -MOFs) exhibit the characteristics of pre-designable and tunable structures, excellent crystallinity, intrinsic porosity and superior conductivity. During the past decade, 2D c -MOFs have been rapidly developed in electronics, sensors, energy storage devices, etc . In this review, the electrical, magnetic and quantum properties of 2D c -MOFs are surveyed in detail. Their applications in semiconductor, metal, superconductor, topological insulator and porous magnet are highlighted. We envision that the combination of 2D c -MOFs with quantum materials could evoke rich physics, flexible chemistry and potential applications in both electronics and spintronics.

Application of the New 400 MHz High-Temperature Superconducting (HTS) Power-Driven Magnet NMR Technology for Online Reaction Monitoring: Proof of Concept with a Ring-Closing Metathesis (RCM) Reaction

Monitoring chemical reactions by nuclear magnetic resonance (NMR) is an established and valuable approach for process understanding, robustness, scalability, and control in the pharmaceutical industry. Understanding speciation, reaction rates, and reaction completion times provides information on how to improve a chemical process, leading to increased quality and quantity of the desired product. An important consideration for online monitoring is to have an NMR instrument colocated with a chemical reactor. The standard commercial medium- to high-field NMR instruments are normally installed in isolated locations due to facility and safety restrictions. Low-field NMR instruments suffer from low resolution and sensitivity, requiring chemometric analysis for medium to complex chemical structures. Reactions are typically monitored using NMR tubes and deuterated solvents. Therefore, reaction analysis may not provide the same kinetic information as when the reaction occurs in a reactor at a larger scale. To overcome these factors, we have tested a prototype NMR instrument with a 400 MHz cryogen-free power-driven high-temperature superconducting (HTS) magnet installed in a chemistry laboratory fume hood for online monitoring of reactions. We have tested the HTS NMR system with a ring-closing metathesis (RCM) reaction of diethyl diallyl malonate with Grubbs 2nd Gen catalyst in a reactor with a protonated solvent. The reaction was monitored online with a Bruker InsightMR flow cell, and data was acquired in automation, yielding a kinetic time-course of the transformation and reaction rate values. This work demonstrates that NMR instruments with HTS magnets can be integrated into the chemistry laboratory with other equipment and are a valuable tool for reaction monitoring under typical reaction conditions and in protonated solvents.

Pauli limited d-wave superconductors: quantum breached pair phase and thermal transitions

We report a quantum phase transition in Pauli limited d-wave superconductors and give the mean field estimates of the associated quantum critical point. For a population imbalanced d-wave superconductor a stable ground state phase viz quantum breached pair phase has been identified which comprises of spatial coexistence of gapless superconductivity and nonzero magnetization. Based on the thermodynamic and quasiparticle indicators we for the first time analyze this phase, discuss the thermal behavior of Pauli limited d-wave superconductor, give accurate estimates of the thermal scales associated with such systems and map out the pseudogap regime. Our work shows that while the Pauli limited superconductors are known to exhibit exotic modulated superconducting phase at large imbalance of fermion populations; in the regime of weak imbalance an intriguing phase of competing orders is realized. We have established that rather than the superconducting pairing field, it is the average magnetization of the system that quantifies this quantum phase transition. Given that the existing Pauli limited superconductors possess unconventional pairing state symmetry of the superconducting order, our work promises to open up new avenues in the experimental research of these materials. We have also demonstrated an alternate scenario wherein the quantum breached pair phase is a natural outcome for a d-wave superconductor with unequal effective masses of the fermion species.

Observation of the Current-Decay and Force-Variation of a Flux-Pumped HTS Magnet Subjected to Traveling Magnetic Fields

A variety of applications with high-temperature superconducting (HTS) magnet require the same characteristic which is the stable magnetic field generated by the persistent current flowing in the magnet. Regretfully, in the linear motor system of magnetic levitation transportation, HTS magnet will suffer from a current decay both internally and externally. On the one hand, joints loss represents the main cause internally. On the other hand, ac losses resulted from external ac circumstance can lead to the worse situation. To alleviate negative factors resulting in current decay, it is necessary to study the effects they will make. In this paper, in order to clarify the current-decay problem, a flux pump system based on transformer-rectifier principle was established for charging the no-insulation racetrack-coated superconductor coil, whose saturated current can be over 70% of the critical current. The variation of current, self-field, and electromagnetic forces of HTS magnet subjected to the traveling magnetic fields generated by a copper-based three-phase stator were observed experimentally. Our conclusion shows that, when the HTS magnet is placed above the three-phase stator, the characteristics of current decay have a strong dependence upon the amplitude and frequency of stator current.

Isostructural Spin-Density-Wave and Superconducting Gap Anisotropies in Iron-Arsenide Superconductors.

When passing through a phase transition, electronic system saves energy by opening energy gaps at the Fermi level. Delineating the energy gap anisotropy provides insights into the origin of the interactions that drive the phase transition. Here, we report the angle-resolved photoemission spectroscopy (ARPES) study on the detailed gap anisotropies in both the tetragonal magnetic and superconducting phases in Sr_{1-x}Na_{x}Fe_{2}As_{2}. First, we found that the spin-density-wave (SDW) gap is strongly anisotropic in the tetragonal magnetic phase. The gap magnitude correlates with the orbital character of Fermi surface closely. Second, we found that the SDW gap anisotropy is isostructural to the superconducting gap anisotropy regarding to the angular dependence, gap minima locations, and relative gap magnitudes. Our results indicate that the superconducting pairing interaction and magnetic interaction share the same origin. The intraorbital scattering plays an important role in constructing these interactions resulting in the orbital-selective magnetism and superconductivity in iron-based superconductors.

Numerical Analysis of Eddy Current Induced by z-Gradient Coil in a Superconducting MRI Magnet

In magnetic resonance imaging (MRI), pulsed magnetic gradient fields induce eddy currents in surrounding conducting components of the MRI scanner. These eddy currents distort MRI image and introduce thermal load in cryostat. Ohmic heating of superconducting magnet due to eddy currents changes the operating temperature of magnet, which can lead to system quench. Numerical simulation of these eddy current is important to compensate/control their adverse effects. In this paper, a coupled circuit network method is outlined to model eddy currents in a superconducting magnet that induced by z-gradient coil. According to the axial symmetry, the cylindrical cryostat vessels are divided into several concentric slices with a finite thickness. Each slice is modeled as a series circuit composed of a resistor and an inductor, and inductively coupled with other slices and gradient coil. To improve the calculation efficiency, the Eigen matrix technique is used to simplify the coupled differential matrix equation. The undesired eddy current effects including time-varying secondary magnetic field and power losses are analyzed. In addition, the eddy current behavior due to mechanical errors of gradient coil is discussed. Compared with the simulation results of ideal gradient coil design, the misalignment between primary and secondary layer of z-gradient coil can cause asymmetric secondary magnetic field along z-axis and further introduce $B_{0}$ -shift in imaging area.

Design Optimization of a 2G HTS Magnet for Subsonic Transportation

Motivated by the growing interest in the Hyperloop system, we have researched a new mode of transportation that runs inside a vacuum tube at more than 1,000 km/h. A 2G high-temperature superconducting (HTS) magnet with a detachable cryocooling system has been developed to thrust and levitate the capsule train efficiently; it also reduces its weight. To compensate for performance losses due to increased operating temperature, the HTS coil shape of the on-board superconducting magnet is topologically optimized with respect to cost and performance. With a number of linear constraints converted from nonlinear superconductivity conditions, many linear topology optimization problems are solved, and then the most preferred design is determined by considering its shape, cost, and performance.

Dynamic Response of a Superconducting EDS Train With Vehicle/Guideway Coupled Dynamics

As a promising candidate for future high-speed transportation, the superconducting electrodynamic suspension (EDS) train have drawn a lot of attentions from the academic and industrial communities in the past decades. The dynamic response of EDS train is one of the key aspects that determines the safety and ride quality. In this work, the essential issue of EDS vehicle/guideway system dynamics is studied, including the magnet/rail interaction and the dynamic characteristics. The electromagnetic forces were provided by electromagnetic interaction between on-board superconducting magnet (SCM) and ground levitation/guidance coils in a null-flux superconducting EDS system. These forces were studied as a research premise for dynamic characteristics analysis of EDS train based on the vehicle/guideway coupling dynamics. A dynamic numerical model, with vehicle/guideway coupling dynamics of EDS system, was established to investigate its dynamic characteristics. The frequency and acceleration responses of the carbody in terms of the electromagnetic forces irregularity at different traveling speeds are presented and analyzed. The result indicates that superconducting EDS train can steadily run over the straight line at a speed of 600 km/h, meeting the riding comfort standard ISO2631.

Preliminary Study of the High Temperature Superconducting Solution for 2 GeV CW FFAG Magnet

Proton beam with an average power of 5 MW-10 MW have important applications in particle physics towards the intensity frontier, as well as in the advanced energy, and material science. The fixed field alternating gradient (FFAG) accelerator combines the advantages of existing accelerators, which has a higher limitation of beam energy than high power cyclotron and has a higher beam-to-grid efficiency than existing high power linac and synchrotron, thus is considered as a good candidate for high power proton machine. By utilizing the strong focusing and large acceptance features of FFAG in the theoretical framework of the fixed field and fixed frequency of isochronous cyclotron, a continue wave FFAG capable of producing 2 GeV/3 mA protons (with an beam power of 6 MW) has been proposed in China Institute of Atomic Energy (CIAE). Due to the beam loss of high power proton beams, the resulting high radiation will deposit a large amount of radiation dose and heat load on the superconducting (SC) magnet. As the high temperature superconductors (HTS) have a much larger thermal margin due to high critical temperature (> 90 K) and high upper critical field (>100 T) than the traditional low temperature superconductors, and have been also considered have the lower overall construction costs and power consumption than the conventional magnet, currently the HTS magnet is the favorable solution for the 2 GeV FFAG magnet design. In this paper, the lattice design along with the requirements on the F-D-F magnet of the 2 GeV FFAG design is briefly introduced first. Then, the design of the F-D-F magnet is outlined. The details of the HTS coil design utilizing ReBCO conductor and operating at ~30 K is also included.

Magnetic Field Leakage of HTS Medical Accelerators

A high temperature superconducting (HTS) magnet used in medical accelerators was designed with combined function for bending and focusing/defocusing, through a layer-by-layer design process. The six-layer combined function magnet produced by the layer-by-layer design process reduces the total volume of HTS material required compared to existing designs, in addition to achieving a precise field. The magnetic field distribution and leakage from the coil ends of the designed combined function magnet were studied, and the effects of changing the length of the coil ends as well as the radius of the layers were compared to the original design, to determine what influence this would have. Results show that shorter coil ends have the smallest leakage magnetic field, and is thus preferred, however due to mechanical constraints of HTS tape there will be a minimum length requirement depending on the design.