Superconducting Magnet Research Articles

Preliminary Design Study on Non-Twisted HTS Conductor for Fusion Applications

Full transposition of superconducting wires within a conductor may not be a requirement for high temperature superconductor (HTS) magnet for fusion applications. Already, conduction-cooled 20 T model coil built with non-transposed HTS stacks has been demonstrated by Commonwealth Fusion Systems (CFS). Here, we discuss 3 possible stacked HTS cryogen-cooled conductor concepts for toroidal field (TF) magnet applications. The first one is somewhat like LTS conductors, the second, HTS stacks are capped by copper stabilizer, the last one, only stacks without copper. We simulate a simplified TF magnet model, 12 T, size of KSTAR, using the 3 conceptual HTS conductor designs. A comparative thermo-hydraulic analysis for a fast charging case has been carried out and its implications on HTS conductor design are further discussed.

Status of the High Field Cable Test Facility at Fermilab

Fermi National Accelerator Laboratory (FNAL) and Lawrence Berkeley National Laboratory (LBNL) are building a new High Field Vertical Magnet Test Facility (HFVMTF) for testing superconducting cables in high magnetic field. The background magnetic field of 15 T in the HFVMTF will be produced by a magnet provided by LBNL. The HFVMTF is jointly funded by the US DOE Offices of Science, High Energy Physics (HEP), and Fusion Energy Sciences (FES), and will serve as a superconducting cable test facility in high magnetic fields and a wide range of temperatures for HEP and FES communities. This facility will also be used to test high-field superconducting magnet models and demonstrators, including hybrid magnets, produced by the US Magnet Development Program (MDP). The paper describes the status of the facility, including construction, cryostat designs, top and lambda plates, and systems for powering, and quench protection and monitoring.

Design Study of a Superconducting Quadrupole Magnet System Sustainable Under Dark Current Heating in ILC Main Linac

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Control of the Magnetic Interaction between Single-Molecule Magnet TbPc2 and Superconductor NbSe2 Surface by an Intercalated Co Atom.

We demonstrate that an intercalated Co atom in superconductor NbSe2 could control the magnetic interaction between the adsorbed magnetic molecule of TbPc2 and the NbSe2 substrate. An intercalated Co atom enhances the magnetic interaction between the NbSe2 and the TbPc2 spin to cause Kondo resonance at the TbPc2 position, a spin-singlet state formed by the itinerary electron. By applying a surface-normal magnetic field, we change the molecule's spin direction from the initial one directed to the Co atom to the surface normal. The change appears as a split Kondo resonance at the TbPc2, one of which is enhanced at the Tb site, which disappears when the outer magnetic field normal to the surface is applied and never appears, even if we return B to 0 T. The phenomenon suggests that the intercalated magnetic atoms can control the magnetic interaction between a magnetic molecule and the superconductor NbSe2.

Vacuum Brazing and Performance Evaluation of T2 Copper Block and 316L Stainless Steel Tube

The International Thermonuclear Experiment Reactor (ITER) Thermal shield (TS) serves as a cryogenic heat exchanger to maintain the thermal stability of the ITER superconducting magnet coil, which is critical to the control of the plasma during the operation of the ITER device. The TS is composed of long-length 316L stainless steel (SS) and copper as brazed joints. In this case, a feasible fabrication design for the CCS TS is presented, accomplished by three kinds of joining processes (vacuum brazing, friction stir weld, and TIG weld). In the reliable fabrication design, the brazing quality of the as-brazed long-distance 316L SS and copper joints plays a critical role in the thermal conductivity performance of the ITER thermal shield. Therefore, a high-quality vacuum brazing process of long-length SS/Cu joints applied in a low-temperature superconductor magnet system was first studied. The macro metallography analysis demonstrates the braze ratio of the samples is 100%, and no crack or defect is observed in the samples. The microstructural characterization reveals the brazing seams are composed of silver-based Ag-rich eutectic. The micro-shear test indicates that the shear strength of the 316L tube and copper joint is 205 MPa, with the fracture position located on the copper side; this zone will be the most vulnerable zone of the joints. In addition, the SEM results illustrated that the shear fracture morphology displayed a ductile fracture feature. The test results demonstrated that the highly precise depth drilling employed in this paper ensured a good control of the brazing clearance, resulting in a 100% braze ratio for the long-length SS/Cu joints. Therefore, it can be concluded that the brazing process can be applied in the ITER TS for the good thermal conductivity performance of long-length SS/Cu-brazing joints.

Fundamental Study of the Removal of Microplastic Fibers Using Swirling Flow and Magnetic Field

Microplastic fibers (MPFs) derived from laundry wastewater and fishing nets, etc. have been found in the environment around the world, raising concerns about their ecological impact. Particularly for laundry wastewater, some of the MPFs are not removed by sewage plants, and are discharged into the environment. We have studied the removal method of MPFs by magnetic separation in sewage plants, but there were some problems for practical applications such as the desorption of magnetite from MPFs and blockage of magnetic filters. To solve the problems, we proposed a novel magnetic separation system combining swirling flow and opposing magnets. In this study, we investigated whether MPFs can be concentrated by particle trajectory simulations and lab-scale experiments. The simulations and experiments showed similar trends that MPFs can be concentrated by the proposed magnetic separation system. The magnetic separation performance will be improved by using a higher magnetic field of opposed superconducting bulk magnets.

Stable operation characteristics and perspectives of the large-current HTS STARS conductor

The High-Temperature Superconducting (HTS) magnet option has been explored for fusion reactors as well as for next-generation fusion experimental devices. The Stacked Tapes Assembled in Rigid Structure (STARS) conductor uses HTS tapes with simple stacking without twisting and transposition. A practically applicable STARS conductor is presently being developed with an operating current of 18 kA at 20 K temperature and ~15 T magnetic field. The conductor is required to have a high current density of 80 A/mm2. For the second stage of the conductor development, internal electrical insulation is applied between the copper stabilizer casing and the outer stainless-steel jacket, and a 6-m conductor sample was fabricated in a solenoid coil shape with a 600-mm diameter and three turns. The coiled sample was tested in 8 T, 20 K using a facility equipped with a maximum 13-T, 700-mm bore solenoid coil. Excitation up to the rated current of 18 kA was successfully attained with stable operation. The characteristics of the conductor observed during the excitation test are discussed.

Optimization of surface bonding methods for fiber Bragg grating sensors at cryogenic temperatures

Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements.

Magnetic Shielding Design of Onboard Superconducting Magnet for Electrodynamic Suspension Train

The superconducting electrodynamic suspension (EDS) train has been expected to be a promising candidate for the future ultrahigh-speed transportation due to its excellent levitation and guidance stabilities, high lift-to-drag ratio, and being free-of of active control. As a core component of the EDS train, the onboard superconducting magnet (SCM) is usually required to generate a strong magnetic field, however, which may threaten the safety of passengers. Therefore, it is indispensable to design a magnetic shielding for onboard SCM, aiming at minimizing the magnetic field in the coach of the vehicle. In this paper, a three-dimensional (3-D) finite element method (FEM) model for solving the electromagnetic problems of magnetic shielding system is established and validated by experimental results. Prior to global optimization, the effects of thickness, multi-layer and partition structure of shielding plate on the magnetic shielding are discussed primarily. It was found that the thickness has a great impact on the shielding effect, while the influence of multi-layer structure is weak. Moreover, the weight of the magnetic shielding plate can be effectively reduced by partitioning it into several blocks with different thicknesses. Based on the above studies, magnetic shielding plate is finally optimized in consideration of the thickness, multi-layer structure and partition. Consequently, the weight of the magnetic shielding plate can be reduced by 20% compared with the original one.

Influence of Interturn Contact Stress on the Quench Behavior of Insulated YBCO Pancake Magnets: A Study Based on Numerical Simulations

Insulated high temperature superconducting (HTS) magnets are still widely used in low field applications, such as HTS motors and generators. In this paper, a two-dimensional/three-dimensional (2D/3D) mixed electrothermal model and a homogeneous 3D electrothermal model are employed to study the influence of inter-turn contact stress on the minimum quench energy (MQE), longitudinal and transverse normal zone propagation velocity (NZPV) of insulated YBCO magnets for the temperature range 30 K – 60 K. This temperature range is of interest usually in applications, such as HTS motors, generators and transformers, because it allows the use of low cost cooling system. It is found that MQE increases significantly with increasing inter-turn contact stress. The inter-turn contact stress has small influence on the longitudinal NZPV of insulated YBCO pancake magnet. However, the transverse NZPV rises significantly with the increase of inter-turn contact stress. The transverse NZPV is nearly proportional to the inter-turn contact stress. The 3D homogeneous model gives results that are in agreement with those of 2D/3D mixed model, especially for the temperature range 45 K – 60 K; however, the MQE and transverse NZPV given by the two models are different for the temperature range 30 K – 45 K. That means for insulated HTS YBCO pancake magnets operating at the temperature range 45 K – 60 K, the 3D homogeneous electro-thermal model can be a good replacement of the 2D/3D mixed model to study the quench behavior, which can achieve better computational efficiency without significant loss of accuracy.

Symmetries, length scales, magnetic response, and skyrmion chains in nematic superconductors

The structure of vortex lattices for unconventional superconductors is mostly unknown and a mathematically challenging problem. The authors propose a general method to find vortex lattices for any superconducting system. They focus on the example of nematic superconductors, demonstrating that in an external magnetic field such superconductors form chains of fractional vortices. Their approach, by describing the geometry of the unit cell, directly maps to the signal such lattices exhibit in muon spin rotation experiments, producing a powerful tool to study the superconducting order parameter.

Emerging Enhancement and Regulation Strategies for Ferromagnetic 2D Transition Metal Dichalcogenides.

Two-dimensionaltransition metal dichalcogenides (2D TMDs) present promising applications in various fields such as electronics, optoelectronics, memory devices, batteries, superconductors, and hydrogen evolution reactions due to their regulable energy band structures and unique properties. For emerging spintronics applications, materials with excellent room-temperature ferromagnetism are required. Although most transition metal compounds do not possess room-temperature ferromagnetism on their own, they are widely modified by researchers using the emerging strategies to engineer or modulate their intrinsic properties. This paper reviews recent enhancement approaches to induce magnetism in 2D TMDs, mainly using doping, vacancy defects, composite of heterostructures, phase modulation, and adsorption, and also by electron irradiation induction, O plasma treatment, etc. On this basis, the produced effects of these methods for the introduction of magnetism into 2D TMDs are compressively summarized and constructively discussed. For perspective, research on magnetic doping techniques for 2D TMDs materials should be directed toward more reliable and efficient directions, such as exploring advanced design strategies to combine dilute magnetic semiconductors, antiferromagnetic semiconductors, and superconductors to develop new types of heterojunctions; and advancing experimentation strategies to fabricate the designed materials and enable their functionalities with simultaneously pursuing the upscalable growth methods for high-quality monolayers to multilayers.

Current status of In-flight fragment separator for RAON

The in-flight fragment (IF) separator of RAON, the main device for producing rare isotope (RI) beams for nuclear science research and applications, is under development. The IF separator consists of a target, beam dump, magnets, and detector systems. The high-power target and beam dump for the 80 kW primary beam were fabricated using graphite. The IF magnet system consists of a total of 8 dipole magnets, 15 sets of quadrupole magnet triplet, 2 sextupole magnets, and power supply systems. In the high radiation region near the target and beam dump, warm iron high temperature superconducting (HTS) magnets and radiation-resistive dipole magnet are used. In the other region, cold iron low temperature superconducting (LTS) quadrupole magnet triplets and normal conducting (NC) dipole and sextupole magnets are used. Fabrication of all IF magnets have been completed and on-site installation is in progress. Also, detectors for particle identification (PID) and data acquisition (DAQ) systems are currently being installed at the focal planes of the IF separator. The current status of IF separator is briefly introduced.

고온초전도 자석 개발을 위한 진공에서의 절연파괴 특성 분석

The high-temperature superconducting (HTS) magnet used in particle accelerators for cancer treatment exhibits a large current change for a short period of time due to its fast ramp-up characteristics, which results in high voltage and instant vulnerability to electrical insulation. This research introduces the saddle-coil, which, due to its complex shape and small bending radius, allows for the local concentration of electric field intensity. Reports from the European particle physics laboratory, CERN have revealed dielectric breakdown due to 1000V between layers during low-temperature superconducting (LTS) magnet experiments for accelerators, yet no research has been conducted to solve this problem. HTS magnets operating in vacuum may be susceptible to electrical breakdown due to high induced voltage and local quench due to rapid-ramp characteristics. Therefore, this paper deals with the dielectric characteristics of vacuum to develop an electrically stable HTS magnet. Specifically, dielectric experiments on penetration and creepage discharge were conducted under various vacuum conditions, and empirical formulae for calculating the electric field intensity at sparkover for the insulation design of HTS magnets was derived.

THE THIRD CRITICAL FIELD (HC3) OF SINGLE-CRYSTALLINE K0.73Fe1.68Se2 BY GINZBURG-LANDAU APPROACH

In this paper, two-band Ginzburg-Landau (GL) equations for magnetic superconductors were solved analytically to determine the temperature dependence of surface critical magnetic field (Hc3). By variation method analytically from 1st GL equations and modified to the Changjan &amp; Udomsamuthirun’s temperature dependence model. We found that, our model could find a good agreement with the experimental data of single-crystalline K0.73Fe1.68Se2 superconductors vicinity the critical temperature scenery and the ratio Hc3/Hc2 (where Hc2 is the upper critical field of single-crystalline K0.73Fe1.68Se2) indicated powerful temperature dependence.

R&D of on-board metal-insulation REBCO superconducting magnet for electrodynamic suspension system

Metal-insulation (MI) REBCO high-temperature superconducting (HTS) magnet has the advantages of short charging delay, low contact losses, and self-protection abilities, which shows important application prospects in many fields. Recently, Institute of Electrical Engineering, Chinese Academy of Sciences developed a prototype on-board MI HTS coil used for electrodynamic suspension (EDS) system working with an acceleration of beyond 8 g. To design a reliable engineering on-board magnet, specific Ic angle dependence data of short sample under different fields were fully considered to estimate the operating margin and losses accurately of the on-board magnet. A semi-analytical method, combination of the dynamic circuit method and finite element method, was used to analyze the dynamic electromagnetic force distribution inside the on-board magnet under motion state, in which the effect of the propulsion coils and the figure-eight-shaped coils were taken into considerations. Finally, a full-size MI double pancake racetrack coil was fabricated and tested in a liquid nitrogen bath. The test results meet the design requirements and verify the feasibility of the design, which provides an important reference for the experiment of on-board EDS with high acceleration in the future.

Superconducting Stiffness and Coherence Length of FeSe0.5Te0.5 Measured in a Zero-Applied Field

Superconducting stiffness ρs and coherence length ξ are usually determined by measuring the penetration depth λ of a magnetic field and the upper critical field Hc2 of a superconductor (SC), respectively. However, in magnetic SC, which is iron-based, this could lead to erroneous results, since the internal field could be very different from the applied one. To overcome this problem in Fe1+ySexTe1−x with x∼0.5 and y∼0 (FST), we measured both quantities with the Stiffnessometer technique. In this technique, one applies a rotor-free vector potential A to a superconducting ring and measures the current density j via the ring’s magnetic moment m. ρs and ξ are determined from London’s equation, j=−ρsA, and its range of validity. This method is particularly accurate at temperatures close to the critical temperature Tc. We find weaker ρs and longer ξ than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg–Landau theory.

Weak ferromagnetism at 48.5 K in rubidium-intercalated biphenyl

Aromatic hydrocarbon molecules with π-conjugated structure have great potential for the search of exotic states such as high temperature superconductivity and novel quantum magnetism. In this work, we found that high temperature weak ferromagnetism can be achieved by intercalating rubidium into the shortest poly(para-phenylene) molecule—biphenyl. Both dc and ac magnetic susceptibility measurements showed that the synthesized material exhibits a typical antiferromagnetic transition at 48.5 K, accompanying a ferromagnetic character in the magnetically ordered region. Combining theoretical calculations and Raman spectroscopic studies, it was found that the mole ratio between rubidium and biphenyl is 1:1, and the molecular magnetic moments are formed by transferring the Rb-5s electrons to the C-2p orbitals. The weak ferromagnetism is produced by intralayer spin-canted antiferromagnetic and interlayer ferromagnetic spin couplings. Our finding enriches the physicochemical functionality of biphenyl and opens a window for the search of π-electron organic magnets in poly(para-phenylene) molecules.

CRITICAL CURRENT DENSITIES IN MAGNETIC SUPERCONDUCTORS Dy1–xErxRh3,8Ru0,2B4 (x=0,2;0,4)

Rhodium borides of the compositions Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4 were synthesized on the basis of the compound DyRh3,8Ru0,2B4 by partial substitution of dysprosium (Dy) with erbium (Er) with a LuRu4B4 type structure. The dependences of χ-1(T), M(B) and Bc2(T) were studied for these compounds. It is established that the compounds Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4 are magnetic superconductors with a critical temperature Tc ≈ 5.1 and 5.8 K, respectively; at T ≈ 3 K the magnetic subsystem of these compounds goes into the antiferromagnetic state. Based on measurements of the magnetic moment from the field (M(B)) for samples of compounds Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4, using the Bean model, dependences on the field of the critical current density jc(B) and the reduced pinning force fp(h) are obtained. The value of jc in the studied samples did not exceed 400 A/cm2 at T ≈ 2 K. It is established that in the case of magnetic superconductors with antiferromagnetic ordering (Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4) at h 0.4 there is a significant deviation from the law of similarity.

Evolution of magnetic stripes under uniaxial stress in La1.885Ba0.115CuO4 studied by neutron scattering

We present the effect of uniaxial stress on the magnetic stripes in the cuprate system ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ with $x=0.115$, previously found to have a stress-induced enhancement in the superconducting transition temperature. By means of neutron scattering, we confirm that the static stripes are suppressed by stress, pointing towards a trade-off between superconductivity and static magnetism, in direct agreement with previously reported muon spin rotation measurements. Additionally, we show that some of the reduced weight in the elastic channel appears to have moved to the inelastic channel, while we can exclude the opening of a spin gap down to an energy of 1 meV. Moreover, a stress-induced momentum shift of the fluctuations towards the typical 1/8 value of commensurability is observed, while no change in periodicity is seen in the static stripe signal. These results impose a strong constraint on the theoretical interpretation of stress-enhanced superconductivity in cuprate systems.