High-temperature Superconducting Field Research Articles

Feasibility Study of the Impregnation of a No-Insulation HTS Coil Using an Electrically Conductive Epoxy

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using an electrically conductive epoxy resin. Recently, several studies of HTS coils without turn-to-turn insulation have been reported for field coils used in rotating machines such as motors and generators. The NI winding technique enhances the thermal stability of the HTS coil without requiring complicated protection techniques because the quench current is automatically bypassed through the turn-to-turn contacts within the HTS coil. Nevertheless, there is still a question as to whether the NI technique can be applied to rotating machines. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. For HTS field coils intended for use in rotating machines, epoxy impregnation is generally necessary to protect the HTS field coil from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability [1] , [2] . However, the NI HTS coil cannot be fabricated by wet winding using epoxy resin because epoxy resins such as Stycast 2850 FT and CTD 521 are electrically insulating materials. This study examines the electrical stability of an NI HTS coil impregnated with an epoxy resin containing electrically conductive particles. The results are likely to present useful data for the application of electrically conductive epoxy impregnated NI HTS coils to rotating machines.

Structural Design of a Module Coil for a 12-MW Class HTS Generator for Wind Turbine

Recently, high-temperature superconducting (HTS) generators are being studied to reduce the weight and size of large-scale gearless generators. However, the large-scale HTS generators have problems relating to high electromagnetic force because of their high current density and magnetic field. Therefore, the forces acting on HTS field coils should be considered as well as their support structures. This paper discusses the structural design of a module coil for a 12-MW class HTS generator for wind turbine. The 12-MW HTS generator module was designed based on electromagnetic analysis results obtained using the 3-D finite-element method. The detailed structure of the module coil was investigated considering the HTS coil bobbin, support, and cooling system. For secure design of the module coil, stress and deformation analyses were performed. Structural analysis results were compared with safety factors and critical yield stresses for each material. As the results, electromagnetic force and maximum stress of the module coil were calculated, and pole-type supports were found to be the most suitable for the 12-MW class HTS generator.

Electromagnetic properties of REBaCuO superconducting tapes considered for magnets of fusion reactors

The RE-123 tapes represent the most advanced product in the field of high temperature superconductors, developed for transport of high electric currents. Although this product has already proceeded to the stage of industrial production, the variety in techniques employed by different manufacturers make the final products by far not identical. We studied electromagnetic properties of a series of 21 tapes from main global wire manufacturers. The tapes were tested by magnetic induction technique (vibrating sample magnetometer, VSM) and by a transport current measurements. The aim was to find the best candidates for superconducting coils for fusion reactors. Engineering currents were deduced from magnetic hysteresis loops measured by VSM at 10K, 50K, and 77K. The data for 77K were compared with the results of direct transport experiments done at the same temperature. Current transport experiments also provided information on the current anisotropy. The VSM tests were repeated after neutron irradiation of the tapes by neutron fluence of 2.12×1018cm−2. A significant enhancement of engineering currents was observed in magnetic fields above 0.5T in all investigated tapes and temperatures. The enhancement, similarly as the engineering current, was roughly exponential function of temperature.

Electromagnetic Analysis of a HTS Linear-Rotary Permanent Magnet Actuator

In this paper, a novel linear-rotary permanent magnet actuator (LRPMA) is proposed with high-temperature superconducting (HTS) field winding imbedded in the stator. In order to show the merits of the HTS LRPMA, a same structure without HTS field windings and one with permanent magnet mounted on the surface of the mover with same magnetization direction are taken as the comparison. The electromagnetic characteristic and harmonic analysis of the three topologies are analyzed using finite element method, including flux density, back electromotive force, cogging torque and cogging force, torque, and thrust. The air-gap flux density can be easily controlled by adjusting dc current in the HTS field winding. Compared with the other two structures, the torque density of the proposed LRPMA can be easily increased. The computed results show that the performance of the HTS LRPMA, such as the torque density, can be greatly improved.

A Novel Arrangement for an HTS Field Coil for Large-Capacity Superconducting Machines

A high-temperature superconductor (HTS) field coil with a novel, step-shaped cross section is proposed for large-capacity superconducting machine applications. The design of the structure is explained in detail, and comparisons are made with conventional coils. The electromagnetic characteristics are also analyzed in comparison with the conventional alternative. From the analysis, the overlapping topology can reduce the volume of the generator and achieve better electromagnetic characteristics by eliminating the magnetic flux sag produced by the adjacent field coils, while also decreasing the iron loss.

A New Double-Sided HTS Flux-Switching Linear Motor With Series Magnet Circuit

A new double-sided high-temperature superconducting (HTS) linear flux-switching motor for transportation system is proposed in this paper, in which the yokeless iron, armature, and HTS field windings are located in its mover, whereas its double-sided long stator only consist of segmented and low-cost iron. Thus, this type of linear motors exhibit robust and low-cost stator, high efficiency, and high-force capability. First, the general design rule of this kind of motor is proposed. Then, based on the proposed design rule, two double-sided HTS flux-switching linear motors with series and parallel magnetic circuits are investigated and calculated based on the finite-element method. The research results show that the series magnetic circuit motor has the advantages of a more sinusoidal back EMF, high thrust-force density, and high-load capacity.

Comparison of Excitation Topologies for Fully Stator-HTS Fault-Tolerant Machines

This paper proposes four topologies for high-temperature superconductor (HTS) fault-tolerant machines. Since the excitation ways of HTS field coils have significant effect on electromagnetic performances, four HTS machines with different HTS field coils are proposed, analyzed, and compared. For a fair comparison, their same rotors and dimensions are designed. Although their stators and field coils are different, these four machines have some common features. Namely, only one cooling system is required. This is because their HTS armature coils and HTS field coils are placed in the stator. Meanwhile, the fault-tolerant teeth (FTT) are adopted. These FTT not only provide extra flux path to improve torque capability but also decouple adjacent phases physically and magnetically, thus enhancing fault-tolerant capability. Finally, the characteristics of the HTS are analyzed and finite-element results are given for verification.

Operational Characteristics of HTS Coils With Flux Diverters in Semipersistent Mode Under Alternating Magnetic Field

This paper deals with the effect of magnetic flux diverters on the current decay behaviors of high-temperature superconducting (HTS) coils which operate in semipersistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machines, the HTS field coil can be designed to operate in PCM to obtain a steady magnetic field during operation. Due to the alternating magnetic field from the armature winding, the HTS field winding experiences an external magnetic field. As a result, the additional magnetization ac loss in the field coil would be induced, and it affects the current decay characteristics. In order to improve the current decay behavior of the HTS field coil, the use of flux diverter made of magnetic material can be considered. In this paper, we experimentally investigated the current decay characteristics of HTS field coils with magnetic flux diverters under alternating magnetic field. The measurement setup, which is composed of HTS field coil and armature coil, was fabricated. The measurement test of the current decay on the HTS field coil in PCM operation was carried out as functions of the amplitude and frequency of the armature coil.

Modularization of the HTS Field Coils for a Large-Scale Superconducting Generator

In general, when a high-temperature superconducting (HTS) field coil breaks down, the overall field coils of a superconducting synchronous generator (SCSG) also stop working because the HTS field coils are connected in series. However, the modularized HTS field coil is operated individually, and thus, if the HTS field coils break down, the generator still operates even under the fault conditions. This paper deals with the modularization of the HTS field coils for a large-scale superconducting generator. The conceptual design of the modularized large-scale HTS generator was designed by using a 3-D CAD program based on electromagnetic and force analysis results. The output characteristics of the HTS generator were analyzed using a 3-D FEM program. The conceptual design of the modularized HTS field coils and fault characteristics analysis results of a 12-MW-class superconducting generator are discussed in detail.

Design of a 12-MW HTS Wind Power Generator Including a Flux Pump Exciter

A flux pump (FP) exciter injects dc current into the high-temperature superconducting (HTS) field coils of an HTS rotating machine without a slip ring and current leads. When designing a large-scale HTS generator with integrated FP exciter, the coil inductance, field current, and time constant need to be optimized for better performance of the machine. In this paper, a 12-MW HTS wind power generator with integrated FP exciter was designed. The essential parameters of a 12-MW HTS generator were optimized using the Taguchi method, targeting the minimization of weight and volume of the generator, the length of HTS wire, and the inductance. In particular, the FP exciter was adopted for supplying dc current to the HTS field coils without the power supply and the slip ring. The magnetic field distribution was analyzed using the 3-D finite-element method. The induced dc current and charging and discharging times of the FP exciter were compared with the metal current leads, for confirmation of the effectiveness of the FP exciter. The detailed results of the HTS generator design were discussed in detail.

A 4 T HTS Magnetic Field Generator, Conduction Cooled, for Neutron Physics Spectrometry

This paper is concerned with the analysis of thermal and magnetic design of a high-uniformity magnetic field (HUMF) electromagnet using high temperature superconducting (HTS) coils in Helmholtz arrangement, for nuclear physics experiments. The sample, exposed to a neutron pulse, is placed in the HUMF, and detectors are registering the emerging radiation. The 4-T dipole HUMF electromagnet system is conduction cooled using a closed-cycle Gifford-McMahon cryocooler. A second cryocooler is used to keep the sample temperature in the 4.2-300 K range. The Helmholtz coils are made of high temperature superconductor (HTS) tape of YBCO type, and are mounted in a special-shaped cryostat. Numerical experiments are performed to assess the magnetic field uniformity. This paper relies on a 3-D computer-aided design model of the electromagnet prototype. They unveil the magnetic field spectrum and the heat paths within the aggregate structure of the magnet system. The "warm” channel crosses the magnet axially and the HTS field winding must be kept within safe temperature limits or the HTS may exit the superconductive state. Therefore, cryocooling is used to remove the heat influx from the ambient. The numerical modeling of the heat transfer phenomena inside the magnet system reveals the temperature distribution and thermal loads for cryocoolers.

Study of HTS Machine System Cooling With a Closed-Loop Thermosyphon: Stability of Unsteady Heat Load and Transient Conduction

Thermosyphoning (TS) is a heat exchange mechanism that uses a circulating refrigerant based on natural convection with no need for a mechanical pump. This cooling technique has been employed to circulate liquid coolant and gaseous coolant for high-temperature superconductor (HTS) rotating machine systems. These machines operate with an HTS field pole in the temperature range from 25 to 40 K, which are suitable temperatures for the survivability of HTS wires. The cryomechanical components are simple; there are a condenser, an evaporator, and adiabatic tubes located between the condenser and the evaporator. For applications to ship propulsion motors or generators of the radial gap type, the liquid/gas coolant flow along the adiabatic tube inside the rotating shaft may be considerably affected by the ship's rolling and/or pitching motion. In this work, we studied the temperature stability and heat load survivability of cryogenic TS, by constructing a 100 W @ 30 K-grade model. We investigated the temperature, thermal resistance, and heat flux, with a maximum load up to 60 W, of ground and onboard tests. The observed TS behavior indicates that there is sufficient survivability to assemble megawatt-class ship propulsion systems.

Scientific issues under high magnetic field

High magnetic field is an important research condition which can provide great opportunities for the development of many subjects. Since 1913, there are 19 Nobel Prizes which are related to high magnetic field. Scientific research under high magnetic field involves a large amount of subjects and various scientific issues. In many research areas such as the quantum transport in semiconductors, Fermi surface measurements, the study of the upper critical field of high temperature superconductors, nuclear magnetic resonance study etc., high magnetic field is an essential condition. Therefore, the scientific research under high magnetic field is always the frontier. In this review, the cutting-edge scientific issues under high magnetic field are presented. The issues in physics include the fractional quantum Hall effect in graphene, the quantum oscillations in black phosphorus, the topological insulator Bi2Se3, the Weyl semimetal NbP, the quantum ordering in non-conventional superconductor, the magnetic field induced new superconducting phase, the magnetic field induced metamagnetism transition, the quantum effect of low dimensional materials, the spin frustration, etc. In chemistry and material science, the focuses are magnetic field induced new chemical reaction, control of reaction routes, the reaction kinetics, synthesis of single molecular magnet and chiral materials, nano-catalysis, etc. Issues related to nuclear magnetic resonance are also mentioned, including the protein structure determination, magnetic resonance imaging for medical science, the structure of melt, geosciences and environmental sciences, etc. Moreover, the present condition and achievement of the high magnetic field facility in China are shown in the following part. Three water-cooled magnets have created three world records. The water-cooled magnet WM1 with a 32 mm bore can generate a steady magnetic field of 38.5 Tesla, which has exceeded the previous record 37.5 Tesla. The water-cooled magnet WM5 with a 50 mm bore has produced a magnetic field of 35 Tesla, which is also the highest for the same type of magnets. The water-cooled magnet WM4 with a 32 mm bore has generated a magnetic field of 27.5 Tesla when inputting 10 MW power. All the magnets have corresponding experimental systems. For example, WM1 has a 3He extreme low temperature experiment dewar installed providing a temperature region from 325 mK to 70 K, which is mainly used for transport measurement. The high-homogeneity water-cooled magnet WM2 has a high field high frequency (82 to 690 GHz) electron magnetic resonance continuous wave system installed. WM5 can cooperate with a Fourier transform infrared spectroscopy (18000 to 4000 cm - 1), a vibrating sample magnetometer for magnetization measurement, and a high pressure experiment system (0–200 GPa). The service of the high magnetic field facilities in China will provide strong support for various cutting-edge research and create advantage for significant original achievement.

On Prefabricated Stress Cone for HTS Cable Termination

The high-temperature superconducting (HTS) cable terminal has been rapidly improved in recent years. One of the hotspots in the HTS field is the prefabricated stress cone, which is widely recognized for its advanced processing technology that brings high insulation resistance and structure compactness. Aiming at designing a robust HTS terminal, an equivalent circuit model of a 35-kV prefabricated stress cone is established. Then, we concentrate on analyzing and optimizing parameters such as the stress cone length and the insulation thickness that affect the electric field distribution with ANSYS. Furthermore, a real 35-kV stress cone is constructed for an insulation performance test. The rationality and feasibility of the prefabricated stress cone design are verified.

Determining the Operating Current of No-Insulation Field Coils in HTS Generators

This paper proposes a new method for calculating the operating current of high-temperature superconducting (HTS) no-insulation coils whose overcurrent characteristics are more stable than those of conventional field magnets in superconducting generators. Problems related to calculating the operating current of conventional high-temperature superconductors have been analyzed, and the conditions for coils to overcome such problems have been proposed. To complement electrical stability issues of conventional HTS field coils, a small pancake coil was constructed from the Bismuth strontium calcium copper oxide (BSCCO) first-generation HTS wire with noinsulation winding. The pancake coil was then tested. The tests confirmed that the no-insulation coil was electrically stable. In addition, it was confirmed that quenching did not occur in electrically stabilized coils, even at higher input currents than the critical current value of the coil. In addition, the magnetic field value having the largest effect on the decrease in the critical magnetic field in the superconducting coil was calculated through finite-element method analysis, thus predicting the effective current of the superconducting coil. This analysis was compared with experimental results to demonstrate the validity of the proposed method.

John R. Clem: Contributions to the Phenomenology of Superconductivity

John Clem, Distinguished Professor of Physics at Iowa State University and Ames Laboratory, who passed away in August 2013, was for many decades one of the world's leading contributors to the phenomenology of superconductivity. Hisfundamental contributions included elucidating domain structures in Type I superconductors, modeling reversiblemagnetization and upper critical field of high temperature superconductors (HTS), introducing the concept of“pancake” vortices in highly anisotropic HTS materials, and developing the theory of flux cutting. He alsotook a great interest in applied superconductivity and consulted for organizations such as IBM, Pirelli Cables and AMSC,deriving the microwave properties of HTS films, elucidating the mechanism of current flow in first generation HTS wires,establishing the transport measurement of ac loss in tapes, and calculating ac loss of fault current limiter coils andHTS power cables. As founder and chief editor of High-Tc Update, he also provided much needed perspective on the torrentof developments during the early years of HTS discoveries.

Comparison Study of Superconducting Wind Generators With HTS and LTS Field Windings

Large-scale direct-driven wind turbines are preferred for offshore wind generation from the viewpoint of cost of energy and system reliability. However, direct-driven generators of traditional technology are very difficult to be scaled up to a rated power of 10 MW or above. Superconducting generators offer a candidate to fill that gap. A feasible superconducting wind generator topology has rotor field windings made of superconductors and traditional copper stator windings carrying alterative currents. Nowadays, two kinds of superconductors are available in the market: the low-temperature superconductors (LTS) and the high-temperature superconductors (HTS). They have great differences in terms of material characteristics, product performance, price, and the relevant cryogenic and refrigeration system, which have a great influence on the design of superconducting generators. In this paper, a comparison study of superconducting wind generators with HTS and LTS rotor field windings is carried out, considering the performance and cost of superconductors and the cryogenic and refrigeration system. From the viewpoint of initial capital, technology maturity, and engineering feasibility, the LTS wind generator is recommended for subscale or full-sized prototype development in the near future.

Design and Performance Analysis of a NI-Type HTS Field Magnet for Superconducting Rotating Machines

The no-insulation-type (NI-type) high-temperature superconductor (HTS) magnet is considered a new and innovative concept for superconducting rotating machines, and it is possible to make an electrically and thermally robust magnet. The purpose of this paper is to investigate the NI-type HTS field magnet performance for superconducting rotating machines. First, specification data for a superconducting rotating machine were acquired based on electromagnetic analysis. Based on the data, the wire type, coil shape, field magnet, and armature were designed. The detailed structure of the NI-type HTS field magnet was designed using a 3-D CAD program. The performance of the NI-type HTS field magnet was tested under charge-discharge and overcurrent conditions. The NI-type HTS field magnet proposed in this paper operated well. Based on the field magnet data, an NI-type HTS field magnet-based superconducting rotating machine was fabricated. The signal condition and generating power of the fabricated HTS rotating machine were examined.

Study of Key Parameters and Cryogenic Vessel Structure of 10-MW Salient-Pole Wind Turbine HTS Generators

A 10-MW wind turbine high temperature superconductor (HTS) synchronous generator operating at 68 K is modeled according to the optimized parameter, and the magnetic flux density (MFD) in various generator parts is simulated. Two key parameters of field leakage coefficient and operating field current are determined based on the MFD simulation results. Moreover, as the cryogenic component, a basic vessel structure used for cooling HTS field coils is proposed, and the required cooling power for heat loads and coolant flow rate are derived.

Enabling High-Temperature Superconducting Technologies Toward Practical Applications

This work covers the high-temperature superconducting (HTS) technologies based on the highlights in recent achievements in the applied HTS field in China. Its comprehensive coverage includes practical HTS material manufacturing and characterization, large-scale applications, and electronic applications. The applied HTS technologies have been well enabled to build applicable devices, and their characteristics have been well verified in the HTS devices developed to be industrialized for practical applications. The highlighted HTS devices and their performance details reveal the trend and the necessary improvement required to reach the goal of industrial applications of HTS technologies.