Bulk High-temperature Superconductors Research Articles

Electromagnetic and Rotational Characteristics of a Superconducting Flywheel Energy Storage System Utilizing a Radial-Type High-Temperature Superconducting Bearing

A 2 kW/28.5 kJ superconducting flywheel energy storage system (SFESS) with a radial-type high-temperature superconducting (HTS) bearing was set up to study the electromagnetic and rotational characteristics. The structure of the SFESS as well as the design of its main parts was reported. A mathematical model based on the finite element method (FEM) was established to research the electromagnetic characteristics of the HTS bearing during the levitation process, which show that a part of the magnetic flux penetrates into the edge of the HTS bulks and then goes back to the opposite pole of the permanent magnet rotor (PMR). The induced current mainly distributes in the edge of the HTS bulks, indicating that larger force acts on the edge part of the HTS bulks and probably causes them to crack. The free rotations of the rotor at different steady-state speeds of 2500–5000 rpm and its radial vibration were displayed. The induced voltage of the stator winding of the motor in this process was analyzed. The rotational characteristics are related to the vibration of the rotor. Below the resonant frequency, the vibration increases significantly with the speed. Enhancing the radial stiffness to limit the vibration amplitude of the rotor is an effective approach to improve the speed.

Lorentz Force Velocimetry using a bulk HTS magnet system

Design of the Lorentz Force Flow-meter (LFF-meter) for weakly-conducting and slow-flowing fluids, e.g., glass melts, place stringent requirements on the magnetic field generation and force measurements. In the scope of the Research Training Group “Lorentz Force Velocimetry (LFV) and Lorentz Force Eddy Current Testing”, a LFF-meter considers: first - a magnetic field source, where NdFeB permanents magnet system is replaced with a bulk high-temperature superconductors (HTS), which promise higher flux densities (> 1 T) and thus higher output force resolution; second - a high-precision force measurements, where electromagnetic force compensation weighing balance (EMFC) setup is replaced with torsion balance based system (TFMS), which claims an increase of the force resolution up to 1 nN. Furthermore, in order to raise the issue of the limiting total mass (which is always an issue for high-precision force measurements), the bulk HTS and TFMS are merged within the cryostat. This work discusses LFV experiments, calculations and numerical simulations, where a magnetic field is generated by the bulk HTS.

A trapped field of 17.7 T in a stack of high temperature superconducting tape

High temperature superconducting (HTS) tape can be cut and stacked to generate large magnetic fields at cryogenic temperatures after inducing persistent currents in the superconducting layers. A field of 17.7 T was trapped between two stacks of HTS tape at 8 K with no external mechanical reinforcement. 17.6 T could be sustained when warming the stack up to 14 K. A new type of hybrid stack was used consisting of a 12 mm square insert stack embedded inside a larger 34.4 mm diameter stack made from different tape. The magnetic field generated is marginally higher than the previous trapped field record achieved by a bulk HTS magnet and 30% greater than previously achieved in a stack of HTS tapes. Such stacks are being considered for superconducting motors as rotor field poles where the cryogenic penalty is justified by the increased power to weight ratio. The sample reported can be considered the strongest permanent magnet ever created.

Numerical Studies on the Dynamic Responses of Levitated High-temperature Superconductor with a Strongly Coupled Thermo-electromagnetic Model

In this paper, we present a 2-D numerical model based on the H-formulation and nonlinear E-J relationship to study the dynamic responses of a bulk high-temperature superconductor(HTSC) levitated above two kinds of permanent magnetic guideway(PMG). Being different from the existing related models and results of this subject, the thermal effect was brought into account by a strong coupled model with electromagnetic and thermal. The levitation forces were calculated by finite element method and then the vertical motion of the levitated HTSC subject to external disturbance is characterized by a second-order dynamic equation which couples the electromagnetic model via the levitation force. We studied the thermal influence on the vertical force and dynamic characteristics of the HTSC levitated above two kinds of PMGs. The obtained results prove that this strongly coupled model can better simulate the dynamic response of HTSC levitation system.

Lorentz force velocimetry using a bulk HTS magnet system: proof-of-concept

This paper presents a proof-of-concept of the idea of using bulk high-temperature superconducting (HTS) materials as quasi-permanent magnets that would form, in the future, an integral part of an advanced Lorentz force velocimetry (LFV) system. The experiments, calculations and numerical simulations are performed in accordance with the fundamental theory of LFV, whereby a moving metal rod passes through a static magnetic field, in our case generated by the bulk HTSs. The bulk HTS magnet system (MS) consists of two Y–Ba–Cu–O samples in the form of bulk cylindrical discs, which are encapsulated in an aluminium holder and wrapped with styrofoam. The aluminium holder is designed to locate the bulk HTS magnets on either side of the metal rod. After field cooling magnetisation with an applied field of 1.5 T at 77 K, the bulk HTS MS provides a quasi-permanent magnetic field over 240 s, enabling Lorentz force measurements to be carried out with a constant velocity of the metal rod. Two sets of Lorentz force measurements with copper and aluminium rods with velocities ranging from approximately 54–81 mm s−1 were performed. The obtained results, which are validated using a numerical model developed in COMSOL Multiphysics, demonstrate the linear relationship between the Lorentz force and velocity of the moving conductor. Finally, the potential of generating very high magnetic fields using bulk HTS that would enable LFV in even weakly-conducting and slow-flowing fluids, e.g., glass melts, is discussed.

Lateral motion stability of high-temperature superconducting maglev systems derived from a nonlinear guidance force hysteretic model

High-temperature superconducting (HTS) maglev, owning to the capability of passive stabilization, is potentially promising for high-speed transportation. The guidance force of bulk HTS materials above a permanent magnetic guideway has a nonlinear response due to the hysteresis effect. As a kind of rail transit, when the vehicle runs along the track, the curve and other disturbances will cause vibrations to the vehicle system. These physical factors will pose dynamic loads on the components, reducing structural reliability as well as affecting the ride comfort. The lateral motion, as an important part of the vehicle system dynamics, needs to be studied in the pursuit of HTS maglev realization. In this paper, we first measured the guidance forces of HTS bulks under different motion conditions, and analyzed the relationship between the lateral displacement, the movement velocity and the guidance force. Then, a mathematical model was built based on these experimental data. The key feature of this mathematical model is that it can describe the hysteresis characteristic of the guidance force. Based on this model, we investigated the lateral motion stability of the HTS levitation system, and found three singular points, one stable focus point, and two unstable saddle points. Lastly, a phase portrait was proposed to indicate the safe working region of the HTS maglev vehicle where the vehicle can automatically return to its equilibrium position. These experimental and simulation results are important to clarify the lateral motion stability under external disturbance or shock, and provide a reference basis for the design of levitation systems.

Improvement of Levitation Properties Owing to High Magnetic Field in a Magnetic Levitation System With Magnetic Shielding Effect of HTS Bulk

We studied a magnetic levitation system incorporating a magnet, a high-temperature superconducting (HTS) bulk, and a ferromagnetic material. We changed a permanent magnet into an HTS magnet to improve the levitation properties of the system. Numerical analysis results showed that the maximum levitation force and maximum restoring force were 1550 and 338 g, respectively, which are 14 and 4.6 times larger than the corresponding values for a permanent magnet system. Next, we determined the levitation properties by changing the distance between the HTS magnet and the HTS bulk. Our data showed that as this distance increased, the peak levitation force decreased, while the restoring force increased. Finally, we tried to improve the levitation properties by increasing the magnetic field of the HTS magnet. The maximum levitation force and restoring force obtained were approximately 30 and 7.6 times larger, respectively, than the values obtained with the permanent magnet system. Thus, we succeeded in improving the levitation properties of the system.

Waveform Control Pulse Magnetization for HTS Bulk With Flux Jump

We improved the performance of single pulse magnetization in order to trap high magnetic field to many bulks in the high-temperature superconducting (HTS) synchronous motor with less number of times. In the proposed waveform control pulse magnetization (WCPM) method, a single pulse magnetic field may effectively magnetize the HTS bulk. In this study, we magnetized Gd–Ba–Cu–O HTS bulk by the WCPM at 77 K using the split-type copper coils without a core. Increasing the predetermined duty ratio in WCPM from 40% to 45%, HTS bulk caused flux jump by pulse field magnetization (PFM), and trapped magnetic flux density greatly increased. WCPM with 50% duty ratio allowed to reach a trapped magnetic flux density 150% higher than the conventional PFM. We always observed flux jumps over 1.7 T for this bulk while using PFM. When we attempted WCPM with the magnetic flux density slightly higher than the observed flux jump as control target for suppressing, the HTS bulk trapped a high magnetic flux density comparable to the field cooling method, only by applying a single-pulsed magnetic field. This bulk successfully trapped high magnetic flux density difficult to realize with the conventional single PFM with low energy loss from the WCPM through feedback control. By finely using flux jump well in PFM, it was shown that the HTS bulk can trap high magnetic field.

Modifying the Pulsed-Field-Magnetization Technique for HTS Bulks in Electrical Machines Without Magnetic Field Sensors

This paper studies the high-temperature superconductor (HTS) YBCO and GdBCO bulks pulse-field magnetization (PFM) in loco , that is, when inserted in an electric machine's magnetic circuit. The PFM typically uses cryogenic hall sensors to monitor the magnetization process. However, to install cryogenic hall sensors in the magnetic excitation circuit of an electrical machine may increase the air gap and increase the machine cost. To surpass that, this paper presents an indirect technique capable of monitoring the superconductor magnetization process through the applied pulse voltage and associated electric current transient waveform. Last, different levels and the numbers of pulses were applied to magnetize the HTS bulks inside the magnetic circuit to analyze the resultant trapped magnetic field. Differences between the YBCO and GdBCO are highlighted and discussed.

Design of High Power Handling Filter Using Cascaded Quadruplet Superconducting Bulk Resonators

We designed transmit bandpass filters that have a four-pole cascaded quadruplet (CQ) structure with high-temperature superconducting (HTS) bulk disk resonators. We developed a trimming device that can change the electric-coupling coefficients simply by adjusting the height of a pair of copper rods. We analyzed the frequency response of the filters using an electromagnetic simulator and found that the trimming device can provide the targeted value for the coupling coefficient. In addition, from simulation of the filters, we obtained frequency responses with a center frequency of 5.04 GHz, a bandwidth of 124 MHz, and a skirt characteristic of 3.23 dB/5 MHz. To our knowledge, this is the first CQ filter structure yet developed using HTS disk resonators and a trimming system for electric coupling.

A Design Method of HTS Bulks Array for Decreasing Rotation Loss in a Superconducting Maglev Microthrust Stand

Rotation losses at low frequency is very crucial to the microthrust stand depending on high temperature superconductors (HTS) maglev. HTS bulks arrays are sources of levitation forces and have an influence on declination angle of the levitation platform during rotation. Changes of declination angle can lead to variation of external magnetic field on HTS bulk arrays, which then affect rotation losses of the microthrust stand. An experimental setup has been established to investigate influence of HTS bulk arrays and eccentric loads on rotation losses. Space ratio is applied to describe the dispersion degree of HTS bulk arrays with different geometrical configurations. Experimental results show that Arrays with smaller space ratio help diminish rotation losses of the HTS maglev stand. Presetting eccentric loads with proper amount and initial circumferential angle could decrease rotation losses for arrays with smaller space ratio. For microthrust measurement using HTS maglev stand, HTS bulk arrays should be optimized to achieve lower rotation losses and better adjustability.

Design Optimization and Experimental Verification of an Electromagnetic Turnout for HTS Maglev Systems

The turnout is a crucial track junction device of the ground rail transportation system, including high-temperature superconducting (HTS) Maglev systems. The permanent magnetic guideway (PMG) employed in the HTS maglev system has a strong magnetic force between the rail segments, which causes moving difficulties and increases operation costs when switching the PMG. In this paper, a type of nonmechanical electromagnetic turnout for a “Y”-shaped Halbach-type PMG is proposed, and the structure of the electromagnet is optimized by finite element software. Simulation results show that the magnetic field of the optimized electromagnet could reach as strong as the replaced permanent magnets (PM), so it is feasible to design and fabricate a nonmechanical electromagnetic turnout. We fabricated a toy-sized “Y”-shaped electromagnetic turnout model and its counterpart with only PM to analyze and compare the magnetic field distribution. A maglev vehicle model with one HTS bulk inside is levitated and used to check the function of the designed turnout. Experiments show that the maglev model could achieve a stable direction switch above the electromagnetic turnout by changing the input current direction of the electromagnet. This work verifies the feasibility of electromagnetic turnouts for the Halbach-type PMG and provides valuable references for the future design of nonmechanical PMG turnouts.

Toward Optimization of Multi-Pulse, Pulsed Field Magnetization of Bulk High-Temperature Superconductors

Pulsed field magnetization (PFM) is the most practical method for magnetizing bulk superconducting materials as trapped field magnets, but the record trapped field achieved by PFM to date is still significantly less than the true trapped field capability of these materials. In this paper, a flexible numerical modeling technique based on the finite element method is used to provide a comprehensive and realistic picture of multi-pulse PFM, which has been shown to be effective in increasing the trapped field/flux over a single pulse. First, the maximum trapped field capability of a representative sample is determined using two types of numerical model simulating field-cooling and zero-field-cooling magnetization. Next, various sets of magnetic field pulses are applied to the bulk to analyze multi-pulse PFM. An increase in the trapped field can be achieved after a second pulse and to do so an increased amplitude of applied field is required to maximize the trapped field fully. The numerical analysis shows that this occurs in subsequent pulses because it is more difficult for the magnetic flux to penetrate the sample and there is a lower temperature rise.

Fracture analysis of bulk superconductors under electromagnetic force

Bulk high-temperature superconductors exhibit priority magnetic properties compared to the conventional permanent magnets. Single-grain bulk GdBaCuO superconductors have significant application potential due to higher critical current density Jc and trapped fields. However, bulk superconductors are subject to larger Lorentz forces under high magnetic fields and their performance is limited by damage to the materials. In this paper, a finite element model based on the H-formulation is used to solve for the electromagnetic force in bulk GdBCO containing defects or inclusions under application of a pulsed field. Strong local electromagnetic force enhancement is observed at the crack tip. A bond-based peridynamic (PD) approach is proposed for analysis of the dynamic mechanical behavior and brittle damage of the sample. Crack initiation and propagation paths can be predicted using PD theory. PD is thus shown to be a suitable method for dynamic fracture problem analysis. As a further demonstration of the proposed model’s capabilities, the bulk fracture process is simulated. The effect of voids, cross cracks and multiple inclusions are presented. Results show that mechanical stability can be enhanced by the presence of silver particles.

Development of HTS Bulk NMR Relaxometry With Ring-Shaped Iron

The compact NMR magnets consisted of the stacked high-temperature superconducting (HTS) GdBCO bulk annuli trapped by field cooling (FC) method have been developed. The target values of strength and homogeneity of magnetic field for proposed NMR relaxometry are 1.5 T and 150 ppm/cm3, respectively, at liquid nitrogen temperature. Since last year, we have been developing a hardware and software to detect the NMR signals by ourselves. Therefore, we needed to determine the final structure of HTS bulk magnet to be used in the NMR relaxometry. We can obtain the trapped magnetic field over 1.5 T at 77.4 K experimentally. However, it is still difficult to obtain the magnetic field homogeneity under 150 ppm/cm3 experimentally, due to their different superconducting properties. Therefore, in this study, four types of HTS bulk magnets to determine the final structure of compact NMR relaxometry were designed and their field properties including the degraded HTS bulk were investigated using three-dimensional finite-element method analysis. The field properties of 10-stacked HTS bulk magnets with the ring-shaped irons for field compensation and the degraded HTS bulk were investigated experimentally.

A Novel HTS Flux-Reversal Linear Permanent Magnet Machine With a Lower Number of Mover Teeth and Higher Thrust Density

Compared to the conventional flux-reversal linear permanent magnet machine (FRLPM), a novel high-temperature superconducting (HTS) FRLPM with a lower number of mover teeth and higher thrust density is proposed in this paper. In order to explain the superiority of the proposed machine, the back electromotive force of the regular FRLPM, the lower mover teeth number FRLPM (LMT-FRLPM), and the LMT-FRLPM with HTS bulks (HTS LMT-FRLPM) are derived in this paper. The analytical results are validated by the finite-element method (FEM). Furthermore, the thrust performances of the regular FRLPM, LMT-FRLPM, and HTS LMT-FRLPM at rated load are also calculated by the FEM, which verifies the outstanding thrust performances of the proposed HTS LMT-FRLPM.

Levitation Force of Bulk YBaCuO and GdBaCuO Under a Low-Pressure Environment

The high-temperature superconducting (HTS) bulk is the core component of HTS maglev systems. For the potential application of evacuated tube transportation, it is necessary to verify the loading capacity of the bulk under a low-pressure environment. Based on a home-made pressure-reducing platform, we investigated the levitation force of two kinds of typical bulks, that is, the same sized YBaCuO and GdBaCuO, above a Halbach permanent magnet guideway under different pressure conditions. The levitation force in the cases of zero-field-cooling (ZFC) and field-cooling (FC) were measured and analyzed. Experimental results show that the reduced air pressure can significantly improve the levitation force of the two kinds of bulks. The levitation force of YBaCuO and GdBaCuO has increased by 11.6% and 4.4% in the FC case, 20.3% and 13.7% in the ZFC case under 20 kPa compared with the atmospheric pressure (100 kPa), respectively. This universal phenomenon was explained by the increasing critical current density Jc of HTS bulks due to cooler liquid nitrogen under the low-pressure condition. It is interesting to find that the YBaCuO bulk was more sensitive to the pressure variation compared with the GdBaCuO bulk. This difference reflects the improvement extent of levitation force of HTS bulks with different Jc performance working in a low-pressure environment.

Stress analysis in high-temperature superconductors under pulsed field magnetization

Bulk high-temperature superconductors (HTSs) have a high critical current density and can trap a large magnetic field. When bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique, they are also subjected to a large electromagnetic stress, and the resulting thermal stress may cause cracking of the superconductor due to the brittle nature of the sample. In this paper, based on the H-formulation and the law of heat transfer, we can obtain the distributions of electromagnetic field and temperature, which are in qualitative agreement with experiment. After that, based on the dynamic equilibrium equations, the mechanical response of the bulk superconductor is determined. During the PFM process, the change in temperature has a dramatic effect on the radial and hoop stresses, and the maximum radial and hoop stress are and respectively. The mechanical responses of a superconductor for different cases are also studied, such as the peak value of the applied field and the size of bulk superconductors. Finally, the stresses are also presented for different magnetization methods.

Dynamic levitation performance of Gd–Ba–Cu–O and Y–Ba–Cu–O bulk superconductors under a varying external magnetic field

We report that the dynamic levitation force of bulk high temperature superconductors (HTS) in motion attenuates when exposed to an inhomogeneous magnetic field. This phenomenon has significant potential implications for the long-term stability and running performance of HTS in maglev applications. In order to suppress the attenuation of the levitation force associated with fluctuations in magnetic field, we compare the dynamic levitation performance of single grain Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) bulk superconductors with relatively high critical current densities. A bespoke HTS maglev dynamic measurement system (SCML-03) incorporating a rotating circular permanent magnet guideway was employed to simulate the movement of HTS in a varying magnetic field at different frequencies (i.e. speed of rotation). The attenuation of the levitation force during dynamic operation, which is key parameter for effective maglev operation, has been evaluated experimentally. It is found that GdBCO bulk superconductors that exhibit superior levitation force properties are more able to resist the attenuation of levitation force compared with YBCO bulk materials under the same operating conditions. This investigation indicates clearly that GdBCO bulk superconductors can play an important role in suppressing attenuation of the levitation force, therefore improving the long-term levitation performance under dynamic operating conditions. This result is potentially significant in the design and application of HTS in maglev systems.

A novel HTS magnetic levitation dining table

High temperature superconducting (HTS) bulk can levitate above or suspend below a permanent magnet stably. Many magnificent potential applications of HTS bulk are proposed by researchers. Until now, few reports have been found for real applications of HTS bulk. A complete set of small-scale HTS magnetic levitation table is proposed in the paper. The HTS magnetic levitation table includes an annular HTS magnetic levitation system which is composed of an annular HTS bulk array and an annular permanent magnet guideway (PMG). The annular PMG and the annular cryogenics vessel which used to maintain low temperature environment of the HTS bulk array are designed. 62 YBCO bulks are used to locate at the bottom of the annular vessel. A 3D-model finite element numerical method is used to design the HTS bulk magnetic levitation system. Equivalent magnetic levitation and guidance forces calculation rules are proposed aimed at the annular HTS magnetic levitation system stability. Based on the proposed method, levitation and guidance forces curves of the one YBCO bulk magnetic above PMG could be obtained. This method also can use to assist PMG design to check whether the designed PMG could reach the basic demand of the HTS magnetic levitation table.