high-Tc Superconducting Coils Research Articles

A Long-Life, High-Capacity and High-Efficiency Cryogenic System Developed for High-Tc Superconducting Magnet Applications

Cryogenic system plays a vital role in the field of high-Tc superconducting (HTS) magnet applications. For a variety of HTS magnet applications in energy storage devices, current limiters, generators, and maglevs, the technology itself is relatively mature. However, the problems associated with the used cryocoolers have hampered the advancement of their practical applications. At present, China is developing a new generation of electrodynamic suspension prototype train with HTS magnets. Compared with the existing commercial electromagnetic suspension technology, it has better suspension stability, larger suspension gap, and lower energy consumption. To meet the aimed cooling requirement of the vehicle-mounted HTS magnet, a long-life, high-capacity and high-efficiency cryogenic system based on the Stirling-type pulse tube cryocooler (SPTC) is designed and developed in the authors’ laboratory. The SPTC driven by the linear compressor without any moving component in the cold head has the intrinsic merits of long life and high reliability. The cryogenic system can achieve a minimum no-load cooling temperature of 12 K. Multiple SPTCs will provide a cooling capacity of 100 W at 45 K with the relative Carnot efficiency of 10.8 % to cool the HTS coils inside the magnet. This paper will describe and analyze the overall design approach, integration with the HTS magnet and the arrangement in maglev of the developed cryogenic system, together with the performance characteristics of the SPTC during the laboratory test.

A new benchmark problem for electromagnetic modelling of superconductors: the high-T c superconducting dynamo

The high-T c superconducting (HTS) dynamo is a promising device that can inject large DC supercurrents into a closed superconducting circuit. This is particularly attractive to energise HTS coils in NMR/MRI magnets and superconducting rotating machines without the need for connection to a power supply via current leads. It is only very recently that quantitatively accurate, predictive models have been developed which are capable of analysing HTS dynamos and explain their underlying physical mechanism. In this work, we propose to use the HTS dynamo as a new benchmark problem for the HTS modelling community. The benchmark geometry consists of a permanent magnet rotating past a stationary HTS coated-conductor wire in the open-circuit configuration, assuming for simplicity the 2D (infinitely long) case. Despite this geometric simplicity the solution is complex, comprising time-varying spatially-inhomogeneous currents and fields throughout the superconducting volume. In this work, this benchmark problem has been implemented using several different methods, including H-formulation-based methods, coupled H-A and T-A formulations, the Minimum Electromagnetic Entropy Production method, and integral equation and volume integral equation-based equivalent circuit methods. Each of these approaches show excellent qualitative and quantitative agreement for the open-circuit equivalent instantaneous voltage and the cumulative time-averaged equivalent voltage, as well as the current density and electric field distributions within the HTS wire at key positions during the magnet transit. Finally, a critical analysis and comparison of each of the modelling frameworks is presented, based on the following key metrics: number of mesh elements in the HTS wire, total number of mesh elements in the model, number of degrees of freedom, tolerance settings and the approximate time taken per cycle for each model. This benchmark and the results contained herein provide researchers with a suitable framework to validate, compare and optimise their own methods for modelling the HTS dynamo.

Design and Evaluation of Prototype High-Tc Superconducting Linear Synchronous Motor for High-Speed Transportation

We developed a prototype of a high-Tc superconducting (HTS) linear synchronous motor (LSM) and evaluated its operational performances to obtain the preliminary design data for a test vehicle having a speed of 518.4 km/h. The HTS electromagnet was fabricated using a non-insulated (NI) GdBCO HTS coil, which was designed to produce a magnetomotive force of 300 kA-turns at a cooling temperature of 20 K. A two-stage Gifford-McMahon cryocooler was used to cool the electromagnet. The prototype was tested up to 2.7 kN sufficient to the required thrust of the test vehicle. The thrust characteristics shown by changing the operating conditions were also consistent with the simulation results. The HTS electromagnet was successfully operated during the test, and there was no fault in the NI HTS coils. Finally, we confirmed the design feasibility of HTS LSM for a test vehicle.

Modeling methodology for a HTS flux pump using a 2D H-formulation

Flux pumps are devices that can magnetize closed superconducting magnets in a gradual manner. High-Tc superconducting (HTS) flux pumps are particularly promising for high-field applications, due to the fact that lossless HTS coils are unavailable. The physics of these devices is also attractive. In this paper, we propose a modeling methodology for a transformer–rectifier HTS flux pump switched by dynamic resistance. A finite element model is built in Comsol and solved by a 2D H-formulation. The simulation result is verified by experimental data. The simulation will give a clear picture of how flux pumping occurs in a superconductor. It will show flux motion across a superconductor by shifting the electric central line, which is a unique feature of type-II superconductors. This work may be interesting in the understanding of magnetization of high-Tc superconductors.

Magnetizing high-Tc superconducting coated conductor stacks using a transformer–rectifier flux pumping method

High-Tc superconducting (HTS) coated conductor stacks have been demonstrated as very promising superconducting permanent magnets (PMs), to be used in applications like rotary motors and levitation devices. In this paper, we present a novel magnetization method to address a key problem in the application of HTS stacks, which is effective magnetization with compact, light and efficient setups. The proposed method, transformer–rectifier flux pumping, uses the mechanism developed for HTS coils to magnetize the modified HTS stacks. The accumulation of flux with time is achieved in the HTS stacks. The results show that the flux pump can effectively magnetize long HTS stacks with a magnetic field applied to a much smaller area. Operational characteristics of the flux pump are experimentally studied and some explanations are proposed for the distinct behaviors observed in the experiments. This study heralds a new direction in the magnetization of superconducting PMs and has the potential to be a practical method for industrial applications.

Design and Test Results of a Cryogenic Cooling System for a 25-T Cryogen-Free Superconducting Magnet

A cryogenic cooling system for a 25-T cryogen-free superconducting magnet (25T-CSM) was developed. The 25T-CSM consisted of 11-T high-Tc superconducting (HTS) coils and 14 T low-Tc superconducting (LTS) coils. The HTS coils, which had about 10-W AC-loss, were cooled to about 10 K by two GM cryocoolers and a helium gas circulation unit. In this system, the gas flow rate had an optimized value and was controlled by a mass flow controller. The LTS coils were cooled to about 4 K by two GM/JT cryocoolers. Each GM/JT cryocooler had two sets of precooling gas lines. These pre-cooling lines were cooled by two single-stage GM cryocoolers and the HTS coil cooling system via a heat exchanger, between the HTS gas circulation line and the LTS cool-down line. As a result of cooling test of the 25T-CSM, the LTS coils and HTS coils were cooled from room temperature to 4.3 K and 4.6 K within 164 h (about seven days), respectively. During the excitation test of the HTS coils, their maximum temperature increased to 7.6 K, which was still sufficiently low.

Feedback Control of a Rectifier Type HTS Flux Pump: Stabilizing Load Current With Minimized Losses

Flux pumps are able to compensate slow current decay in high- T C superconducting (HTS) magnets through noncontact approaches. It is a promising alternative for power sources and thick current leads in operating HTS coils. Following the previous work of a rectifier flux pump, we developed a feedback control system to achieve flexible control of load current. Experimental results show that the flux pump can stabilize load current at a preset level with the help of the control system. Power loss of using the flux pump is also compared with that of using current leads.

Angular dependence of direct current decay in a closed YBCO double-pancake coil under external AC magnetic field and reduction by magnetic shielding

High Tc superconducting (HTS) coils are ideal candidates in the use of high field magnets. HTS coils carrying a direct current, however, suffer a non-negligible loss when they are exposed to an external AC magnetic field. Although this phenomenon is well known, no study concerning AC magnetic field angular dependence of direct current decay has ever been shown. In this work, we experimentally investigate the direct current decay characteristics in a closed double pancake coil made of a YBCO coated conductor under external AC field. AC field of different angles with respect to the coil plane is applied. Results show that the current decay rate presents a strong angular dependence. The fastest decay occurs when the field is parallel to the coil plane, in which case the surface of the tape in the outermost layer experiences most flux variation. To reduce the decay rate, we propose wrapping superconducting tapes around the outermost layer of the coil to shield external AC field. This method significantly reduces direct current decay rate under parallel field, without affecting the perpendicular self-field of the coil.

An Overview of Flux Pumps for HTS Coils

High-Tc superconducting (HTS) flux pumps are capable of injecting flux into closed HTS magnets without electrical contact. It is becoming a promising alternative of current source in powering HTS coils. This paper reviews the recent progress in flux pumps for HTS coil magnets. Different types of HTS flux pumps are introduced. The physics of these flux pumps are explained and comparisons are made.

Mechanism of a high-Tc superconducting flux pump: Using alternating magnetic field to trigger flux flow

High-Tc Superconducting (HTS) magnets operating in persistent current mode suffer a current decay due to flux creep of superconductor and joint resistance. Flux pumps are able to inject direct current into superconducting circuit to compensate the current decay, without the thermal loss caused by current leads. In this work, we proposed a flux pumping mechanism for HTS coils, with an experimental verification and an analytical model. The basic principle we have used is that flux flow can be triggered when the superconductor carrying a direct current is subjected to a perpendicular AC magnetic field. Low frequency alternating current is induced in a loop of YBCO tape using an AC field. A portion of the tape which we refer to as the “bridge” shorts a superconducting coil. A high frequency AC field is applied perpendicular to the bridge tape when alternating current in the tape reaches one polarity. This triggers a net flux flow and results in a current increase in the coil. The proposed flux pump has clear physics and is easily controllable, which may make it promising in practical use.

Quench detection/protection of an HTS coil by AE signals

The normal zone propagation velocities in high Tc superconductors are slow at high operation temperature and therefore local and excessive temperature rise generates at quench occurrence, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in high Tc superconducting (HTS) coils as soon as possible and protect them. The authors have presented a quench detection method for HTS coils by time–frequency visualization of AE signals and have shown its usefulness for a HTS coil with height and outer diameter of 40–50mm. In this paper, the authors present a quench detection/protection system based on superior method in quench detection time to the previous method and show its usefulness for a larger HTS coil (height and outer diameter: 160–190mm) than the previous coil.

HTS 5 Tesla Synchrotron and Neutron Beamline Magnets

Two world-first High-Tc Superconducting (HTS) beamline magnets have been designed, manufactured and commissioned for synchrotron and neutron research facilities. One, for the Hahn-Meitner Institut, is for use with their high-resolution diffraction and resonant magnetic scattering instrument MAGS at the Berlin electron synchrotron facility BESSY. The key features of this 5 tesla split-pair magnet include field performance within a given confined geometry, and low weight. Mounting in a 6-circle goniometer and employing conduction-cooled HTS coils, it operates through 90 degrees of rotation with respect to the beamline axis and scattering plane. The neutron beamline magnet is also a 5 tesla split pair, designed for both neutron reflectometry and small-angle neutron scattering research at the new OPAL neutron facility of the Australian Nuclear Science and Technology Organisation. This much larger magnet offers wide neutron beam accessibility angles in both axial and transverse directions, large (50 mm) sample capability, and mounts on tilt stages operating in two axes. Both magnets use pulse-tube refrigeration for the HTS coils, which have been constructed from high performance BSCCO wire, and both are compatible with separate commercial cryofurnaces providing sample temperatures from 1.5 K (MAGS) or 4 K (OPAL) to over 600 K.

Indirect Measurement of AC Current Distributions in a Bi-2223 Solenoidal Coil by a Pickup Coil Method

In order to investigate AC current distributions in high Tc superconducting (HTS) coils, we have developed a measurement method to evaluate the current distributions in a HTS solenoidal coil. In this method, the current distributions are indirectly found from the results of the field distributions measured by pickup coils located around the sample coil surface. To confirm the accuracy and sensitivity of this method, current distributions in a test coil made of insulated copper wires are measured. Numerical and measured field distributions are compared, and it is found that our method is accurate enough to measure the current distributions for the HTS coils. Using this method, we measured the current distributions in a Bi-2223 solenoidal coil at room temperature and 77 K. The comparison between the experimental and numerical results shows that the current distributions in the HTS coil are not uniform. On the basis of the obtained results, the characteristics of the AC current distributions in the HTS coils are discussed.

Fabrication and test of a 4 kjJ Bi-2223 pulse coil for smes

We designed and fabricated a 4 kJ conduction-cooled high-Tc superconducting (HTS) pulse coil. The coil is wound with an interlayer-transposed 6-strand parallel conductor which is composed of Bi-2223 silver alloy-sheathed multi-filamentary wires. We had developed a complete 3.6 MJ/1 MW low-Tc superconducting (LTS) SMES system for testing on a power line at Imajuku substation. Aiming at the feasible operation of SMES applying a HTS coil, we made a SMES system set-up in which HTS coils were serially connected to 3 LTS coils of the SMES. The SMES including the HTS coil was connected to Imajuku substation's power system, to made operational tests of compensation for load fluctuation at the 6 kV power line. The test results lead to the feasibility of the HTS SMES for practical use in future power systems.

Development of a superconducting linear synchronous motor

High Tc superconducting (HTS) wires were applied to primary coils of a small size permanent magnet linear synchronous motor. Three-phase racetrack shaped test coils were fabricated and tested. The HTS coils were cooled down to 33 K by using a GM-cryocooler in order to overcome the decrease of the critical current density due to high intensity of magnetic field in the coil. A vacuum chamber and thermal shield for reduction of the radiation and the convection heat penetrations were fabricated. The parameters of the linear motor were calculated by using finite element method, and verified by experiment. It is possible to achieve large thrust force compared with that of a conventional machine because large current can flow in HTS primary coil.

Effective thermal conductivity in HTS coils

We present a numerical model to determine the effective thermal conductivity in high- T c superconducting (HTS) coils. The model, based on the heat conduction equation solved with the finite element method, will help us to compute the temperature dependence of the effective thermal conductivity in coils with varying proportions of epoxy in the total winding volume. The conductor parameters in the coils correspond to those in a real Bi-2223/Ag multifilamentary composite tape, where the anisotropy of the Bi-2223 material has been taken into account. We will furthermore discuss about the effects of geometry of the composite structure on effective thermal conductivity. Our result shows that in epoxy impregnated HTS coils at temperatures below 60 K, the effective thermal conductivity is over two orders of magnitude higher in the parallel direction with the conductor axis compared to the perpendicular direction.

AC-induced DC voltage in HTS coil

The interplay between AC and DC currents in a High- T c Superconducting (HTS) coil, made of multifilamentary silver-sheathed Bi-2223 tape, was investigated. We observed that the application of a small sinusoidal current in the frequency range of 50–500 Hz into the coil, while it is already carrying a DC current in the range of 16–22.5 A, caused an increase in the coil DC voltage. The DC voltage increment due to the AC signal is found to increase linearly with frequency and quadratically with amplitude. The DC voltage increment increases as the coil current grows towards its critical value of 22.2 A. This result may be important in some power applications such as fault current limiters (FCL) and superconducting magnet energy storage (SMES) based on HTS coils.

Magnetic field enhancement of high-T c superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T c superconducting coils. The central magnetic fieldB 0, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB 0 for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB 0 value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B 0 was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T c superconducting coils and magnets.