Double Pancake Coils Research Articles

Testing of an MgB2 coil for a wind turbine generator pole

We report on the testing of a 1 m long and 0.5 m wide racetrack coil wound from 4.5 km of MgB2 tape-shaped superconductors with a copper strip soldered to the nickel matrix. The cross-section of the coil corresponds to what is needed for the field windings of a 10 MW direct drive wind turbine generator pole, but where the straight section has been shortened. Ten double pancake sub-coils were wet-wound, with epoxy as the only electrical insulation, and assembled. The entire coil was cooled down to 14 K, and the direct current was ramped up. Voltage measurements were made over the double pancake coils. For eight of the ten double pancake coils, excessive voltages appeared at currents much lower than the design current. These voltages may either be attributed to the wire quality or to the winding process. A quench was observed at 149 A, which is in reasonable agreement with the predicted load line of the coil obtained by scaling wire transport measurements at 1 T and 20 K with hysteretic magnetization measurements in the range 1-5 T and 5-20 K.

Axial Compressive Stress Dependence of Critical Current of REBCO Double-Pancake Coil

This study investigated the axial compressive stress dependence of the critical current of Rare Earth Barium-Copper-Oxide (REBCO) double-pancake coils. We measured the critical currents of REBCO coils cooled by liquid nitrogen while various axial compressive stresses from 0 MPa to 130 MPa were applied. After each stress was applied, the critical currents of the coils were also measured without the stress application to examine if the REBCO coils were irreversibly deteriorated. The experimental results showed that the REBCO coils were deteriorated at more than 100 MPa and the critical currents of the coils decreased by about 30% by applying 120 MPa to the coils. Moreover, reversible reduction of the critical currents of the coils were observed while the stresses of less than 100 MPa were applied. The reversible critical current decrease occurred even at a small stress, and the critical currents of the coils were reversibly reduced by about 15% while the stress of 100 MPa was applied. This study quantitatively clarified the dependence of the critical current of the REBCO coil on the axial compressive stress and the limitation of the axial compressive stress to prevent the deterioration of the REBCO coil.

Development of a Cryogen-Free Compact 3 T Superconducting Magnet for an Electromagnetic Property Measurement System

A cryogen-free portable 3 T high-temperature superconducting magnet for an electromagnetic property measurement system has been developed to serve as a user facility at the Korea Basic Science Institute. The metallic insulation method was adopted to reduce the charging delay without sacrificing the self-protecting feature. A genetic-algorithm-aided optimized design was carried out to minimize the superconducting tape consumption while satisfying several design constraints. After the design, the compact high-temperature superconducting magnet composed of eight double-pancake coil modules was wound with high-temperature superconducting tape and stainless steel tape, and integrated with a two-stage cryo-cooler. The 3 T magnet was successfully cooled to approximately 20 K with a cryo-cooler and reached the target field of 3 T without any problems. Long-term measurements and a range of other tests were also implemented to verity the performance of the magnet. Test results demonstrated the feasibility of a cryogen-free portable high-temperature superconducting magnet system for electromagnetic property measurement experiments.

Development and Testing of 2G High Temperature Superconducting (HTS) Field Coils for HTS Synchronous Machines

High temperature superconducting (HTS) synchronous machines are compact and light weight machines with higher torque to weight ratio, lower noise and better dynamic response for strategic and critical industrial applications. The development of HTS field coil is one of the main technological challenges in realizing the HTS synchronous machines, as it involves the selection of HTS tape, winding technology, handling of the HTS tape while winding on a winding machine and testing of the developed HTS coil at desired cryogenic temperatures. With advancements in HTS tape technology, recent 2G HTS tapes are offered with higher engineering current densities with respect to 1G HTS tapes even at higher magnetic fields. Hence, the replacement of 1G HTS field coils of previously designed 200 kVA HTS synchronous machine with 2G HTS field coils at the same operating cryo-temperature shows a potential to enhance its capacity to 350 kVA (more than 75% increase in machine output). In the present work the details of design, development and testing of a double pancake type 2G HTS field coil are presented for capacity enhancement of earlier 1G based 200kVA HTS synchronous machine.

Investigation of a Six-Pole Stator System Using No-Insulation 2nd Generation HTS Coils for a 10 kW Generator Demonstrator

The enhanced thermal stability of no-insulation superconducting coils, due to the absent turn-to-turn insulation is demonstrated in several publications. With the self-protecting properties of these coils, the application of second generation high temperature superconductors in real energy applications can be simplified, regarding quench detection and protection. Furthermore, it enables a system design with superconducting coils operating closer to their critical current limitation compared to their insulated counterparts. The usage of superconducting materials, improving the power density in wind turbine generators is an ongoing research topic. As a result, the design and construction of a 10 kW laboratory demonstrator is intended. Therefore, six double pancake racetrack coils are manufactured with two different superconducting tape architectures and compared with respect to the critical current, turn-to-turn resistance and transient performance in liquid nitrogen at 77 K. The superconducting coils are joined together to a six-pole stator system for the installation in the 10 kW generator demonstrator. All 12 pancakes reach the expected critical current and show a similar transient response of the magnetic field in the individual measurements. However, the series-connected stator system has a much longer field delay than the individual pancakes, which cannot be sufficiently explained by the typical equivalent circuit diagram for no-insulation coils.

Design Overview of the MIT 1.3-GHz LTS/HTS NMR Magnet with a New REBCO Insert.

We present a design overview of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) with a newly designed 835-MHz REBCO insert (H835) as a replacement for the 800-MHz REBCO insert (H800) that was damaged when it quenched during operation in 2018. The new H835 is designed to contribute 19.6 T in a background field of 10.93 T by an LTS NMR magnet that normally rated at 11.74 T (500 MHz): combined, 1.3G generates a total field of 30.53 T corresponding to a proton resonance frequency of 1.3 GHz. H835 is designed to operate stably while meeting 1.3G design constraints. We have also designed H835 to protect it from permanent damage in an improbable event like a quench. Key design features are: 1) a single-coil formation, composed of 38 stacked metal-co-wound no-insulation and 2 stacked no-insulation double-pancake coils, all with mechanically improved cross-over sections; 2) enhanced thermal stability; and 3) reduced current margin with a detect-and-heat method. This paper includes: 1) electromagnetic and mechanical design of H835; 2) cryogenics overview; 3) quench protection strategy; and 3) discussion on the next steps to successfully complete 1.3G.

Electromagnetic Design and Mechanical Behavior Analysis of an 850 MHz All REBCO Nuclear Magnetic Resonance Magnet

A 20 T/ϕ32 mm all REBa2Cu3O7−δ (REBCO, RE = rare earth) REBCO superconducting magnet for 850 MHz nuclear magnetic resonance spectroscopy is being designed. The high-temperature superconductors, REBCO-coated conductors with both high tensile stress and high critical current density under high magnetic field would allow superconducting magnets to be compact and lighter than their low-temperature superconductor counterparts. The 20-T REBCO superconducting magnet consists of a stack of 28 double-pancake coils with the so-called no-insulation methods. Considering the perpendicular magnetic field to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis, five different width tapes, i.e., 4.1, 5.1, 6.1, 7.1, and 8.1 mm were placed on the midplane to both end positions in an axial direction. To improve the magnetic field homogeneity, the inside notch on the 4.1-mm tapes was adopted. The peak-to-peak magnetic field homogeneity over a 5-mm diameter spherical volume is better than 0.8 ppm. In this article, the design of the REBCO coils, electromagnetic field analysis, and stress analysis of the superconducting coils were presented.

Progress on the Upgrade of EDIPO, a 15 T Large Aperture Dipole

The upgraded magnet assembly of the EDIPO test facility aims at generating 15 T at 4.2 K and 85% of short sample limit in a large rectangular aperture and over a uniform field length of 1000 mm. During 2020 the design studies have concentrated on two flared-end block-coil design options with four double-pancake coils, two per pole. The coils are planned to be produced using Nb3Sn Rutherford cable with 44 × 1.1 mm diameter strands, which results in significantly larger dimensions compared to the typical cables used in accelerator magnets. Two sections of dummy superconducting cable have been produced, and dedicated tooling was developed to study the wind-ability of such high aspect ratio cable and provide input to define the geometrical parameters of the coil flared ends. This paper describes two magnet designs proposed for EDIPO focusing on the magnetic and the mechanical design.

Hoop Stress Tests of an Epoxy-Impregnated REBCO Coil With Fluorine-Coated Polyimide Insulation

REBCO coated conductors (CCs) are suitable for high field magnets due to their high in-field critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and high tensile strength. However, in order to apply the REBCO CCs to a high field magnet, it is important not only to evaluate the characteristics of short samples but also to clarify the behavior of the REBCO coil under strong electro-magnetic forces in high magnetic fields. In this study, we investigated the tolerance to the electro-magnetic force by applying hoop stresses to a REBCO double pancake coil in high magnetic fields. The REBCO coil was wound with BHO-EuBCO CCs and impregnated with epoxy resin. In order to prevent a degradation due to thermal delamination stress, fluorine-coated polyimide tapes were wound on the REBCO CCs. When a maximum hoop stress of 608 MPa was applied to the coil, normal voltage was generated and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation was confirmed in one side of double pancake. As a result of the investigation, the degradation occurred only at lap joint part between CCs in the windings, and the cause of the degradation was thought to be stress concentration at the both edges of lap joint. Since there was no <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation except for the lap joint part, it is considered that the REBCO coil can withstand larger hoop stress if we can reduce the stress concentration at the joint part. We therefore proposed a new joint structure and confirmed that it has higher tensile strength than a conventional lap joint structure.

AC Loss Reduction Through Flux Diverters for Superconducting Wireless Charging Coils at High Frequencies

Wireless power transfer (WPT) for electric vehicles (EVs), also known as wireless charging, is a fast-developing technology with many advantages over conventional cable charging. Novel materials such as high temperature superconductors (HTS) can be used to increase system performance. However, HTS coils have high ac losses at wireless charging frequencies (10-100 kHz) and this is crucial for WPT systems. Flux diverters have been used in the past for dc and low frequency (~50 Hz) superconducting applications but their effectiveness at high frequencies is unexplored. This article investigates the impact of flux diverters on HTS double pancake coils operating at high frequencies up to 85 kHz, commonly used for wireless charging of EVs. Various geometric characteristics of flux diverters are investigated such as air gap between diverter and coil, thickness, and width. A 2-D axisymmetric multilayer model using finite element analysis was used to examine the coil and diverter losses at such frequencies and different load factors (LFs) between 0.1 and 0.8. It is demonstrated that flux diverters are a viable option to reduce the coil losses even at high frequencies and the width of the flux diverter has the biggest impact on the loss reduction. In general, flux diverters are more suitable for applications using high LFs. Lastly, the impact of the diverter in terms of magnetic field distribution above the coil and overall loss distribution in the HTS coil was examined.

Electromagnetic-thermal-mechanical behaviors of a no-insulation double-pancake coil induced by a quench in the self field and the high field

High-temperature superconducting double-pancake (DP) coils wound by the no-insulation (NI) approach have been proved to have a high thermal stability and a self-protecting ability. This paper mainly studies the effect of a quench of one pancake coil on the electromagnetic-thermal-mechanical behaviors of an NI DP coil in the self field and the high field. An electromagnetic-thermal coupling quench model is used to calculate the distributions of current, temperature and electromagnetic field in the coil, and then a three-dimensional homogeneous mechanical model is built to analyze the changes in strain and stress during a quench by considering the distributions of thermal strain and Lorentz force of the coil. The results indicate that the obvious increase in circumferential current and radial current density in the bottom pancake coil is induced by a quench of the top pancake coil due to the electromagnetic coupling effect in the self field and the high field, and that the DP coil still has a negative coil voltage during a quench in different fields. Although the bottom pancake coil has a large circumferential current, the mechanical deformation of the DP coil during a quench is mainly caused by the temperature rise in the self field. The thermal expansion of the top pancake coil has a remarkable effect on the mechanical behaviors of the bottom pancake coil. Moreover, the DP coil has the same temperature rise and mechanisms of bypass current in the self field and the high field. However, the mechanical deformation of the DP coil is based on the combined effects of temperature rise and Lorentz force in the high field. It can be found that the values of the hoop and axial stresses are affected by a large electromagnetic stress.

Испытания ВТСП-2 магнита для нейронных исследований

The article describes the development, fabrication, and test results of a double pancake сoil made using second-generation high-temperature superconductors (HTSC-2G). The HTSC-2G coil is the key element of the cryomagnetic system intended for use as part of an experimental research setup for remotely controlling the expression of neurons by means of constant and low-frequency (up to 100 Hz) magnetic field. This project is a continuation of works [1] carried out on the integrated topic “Electronic Components and Neuromorphic Ccontrol Systems”, which includes, as a constituent part, the development and fabrication of an HTSC-2G cryomagnetic system for studying neuron activity under the effect of external magnetic field. A distinctive feature of the project is the use of a cryomagnetic system with a low energy consumption achieved owing to the use of modern HTSC materials. This will open the possibility to continuously observe the object under study from the start of its exposure to magnetic field to the occurrence of reaction signs. The technology for winding a double pancake HTSC-2G coil is developed and described. For making the coil, an HTSC-2G wire in polyamide varnish insulation was used. The technology of making inner junctions in double pancake HTSC-2G coils with a transition resistance of less than 120 nΩ at 77 K has been developed and successfully tried out. The results from preliminary tests of the HTSC-2G coil in liquid nitrogen are presented.

A New Approach to Measure Magnetic Field of High-Temperature Superconducting Coil Based on Magneto-Optical Faraday Effect

Magnetic field monitoring devices are essential for studying and operating high-temperature superconducting (HTS) magnets. Apart from conventional Hall sensor, passive optical fiber sensor is also an option for magnetic field monitoring. In this article, we proposed a novel access based on yttrium iron garnet magneto-optic (YM) sensor, which is one kind of optical fiber sensors, to measure magnetic fields generated by an HTS double pancake coil. Moreover, we used a magnetic field bypass tube made of silicon steel material to expand the measurement range of YM sensor. The test results of YM sensor in liquid nitrogen demonstrated that the YM sensor was benchmarked against Hall probe up to 24.7 mT at 77 K. Using a silicon steel tube to shield magnetic field could extend the field measurement range to 37.6 mT. YM sensor could be one of the promising methods for monitoring HTS magnet magnetic field.

Preliminary Study on Different Winding Processes of ReBCO Coils

This article intends to explore the optimal electrical characteristics of different winding processes for future high-field accelerator magnet insert. In the study, five double-pancake (DPC) racetrack ReBCO coils were wound by dry winding with no insulation (NI) process, dry winding with Kapton insulation (KI) process, wet winding with KI process, wet winding with NI process, and dry winding cowound with stainless steel tape process, respectively. The five DPC racetrack coils were fabricated and tested at 77 K in the Institute of High Energy Physics, Chinese Academy of Sciences. The article mainly presented the two winding processes, dry winding with NI and cowind with 304 stainless steel tape insulation since their special self-protect character during quench and delay effects during excitation and demagnetization. Through the experiments, we studied the characteristics of different winding processes and quench protection methods, which are important for the future application of High-temperature superconducting (HTS) insert coils.

AC loss measurement and simulation in a REBCO coil assembly utilising low-loss magnetic flux diverters

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

Influence of Preexcitation, High Temperature Magnetization and Combined Excitation Method on Screening Current Attenuation in Conduction-Cooled REBCO Coil

This article investigated a suitable excitation method for a conduction-cooled REBCO coil to reduce the temporal attenuation of the screening current induced field (screening field) for magnetic resonance imaging. We investigated the effect of the preexcitation of the coil and high temperature magnetization method on the screening field attenuation by measuring the variation rate of the magnetic field at the side surface of a conduction-cooled rare-earth barium copper oxide double pancake coil. Experimental results showed that the preexcitation of the coil and high temperature magnetization increased the temporal stability of the coil magnetic field when the preexcitation current is higher than the main operational current, or the temperature change of the high temperature magnetization is over 5 K, respectively. We also clarified that the combination of the preexcitation, overshooting, and high temperature magnetization method greatly suppressed the attenuation of the screening field and also the load factor of the coil than in the case when only the high temperature magnetization is applied.

A Feasibility Study on “Magnetic Dam” to Absorb Magnetic Energy in NI HTS Magnet During Quench

When a quench occurs in a no-insulation (NI) high temperature superconductor (HTS) magnet that consists of a stack of double-pancake (DP) coils, a large amount of current is often induced in an NI DP coil that is electromagnetically coupled with neighboring DP coils. The large induced current leads to an excessive magnetic stress and occasionally damages the magnet. In this paper, we propose a new quench protection concept to reduce the amount of induced current in an NI HTS magnet. The key idea is to use resistive copper in order to absorb a portion of electromagnetic energy that is initially stored in the magnet before the quench. We tentatively name these resistive coils as a “magnetic dam,” as they may slow down the electromagnetic quench propagation speed among the NI DP coils, which may be beneficial to avoid over-stressing by the large over-current.

An HTS NI Magnet Charged by Multiple Flux Pumps: Construction and Test of the Prototype

A high-temperature superconducting (HTS) no-insulation (NI) magnet charged by multiple flux pumps has been constructed and tested. The magnet consists of 5 NI double pancake coils (DPCs), whose inner diameter is 50 mm, outer diameter is about 92 mm, and the axial length of the NI magnet is 50 mm. To avoid the current limitation of single DPC on the whole magnet and fully use the current capacity of each DPC, the 5 DPCs were charged by 3 separate flux pumps to provide 3 different DC currents. The rotor speed of each flux pump was controlled by the LABVIEW program and microcontroller to regulate the charging current and the central magnetic field, which were measured by Hall sensors and displayed on the panel in real-time. The effect of the charging sequence on the central magnetic field had also been studied. Compared to the magnet operating at a single current, the proposed method in this paper has increased the central magnetic field from 290 mT to 390 mT, with an increase of 34.48%.

HTS synchronous machine critical current modelling and comparison with tests

Superconducting rotating machines generate high magnetic fields and for economic application of these machines and to ensure safe and reliable operation accurate assessment of the critical current of the superconductor is required. This paper describes a process to assess the critical current for a HTS synchronous machine with a superconducting rotor using a self-consistent numerical model. The model is used to study the critical current of first generation (1G) Bi-2223 pancake coils under self-field conditions for coil tests as well as within a machine magnetic field under full load conditions. Model results are compared against test measurements on individual HTS pancake coils as well as a double pancake coil in an operating synchronous machine at the Queensland University of Technology (QUT) test facility. Results obtain show that the self-consistent model is fast and accurate in determining the effective critical current of superconductors in such applications. Insights from the modelling assist with the design and optimisation process of the superconducting coils, selection of coil voltage sensing locations to detect the risk of quench and establish safe margins for the superconductor operating current.

Sudden-Discharge Cycling Characteristics and Millisecond Dynamic Behaviors of a HTS Stainless-Steel Insulated Double-Pancake Coil With Thin Copper Plates

We have tested an intermediate-sized HTS stainless-steel (co-wound) insulated double pancake coil ( $\sim$ 132 turns per coil) and discovered dynamic effects during sudden discharge on both millisecond and second scales. For soft break, the power supply is turned off suddenly but the shunt resistor (164 milliohm) is still connected; for hard break, a high voltage contactor is used to suddenly open the circuit so that the leads are completely open. Hard break is dangerous to traditionally insulated LTS magnets, but the SS-insulated HTS DPC retained integrity over $>$ 80 charge-discharge cycles. Voltage decay curves for soft and hard breaks are studied on short and long timescales. On long timescales, we observe the expected exponential decay and measure the time constant to calculate contact resistance. On short timescales, we observe highly dynamic effects: a hyper-exponential decay suggesting that the inductance or contact resistance is suddenly changing with time, likely an indication of the $RL$ circuit forming. Moreover, peak voltage immediately (ms) after the sudden discharge were collected and compared. It is found that the peak voltage in soft-discharge is proportional to the applied current, while the those in hard-break has an unique pattern and tends to stay flat even though the applied current increases.