YBCO Coil Research Articles

Delamination Properties of YBCO Tapes Under Shear Stress Along the Width Direction

In YBCO coils, the increased transport current density and magnetic field will induce larger electromagnetic force, and YBCO coated conductors (CCs) may suffer interlayer delamination by shear forces, which may give rise to performance degradation of the conductor. The delamination characteristics of YBCO CCs under shear stress should be well understood for better application of YBCO tapes to high-temperature superconducting equipment. This paper describes the experimental results for different delamination strength and critical current $I_{c}$ degradation behaviors of the YBCO CCs under shear stress at the unslit edge, center, and slit edge of the conductor. The results show that the average delaminated shear strengths of 4.9, 5.9, and 3.7 MPa are recorded for the unslit edge, center, and slit edge at room temperature, whereas 6.8, 7.2, and 5.1 MPa are recorded for the unslit edge, center, and slit edge at liquid nitrogen. The different degradation behaviors of $I_{c}$ under shear stress are related to the location where delamination occurs in the sample. According to the finite-element method analysis for the stress distribution of the conductor under shear stress, stress concentration leads to initial delamination of the conductor. The decrease of moment arm is suggested to reduce the effect of moment on the delamination test.

Design and Experimental Study of a Model Magnet for Spiral-Sector FFAG Accelerators

A project to develop fundamental technologies for accelerator magnets using high- $T_{c}$ superconductors is currently underway. In this project, conceptual design studies of fixed-field alternating gradient (FFAG) accelerators for carbon cancer therapy and for accelerator-driven subcritical reactors have been carried out. To establish technologies for cryocooler-cooled high- $T_{c}$ superconducting (HTS) accelerator magnets wound with coated conductors, a downscaled model magnet system for a spiral-sector FFAG accelerator has been developed. A model magnet consisting of multiple coils with complicated shapes, including a negative-bend part or a 3-D bent part, was designed to demonstrate the generation of an asymmetrical magnetic field on the midplane of the magnet bore. One issue is how to wind coils for the accelerator magnets by using tape-shaped coated conductors, which have anisotropic bending flexibility between the flatwise and edgewise directions. Another issue is the effects of the characteristic features of the coated conductors on the accuracy of the magnetic field. The field precision of coils wound with coated conductors is considered to be influenced by the dimensional accuracy of the YBCO tapes and coils, as well as the magnetization of the HTS layer. This paper describes the design and fabrication of YBa2Cu3Ox coils for the model magnet, as well as disturbance factors of the field of the HTS magnet and the effect of dimensional variations of the YBCO coil on the magnetic field.

Quench Protection of YBCO Coils: Co-Winding Detection Method and Limits to Hot-Spot Temperature

A quench detection method of a high-temperature superconducting (HTS) coil using a cowinding method is studied. It is experimentally shown using a small YBCO test coil that the cowinding method is highly effective in canceling the inductive voltage component and can detect a quench occurring in an HTS coil with high sensitivity, compared with a commonly used bridge-balance method. An HTS coil is damaged by a quench when hot-spot temperature exceeds a certain threshold. The authors also experimentally investigated conditions to keep the hot-spot temperature of coils wound of YBCO tapes below a safe level, using the highly sensitive cowinding quench detection method.

A flux pumping method applied to the magnetization of YBCO superconducting coils: frequency, amplitude and waveform characteristics

This letter presents a flux pumping method and the results gained when it was used to magnetize a range of different YBCO coils. The pumping device consists of an iron magnetic circuit with eight copper coils which apply a traveling magnetic field to the superconductor. The copper poles are arranged vertically with an air gap length of 1 mm and the iron cores are made of laminated electric steel plates to minimize eddy-current losses. We have used this arrangement to investigate the best possible pumping result when parameters such as frequency, amplitude and waveform are varied. We have successfully pumped current into the superconducting coil up to a value of 90% of Ic and achieved a resultant magnetic field of 1.5 T.

Design of a superconducting linear synchronous motor with YBCO coil magnet

The forces of the flat single-sided linear motor with four superconducting coils on the excitation system were presented by experimental method. The influences of the coil height and number of turns on the forces were investigated by the simulation method. Although the maximum thrust increased monotonically with the value of turns, the corresponding increment of the thrust decreased with the value of turns. The thrust reaches a certain saturation value with increasing the height of the superconducting coil. Continuing to increase the height of the coil could lead to an increase of the normal force. The superconducting linear motor is finally designed with the capability to generate thrust of 1 kN/m.

Simulation of AC Loss in Small HTS Coils With Iron Core

AC loss is one of the key problems in HTS applications. This study focuses on the AC loss in small Bi-2223/Ag and YBCO coils, with an emphasis on the presence of an iron core. HTS coils with an iron core can be used in a number of applications, including induction heaters, motors, fault current limiters, controllable reactors and transformers. The major concern of the study is the interaction between the iron core and the HTS coil. This study first provides a brief introduction of the resistivity-adaption algorithm (RAA) method, which is used to simulate the field penetration, based on a given flux motion theory. A general model is then proposed, with a discussion on the effect of the iron core. Results showed that a well-designed iron core is beneficial for the AC loss reduction.

Impact of Trapped Helium Gas Bubble in Liquid Helium on the Cooling in High Magnetic Field

Liquid helium is used as the coolant for high field superconducting magnets or samples tested in the high magnetic field at the National High Magnetic Field Laboratory. When the magnetic field is ramping up, the boil-off helium gas generated by heat losses in the coil and from external environment of the cryostat can be trapped in the liquid helium bath in a region where Bz · dBz/dz is less than -2100 T 2 due to the weakly diamagnetic property of helium. It results to the surfaces of the magnet or samples being covered partially or even fully with the accumulated helium gas instead of liquid helium. The very low thermal conductivity of helium gas may cause an inefficient cooling for the magnets or samples. In the beginning stage of YBCO coils development, the trapped gas bubble caused a temperature increase in the region of trapped gas bubble and prevented the test coils ramped up to a higher current. This raised an issue for the cooling of the 32-T magnet, which is to be cooled in a liquid helium bath, because the trapped gas bubble may cause inefficient cooling of the magnet in this gas bubble region. Relevant tests were performed, and the trapped gas bubble was observed in recent tests of the 32-T magnet prototype coils. The impact of the gas bubble on the cooling of the 32-T YBCO prototype coils was tested, and the test results are presented.

Design and Fabrication of a Hybrid HTS Magnet for 150 kJ SMES

This paper describes design and fabrication of a hybrid high temperature superconducting (HTS) magnet for a 150 kJ superconducting magnetic energy system. The hybrid HTS magnet, which employs both BSCCO taps and YBCO taps, is composed of 18 double pancake coils (DPC). Six DPC made of BSCCO are respectively installed on the top and bottom with an inner diameter of 240 mm, an outer diameter of 396 mm, and height of 11.5 mm. Six YBCO coils are mounted in the middle of the magnet with the inner and outer diameters of 264, 396 mm and height of 12.1 mm. Copper plates with thickness of 1 mm are arranged between DPC for the cooling of the heat, and epoxy resin plates with thickness of 0.2 mm are arranged between coil and copper plate for the insulation. The inductance of the magnet is about 11.06 H and the resistance is 169.5 Ω at the room temperature. In order to evaluation the performance of the hybrid magnet, each solidified DPC and the assembled magnet were tested in a bath of liquid nitrogen at 77 K. The resistances of 17 joints were also measured and evaluated by the standard four-probe method.

Quench Studies of YBCO Insulated and Noninsulated Pancake Coils

As a result of extremely high upper critical fields Bc2, high temperature superconductors (HTSs) have the potential to be used as high field insert coils in magnet systems where the background field is provided by low temperature superconductors (LTS). However, due to low quench propagation velocity in HTS as compared to LTS, the issue of developing a fast and reliable quench detection and protection scheme for such magnet systems remains a serious challenge. In order to provide a stable operation for HTS inserts, quench studies on YBCO coils were performed. Three single pancake coils were wound with 1.5 m of YBCO coated conductor provided by Superpower. They were equipped with quench initiation heaters and instrumented with type K thermocouples and voltage taps to monitor the development of the quench. Two of the coils were additionally insulated with a thin layer of polyimide film laid on top of YBCO CC. The heat propagation with no transport current was studied at 77 K and 4.2 K without an external magnetic field. Further tests included quench propagation at 77 K. The measurements involved voltages developed around the quenched region, hot spot temperature, and determining the threshold voltage to ensure the successful operation of the quench protection system.

Experiment and Numerical Simulation on Quench Detection in Cryocooler-Cooled YBCO Coil for SMES Application

In the real application of high-temperature superconducting (HTS) coils to a superconducting magnetic energy storage (SMES) system, coated conductors are cyclically subjected to tensile strain due to electrical charging and discharging. A quench is not only caused by failure in the power supply and cooling system but also could be induced by local deterioration of the superconducting characteristics because of the cyclic strain during operation. In such a local deterioration case, the conventional detection method using voltage signal for low-temperature superconducting (LTS) coils is not applicable for the HTS coils because of the significantly slow velocity of normal-zone propagation. Furthermore, the voltage detection method is considered to be extremely difficult because the noise of the converters and other equipment is much larger than the local normal-zone voltage of the HTS coils. Therefore, a new quench-detection method for HTS coils is required. In our previous studies, a current detection method was developed for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of four electrically insulated coated conductors. In the present study, experiments and numerical simulations were carried out on a double pancake model coil that assumes real SMES operation to verify the validity of the current detection method.

Influence of Winding Accuracy on Magnetic Field Distribution in YBCO Pancake Coil for Cyclotron Application

In recent years, YBCO high-temperature superconducting (HTS) tapes have been expected to be applied to devices in power systems or in other industrial applications, owing to their rapidly improving quality and productivity. Our objective is to develop an HTS cyclotron with a compact body, high efficiency of output power, and high performance in heavy particle radiotherapy. In such an application, extremely high precision is required in the range 0.01% to 0.1% of magnetic field, both spatially and temporally. An isochronous field along with an azimuthally varying field is generated by an air-core pancake coil system during construction. Therefore, developing a technology for extremely high coil-winding accuracy is very important. In this study, we constructed an experimental model of a YBCO pancake coil using a high-precision coil winding machine. Then, the winding error in the radial and axial directions was experimentally evaluated using a pair of laser displacement meters and a surface roughness tester. We also developed a numerical simulation model to analyze the influence of winding error on the magnetic field distribution. The relationship between the winding error and the magnetic field of the meter-class YBCO coil assuming a real system was also evaluated by numerical simulation.

Output Characteristics of Linear Switched Reluctance Motor with YBCO Tape Conductors

This paper presents output characteristics of a linear switched reluctance motor (LSRM) with excitation windings wound by using high-temperature superconducting (HTS) tapes. In a double-sided LSRM, Bi-2223 or YBCO tape conductors are used for the excitation windings. The characteristics of the LSRM are obtained by a finite element method analysis. We can obtain large thrust compared with a conventional LSRM by using YBCO tape conductors. Also, the effect of a configuration of the YBCO coil on the thrust is calculated. We discuss a suitable coil configuration for the average thrust upgrading.

Development of a Combined YBCO/Bi2223 Coils for a Model Fault Current Limiter

Recently, the high-temperature superconducting (HTS) devices and superconducting technologies have become widespread in the field of electric power. The 2G wires are expected to be used in future power applications because it has high properties in high temperature and high magnetic fields. One of the most promising applications for HTS coils in electrical engineering is for high-temperature superconducting fault current limiter. In this paper, the design, fabrication, and experiment of the combined YBCO/Bi2223 coils were described. Nine coils wound with 122 m YBCO tapes and three coils wound with 376 m Bi2223/Ag tapes, were assembled into the combined coils. The YBCO coils were in a noninductance configuration and the Bi2223/Ag coils were in a solenoid configuration. For getting higher resistances and rapid response to the fault current, the structure of the combined coils, the heat stability, and the insulation of the YBCO coils were the main targets in the development of the YBCO coils.

Quench Detection Method for Cryocooler-Cooled YBCO Pancake Coil for SMES

In this study, we focus on a quench detection method for a cryocooler-cooled double pancake coil wound with an YBCO bundle conductor for superconducting magnetic energy storage. Because the local normal-zone propagation velocity is significantly slower in a high-temperature superconducting coil than that in a low-temperature superconducting coil, it is difficult to detect the local normal transition using a voltage between both coil ends. In the application of an YBCO coil for superconducting magnetic energy storage, quench detection is considered to be more difficult because of the noise of the converter or other equipment. Therefore, the normal transition and quench must be detected using some other method. A current detection method, which measures the current redistribution in the YBCO coil with hall probes, was proposed and verified to be able to determine the normal transition by the numerical simulations in the previous paper. In this study, on the basis of the current detection method, the experiments and numerical simulation were performed on a cryocooler-cooled small model double-pancake coil wound with an YBCO bundle conductor composed of four electrically insulated YBCO tapes. As the results, the current detection method was verified to be effective in detecting the local normal transition; Hall probes were used to measure the magnetic field and to estimate the current in individual YBCO tapes making up the bundle conductor.

Quench Propagation in YBCO Pancake: Experimental and Computational Results

High-temperature superconductors are promising materials for future applications such as high field magnets thanks to their ability to carry high current densities. Nevertheless, their protection still remains a key issue mainly due to the slow velocity of quench propagation. To understand the quench behavior of a YBCO coil, a simulation code has been developed using the CASTEM-CEA finite element software. Simulations can be performed considering constant current and magnetic field. Results of those simulations will be displayed and a way for improving the protection by adding a stabilizer will be discussed. Two well instrumented YBCO coils were fabricated in order to obtain experimental data on quench propagation in pancake configuration. Their design and some measurements are reported in this paper along with another experiment on a double pancake made by and tested at CNRS Grenoble. Finally, we compare the numerical and experimental results and discuss the accuracy of our simulations.

Quench Considerations and Protection Scheme of a High Field HTS Dipole Insert Coil

The large scale particle accelerators of the future in the 20 T regime are enabled by high temperature superconducting magnets. The dipole magnets needed in new high-field accelerators can be constructed with an YBCO insert and a Nb3Sn outsert. Such a configuration makes the quench analysis and magnet protection challenging because the quench behavior in both of these coils is different and there is very strong inductive coupling between the coils. The Nb3Sn coil is characterized by high energy and current and relatively fast quench propagation velocity. However, quench propagates slowly in YBCO coils because of typically wide spread large temperature margin. Currently, in the EuCARD project, a European collaboration is targeting to construct a small-scale high field YBCO-Nb3Sn hybrid magnet. In this paper, we scrutinize quench in the YBCO insert. We utilized an approach based on a solution of the heat diffusion equation with the finite element method. Additionally, we present a protection scheme for the coil.

Comparison Between the Fault Current Limiting Performance of Bi-2212 Bifilar Components and 2G YBCO Coils

Short-circuit tests with commercial Bi-2212 and with a YBCO 2G tape components were performed. The Bi-2212 bulk coil has been recently improved by the manufacturer, exhibiting higher homogeneity and better transport properties than previous ones. Such bulk components consist of a shunted bifilar coil (Ic = 400 A at 77K). The YBCO 2G tape component consist of two antiparallel 2G shunted coils (Ic = 1200 A at 77 K). Fault current limiting tests were performed under asymmetrical prospective currents varying from 1 kArms (2.7 kApeak) to 30 kArms kArms (80 kApeak). The source voltage (Vo) was also varied in order to evaluate the influence of Vo on the limited current (138-400 Vrms). For the same Vo value (200 Vrms), the Bi-2212 components limited prospective currents as high as 80 kApeak to about 6.5 kApeak (first peak) and less than 1.5 kArms , whereas the YBCO 2G tape component limited the same prospective currents to less than 5 kApeak (first peak) and less than 2.5 kArms. The YBCO 2G component presents a faster actuation than the Bi-2212 component.

Development of a High-Efficiency Conduction Cooling Technology for SMES Coils

YBCO(YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> ) wires have superior characteristics such as high energy densities, thus allowing the fabrication of large-scale, high power-output superconducting equipment applications. A high voltage is desired to achieve a large power output from a superconducting coil, and is realized by improving the insulating properties of the coil with the introduction of solid insulation. However, because high temperature superconducting wires have high critical temperatures and thermal tolerance compared to metal-based superconducting wires, the benefits offered allow the insulated superconducting coil to be directly cryocooled. In a helium circulation cryocooling system, the aluminum-fabricated transmission heat shields are coated around the surface of the coils to cryocool equally, and are embedded into the transmission heat shields with the conducting pipe allowing helium gas to circulate. We manufactured a conduction cooled model coil of 600 mm class diameter, and have verified the ability to realize a cooling of more than 3 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for coil surface heat flux at an operating temperature of 20 K using the demonstration cooling system. In addition, we have also evaluated the electric insulation performance, which is in contradiction to the cooling performance. The conduction-cooled YBCO coil withstood up to DC 13 kV voltages at 20 K.

YBCO and Bi2223 Coils for High Field LTS/HTS NMR Magnets: HTS-HTS Joint Resistivity.

This paper presents our latest experimental results on high-temperature superconducting (HTS) splice joints for HTS insert coils made of YBCO and Bi2223, that comprise a 1.3 GHz low-temperature superconducting/HTS nuclear magnetic resonance magnet currently under development at Francis Bitter Magnet Laboratory. HTS splice joint resistivity at 77 K in these insert coils must be reproducible and < 100 nΩ cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice·joint samples were fabricated, and their resistivity and I c were measured at 77 K. First, we describe the joint splicing setup and discuss the parameters that affect joint resistivity: pressure over joint surface, solder, and YBCO spool batch. Second, we report results on YBCO-YBCO joints at 77 K in zero field. Measurements have shown that spool batch and solder are primary sources of a wide range of variation in YBCO-YBCO joint resistivity. By controlling these parameters, we expect to reproducibly achieve HTS-HTS resistive joints of resistance < 100 nΩ cm2.

高温超電導コイルの安定性評価基準とクエンチ検出・保護

Since high-temperature superconducting (HTS) conductors have a greater volumetric heat capacity at operating temperatures envisaged for practical applications, the possibility of quench caused by a mechanical disturbance such as friction due to wire movement is much lower than that for low-temperature superconducting (LTS) coils. In real applications such as superconducting magnetic energy storage (SMES) systems, however, electrical charging and discharging are repeated and the HTS conductor may deteriorate locally due to cyclic mechanical strain. We also have to consider the possibility of quench caused by a failure in the power supply, cooling system or similar problems. Therefore, a protection scheme assuming quench is also required for HTS coils. In this report, we describe a method of determining the appropriate stabilizer thickness of the YBCOcoated conductors from the perspective of the quench protection of the YBCO coil, which dumps the stored magnetic energy in the coils via external dump resistance. We have been developing a quench detection method for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of an electrically insulated YBCO-coated conductor. Since the normal-zone propagation velocity is quite slow in HTS coils, the detection of a non-recovering normal zone using a voltage signal is quite difficult. Therefore, quench must be detected using some other method. We numerically show that quench in the coil can be detected by observing the transposition of current in the bundle conductor caused by local normal transition. Furthermore, we show the results of experiments on a small model coil wound with a four-YBCO-strand bundle conductor to examine the possibility of a quench detection method by observing the transposition of current in the bundle conductor. This work was mainly supported by the New Energy and Industrial Technology Development Organization (NEDO) as part of the technological development of yttrium-based superconducting power equipment.