High-temperature Superconducting Pancake Coils Research Articles

Analysis of a No-Insulation HTS Pancake Coil Including Multiple Resistive Joints

One of the technical issues in the application of High Temperature Superconducting (HTS) tapes to magnet technology is the need to wind long tape sections with uniform electric properties along the length. It has been reported that a No-Insulation (NI) coil can properly work even in the presence of defective super-conductive regions, with a minimal drop of performance as com-pared to its “defect-free” counterpart. This could open up the possibility of manufacturing coils by jointing together several tape segments of limited length, lowering the conductor cost. In this work, a single pancake NI HTS coil is wound using several tape segments cut from the same lot, jointed together. The electrical resistance of each joint is set independently, realizing either high or low resistance joints. Thus, the coil is designed to include multiple defective sections at specific locations. The coil is refrigerated by conduction-cooling, and tested at different temperatures and charging rates. The coil instrumentation allows measuring the voltage over the whole winding, as well as the magnetic field in the central bore. The measurements are used to study the defect-irrelevant behavior of the coil. A simple equivalent lumped parameter circuit is applied to derive the coil effective parameters and analyze its electromagnetic behaviour.

The Influence of Metal Plates on Quench Protection of High Temperature Superconducting Pancake Coils

Quench has always been an urgent problem for high temperature superconductor (HTS) magnets. Fast discharge of the coil current is an effective method to protect the HTS magnet from quenching. In HTS magnets, metal plates are originally used to provide conduction cooling and mechanical support for the coil. During fast discharging operations, the metal plates can rapidly absorb part of the magnetic energy stored in the HTS coils through electromagnetic coupling. Previous studies are based on copper plates. This paper studies the influence of several promising metal materials on the discharging process of the HTS coils coupled with metal plates by experiments and simulations. The results show that in the explored resistance range, the higher the electrical conductivity of the metal plates, the better the acceleration of the current attenuation of coil at the early stage of discharging process. The effects of different metal plates on accelerating current attenuation of the coil are ranked as follows: SS 304L < Al 6061-T6 < Al (RRR = 30) < Cu (RRR = 30) < Au < Cu (RRR = 300) < Ag. However, Cu (RRR = 300) plates can absorb more energy from the HTS coil than the Ag plates during the discharging process. Copper with higher purity (Cu (RRR = 300)) and silver are promising alternatives for metal plates of HTS magnets. Generally, a fast current drop is always the most effective method for avoiding tape turn-out during a local quench; therefore, silver with the highest electrical conductivity is the best choice in view of quench protection.

The Effects of Ferromagnetic Disks on AC Losses in HTS Pancake Coils with Nonmagnetic and Magnetic Substrates

Performance improvement of high-temperature superconducting (HTS) pancake coil is of paramount importance for its application in electrical facilities. In this paper, the utilization of ferromagnetic disks including weak magnetic material and strong magnetic material is investigated through numerical simulations for the aim of reducing the AC loss. Both the nonmagnetic and magnetic substrates are considered for the rare-earth-barium-copper-oxide coated conductor tapes wounded into the pancake coil. In order to analyze the influences of ferromagnetic disks on the maximum allowable currents and AC losses of nonmagnetic substrate-based coil and magnetic substrate-based coil, a self-consistent model based on the A-formulation is adopted to calculate the critical current and a 2D axisymmetric model built on the H-formulations is established to calculate the AC loss. According to the simulation results, it is found that ferromagnetic disks especially the strong magnetic disk are slightly detrimental to the critical currents of the pancake coils. Nevertheless, the presence of large background field weakens the effects of ferromagnetic disks. The simulation results also indicate that strong magnetic disks achieve about 50% reduction of the AC loss in the nonmagnetic substrate-based coil, and the AC loss of the magnetic substrate-based coil can be increased or reduced with strong magnetic disks depending on the applied current amplitude. The utilization of strong magnetic disks leads to the enhancement of the AC loss in the magnetic substrate-based coil at lower current and to the reduction of it at larger current.

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.

Fast Algorithm for Evaluating Critical Current of High-Temperature Superconducting Pancake Coil

Second-generation (2G) high-temperature superconducting (HTS) tapes are now capable of carrying very high transport current and promising for a wide range of applications. The critical current of HTS coils is important for applications, such as superconducting electric machines, superconducting magnetic energy storage, and superconducting magnets. Therefore, precisely and quickly calculating critical current of HTS coils is very important for designing HTS devices. This paper provides a fast algorithm for evaluating critical current of HTS pancake coil. The fast algorithm is realized through a stationary model, which is based on finite element method (FEM) software. The stationary model means that the model is solved by stationary study instead of time-dependent study. To validate this method, a pancake HTS coil was wound and its critical current was measured. Meanwhile, an axial symmetric stationary model was built according to the geometry of the measured HTS coil. By comparing measured and calculated results, the effectiveness of the stationary model was demonstrated. Moreover, the stationary model is compared with H formulation model. The calculated results by the two models are nearly the same. However, by using stationary calculation, the stationary model can remarkably speed up the computational process. Due to the advantage of calculating speed, the stationary model can be used to characterize and design large-scale HTS applications.

The Development of HTS Solenoid Lens for Electron Microscope

The electron microscope has higher resolving power than a light microscope and can reveal the structure of smaller objects. This paper describes the development of the lens of an electron microscope with high-temperature superconducting (HTS) solenoid. The lens consists of double Bi-2223 HTS pancake coils and a ferromagnetic container. The cryogen-free HTS coils are refrigerated by a cryocooler and operated at the temperature of 43 K. The diameter of the warm bore is 10 mm, and the peak central magnetic field is about 2 T.

Persistent Current Mode of a 1-T-Class HTS Pancake Coil for NMR/MRI Applications

We had proposed a winding method, the so-called “wind-and-flip,” which enables a persistent current-mode operation of a high-temperature superconducting (HTS) pancake coil. Two pancake coils had been fabricated using this method and tested in a liquid nitrogen bath with an external magnetic field. The result showed that the time stability was not good enough to use for magnetic resonance imaging or nuclear magnetic resonance when they were magnetized by field cooling at 77 K. In this paper, the test results of a stacked HTS pancake coil for a center magnetic field of 1 T are presented. The coils were cooled to 20 K by conduction cooling using a Gifford-McMahon cryocooler. We measured the time stability of the persistent current-mode operation at 20 K, which shows a strong possibility of a high-field HTS magnet using this method.

Wireless Power Transfer Using High Temperature Superconducting Pancake Coils

It is well known that the efficiency of a wireless power transfer (WPT) system depends on the resistance of the transmitting and receiving coils, especially for the case of low frequency. Since the ac loss of high temperature superconductors (HTS) is much lower than that of conventional conductors, using high temperature superconductors for WPT system is a good way to increase power transfer efficiency. Here, several kinds of WPT experiments were designed and conducted by use of HTS and conventional conductor (copper) coils, respectively. The results are presented and analyzed. It is shown that the behavior of HTS WPT system is similar to that of a conventional WPT system, but the efficiency of WPT system is much higher by use of HTS coils and HTS coil is more suitable for being as a transmitting coil than as a receiving one.

Measurement and Simulation of Magnetic Field Generated by Screening Currents in HTS Coil

In coils wound with high-temperature superconducting (HTS) tapes, large screening currents are induced in the tape by the radial component of the magnetic field. Furthermore, when a transport current is supplied to the HTS tape under a magnetic field, the total current (which consists of the transport and screening currents) flows inhomogeneously. The magnetic field generated by the screening currents affects the magnetic field distribution in cyclotron applications. Investigating the effects of the screening currents on the reduction, hysteresis, and drift of the magnetic field in the HTS coil is therefore necessary. In our previous studies, we have developed a novel simulation code for a three-dimensional electromagnetic field with the aim of clarifying these effects. In the present study, we measured the reduction and hysteresis of the magnetic field generated by the screening currents for a small model of HTS pancake coil. Additionally, we numerically investigated the influence of the screening current in the HTS coil on the spatial and temporal behavior of the magnetic field using our developed simulation code, which considers the nonlinear voltage-current characteristics.

Stability analysis of an insulationless HTS pancake coil under the magnitude of external magnetic field

An HTS (high temperature superconducting) coil without insulation has been investigated since a metallic insulation was suggested in the mid-1980s. The advantage of an insulationless HTS pancake coil is that it is more stable than an insulated HTS pancake coil. This paper focuses on the various characteristics of the insulationless HTS pancake coil related with stability, especially under the external magnetic field. Because HTS pancake coil may be influenced by the external magnetic field in a real operational environment of electrical devices. First, charge-discharge test was performed for the characteristics evaluation of the insulationless HTS pancake coil as compared with insulated HTS pancake coil in liquid nitrogen at 77 K. Based on the experiment results, characteristics evaluation of the insulationless HTS pancake coil was implemented under the external magnetic field. In order to carry out the experiment, we have fabricated a cylindrical solenoid coil to apply the magnetic field. The various characteristics of the insulationless HTS pancake coil were evaluated for charge-discharge and over-current conditions. This paper proves that the insulationless HTS pancake coil is more stable than the insulated HTS pancake under the external magnetic field.

The Characteristics of the Normal-Zone Propagation of the HTS Coils With Inserted Cu Tape Instead of Electrical Insulation

In the case of motors and generators, the benefits of using superconductors could be represented by the reduction of 50% in both losses and sizes compared to the conventional machines of the same rating. The high temperature superconducting (HTS) coils wound with HTS conductors have very low normal zone propagation velocity (NZPV) and complicated quench behaviors because the conductors have a low thermal conductivity and large specific heat. According to these reasons, in case of HTS coils and magnets, it is difficult to expect the self-protection like as low temperature superconducting (LTS) coils. In this paper, we suggest a method which inserts the Cu tape between turn-turn winding in the HTS coil instead of electrical insulation to improve the self-protection ability of HTS coils. Because the thermal and electrical contacts along the transverse direction are enhanced by inserted Cu tape instead of insulator and the whole thermal stability of HTS coils would be increased. To clarify the quench behavior of this suggested coil, we investigate the normal-zone propagation properties of the fabricated HTS pancake coils wound with 2G conductor and Cu tape under the cryocooled operation using GM cryocooler. The effects of inserted Cu tape and the improved the self-protection ability for the suggested HTS coil will be discussed.

The study on improving the self-protection ability of HTS coils by removing the insulation and lamination of the various metal tapes

The high temperature superconducting (HTS) coils have very low normal zone propagation velocity (NZPV) and complicated quench behaviors. Because of these reasons, it is difficult to expect the self-protection. In this paper, two methods are suggested to improve the self-protection ability of HTS coils. One is to remove the insulation between turn–turn winding in HTS coils to enhance the thermal and electrical contacts along transverse direction and to increase the whole thermal stability of them. The other is Cu or Brass tapes are wound with HTS tape wire instead of the insulation to improve the thermal, electrical and mechanical stability of the HTS coils. To clarify the quench behaviors of the suggested coils, the normal zone propagation properties of the fabricated non-insulated HTS pancake coils were shown by measured voltage and temperature traces during the quench as a function of the operating temperature. And the characteristics of the normal transition and amount of the shared current to adjacent layers were quantitatively measured by a Hall sensor located at the center of HTS coils to measure the change of the self-magnetic field by operating current. The current sharing behaviors in the non-insulated HTS coil and the coil with Cu tape inserted were observed. The minimum quench energy (MQE) of the HTS coil with Cu tape inserted was the largest in the tested coils. It means that the HTS coil with Cu tape inserted had the highest transient stability against the thermal disturbances among the tested coils and the improvement of self protection abilities was proved.

Medium factors of electrical insulation systems in high temperature superconducting power apparatus with coil structure for equivalent ac withstand voltage test at room temperature

Equivalent insulation test voltage at room temperature of high temperature superconducting (HTS) power apparatus with coil structure for power frequency withstand voltage tests is discussed based on medium factors of insulation weak parts in electrical insulation elements of the HTS power apparatus. Uniform and non-uniform field gaps, triple junction and solid insulator surface are selected as the insulation weak parts, and their medium factors are theoretically and experimentally calculated as the ratio of partial discharge (PD) inception or direct-breakdown voltages in liquid nitrogen or cryogenic nitrogen gas to those in atmospheric air at room temperature. The results show that the type of insulation weak part affects the relationships between the medium factor and the gap length between electrodes as well as the state of cryogenic surrounding medium. These medium factors are compared with the medium factor of a coil-to-coil insulation element of HTS pancake coils that contains some of the insulation weak parts, and it is verified that the medium factor of the coil-to-coil insulation element can be obtained by using that of triple junction or solid insulator surface. Finally, a selection method of a significant unique medium factor is discussed to determine the equivalent insulation test voltage at room temperature for the confirmation of the insulation reliability of all the parts of the HTS apparatus.

Thermal stability of a cryocooler-cooled HTS pancake coil wound with Bi2223 tape

We fabricated and tested a cryocooler-cooled high temperature superconducting (HTS) single pancake coil wound with a Bi2223 tape in order to investigate thermal stability of the HTS coil. The inner diameter of the coil is ∅100 mm and the outer diameter is ∅257 mm. The coil is impregnated with epoxy resin and a conduction plate made of aluminum is fixed on the surface of the coil through an AlN plate for insulation. We measured thermal runaway currents at several temperatures. In this paper the thermal stability of the coil is discussed.

Quench propagation properties in HTS pancake coil

In this paper, the authors present the results of experiments and computer simulations to make clear the quench propagation properties in Bi-2223/Ag superconducting multifilament tape. They developed a computer code based on the two dimensional finite element method (2-D FEM). Computed voltage and temperature traces during a quench agree well with the experimentally recorded voltage and temperature traces. Good agreement validates the computer code, making a useful tool in developing protection strategies for high-temperature superconducting (HTS) coils. Therefore, they simulate the quench process in a single pancake coil wound with Bi-2223/Ag superconducting multifilament tape with background magnetic field up to 10 T at 20 K using the developed computer code. The quench propagation properties and stability in HTS coils are discussed and compared with those in low-temperature superconducting (LTS) coils.