High-temperature Superconducting Coil 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.

Thermal Conductivity of NI HTS Coil Fabricated by Diffusion Bonding Technique

The no-insulation (NI) winding technique shows superior thermal protection performance when quenching of a high- temperature superconducting (HTS) coil occurs compared to the insulation winding technique. However, the thermal conductivity of an NI HTS coil are not clearly known because the HTS tape has various surface conditions such as roughness and oxidation. In this study, the diffusion bonding technique was applied to predict the thermal conductivity of an NI HTS coil, and the core of this technique is to remove the interface between turns by applying Ag diffusion bonding. To investigate the thermal conductivity of an NI HTS coil, an NI HTS coil was fabricated using a diffusion bonding technique, and a stainless-steel 304 block was fabricated to verify the experimental results. The thermal conductivity of the NI coil was measured using a conduction cooling experimental apparatus by applying heat loads to the outside of the coil. The measured thermal conductivity was compared to the values calculated using the thermal resistance circuit at temperatures from 20 K to 77 K.

Influence of Copper Rings and Plates on Eddy Losses of No-Insulation High Temperature Superconductor Coil in Synchronous Machines

No-insulation (NI) high temperature superconducting (HTS) machine is a semi-superconducting machine with improved thermal-stability and power density, in which the rotor windings are replaced with NI HTS coils. However, the AC background field from stator windings leads to significant eddy current in NI HTS windings. The induced eddy current and loss affect the operating efficiency and safety of HTS machine. Traditionally, copper discs and rings are used for conduction cooling and mechanical support of NI HTS coil. In this paper, the influence of copper discs and rings on reducing the eddy current and loss of double pancake NI HTS coil is studied. Three structures of copper discs, copper ring, and both copper discs and ring are analyzed, by both simulation and experiment. The results show that all these three structures of copper shielding can effectively reduce the eddy loss on NI HTS coil within the HTS machine operating temperature of 20–30 K. The combination of copper discs and ring has the best shielding performance.

Test and Analysis of Laboratory-Scale D-Shaped Co-Wound No-Insulation HTS Single Pancake Coil for TF Coil Application

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.

Fabrication and Test of a 1:4 Scaled HTS Coil for the Defocusing Magnet of 2 GeV FFAG Accelerator

The basic design of a 2 GeV fixed field alternating gradient (FFAG) accelerator was proposed by China Institute of Atomic Energy (CIAE) in 2019. A preliminary study of high temperature superconducting (HTS) magnet for the 2 GeV FFAG accelerator lattice scheme was carried out, considering the beam loss of high energy proton beams and energy efficiency of the main machine. In order to demonstrate the feasibility of manufacturing a spiral-shaped HTS magnet with concave edges based on ReBCO tapes, a 1:4 scaled HTS model magnet is being developed in CIAE. First, the design and fabrication of the 1:4 scaled prototype HTS magnet with concave edges will be described in detail. Then, each double pancake (DP) was tested by liquid nitrogen (LN) immersion. The terminal voltage and magnetic field variation were measured and simulated. Finally, the whole assembled HTS magnet was cooled down to 23 K using helium gas pipeline contact cooling and operated on 300 A for over 24 hours. The central magnetic field reached 719.5 mT, without iron. More detailed electromagnetic characteristics of the magnet during the excitation will be illustrated, which is much helpful for the fabrication of the final full-size HTS magnets.

Study of Refrigerant Circulation System and Cryofan for Cooling High Temperature Superconducting Coil

A circulation cooling system is mentioned as a method for efficiently cooling high-temperature superconducting coils (HTS coil), and the purpose is to keep the temperature of the HTS coil to about 50 K or less. In addition, it is necessary to clarify what kind of change occurs if a room temperature compressor or a cryofan are used for the circulation device as the cooling performance of the circulation cooling system. In this study, we analyzed the cases where multiple HTS coils were cooled, using a room temperature compressor and cryofan as the circulation device, and using helium and hydrogen as the refrigerant, and compared their characteristics. As a result, it was shown that the cooling capacity the system that uses the cryofan is higher than that of using the compressor. Focusing on the refrigerant, it was shown that hydrogen has better cooling performance than helium.

Mechanical and Thermal Analysis of an HTS Superconducting Magnet for an Achromatic Gantry for Proton Therapy

The mechanical and thermal analysis of the superconducting magnets, forming an innovative fixed-field bending section for use in a proton therapy gantry, is presented here. The design concept has a large momentum acceptance, covering the full proton energy range of 70 to 220 MeV with fixed field in the superconducting magnets, and uses a simple magnet geometry based on double-pancake high temperature superconductor coils. The main planned steps for the assembly and fabrication of the magnets are discussed. The mechanical analyses of the magnet, with emphasis on the stress state of the conductor during the powering of the magnet, is here described. The thermal analysis investigating the adequacy of the copper structure surrounding the conductor to extract the heat deposited in the conductor during powering is also discussed in detail.

Numerical and Experimental Investigations of HTS Coils Inductance

In this article, the inductance variation with the current in high-temperature superconducting (HTS) pancake coils is investigated numerically and experimentally. Both dc and ac modeling approaches are developed. The results show that the dc modeling approach, even though it considers the non-uniformity of the current density distribution using a power minimization criterion, is not sufficient to characterize the inductance variation, which is affected by the dynamic behavior of HTS coils. The ac modeling approach reproduces a variation of the inductance similar to the experimental results.

Design, Fabrication and Test of a 1.5 T Cryogen-Free HTS Dipole Magnet for the Heavy Ion Spectrometer

A 1.5 T high-temperature superconducting (HTS) dipole magnet for the heavy ion spectrometer has been fabricated and tested. It mainly consists of four double pancake HTS coils and a warm iron yoke with two cylindrical poles. The gap between the poles is 120 mm. A time projection chamfer (TPC) with the dimension of 100 mm × 100 mm × 100 mm will be placed in the gap. The four coils wound with a 12 mm wide and 0.28 mm thick HTS tape have an inner diameter of 480 mm. Each of the pancake coils has 176 turns. The coils will be cooled down below 10 K by a GM cryocooler and generate a central field of 1.5 T at an operation current of 280 A. In this paper, the design and construction of the HTS magnet are described and the test results are also reported and discussed.

The Effect of Different Copper Discs on the Discharge of Superconducting Coils

High temperature superconducting (HTS) magnets often work at high energy density and have slow quench propagation speed, so a quench will present a serious risk to the safety of magnets. The quench protection method based on the dump resistance can effectively reduce the current and release the energy in the HTS magnets. However, too large dump resistance may cause excessive voltage across the magnets. A quench protection system consisting of dump resistances and metal discs has been proposed. Copper discs are often embedded in HTS magnets for conducting cooling and mechanical support. In the discharging process of HTS magnets, the copper discs can absorb energy from the magnets through magnetic coupling, thus accelerating the current decay of the magnets. This quench protection method is more effective than using dump resistance alone. In this paper, the effect of different copper discs on the discharging process of HTS coils is discussed. Eight types of copper with different residual resistivity ratios (RRR) are applied. The results show that with the increase of the RRR of the copper disc, the current decay rate of the coil increases, and the energy absorbed by the copper disc from the coil increases. The role of different copper discs in the fast quench protection of the coil can be sorted as: RRR = 300 > RRR = 100 > RRR = 80 > RRR = 60 > RRR = 40 > RRR = 30 > RRR = 20 > RRR = 10. The copper disc with RRR of 300 shows the best performance in quench protection of HTS coils.

Current control and tuning in a closed HTS coil

Up to today, a coil made of high temperature superconductor (HTS) usually needs a soldered joint to form a closed circuit. It is very difficult to have the joint fully superconducting. The Ohmic joint will cause Joule loss when the coil carries a quasi-persistent current. As a result, the electromagnetic energy stored in an HTS coil declines with time. We propose an approach to reduce the Joule loss of an HTS coil during the energy storing stage. The principle of the approach is to tune the current of the HTS coil smaller by introducing an iron core into the coil to increase the inductance of the coil. With this approach, the Joule loss on the HTS coil will be significantly reduced and the initially stored energy in the coil will be kept with little attenuation in a relatively long energy storing period. When a larger current is required for some functions, the current can be almost returned to the initial charged value by removing the iron core from the HTS coil. In this paper, we report our results in analytical deduction and experimental verification of this principle. Besides, the current value can be tuned to any value in a certain range by controlling the position of the iron core inside the HTS coil. This feature may be useful for some applications in which the current or magnetic field needs to be adjusted.

Performance evaluation of a superconducting flywheel energy storage system incorporating an AC homopolar motor layout

In this paper, a novel high-temperature superconducting flywheel energy storage system (SFESS) is proposed. The SFESS adopts both a superconducting magnetic bearing and a superconducting alternating current (AC) homopolar motor. The superconducting AC homopolar motor has structural advantages in high-speed operation, however performance of the high-temperature superconducting (HTS) field coil is easily affected by the external magnetic field generated by armature windings in the process of flywheel charge and discharge, and should be paid attention. We established an electromagnetic-thermal co-calculation model of the HTS coil to analyze the coil characteristics in this application. Simulation results demonstrate that the magnetic flux density is small near the central region of the coil and is the largest at the edges of the coil. The temperature rise caused by the change of magnetic field distribution reduces the critical current density of the tapes and increases AC loss. Furthermore, we studied the grid connection control strategy of the SFESS, which adopts double closed-loop control and direct power control, and verified the feasibility of the control strategy by multi software real-time and dynamic co-simulation. Works in this paper give some guidance to further development of a 50 kW·h SFESS prototype. All the configurations of the SFESS, the method of calculating AC loss of HTS field coil, and the control strategy of the SFESS system could be extended to large commercial units.

Fabrication and Test of HTS Magnet for Induction Heating Device in Aluminum Extrusion Processing

This paper reported the fabrication and the test results of the high temperature superconducting (HTS) magnet developed for the aluminum billet heater for aluminum extrusion processing. In this magnet, the HTS coils were combined with iron cores, because the required magnetic field in the working region was about 1T. Two HTS coils wound directly on the convex shape iron cores were fabricated. The HTS coils were made by the no-insulation winding of REBCO tape with the YOROI-coil technique. Each of them was separately installed in an individual cryostat. Two HTS coils with iron cores were disposed oppositely to each other and were magnetically connected by a C-shape yoke placed in room temperature. The HTS coil was cooled down to about 15 K by the GM cryocooler in the initial cooling. The magnetic field of 1.06 T at the center of the heating region was stably generated with the rated current of 200 A. It was also demonstrated that the HTS magnet could be charged up to 300 A without quench. According to the observation of the electromagnetic force direction from the outside of the cryostat, it was confirmed the validity of the force balance design of the magnet.

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.

Modeling of an Axial Field Machine (AFM) With Superconducting Windings

The development of high-temperature superconducting (HTS) electrical machines results in applications of various fields, as they have higher efficiency and power density. Numerical modeling of superconducting electrical machines is usually performed by finite element method based their electrical behavior, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> -Formulation of the Maxwell's equations is one of the models that are used tremendously due to its high flexibility and simplicity. Axial field machine (AFM) is a machine topology with higher efficiency and power density, but with more complexity when modeling. This paper proposed a new 2D model for the superconducting AFM, where the superconducting coil is simulated with a 2D infinitely long model, this model also divides the electrical machine into two parts: the superconducting parts (HTS coils, bulks, etc.) and non-superconducting parts (e.g., conventional conductors, machine structure). This paper compares the results of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H-A</i> Formulation and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T-A</i> Formulation with this approach, which requires appropriate boundary conditions and continuity between the two regions, along with correct modeling of fixed and moving parts of the electrical machine, where the geometry of this model is based on a linear synchronous machine, which means the rotor is moving linearly instead of rotation.

Design of a 60 kA Flux Pump for Fusion Toroidal Field Coils

High-temperature superconducting (HTS) magnets offer a novel pathway to magnetically confined fusion energy generation. Reactor-scale fusion devices require tens of kA of current to generate large toroidal fields for magnetic confinement. Operating HTS magnets at kA-current levels is difficult due to unavoidable joint resistances that require active driving of current. The large footprint and energy demand of a solid-state power supply and multi-kA current leads present significant constraints on the size, cost and efficiency of a fusion tokamak. An alternative approach is to use an HTS flux pump, which can generate and sustain persistent currents in HTS coils at the kA-levels required by fusion magnet systems. Here, we outline the design of a 60 kA transformer-rectifier flux pump for energizing and sustaining an 80 mH HTS fusion magnet. The flux pump is driven by an iron-core transformer with copper primary and HTS secondary windings. This is rectified by switching elements in the superconducting circuit into a uni-directional dc voltage output which charges the inductive load. To charge the load coil to 60 kA in 24 hours, an output dc voltage of 55 mV is needed. However, only 6 mV is required to maintain a long-term steady current of 60 kA. Such a device can significantly reduce the cost and footprint of future fusion reactor designs.

Experimental Study and Frequency Domain Analysis on Metal-Insulation HTS Coil

This work reports a frequency domain analysis on alternating current (AC) response of a metal-insulation (MI) high temperature superconducting (HTS) coil. An 85-turn MI test coil was wound and operated in a liquid nitrogen bath at 77 K. Sinusoidal current was applied to the test coil to measure the AC responses. The test coil’s total voltage and the central magnetic field were measured from 0.01 Hz to 5 Hz. The phase difference between the total voltage and the azimuthal current was calculated from the measured data. To explain the phase difference between the total voltage and the azimuthal current, the additional frequency-dependent resistive component was adopted. The results demonstrate that the discrepancy between the measurement and the simulation results from the lumped-circuit model tends to increase with frequency.

Charging Characteristics of 2G HTS Coils With Insulation, Metal-Insulation, Non-Insulation, and Smart-Insulation Using Circuit Simulations

This study aimed to compare and analyze the output characteristics of second-generation high-temperature superconducting racetrack coils with insulation, metal-insulation (MI), non-insulation (NI), and smart-insulation (SI). First, the output characteristics of each coil were experimentally validated, and circuit equations were examined to analyze these characteristics. The output characteristics were found to be affected by the turn-to-turn resistance. Circuit analysis was used to identify the nature of dependence of output characteristics on turn-to-turn resistance in addition to internal behavior and other mechanisms. In particular, an equation relating the output characteristics of the SI coil with temperature change was derived, and each coil model was simulated using the circuit equation. Then, the simulation results were analyzed, the advantages and disadvantages of each insulation method were discussed, and the validity of the simulation was confirmed. The circuit simulation proposed in this study is expected to facilitate advanced predictions in smart insulation material candidate groups.

Reel-to-Reel Fabrication of HTS CroCo Strands and Test of HTS CroCo Demonstrator Coils

The High Temperature Superconductor (HTS) CrossConductor cable concept (HTS CroCo) was developed by the Karlsruhe Institute of Technology. Meanwhile the HTS CroCo manufacturing process is advanced to produce CroCo strands in a reel-to-reel process, with which a HTS CroCo was produced over 50 meters in length. In order to characterize the fabricated whole length of conductor, it is wound in a single layer solenoid geometry with a diameter of ∼97 cm that corresponds roughly to that from the production reel. The windings were arranged on the support structure of the coil in 13.75 turns first in a non-insulated manner. The assembly was cooled down to T = 77 K in a liquid nitrogen bath and then energized with current. After warming-up, the turns were insulated against one another by introducing a defined distance between the windings and the test procedure was repeated. In this contribution the results of current tests with the different demonstrator coils are presented. The current test program included charging and discharging, transient response and steady state operating conditions of both coil configurations. As the electrical behavior of a non-insulated HTS coil is different from an insulated one, further investigations are carried out by modelling of the electric performance of the coil.

Metal-as-insulation HTS coils

:In this article, we summarize what we have learned about metal-as-insulation (MI) winding behavior and technical challenges. High-temperature superconductor (HTS) materials are highly thermally stable. This feature compared to classical low temperature superconductor (LTS) enables the use of MI technology to improve the protection against quenches. We firstly present a review of what has been done worldwide since 2011, which is the first time that a co-winding of a metallic tape without insulation was used in HTS pancake coils. In this review we also focus on the turn-to-turn contact resistivity parameter and the ranges obtained by groups worldwide. Then, we give details of our NOUGAT insert, a LNCMI-CEA-Néel Institute MI HTS insert built in 2018. We describe the specific conception of the magnet including the compact magnetic shielding. The high number of tests performed in high resistive background fields between 2018 and 2021 gives us a unique data set on this technology for ultra-high field (UHF) magnets; even in case of a background field fast drop at 19 T. This magnet is the first REBCO solenoid of this size using this technology, and tested intensively at such high magnetic fields (up to 32.5 T) so far. Finally, we discuss about the self-protection feature of MI coils and we propose a passive protection way for high turn-to-turn resistivity values.