Double Pancake Coils Research Articles

Investigation of the energizing characteristics of a metal-insulation iron-based superconducting coil

With the progress of iron-based superconducting wire and tape fabrication technologies, extensive research has been conducted on iron-based superconducting coils. A deep understanding of the energizing characteristics of these coils is crucial for their large-scale application. This study investigates two metal-insulated double pancake coils, each having a different number of turns and co-wound with Ba0.6K0.4Fe2As2 iron-based superconducting tape and stainless-steel tape. We conducted critical current tests, sudden-discharge experiments, and charge-discharge cycles at 4.2 K to obtain the time constants and characteristic resistances of the coils. Additionally, the impact of stainless-steel as co-wound material on coil performance was analyzed. Theoretical models were employed to calculate the charging processes and validate them against experimental results. This research offers valuable insights for the practical application of iron-based superconducting materials.

Double-pancake superconducting coil integrated with DFIG-based wind power systems under voltage-dip events: Design and performance analysis study

The doubly fed induction generator (DFIG)-based wind turbine provides massive development in power generation. Uncertain power oscillations and voltage-dip events affect the stability of the DFIG. To mitigate the effect of voltage-dips in short period, superconducting magnetic energy storage (SMES), which possess rapid energy exchange and high power density, is integrated with large-scale renewable power systems. SMES is integrated at the DC link of the DFIG to ensure the power balance and fault ride-through (FRT) capability. During this operation, frequent current changes occur in the high-temperature superconducting (HTS) coil, which leads to an increase in AC losses. As a remedy, an additional power converter structure controlled with a model-predictive controller (MPC) is proposed. Firstly, a scale-down HTS double-pancake coil model is designed using a finite element model by which transport current loss and magnetic field formulation are studied. Additionally, the critical coil current is also verified through experimental studies. The effectiveness of the proposed MPC-based SMES integrated DFIG on DC link voltage and grid current under IEC-61000-4-11 standard, symmetrical and asymmetrical voltage-dip cases are investigated. The obtained results demonstrate that the proposed system effectively reduces transients in DC link voltage and improves the FRT capabilities of the grid.

Performance Evaluation of a Double Pancake Modular Coil Wound by Multi-Layer Spiral HTS Cable

Performance Evaluation of a Double Pancake Modular Coil Wound by Multi-Layer Spiral HTS Cable

26.86-tesla direct-current magnetic field generated with an all-REBCO superconducting magnet

This paper reports an all REBCO (REBa2Cu3Ox, where RE = Y, Gd) superconducting magnet that generated a direct-current magnetic field of 26.86 T at 4.2 K in self-field, which is the highest magnetic field achieved by the high temperature superconducting (HTS) magnet. The magnet consists of a stack of 10 double pancake (DP) coils wound with two different width REBCO conductors, and the no-insulation (NI) winding technology was applied to increase its engineering current density. The inner and outer winding diameters and overall height of the magnet are 30 mm, 103.98 mm, and 102.6 mm, respectively. In order to control the stress risk in HTS magnet, the over-banding made of high elastic modulus stainless steel tapes was employed to the magnet. In addition, no obvious degradation was found in the performance of the magnet after quench at 26.86 T of 295.5 A. These test results demonstrate the great potential of REBCO magnets in high field applications.

Investigation of the effect of difference in the characteristic resistance of DP coils on the field and losses of MI HTS magnets

We found a significant difference between the central and end magnetic field delays during the testing of a metal-insulation (MI) high-temperature superconducting (HTS) magnet. In this paper, we first analyzed the reasons for the difference in the field, and the results show that it is mainly due to the difference in the characteristic resistance of each double pancake (DP) coil. We further evaluated the effect of the difference on the electromagnetic analysis results of the MI HTS magnet. Subsequently, we developed an equivalent circuit model that considers the difference in the characteristic resistance of the DP coils and used the model to analyze the effect of this difference on the fields and losses generated by the MI HTS magnets, especially for AC conditions. Finally, based on our analytical results, a coil arrangement strategy considering the characteristic resistance of each DP coil was proposed. This work can guide us in evaluating the magnetic field more accurately in future engineering applications and provide a coil arrangement strategy for the MI HTS magnet, which requires to excite rapidly or generate an alternating magnetic field.

Energization test apparatus of HTS coils cooled by liquid hydrogen and manufacture of split-type REBCO external field coil

Abstract Liquid hydrogen (LH2) is a potential coolant of high-temperature superconducting (HTS) devices. However, owing to the high flammability of hydrogen and the risk of hydrogen embrittlement in materials, studies on LH2-cooled superconducting devices are rare. We developed an LH2 test apparatus to analyze the basic characteristics of LH2 as a coolant and evaluate the energizing characteristics of LH2-cooled superconducting wires. A REBCO external field coil was designed and manufactured for conducting cooling stability tests on various HTS coils cooled by LH2. The field coil, comprising eight single pancake coils of 4 mm wide REBCO wires, with inner diameter 106 mm and outer diameter 250 mm was divided into upper and lower sections. The test coil was placed in the central space. A magnetic field perpendicular to the test coil’s wire surface was generated by running currents in opposite directions through the upper and lower sections. Each double pancake coil was securely placed in a stainless-steel housing to withstand repulsive electromagnetic forces. The manufactured field coil was cooled using LH2, energized to 150 A, and successfully generated the intended magnetic field of 1.75 T. No increase in voltage was observed in any of the double pancake coils, and there was no mechanical degradation due to electromagnetic forces. Subsequently, we initiated thermal runaway tests on various HTS coils using the manufactured field coil to assess the cooling stability of LH2-cooled HTS coils. The study will facilitate the development of explosion-proof designs and safety technologies for LH2-cooled superconducting devices and cooling systems.

Calculation on Current Distributions among Transposed Three- and Six-parallel Conductors in Superconducting Armature Coils for Superconducting Synchronous Motors

Abstract The armature coil of the superconducting synchronous motor for electric aircraft requires a current of 1,000 A or more. However, a single superconducting tape at liquid nitrogen temperatures is insufficient for this operating current. Therefore, the armature coil should be composed of multi-strand parallel conductors. If the coil is constructed from parallel conductors, the inductance balance between the parallel conductors will be collapsed, so the current distributions will be non-uniform. As a solution for this issue, the uniform current distribution can be achieved by the transposition technique that changes the order in which tapes are stacked. Our previous research has developed the transposition method for the 2 n -strand parallel conductors (n = 1, 2, 3, and so on). However, some of the superconducting synchronous motors are actually designed using a number of parallel conductors other than 2 n . This study focused on the three- and the six-strand parallel conductors. We analytically clarified the unique transposition configuration for the three-strand parallel conductors that achieves uniform current distribution, and the transposition configuration for the six-strand parallel conductors by combining the transposition configuration for the three- and two-strand parallel conductors. As a result, the current distribution ratio became less than ±5% when the number of turns of the double pancake coil was a multiple of six by applying the transposition.

23 Tesla high temperature superconducting pocket magnet

We present a compact 23 T no-insulation (NI) magnet that was wound with 60 m of 10 mm wide high temperature superconducting (HTS) tape. The magnet consists of only one pocket-sized double pancake (DP) coil with an inner diameter of ∼6 mm, a height of 20 mm, and an outer diameter of 41.6 mm. Another NI coil of similar size but with a larger inner diameter of 8 mm reached a slightly lower magnetic field of 21 T. We also present a smaller coil which was wound with only 20 m of HTS tape and still achieved a magnetic field of 16 T. During the experiments in liquid helium, each coil was charged to a current between 690 A and 850 A, corresponding to a high current density of 1500–1900 A mm−2. The small bore size and high current density contributed to the high fields generated by these coils. We present the fabrication details, helium tests and repeatability analysis of these ‘pocket’ magnets.

An open-source 3D FE quench simulation tool for no-insulation HTS pancake coils

A new module, Pancake3D, of the open-source finite element quench simulator (FiQuS) has been developed in order to perform transient simulations of no-insulation (NI) high-temperature superconducting pancake coils in three-dimensions. FiQuS/Pancake3D can perform magnetodynamic or coupled magneto-thermal simulations. Thanks to the use of thin shell approximations, an H→−ϕ formulation, and anisotropic homogenization techniques, each turn can be resolved on the mesh level in an efficient and robust manner. FiQuS/Pancake3D relies on pre-formulated finite-element (FE) formulations and numerical approaches that are programmatically adjusted based on a text input file. In this paper, the functionalities and capabilities of FiQuS/Pancake3D are presented. The challenges of FE simulation of NI coils and how FiQuS/Pancake3D addresses them are explained. To highlight the functionalities, the results of a magneto-thermal analysis of a double pancake coil with 40 turns per pancake with local degradation of the critical current density are conducted and discussed. Furthermore, a parameter sweep of the size of this local degradation is presented. All FiQuS input files for reproducing the simulations are provided.

Turn-to-Turn Contact Resistance Measurement of NI REBCO Double Pancake Coils

Turn-to-Turn Contact Resistance Measurement of NI REBCO Double Pancake Coils

Experimental AC loss study on REBCO coil assemblies coupled with an iron cylinder

In high-temperature superconducting (HTS) power devices, the presence of iron cores changes the magnetic field profile around the HTS coil windings, potentially affecting their AC loss characteristics. AC loss measurements for HTS coil windings coupled with an iron core using the electrical method can lead to a significant error, owing to the indirect estimation of the iron core loss through using a copper test coil. To investigate the cause of the experimental error and the influence of an iron core on coil AC losses, transport AC losses of REBCO double pancake coil (DPC) assemblies coupled with an iron cylinder were measured. A 40-turn 1DPC and an 80-turn 2DPC assembly wound with 4 mm SuperPower wire were employed in the measurements. To ensure the same iron core loss using the HTS coil assembly and the copper coil, 2D finite element method simulations were conducted iteratively to design the iron core and the copper coil to get the same local magnetic field distributions in the designed iron core for the two cases. The main cause of the error is due to the difference in local magnetic flux densities in the iron core generated by the HTS coil assembly and the copper coil even when the ampere-turns of the coils are identical. We showed that the simulation-guided measurement method can assure accurate AC loss measurement in the HTS coil assemblies coupled with iron cores. Compared with the AC losses in the 1DPC and 2DPC coil assemblies without the iron cylinder, the presence of the iron cylinder significantly increases the coil losses. Frequency dependence is observed in the coil AC losses of the 1DPC and 2DPC assemblies when coupled with the iron cylinder. This is due to the eddy current induced in the iron cylinder generating a magnetic field, which influences the coil AC loss.

A numerical study on homogeneity and central field with axial spacing between DP coils of HTS magnet at 65 K

A numerical study is performed with COMSOL multi-physics, to analyze the effect of axial spacing between double pancake (DP) coils on central magnetic field and field homogeneity of High temperature Superconducting (HTS) magnet. The magnet is wound in the form of DP coils having, inner winding diameter as 100 mm and is operated at 65 K. The field homogeneity is calculated over a Diameter of Spherical Volume (DSV) of 40 mm for the entire study. The analysis is carried out by considering a constant length of HTS tape of 750 m and varying the number of DP coils from 4 to 14. The number of turns in all the DP coil is equal for a given set of DP coils to maintain the length of HTS tape to be constant. The coil separation within DP coils is of constant gap from 0.5 to 2 mm, while the axial spacing between the DP coils is varied from 0 to 50 mm for all the set of DP coils. The optimum gap between DP coils was evaluated using Exhaustive search optimization techniques and decreased with the number of DP coils. The best field homogeneity of 662.5 ppm was achieved with 14 DP coils having gDP=20.3 mm and gSP=2 mm for the chosen solenoid configuration.

Fast and accurate electromagnetic modeling of non-insulated and metal-insulated REBCO magnets

REBCO high-temperature superconductors are promising for all-superconducting high-field magnets, including ultra-high field magnets. Non-insulated (NI) and metal-insulated (MI) windings are a good solution for protection against electro-thermal quench. Design and optimization requires numerical modelling of REBCO inserts for high-field magnets. Here, we detail a fast and accurate two-dimensional cross-sectional model for the electromagnetic response of NI and MI coils, which is based on the minimum electro magnetic entropy production. Benchmarking with an A − V formulation method on a double pancake coil shows good agreement. We also analyse a fully superconducting 32 T magnet with a REBCO insert and a low-temperature superconducing outsert. In particular, we analyse the current density, the screening current induced field (SCIF), and the AC loss. We have shown that metal-insulated coils enable transfer of angular current in the radial direction, and hence magnet protection, while keeping the same screening currents and AC loss of insulated coils, even at relatively high ramp rates of 1 A s−1. Surprisingly, soldered coils with low resistance between turns present relatively low AC loss for over-current configuration, which might enable higher generated magnetic fields. The numerical method presented here can be applied to optimize high-field magnets regarding SCIF in MI or NI magnets. It also serves as the basis for future electro-thermal modelling and multi-physics modelling that also includes mechanical properties.

A quench detection method for parallel co-wound HTS coils based on current redistribution

High-temperature superconducting (HTS) coated conductors (CCs) have become the preferred material for superconducting magnet applications due to their high engineering current density and high mechanical strength. However, due to the low quench zone propagation velocity of CCs, magnets wound with CCs suffer from severe quench risks. Therefore, a rapid, sensitive, and reliable quench detection method is crucial for the safe operation of such HTS magnets. In this paper, we propose a quench detection method based on current redistribution, in which two pieces of HTS CCs are soldered together at each end and insulated in the middle part, which are then parallel co-wound into a double-pancake coil. The two tightly coupled windings and low resistance joints form a very low inductance current loop, resulting in fast current redistribution between the two co-windings even at the inception of quench (with still low quench voltage). We deduced analytical solutions of the current redistribution process under different magnet operational scenarios, including constant current operation, charging and discharging, and proposed quench detection criteria. Corresponding quench tests were performed on a small scale co-wound HTS coil, and the results well verified the analytical solutions and the effectiveness of the quench detection method. Our work may be useful for lowering the risks in HTS magnet quench in various applications.

Study on the Arrangement of REBCO Double-Pancake Coils with Different Critical Currents in 30-T Superconducting Magnets

Study on the Arrangement of REBCO Double-Pancake Coils with Different Critical Currents in 30-T Superconducting Magnets

Development of metal-insulated iron-based superconducting coils and charging tests under high magnetic fields up to 32 T

To explore the potential of iron-based superconducting (IBS) tapes in high-field applications, IBS pancake coils were dry-wound using the uninsulated IBS tape, supplemented with cowound stainless steel tapes for reinforcement. We have successfully fabricated a double pancake coil (DP-1) and a series-connected coil consisting of six DP coils (DP-6). These coils underwent multiple charging tests in a 35 T resistive magnet and retesting in a 14 T superconducting magnet. After conducting multiple rounds of retesting in two different background field magnets, it was found that the IBS coils did not show any degradation in their current-carrying performance. The critical current (I c) value of the DP-1 at 4.2 K and 32 T reached 60 A. To systematically evaluate the I c performance of the six coils in the DP-6, we monitored the V–I curves between 26 voltage taps. The resistive voltage rise was observed at low charging currents, indicating that each coil has suffered different levels of damage. Nevertheless, the overcurrent tests demonstrated that the IBS coil can still carry a current far exceeding its I c without irreversible degradation. We concluded the possible reasons for these phenomena by analyzing the experimental details and test data. In addition, the coil’s internal structure was observed to assess the quality of coil winding, joint soldering, and epoxy resin impregnation. This study will provide references for the electromagnetic and mechanical properties of IBS tapes under ultra-high magnetic fields and contribute to the future development of higher-performance practical IBS coils.

Development of the first Tesla class iron-based superconducting coil for high field application

This study has, for the first time, combined iron-based superconductor (IBS) with high field insert coil technology to generate a higher magnetic field. Nine large-sized iron-based superconducting double pancake coils (DPCs) were fabricated, and their performance was evaluated at 4.2 K. Subsequently, seven DPCs with superior performance were assembled to form an IBS insert coil and tested at the 20 T background field. The test results indicated that the insert coil successfully generated above 1 T magnetic field under the 0 T, 10 T, and even 20 T background field, respectively. Notably, the poorest IBS DPC still had a critical current of about 84 A at the background field of 20 T, which outperformed all previously reported performance tests of IBS coils. This paper provides a detail introduction to the design and fabrication process of the IBS insert coil, presents the test results, and discusses them.

AC Loss Reduction in HTS Coil Windings Coupled With an Iron Core

Rapid-cycling synchrotron (RCS) magnets based on a superferric design consist of high temperature superconducting (HTS) coil windings coupled with iron cores. However, the presence of iron cores significantly increases AC loss in HTS coil windings, making AC loss reduction a critical issue for applying HTS technology in RCSs. Enlarging the distance between iron cores and coil windings may reduce AC loss. In addition, magnetic materials with different saturation magnetic fields also may influence AC loss in HTS coil windings coupled with iron cores. To investigate the distance dependence, AC losses of 1DPC (double pancake coil)-, 2DPC-, 4DPC-, and 8DPC assemblies with various distances are simulated using the 3D <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> homogenization method with/without an iron core. Two magnetic materials with different saturation magnetic fields are chosen as the iron core to evaluate AC loss in HTS coil assemblies. The results show that the AC loss values in HTS coil assemblies coupled with the iron core decrease significantly with growing distance. For a given distance and current, when the iron core has a lower saturation field, the AC loss values of the 4DPC assembly are smaller than those with a higher saturation field.

Computation of the Strain Induced Critical Current Reduction in the 16 T Nb3Sn Test Facility Dipole

A test facility dipole is being developed at LBNL, targeting a 16 T field in a 144 mm wide aperture. The magnet uses a block design, with two double-pancake coils. In order to minimize motion under the large Lorentz forces, the coils are preloaded against a thick aluminum shell and iron yoke using bladder and key technology. It is then crucial to verify that the performance of the magnet is not degraded due to strain induced on the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor during assembly, cool-down and powering. The critical current of extracted strands was measured in a varying background magnetic field and as a function of the applied longitudinal strain. Finite element analysis was used to extract the strain state inside the superconducting strands during magnet assembly and operation. This strain was then compared to the measurements to evaluate potential reversible and irreversible effects on the magnet performances. The results suggest that the magnet can reach 16 T with sufficient margin, with no irreversible degradation in the high field region.

Basic Concept for Uniform Current Distribution in Parallel Conductors by Introducing a Small Number of Transpositions in REBCO Armature Coils

We propose the concept of uniform current between groups (CUCG) in superconducting armature coils composed of two or more strands of transposed parallel conductors. We validate the proposed concept by measuring the current distributions in several double pancake test coils comprising two and four parallel conductors using Rogowski coils by applying AC signals of varying frequencies and amplitudes. Accordingly, uniform transport currents across four strands were confirmed. The proposed concept produced uniform current distributions across four groups of parallel conductors with a small number of transpositions. This work paves the way for developing armature coils composed of more than eight strands in parallel conductors by introducing a small number of transpositions based on CUCG.