REBCO Coil Research Articles

Effects of lateral critical current nonuniformity on stresses in dry-wound high-field REBCO coils

Abstract The distribution of critical current density (jc) in REBCO coated conductors affects the magnetization behaviors and subsequently screening-current-induced stresses, particularly for solenoid magnets in high fields. This paper studies numerically the correlation between lateral jc profile across conductor width and stress distribution in pancake coils. The modeling framework considers bending, winding, thermal contraction, and magnetic forces including coupled electromagnetic-mechanical behaviors, i.e., the deviation of the perpendicular field away from axial direction due to tilting deformation. The lateral nonuniformity is introduced using trapezoidal functions, emulating typical jc profiles originated in pristine tapes and those caused by slitting. First, parametric studies are carried out on the small test coils previously reported in the 'Little Big Coils' (LBC) paper. It is shown that while slitting edge defects have a moderate impact on peak strains, imperfections in the pristine tape with a larger shoulder width can accelerate the penetration process, shifting peak force to the structurally resilient middle section. Similar behaviors are found in the LBC3 case study, suggesting that lateral jc nonuniformity may have contributed to the observed degradation states in pancakes with different slit-edge orientations. Furthermore, the manipulation of lateral jc profile is proposed as a strategy to manage stresses in high field solenoids. This is demonstrated in the design study of a hypothetical REBCO insert. By adjusting the lateral jc distribution and the overall scaling factor, magnet designs with a reasonable current margin and moderate peak strain can be found. The multi-width concept is then applied to allow for a higher operating current and a larger margin for the end pancakes. Albeit being a generic case, this study highlights the sensitivity of peak stresses in high-field magnets to jc distribution. This feature may be taken into account to fine-tune magnet designs and adjust coil assemblies for better overall performance. It also emphasizes the need for careful characterization and effective control of lateral jc distributions in REBCO coated conductors.

Strain analysis and preliminary test of an all-superconducting high-field magnet generating 32.4 T direct-current magnetic field

Abstract A higher available magnetic field can give more flexibility for significant research and constructions. Inserting feasible high-temperature-superconducting (HTS) magnet inside low-temperature-superconducting (LTS) external is becoming an efficient method to build ultra-high-field (UHF) magnets. However, HTS insert magnets exhibited concentration of magnetic strain during the energization, significantly owing to the screening-current effect. This phenomenon affect the structural integrity of HTS magnets, posing a difficulty for UHF magnets to reach a center field larger than 30 T. In this paper, we report the recent progress of the 35 T all-superconducting UHF magnet project. The full-cycle development of this homemade UHF magnet was a strong collaboration with multiple domestic institutions in China. We used commercial REBa2Cu3O7-x (RE = rare earth element, REBCO) conductors to wind the 20 T HTS insert. Specifically, we optimized the electromagnetic design of this HTS insert with consideration of screening-current induced strain. We applied the no-insulation and metal-as-insulation winding techniques simultaneously to balance the time constant of the two nested REBCO coils. During the excitation test, the HTS insert maintained a 256 A operation current as designed for nearly 18 hours. With the energization of LTS external magnet, the entire facility reached a center field larger than 20 T for 16 hours, 30 T for 6.35 hours and 32 T for 2 hours. It finally reached a 32.4 T center field, at which point the LTS external quenched, inducing the HTS insert to quench as well. After quench, the HTS magnet can maintained 229 A operation current during the postmortem standalone test. This 32.4 T result exhibits effectiveness of considering the screening-current induced strain to enhance the structural integrity of REBCO magnets, and verifies the high reliability of REBCO magnets by generating a long-duration high field environment without any quench accident originating from the REBCO magnet.

A FEM Simulation Model to Calculate Local Currents and Voltages of NI REBCO Coil With Both Screening Current and Transverse Current Considered

A FEM Simulation Model to Calculate Local Currents and Voltages of NI REBCO Coil With Both Screening Current and Transverse Current Considered

Elastic-plastic conductor damage evaluation at over 0.4% strain using a high-stress REBCO coil

Recent reports on screening current stress simulations of high-field REBCO magnets frequently present peak stresses over 1 GPa. However, this result is probably an unrealistic artifact of purely elastic calculations, considering the macroscopic yield and fracture stresses of approximately 900 MPa and less than 1.1 GPa for Hastelloy substrate-coated conductors. Here, we evaluate elastic-plastic conductor damage at over 0.4% strain using a high-stress REBCO coil exposed to a high field to explore this elastic-plastic regime. The coil was located off-center in a low-temperature superconductor magnet so as to induce a significant screening current in the enhanced radial field. Voltage taps, a Hall sensor, and two strain gauges were used for the instrumentation. We obtained strains exceeding 0.4% near the outward edge during the coil current charge from 350 A to 390 A, where the coil was exposed to external axial and radial magnetic fields of 13 T and 0.5 T. Post mortem results showed wavy plastic deformation, electrical damage, and REBCO defects. An elastic-plastic simulation reproduced the measured strains and predicted that ∼1 GPa stress is sufficient to induce ∼0.9% strain, thus validating our initial concerns with purely elastic models. This paper provides our experimental and simulation results.

Comparison Between Results of Quench Simulation and Tests of a 13 T REBCO Coil in a Strong Background Magnetic Field

Comparison Between Results of Quench Simulation and Tests of a 13 T REBCO Coil in a Strong Background Magnetic Field

Development of an Intra-Layer No-Insulation (LNI) REBCO Coil Implemented With a Resistance-Controlled (RC) Interface

Development of an Intra-Layer No-Insulation (LNI) REBCO Coil Implemented With a Resistance-Controlled (RC) Interface

Screening Current Induced Stress/Strain Analysis of High Field REBCO Coils With Co-Winding or Over-Banding Reinforcement

Screening Current Induced Stress/Strain Analysis of High Field REBCO Coils With Co-Winding or Over-Banding Reinforcement

Test of High Temperature Superconducting REBCO Coil Assembly for a Multi-Frequency ECR Ion Source

Test of High Temperature Superconducting REBCO Coil Assembly for a Multi-Frequency ECR Ion Source

Experiment and Simulation on Mechanical Behavior in 1/2-Scale Demonstration REBCO Coil System of Skeleton Cyclotron for Cancer Therapy

Experiment and Simulation on Mechanical Behavior in 1/2-Scale Demonstration REBCO Coil System of Skeleton Cyclotron for Cancer Therapy

Numerical Evaluation of Screening Current-Induced Magnetic Field in 1/2-Scale No-Insulation REBCO Coil System of Skeleton Cyclotron for Cancer Therapy

Numerical Evaluation of Screening Current-Induced Magnetic Field in 1/2-Scale No-Insulation REBCO Coil System of Skeleton Cyclotron for Cancer Therapy

The Effect of Field-Dependent $n$-Value on Screening Current, Voltage, and Magnetic Field of REBCO Coil

The Effect of Field-Dependent $n$-Value on Screening Current, Voltage, and Magnetic Field of REBCO Coil

Comparison Between Insulated and NI Layer-Wound ReBCO Coils During Transients

Comparison Between Insulated and NI Layer-Wound ReBCO Coils During Transients

Numerical Investigation on Current Behaviors of NI/PI REBCO Coils Wound with Many Bundled Tapes for Sudden Discharge

Abstract For future applications such as compact fusion reactors, it is necessary to investigate some characteristics of meter-class D-shaped no-insulation (NI) REBCO coils. In recent years, several kinds of NI winding techniques, such as multi-tape-bundled REBCO tapes and a partial-insulation winding technique, have been proposed and experimented to improve the current characteristics of REBCO coils, such as an excitation delay and a large induced current. In this paper, the sudden discharging has been simulated to investigate not only the current behaviors but also the Joule heats for meter-class D-shaped REBCO coils with a few different NI winding techniques: (1) complete no-insulation between tape-to-tape/turn-to-turn for multi-tape-bundled windings and (2) partial-insulation for turn-to-turn of multi-tape-bundled windings without tape-to-tape insulation. The simulation results show that the amount of Joule heats emitted in a short time is large for many-tape-bundled NI REBCO coils. Also, the simulation result show that the greater number of tapes bundled on an NI REBCO coil, the more unbalanced current distributions are observed among the bundled tapes. Furthermore, the partial-insulation REBCO coils show a particular current distribution. These results reveal that cooling systems should be considered depending on the winding techniques.

Experimental study on screening-current induced strain in REBCO insert coils: under critical current control and operation current cycles

The screening-current effect is an inhomogeneous distribution of current density inside the REBCO conductors. The additional strain induced by the screening current, known as screening-current induced strain (SCS), is considered to affect the structural integrity of REBCO windings, especially when operating high-field REBCO insert magnets. In this work, we wound and tested a series of 50-turn REBCO coils inside a 10 T LTS external to investigate the influencing mechanism of multiple electromagnetic factors on SCS. We varied the critical current in different coils by different heat treatment procedures. Each coil was tested individually, experiencing a external field cycle and multiple operation current cycles at constant external fields. The extreme scenario for each coil was being energized to 400 A while the external field was 10 T. We adapted the discrete-coupled model to estimate the hoop strain distribution, monitored the experimental results by multiple strain gauges at the outermost turn. Test coil with a lower critical current endured a lower maximum hoop strain. When we were energizing the test coils, hoop strain increased at the edge of REBCO tapes while remaining nearly constant in the middle region. Additionally, the maximum hoop strain at the outermost turn decreased after each excitation cycle. This work could be an experimental reference for optimizing the electromagnetic design and the excitation scheme during the development of high-field REBCO magnets.

Numerical Simulation on AC Loss in REBCO Stacks and Coils Carrying Non-Sinusoidal Currents

Numerical Simulation on AC Loss in REBCO Stacks and Coils Carrying Non-Sinusoidal Currents

Delamination Avoidance in Epoxy Wet-Wound REBCO Coils by Release Agent Coating

Delamination Avoidance in Epoxy Wet-Wound REBCO Coils by Release Agent Coating

The Possible Control Method of Turn-to-Turn Contact Resistivity of REBCO Coil by Surface Coating

The Possible Control Method of Turn-to-Turn Contact Resistivity of REBCO Coil by Surface Coating

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.

Electromagnetic and Thermal analysis of an Intra-layer no-insulation coil during Quench

The intra-layer no-insulation (LNI) layer-wound REBCO coil has been widely studied for its excellent high thermal stability in the research of nuclear magnetic resonance superconducting magnets. Due to the complex structure of the LNI layer-wound coil and the numerous factors that affect thermal stability, its quench characteristics are not clear. In this paper, a numerical multiphysics coupling model is established to study the transient electromagnetic and thermal behaviors of the LNI layer-wound coil during quench. The results indicate that temperature follows the diffusion law, while the currents evolve through electrical connection and electromagnetic induction. Due to the high contact resistance, the azimuthal current and axial current vary almost in units of the entire coil layer. The direction of the radial current upstream of the turn where a quench occurs is positive due to the bypassing behavior, while the radial current direction is negative downstream. The insulation material inside the LNI layer-wound coil strongly affects the quench propagation. When its thickness decreases, the equivalent radial thermal conductivity increases, thereby increasing the quench propagation velocity and current peak, and shortening the duration of the current response during quench.

Design and Analysis of a Cooling System for the REBCO Coil Based on GM/JT Cycle

High‐field magnets have been used in a wide range of scientific and industrial areas. REBCO (REBa2Cu3Ox, where RE = Rare Earth) coated superconductors (CCs) have been gaining increasing interest due to the potentialities of using them in high‐field magnets. However, the thermal stability of the REBCO CCs coils is limited by longitudinal heat conduction inside the coils and bubbles generated by evaporating helium. So as to enhance the heat transfer in REBCO coil, a cooling system based on GM/JT cycle was designed. In this paper, the design, thermodynamic calculation, and analysis of the cooling system for the REBCO coil are provided systematically. It is found that when the mass flow rate is 0.75 g/s at a pressure of 10 bar, the cooling capacity of the system is up to 10.16 W. Meanwhile, the inlet pressure and counterflow heat exchangers efficiency are key factors for significantly improving the cooling capacity of the system. An open loop experiment also has been carried out and the cooling capacity of 0.79 W at 4.5 K is obtained finally when He mass flow rate is 0.1 g/s. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.