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.

A higher available magnetic field can provide more flexibility for significant research and construction. Inserting a feasible high-temperature-superconducting (HTS) magnet inside a low-temperature-superconducting (LTS) external is becoming an efficient method to build ultra-high-field (UHF) magnets. However, HTS insert magnets exhibited a concentration of magnetic strain during the energization, significantly owing to the screening-current effect. This phenomenon affects 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 REBa₂Cu₃O 7 − 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 h. With the energization of LTS external magnet, the entire facility reached a center field larger than 20 T for 16 h, 30 T for 6.35 h and 32 T for 2 h. 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 maintain 229 A operation current during the postmortem standalone test. This 32.4 T result exhibits the 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.

REBa ₂ Cu ₃ O _7-x (REBCO) coated conductor has emerged as a promising material for the development of ultra-high-field (UHF) magnets. However, the presence of screening-current induced strain within this tape-shaped conductor exerts a huge impact on the operational stability of REBCO coils. We focused on the design of an all-superconducting 36 T / 40 mm UHF magnet, which included a 16 T insert magnet consisting of two nested REBCO coils. In this study, the screening-current induced strain in the 16 T magnet was numerically studied using the coupled electromagnetic-mechanical model, which considered the tilting angle of REBCO tapes and the strain dependency of the critical current. We calculated the screening-current induced strain of each pancake individually and compared these results with those calculated by the sequential model. According to the coupled model, the maximum hoop strain was 0.52 %, relatively lower than that calculated by sequential model. The most dangerous pancake was estimated to be the outermost pancake of each coils. Additionally, we varied the critical current value of the REBCO tapes and studied the relationship between the critical current value and screening-current induced strain. This work provides a feasible way to calculate the screening-current effect in large-scale HTS magnets.

A 20-T/17-mm High-Temperature-Superconducting (HTS) magnet was developed and tested, comprising a 15-T no-insulation (NI) REBCO insert (Coil 1) nested in a 5-T metal-insulation (MI) REBCO coil (Coil 2). Coil 1 consists of 20 NI double pancakes (DP), while Coil 2 consists of 28 MI DPs. The 20-T magnet was cooled in liquid helium and charged for five runs, the maximum power supply current reached 228.7 A. Multiple quenches occurred during the charging process, causing the power supply current to drop to zero within one second. The rapid current variation induced a large voltage inside the coil, which damaged Coil 1. After re-warming, Coil 1 was tested in liquid nitrogen, and its critical current was significantly degraded. To investigate the degradation, three representative DPs from Coil 1 were examined using continuous I _c measurement and observed with optical microscope and scanning electron microscope (SEM). Most degradation areas consisted of a bubble with a current breakdown point distributed throughout Coil 1. Probable causes were proposed in this article.

A 20 T REBCO insert magnet has been designed considering a 15 T/150 mm background field generated by an LTS magnet. A two-nested-coil structure was chosen. The target of this project is to generate a 20 T/80 mm user field by inserting the outer MI-REBCO coil (Coil 2) first, then try to reach 35 T by inserting the inner NI-REBCO test coil (Coil 1). Coil 2 will be wound by copper packed, 185-μm thick REBCO tapes co-wound with 50-μm thick Hastelloy tapes. Coil 1 will be no-insulated wound by 65-μm thick REBCO tapes. Two mechanical models were built to estimate the stress distribution inside the HTS coils during operation. The influence of the screening current distribution on stress was discussed. The unbalanced force caused by the coil misalignment was also simulated. The 20 T HTS insert magnet is planned to be built and tested in 2021. The progress of coil winding and preliminary test results at 77 K were presented.

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.