The nonuniform superconducting current distribution in a REBCO coated conductor, including a varying-field-induced screening current, is responsible for a significant magnetization effect that not only degrades the field quality of REBCO magnets, but introduces risks of overstressing the conductor. This paper presents our experimental and simulation studies on the screening current effect on an 800 MHz (18.8 T) REBCO insert (H800) that together with a 500 MHz LTS nuclear magnetic resonance (NMR) magnet (L500) constitutes the MIT 1.3 GHz LTS/HTS NMR magnet (1.3 G). To develop our simulation model, which was subsequently validated by a good agreement between simulation and experiment, we chose H800, Coil 1 of the 3-coil assembly operated alone and the entire H800, for the sources of experimental data, specifically their remnant fields after current discharge and diminished axial fields during operation. Armed with this valid model, we examined in detail the negative effects of screening current on H800, an important 1.3 G component. Our simulation indicates that the screening current, nonuniformly distributed in the REBCO conductor, not only deteriorates H800 field, both strength and homogeneity, thus that of 1.3 G, but may overstress the REBCO conductor.

In recent years, remarkable progresses have been made in the R&D efforts for high temperature superconducting (HTS) high-field magnets. The screening-current-induced magnetic field and mechanical stress/strain in rare-earth barium-copper-oxide (REBCO) coils are raising growing concerns. This paper presents experimental and theoretical analyses on the effects of screening-current-induced mechanical strains in a REBCO insert setup without transport currents. Strain measurements on two REBCO coils, with and without over-banding structures, were carried out in an low temperature superconducting (LTS) background field magnet. Electromagnetic-mechanical models were developed, coupling the tilting angles of the superconducting tapes and the strain dependency of the critical current. The discrete-coupled model with turn-to-turn contacts, the discrete-sequential model and the block model were implemented and compared against the measured hoop strains. Combining experimental data with simulation models, it is shown that moderate over-banding structures are effective in providing reinforcements for the superconductors. A new method of edge-bonding was proposed and tested, which could reduce hoop strains in low applied fields but failed in higher fields. This work could be useful for the design and analysis of future high-field REBCO magnets.

The shielding (or screening current) effect in high- T _c coated conductors has a strong influence on its stress/strain distribution in a coil winding, especially during high-field operations. To demonstrate this phenomenon, a special experimental setup was designed. With an LTS background magnet and a small HTS insert coil, we were able to carry out direct observations on the hoop strains of a 10 mm wide REBCO sample. Measured data was compared with numerical solutions obtained from models based on T – A formulation and elastic models, showing good agreement. An analytical expression was proposed to estimate the maximum radial Lorentz force considering the screening current effect. Using the developed numerical models, we further studied two methods that are utilized for reducing the screening-current-induced magnetic field, i.e. use of multi-filamentary conductors and current sweep reversal approach.

Rare-earth-based barium copper oxide (REBCO) coated conductors are promising candidates for the development of ultra-high-field (UHF) magnets, due to its high in-field performance, engineering current density, tensile strength and commercial availability. However, technological challenges pertaining to the large screening currents still remain. The major issues caused by the screening currents in REBCO conductors in UHF applications involve two aspects: the screening current induced magnetic field (SCF), and the screening current induced stress (SCS). In the past decades, extensive research has been devoted to the SCF, offering a variety of possible remedies. With latest advances in the construction and testing of UHF magnets, new observations of the SCF involving REBCO coils were reported. The SCS was identified in recent years and has raised growing concerns. The excessive and highly concentrated Lorentz force, rooted in the high magnetic field and the screening currents, poses threats to the mechanical strength of REBCO coated conductors. The aim of this paper is to review recent research efforts in understanding and tackling the screening current related technological issues. For the SCF, we focus on the latest observations in high-field experiments and its various mitigation methods. For the SCS, we present recent studies including experimental characterizations, numerical modelling and possible countermeasures. It is still an open question to precisely predict the SCS in large-scale HTS magnets. How to minimize the influence of SCF and SCS is one of the key technical challenges for the design of future UHF magnets.

With recent advances in second-generation high temperature superconductors (2G HTS) and cable technologies, various numerical models based on finite-element method (FEM) have been proposed to help interpret measured AC loss and assist cable design. The T-A formulation, implemented in COMSOL, shows great potential for reducing the overall computation costs. In this paper, the performance of the T-A formulation for calculating the AC loss of coated superconductors and cables were assessed and compared against the widely accepted H formulation, with benchmark model of a single REBCO tape in 2D/3D and a 14-strand Roebel cable. Evaluation and comparison on key metrics including the computation time, the number of degrees of freedom and the numerical accuracy were presented, which could provide a reference for researchers in applying the T-A formulation for AC loss calculation.

Considering coil stress management, conductor thermal stability, and susceptibility to delamination, the dry-winding approach has been gradually applied and tested in high-temperature-superconducting coils wound with REBa₂Cu₃O 7 − x (REBCO) coated conductors (CCs). In dry-wound coils, the winding tension can be a useful tool for managing the mechanical stresses and maintaining the structural stability. It is also related to the contact resistivity characteristics of no-insulation and metal-as-insulation coils. In this paper, we present a study on the winding stresses in dry-wound pancake coils considering nonlinear compressive behaviors in the radial direction. Coils wound with stainless-steel (SS) tapes and REBCO CCs were prepared, respectively, with various sizes (90–140 turns) and winding tensions (26–61 MPa). The radial pressure was obtained by applying the pressure measurement films, which cover a pressure range up to 50 MPa. A relatively moderate radial pressure buildup was observed in coils wound with SS tapes, indicating that the effective radial modulus of the coil as an entity is considerably lower compared to the tensile modulus in the azimuthal direction. This is consistent with the nonlinear stress–strain relationship of stacked thin sheets under transverse compression, which can be further explained by the contact behaviors between two rough surfaces. The average radial pressure in CC coils shared similar characteristics as that in SS coils, whereas the radial stress along the conductor edges were significantly higher compared to the middle. Cross-sectional microscopy of the CCs suggests that this can be primarily attributed to the thickness variations across the conductor width. These results suggest that the microscopic rough-surface contact and thickness variation of the wound tape play a critical role in the coil stiffness as well as the winding stress distributions.