(RE)BCO commercial coated superconductors have been gaining an increasing interest due to the potentialities of using them in high-field magnets. The leading project is the 32 T user magnet recently tested successfully at full field at the NHMFL, Florida, USA. This state-of-the-art high field all-superconducting magnet, bath-cooled at 4.2 K, is comprised of a two-nested-coil insert pancake-wound with (RE)BCO tapes supplied by SuperPower Inc. and a multi-coil LTS outsert. To ensure the magnet reliable operation, it is important to estimate the hysteresis losses. Such an estimate will allow implementing safe operational procedures to avoid premature quenching and, in the worst case scenario, the insert failure. The insert coils have thousands of turns, with notable variations in the critical current. Therefore, estimating the losses in such a large superconducting magnet presents a significant challenge. We propose here a new approach relying on a multi-scale scheme to achieve a high computational efficiency. This new method is flexible enough to simulate different sections of the entire insert with the right level of detail while providing a larger computational speed than other approaches using the finite element method. Estimates of the hysteresis losses in the insert coils for a ramping operation sequence are presented.

The most detailed, first-principle simulation model of a quench in a high-temperature superconducting metal-insulation multi-module/multi-double-pancake coil, wound with a REBCO coated superconductor/tape and normal-metal tape (co-wind), was developed. It enables one to simulate a normal zone propagation, or non-propagation, in the subtle multi-turn structure of the coil's pancakes due to the inductive coupling and transverse electric contact between all the REBCO turns and co-wind turns along with the external circuit and active and passive quench protection systems. In doing so, the multi-module coil's structure is represented as a “rectilinear equivalent” electric circuit, a branched multi-decker multi-rectangle planar grid, which facilitates the mathematical formulation, using the standard electric circuit differential equations and the Kirchhoff's rules. The transient non-linear comprehensive heat conduction equations are employed to model the thermal part of the phenomenon at the same level of detail. All this turns the model into an effective design tool for metal-insulated, -stabilized and/or -reinforced multi-module coils. A parametric study of quench in a 6-module no-insulation (NI) test coil, using a stainless-steel thin co-wind, was conducted to identify the possibilities of the model. The profound effect of the transverse contact resistivity on an NI-coil quench behavior is confirmed and detailed.

Inclusion of screening current induced stress (SCIS) is becoming more commonplace in the design of ultra-high field magnets with REBCO tapes. It is desired to comprehensively manage tensile hoop stress, compressive hoop stress, and tilting angle caused by the SCIS. In this study, we have implemented effects of winding and cooldown processes to an already developed FEM model and analyzed the stress/strain behavior considering the SCIS on REBCO pancake coils in a 35 T LTS/HTS magnet with co-winding or over-banding reinforcement. Effects of thermal contraction and winding tension, and advantages/disadvantages of the co-winding or over-banding method are discussed.

The National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL, USA, has developed a 32-T all-superconducting user magnet system combining two series-connected high-field high-temperature superconductor (HTS) nested inner coils (insert) wound with SuperPower REBCO tapes and a low-temperature superconducting (LTS) outer magnet (outsert) composed of five coils (broken in 17 electrical sections). As a part of the magnet development work, a number of quench tests were done on the HTS insert dual-coil prototype in the actual LTS outsert, mimicking the actual magnet configuration and operation. The tests on the coupled insert/outsert system were performed at 4.2 K, using a set of tailor-made quench protection heaters to be employed in the actual insert. In these tests, in the transition to the normal state, a noticeable part of the outsert stored energy could be transferred to the insert through the inductive coupling. The experimental results are analyzed here by means of a quasi-3-D COMSOL FEM model. The model, developed at the University of Bologna in collaboration with the NHMFL, enables one to calculate the coil temperature distributions, the internal voltages, and current evolution through the increase of normal zone resistances over the insert coils. A lumped parameter equivalent model of the magnet electric circuit permits including appropriately the effect of mutual inductive coupling between the insert and the outsert coils on the currents in them. Also, the model accounts for a time-varying 2-D distribution of the magnet field generated by both the insert and outsert within their coils in the event of a quench. The REBCO insert coils are described in terms of an anisotropic homogenized medium, albeit with due regard for the essentially nonlinear characteristics of the superconducting tapes and for the interturn stainless steel reinforcement and sol-gel alumina insulation. A detailed comparison between the numerical and experimental results provides detailed insights into the modes of propagation of the normal zones.

High-temperature superconducting magnet coils made with Sumitomo Type HT-NX are complex composite structures composed of Bi-2223 conductor filaments, silver matrix, solder, nickel-alloy laminations, polymer insulation, and epoxy or wax. The mechanical properties of these composites are required inputs to a correct stress analysis. Measurements of the desired properties are performed on representative model test specimens. Mechanical test specimens composed of several layers of insulated conductor are prepared by cutting to length, stacking and epoxy impregnation. Mechanical tests are performed in liquid nitrogen and liquid helium. Elastic constants are found from tensile strain measurements in the conductor longitudinal, or coil hoop, direction and from compressive strain measurements in the conductor transverse, or coil radial and axial directions.

The National High Magnetic Field Laboratory (NHMFL) has completed testing and commissioning of a unique ultrahigh field magnet that provides 36 T in a 32 mm bore with field inhomogeneity and stability better than 1 ppm over a cylinder of 1 cm diameter and length and a duration up to a few hours. While the magnet meets all its primary performance goals, there were some unexpected observations during the testing and commissioning activities. Primarily, while the magnet reaches full current and field, we are unable to charge the magnet as quickly as was anticipated. Also, the magnet occasionally quenches for no apparent reason. Charging the magnet at slower rates reduces the probability of quenches.