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

Abstract

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 (2D) cross-sectional model for the electromagnetic response of NI and MI coils, which is based on the Minimum Electro Magnetic Entropy Production (MEMEP). 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 (LTS) outsert. In particular, we analyze the current density, the screening curren 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. 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 modeling that also includes mechanical properties.

Modelling the mechanics of 32 T REBCO superconductor magnet using numerical simulation

“…As shown in [47], the radial currents are negligible in metal-insulated magnets under regular operation. Thus, we can assume that the Lorentz forces in metal-insulated magnets are the same as in the insulated case.…”

“…The REBCO insert design is for a metal-insulated coil, where a stainless-steel (SS) tape is co-wound with the superconductor. Taking into account the high turn-to-turn surface resistance through the SS (around 10 −6 -10 −7 Ωm 2 ), the radial currents can be neglected [47]. Thus, the tapes can be treated as electrically insulated.…”

“…In this work, we compute the local current density, taking screening currents into account by the MEMEP, as described in [39,43,47]. For simplicity, we assume an axi-symmetrical geometry for the pancake coils (see figure 2), which is justified because the tape and insulation thickness is much smaller than the coil inner radius.…”

“…As input, we use J c (B, θ) according to the analytical fit of the experimental data in [51] for a Fujikura tape. We also consider the resistance of the metal stabilization, substrate and isolation by taking a homogenized anisotropic resistance into account, as in [47]. In essence, this homogenization assumes that the superconducting layer and the metals are connected in parallel for the angular current density, J φ , while they are connected in series for the radial current density, J r .…”

“…In this work, we assume J r = 0 for simplicity (noninsulated coil). The reason is that the screening current after several minutes of charging the magnet in metal-insulated and non-insulated windings is the same as in the insulated case [47]. For axi-symmetric problems with insulated coils and current constraints, it is sufficient to minimize the following functional [43]:…”