Experiment and Numerical Modeling of Current Distributions Inside REBCO Tapes of No-Insulation Superconducting Magnet Coils Under Time-Varying Fields

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale no-insulation, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10-4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil's outer surface.

Section: Introductionmentioning

confidence: 87%

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

Harmsel

Otten

Dhallé

et al. 2023

“…These copper coils were powered by an AC transformer connected to the power grid and were calibrated by a Hall sensor measurement and simulation to generate 1.55 mT A −1 in the HTS coil volume with less than 3.7% discrepancy. The HTS coil used in this experiment has already been used and introduced and tested in detail in our previous work [61], which indicates a contact resistivity of 32.4 µΩ * cm 2 obtained from the sudden discharge test. The other parameters of the HTS coil, like the geometry, critical current and J c (B) parameters, are all as shown in table 1 [61].…”

Section: Methodsmentioning

confidence: 99%

“…Schnaubelt et al [54] built an NI HTS coil model based on the H−ϕ formulation with the turn-to-turn contact layers modeled as the shell elements. Mataira et al [55][56][57] proposed a method that models the NI coil as a homogeneous bulk set with an anisotropic resistivity; this method has been used and verified by quite a few groups in the world [58][59][60][61][62][63][64][65]. As a pure FEM model, this model is easy to realize, and very convenient for cohesive multi-physics coupling with other physical fields (e.g.…”

Section: Introductionmentioning

confidence: 99%

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

Zhong,

Wu,

et al. 2024

Self Cite

The no-insulation (NI) winding technique is promising to be applied in the persistent-current mode (PCM) operation of high-temperature superconducting (HTS) coils to provide those advantages. To apply the NI PCM coil, its demagnetization behaviour (i.e., decay of persistent DC current) by external AC field, which occurs in maglev trains, electric machines, and other dynamic magnet systems, is essential to be understood. For this purpose, a 3D FEM model, capturing the full electromagnetic properties of NI HTS coils, is established. This work has studied three kinds of AC fields, observing the impact of turn-to-turn contact resistivity on demagnetization rates, which is attributed to current distribution modulations. Under transverse AC field, the lower contact resistivity attracts more transport current to flow in the radial pathway to bypass the ‘dynamic resistance’ generated in the superconductor, leading to slower demagnetization. Under axial AC field, the demagnetization rate exhibits a non-monotonic relation with the contact resistivity: (1) the initial decrease of contact resistivity leads to concentration of induced AC current on the outer turns, which accelerates the demagnetization; (2) the further decrease of contact resistivity makes the current smartly redistribute to avoid to flow through the loss-concentrated outer turns, thus slowing down the demagnetization. Under the rotating DC field, a hybrid of the transverse and axial fields, the impact of contact resistivity on the demagnetization rate exhibits combined characteristics of the transverse and axial components. Besides, quantitative prediction on the demagnetization rate of NI PCM coil under external AC field is instructive for practical designs and operations, which is tried by this 3D FEM model, and a comparison with experimental result is conducted.

“…The equivalent anisotropic homogeneous model in [38] for finite-element model H-formulation in COMSOL software can reduce the 3D problem into its 2D cross-section. This 2D model enabled to describe screening currents in single NI pancake coils [38], and stacks of 4 [39] and 8 [40] pancake coils with relatively low number of turns, around one order of magnitude lower than in HTS inserts in ultra-high field magnets. The reason seems to be still high computing times.…”

Section: Introductionmentioning

confidence: 99%

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

Pardo,

Fazilleau

2024

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.