Manipulation of screening currents in an (RE)Ba 2 Cu 3 O 7− x superconducting magnet

REBCO coated conductors are now being used for building very high-field magnets with large electromagnetic stresses, both expected ones due to transport current ⃗ J × ⃗ B stresses and additional stresses resulting from the large screening currents inherent in wide tapes. Post mortem analyses of several recent test coils operated above 40 T show that significant conductor plastic deformation occurs, even for JBR stresses well below the ∼1 GPa yield of the Hastelloy substrate of the conductor. To investigate these deformation mechanisms, conductors were unwound after coil test and carefully examined with respect to their length-wise I c which revealed many areas of local damage. Regions of interest were examined by metallographic cross-section, Hall microscopy, magneto-optic imaging and scanning electronic microscopy. Important damage frequently occurred to the outer edges of pancakes in the coil ends, which were often plastically deformed over the whole turn circumference, especially when this outer edge was a slit edge. Internal conductor damage was also seen, especially delamination between the buffer and REBCO layers at slit edges. Careful sectioning of the tape at ∼10 mm intervals showed that the plastic deformation of the turns was complex and variable around the turn circumference, with tape cross-sections that exhibited continuous shape change in the outer turns. The bending center line of tapes often shifted from the tape center line toward the edge closest to the coil center, indicating asymmetric effects of transport and screening current stresses across the conductor width. A surprising and vital result is that damage was prevalent when the slit edge was also the edge at which transport current flowed. This damage was absent when the transport current flowed at the not-slit edge, implying great sensitivity of the effect of screening current stresses to localized conductor damage.

Section: The Beneficial Effects Of Incorporating Screening Current St...mentioning

confidence: 99%

Analyses of the plastic deformation of coated conductors deconstructed from ultra-high field test coils

Small

Kim

et al. 2020

“…The SCMFs calculated by the modified model and FEM along the central axis of the magnet 1 are in agreement, as shown in the inset. It can also be found that the SCMF is not centrally symmetric, as can be observed in [42]. Figure 7 shows the change in the screening current in four SPs of magnet 1.…”

Section: Comparison With the Finite Element Model And Original Modelmentioning

confidence: 63%

A modified model to estimate the screening current-induced magnetic field of a REBCO magnet

Tang¹,

Liu²,

Li³

et al. 2022

The experiments have showed that the actual value of magnetic field in the magnet wound by rare-earth (RE) Ba2Cu3O7−x (REBCO) tape during the operation is less than its design value due to the effect of screening current-induced magnetic field (SCMF). Some simulation methods have been used to calculate the SCMF. In this paper, a modified model is proposed to estimate the SCMF of the magnet based on the previous simple model. The inductance correction and field-dependent critical current density are considered in the modified model. Thereinto, two parameters, named reversal coefficient and Nagaoka coefficient, are introduced in the model. The former is used to track the location of the minimum SCMF value in the charging process, and the latter is applied to correct the induction of the magnet. The numerical results indicate that the SCMF estimated by the modified model is in agreement with those from experiments and finite element method (FEM). Moreover, the effects of electromagnetic and geometrical parameters on the reversal coefficient and Nagaoka coefficient are also investigated. Finally, the model is extended to estimate the SCMF of no-insulation (NI) magnet.

“…However, a full resolution model of such NI coils is practically difficult to implement due to the high aspect ratio of the superconductor and the need to consider the currents flowing normal to the conductor's surface. While progress has been made using circuit grid models (CGM) [18,[24][25][26][27], these models lose axial fidelity and cannot simulate the associated shielding currents, and associated stress [28], within the HTS windings [23,[29][30][31].…”

Section: Introductionmentioning

confidence: 99%

Finite-element modelling of no-insulation HTS coils using rotated anisotropic resistivity

Mataira¹,

Ainslie²,

Badcock³

et al. 2020

The no-insulation (NI) winding method is an effective technique for winding coils from high-T c superconductors (HTS). NI coils are electrically and thermally robust due to their ability to radially bypass current away from the fragile superconducting path when necessary. This avoids stored magnetic energy being entirely discharged on local defects in the HTS tape. However, the increased degrees of freedom for the current distribution makes finite-element modelling of these coils a complicated and multi-level problem. Here we present and validate a 2D axially symmetric model of an NI (or partially insulated) coil that captures all the inherent electromagnetic properties of these coils, including axial vs radial current flow and critical current suppression, and also reproduces the well-known charging and discharging characteristics. The model is validated against previously reported discharge measurements, and is shown to produce results consistent with the expected equivalent-circuit behaviour. Only by solving the NI coil problem with both axial and radial fidelity can the interplay of critical current anisotropy and turn-to-turn current be properly accounted for. The reported FE model will now enable coil designers to simulate key complex behaviours observed in NI coils, such as shielding currents, magnetic field inhomogeneity and remnant field effects.