A simple screening current-induced magnetic field estimation method for REBCO pancake coils

Self Cite

The screening current induced in rare-earth barium copper oxide (REBCO) tape generates an unwanted irregular magnetic field. The screening current-induced field (SCIF) is a challenging issue for MRI, NMR, and accelerators magnet composed of REBCO coils. A few FEM-based simulation methods have been proposed to estimate the SCIF; however, they require a long computation time.Recently, we have proposed a simple SCIF computation method based on the self and mutual inductances of REBCO pancake coil and screening current radial paths on the top and bottom of pancake coils. Accuracy of the proposed method is not excellent; however, computation time is quite short. In this paper, we report an equivalent circuit model that includes the self and mutual inductances of a REBCO pancake coil and screening current radial path. Moreover, with this proposed method, we can compute SCIF of no-insulation (NI) REBCO pancake coils, while it is not the case with the previously proposed FEM-based simulation method. The proposed method has been validated by experiment. The proposed method is available online.

Section: Screening Current-induced Magnetic Field Simulation Methodsmentioning

confidence: 99%

Section: Two Double-pancake and Four Single Pancake Coils (All Turns Insulated)mentioning

confidence: 99%

A simple screening current simulation method using equivalent circuit model for REBCO pancake coils

Noguchi¹,

Imai²,

Park³

et al. 2020

Self Cite

“…The THD factor, defined in (8), is calculated for the field error results for the first and second prototype (Fig. 15 and 16).…”

Section: Field Error Cancellationmentioning

confidence: 99%

High-temperature superconducting screens for magnetic field-error cancellation in accelerator magnets

Bortot

Mentink

Petrone

et al. 2021

Accelerators magnets must have minimal magnetic field imperfections for reducing particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the field imperfections from persistent currents need to be carefully evaluated. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field components, such that only the undesired field components are compensated. The screens are passive, self-regulating, and do not require any external source of energy. Measurements in liquid nitrogen at 77 K show for dipole-field configurations a significant reduction of the magnetic-field error up to a factor of four. The residual error is explained via numerical simulations, accounting for the geometrical imperfections in the HTS screens, thus achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at 4.5 K, field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets.

“…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.