Screening current effect on the stress and strain distribution in REBCO high-field magnets: experimental verification and numerical analysis

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

“…Plastic deformation was observed on the HTS tapes during post mortem analyses of the LBCs [6,18]. Subsequently, the effect of SCIS was demonstrated quantitatively on small coils by direct measurements of the in-field hoop strains [19][20][21]. Theoretical models taking into account the turn-to-turn contacts in the dry-wound structure and the distribution of the screening currents were developed and applied to large-scale magnets [22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Screening-current-induced mechanical strains in REBCO insert coils

Yan

Song

Xin

et al. 2021

Self Cite

In recent years, remarkable progresses have been made in the R&D efforts for high temperature superconducting (HTS) high-field magnets. The screening-current-induced magnetic field and mechanical stress/strain in rare-earth barium-copper-oxide (REBCO) coils are raising growing concerns. This paper presents experimental and theoretical analyses on the effects of screening-current-induced mechanical strains in a REBCO insert setup without transport currents. Strain measurements on two REBCO coils, with and without over-banding structures, were carried out in an low temperature superconducting (LTS) background field magnet. Electromagnetic-mechanical models were developed, coupling the tilting angles of the superconducting tapes and the strain dependency of the critical current. The discrete-coupled model with turn-to-turn contacts, the discrete-sequential model and the block model were implemented and compared against the measured hoop strains. Combining experimental data with simulation models, it is shown that moderate over-banding structures are effective in providing reinforcements for the superconductors. A new method of edge-bonding was proposed and tested, which could reduce hoop strains in low applied fields but failed in higher fields. This work could be useful for the design and analysis of future high-field REBCO magnets.

“…Meanwhile, the screening current (SC) induced in the REBCO CCs generates an undesired magnetic field [12,13,14,15]. Recently, it was pointed out that the screening current may substantially enhance magnetic stress in REBCO magnets [16,17,18,19,20,21]. To estimate the peak stress accurately, the importance of the accurate computation of the screening current distribution must be enhanced.…”

Section: Introductionmentioning

confidence: 99%

A newly developed screening current simulation method for REBCO pancake coils based on extension of PEEC model

Noguchi¹,

Hahn²

2022

Since the screening current (SC) in rare earth-barium-copper-oxide (REBCO) coated conductor (CC) generates an undesired magnetic ﬁeld, it must be accurately estimated, especially for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). Moreover, in recent years, it was pointed out that the screening current enhanced the stress/strain in REBCO CC, when an REBCO magnet was operated as an insert under an ultrahigh magnetic ﬁeld. The previously reported SC simulation methods may be roughly categorized into ﬁnite element method (FEM) and equivalent circuit method. The FEM-based method often adopted an axisymmetric model or a thin ﬁlm approximation model, while the circuit-based are the simple equivalent circuit model and the network equivalent circuit model, so-called the partial element equivalent circuit model. The latter is newly proposed in this paper. Features of those SC simulation models are brieﬂy compared to each other in this paper. Each SC simulation models have pros & cons. We have to adequately chose an SC simulation model depending on a purpose. In addition, we propose a new developed SC simulation model, named the advanced partial element equivalent circuit (A-PEEC) model. It is extendable to an SC simulation of no-insulation REBCO pancake coils. To simulate the SC of a simple coil model and the LBC3 magnet, we investigated the screening current distribution maps, and the simulated screening current-induced ﬁelds were compared with the measurements. We have conﬁrmed the validity of the newly proposed A-PEEC model.