38 mm diameter cold bore metal-as-insulation HTS insert reached 32.5 T in a background magnetic field generated by resistive 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: Introductionmentioning

confidence: 99%

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

Small

Kim

et al. 2020

“…It possesses the integrated properties of fast charging speed as the insulation coils and self-protection as the no-insulation coils [17]. Due to these merits and the mechanical robustness of the coil itself, as well as the alleviation of the transient unbalanced stress in the quench issue, metal-insulation coated conductor coils have been gradually adopted in building high-field magnets [18][19][20]. However, generally, coils with single-layer metal-insulation can barely have both large contact resistivity and strong thermal conductivity.…”

Section: Introductionmentioning

confidence: 99%

Performance enhancement of coated conductor magnet with double-layer metal insulation

Wang

Zhou

et al. 2023

The double-layer metal-insulation method using the brass sheets as the double-layer insulators was proposed in this paper. It can enhance the contact resistivity while reserving greater thermal conductivity merit. The underlying mechanism of the contact resistivity enhancement is to increase the number of contact surfaces and to degrade the contact quality between the insulators. Then, we wound a single-layer brass-insulation coil and a double-layer brass-insulation coil to compare their contact resistivities and confirmed the effectiveness of the double-layer metal-insulation method. Besides, since the capacity withstanding the overcurrent is weakened with the increscent contact resistance of the metal-insulation coil, we further investigated the influence of the contact surface resistivity distribution on the coil performance under different scenarios to optimize the double-layer metal-insulation coil for receiving a superior thermal stability. Simulation results indicated that the coil possessed the dominated 2nd contact surface resistivity and minimum 1st and 3rd contact resistivity is the optimal design for the double-layer metal-insulation coil to receive the best thermal stability, irrespective of the cooling environment, contact resistivity magnitude, operating current and coil dimension. In addition, in regard to the thermal performance differences caused by the contact surface resistivity distribution, we found that the increment of contact surface resistivity and the overcurrent would enlarge the distinctions at different levels.

“…This risk can be significantly lowered by the adoption of the dry-winding approach, besides the added benefits of simplified manufacturing process and increased engineering current density. It has been used in many prototype coils [6][7][8] and magnet applications [9][10][11][12][13][14][15] in recent years.…”

Section: Introductionmentioning

confidence: 99%

Measurement and analysis of winding stresses in dry-wound pancake coils considering nonlinear compressive behaviors

Yan,

Kim,

Bong

et al. 2023

Considering coil stress management, conductor thermal stability, and susceptibility to delamination, the dry-winding approach has been gradually applied and tested in high-temperature-superconducting coils wound with REBa2Cu3O 7 − x (REBCO) coated conductors (CCs). In dry-wound coils, the winding tension can be a useful tool for managing the mechanical stresses and maintaining the structural stability. It is also related to the contact resistivity characteristics of no-insulation and metal-as-insulation coils. In this paper, we present a study on the winding stresses in dry-wound pancake coils considering nonlinear compressive behaviors in the radial direction. Coils wound with stainless-steel (SS) tapes and REBCO CCs were prepared, respectively, with various sizes (90–140 turns) and winding tensions (26–61 MPa). The radial pressure was obtained by applying the pressure measurement films, which cover a pressure range up to 50 MPa. A relatively moderate radial pressure buildup was observed in coils wound with SS tapes, indicating that the effective radial modulus of the coil as an entity is considerably lower compared to the tensile modulus in the azimuthal direction. This is consistent with the nonlinear stress–strain relationship of stacked thin sheets under transverse compression, which can be further explained by the contact behaviors between two rough surfaces. The average radial pressure in CC coils shared similar characteristics as that in SS coils, whereas the radial stress along the conductor edges were significantly higher compared to the middle. Cross-sectional microscopy of the CCs suggests that this can be primarily attributed to the thickness variations across the conductor width. These results suggest that the microscopic rough-surface contact and thickness variation of the wound tape play a critical role in the coil stiffness as well as the winding stress distributions.