Strain Distribution in REBCO-Coated Conductors Bent With the Constant-Perimeter Geometry

Abstract

Abstract-Cable and magnet applications require bending REBa2Cu3O 7−δ (REBCO, RE = Rare Earth) tapes around a former to carry high current or generate specific magnetic fields. With a high aspect ratio, REBCO tapes favor the bending along their broad surfaces (easy way) than their thin edges (hard way). The easy-way bending forms can be effectively determined by the constant-perimeter method that was developed in the 1970s to fabricate accelerator magnets with flat thin conductors. The method, however, does not consider the strain distribution in the REBCO layer that can result from bending. Therefore, the REBCO layer can be over-strained and damaged even if it is bent in an easy way as determined by the constant-perimeter method.To address this issue, we developed a numerical approach to determine the strain in the REBCO layer using the local curvatures of the tape neutral plane. Two orthogonal strain components are determined: the axial component along the tape length and the transverse component along the tape width. These two components can be used to determine the conductor critical current after bending. The approach is demonstrated with four examples relevant for applications: a helical form for cables, forms for canted cos θ dipole and quadrupole magnets, and a form for the coil end design. The approach allows to optimize the design of REBCO cables and magnets based on the constantperimeter geometry and to reduce the strain-induced critical current degradation.

Finite-Element Analysis of the Strain Distribution Due to Bending in a REBCO Coated Conductor for Canted Cosine Theta Dipole Magnet Applications

“…Additionally, in the FEA a tension of 2 N was applied to numerically execute the winding, while experimentally the tape was wound by hand without measuring the tension applied. However, the FEA results agrees with the strain calculated analytically using equations in [12]. Additionally, the FEA results show that the longitudinal strain is not constant across the tape's width.…”

“…A uniform load of 2 N is applied on the opposite edge of the tape to maintain tension during winding. On the same edge, a displacement of 6.6 mm (winding pitch from [12]), along the width's direction of the tape is applied. The former is modelled as an elliptical surface.…”

“…The effect of pure bending on specific cables configurations has been studied in [9]- [11]. In [12], a curvature-based analytical model was developed to predict the strain generated in the REBCO layer due to bending but it does not account for the variation in the strain distribution caused by the local deformations of the tape and the Poisson's effect.…”

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…The winding strain on the REBCO layer depends on the conductor architecture and magnet design. Conductors with a high aspect ratio such as the Roebel cable can minimize the strain in REBCO layers by following the constant-perimeter principle for magnet design and conductor bending [100][101][102][103][104]. From this aspect, round REBCO conductors are more flexible and can work with diverse magnet designs, although to understand the critical current degradation and reduce the strain development is still critical [30,[105][106][107][108][109][110].…”

“…Understanding how strain develops in REBCO layer during the cabling and magnet winding processes will be critical [32,104,106,[108][109][110]195,196]. The design tools based on the understanding of the conductor mechanics will help guide the development of more flexible round REBCO conductors and the optimized REBCO tapes with thinner substrates and narrower widths [34,38,160].…”