Understanding Magnetization Losses of Roebel Cables With Striated REBCO Strands

Abstract

High current superconducting cables are essential for past and future accelerator and fusion magnets. The low temperature superconducting (LTS) cables for the LHC and ITER machine owe their success to the effective minimisation of the magnetisation in the LTS wires by incorporating twisted fine filaments. In contrast, the magnetisation of REBCO tapes remains significant in assembled strands of Roebel cables and twisted stacks cables. The quantitative details the magnetisation loss have not been sufficiently elaborated due the 3D nature of the strand assembling and/or twisting. Although full 3D modeling of Roebel cables has been made, the separation of loss components is less straightforward due to the complexity of their interplay. By using simplified 1D models based on conceptual reasoning, it was shown in our previous studies that (a) Roebel cables with $(2m+1)$ REBCO strands of critical current $I_{c}$ are essentially two side-by-side stacks of $m$ transposed strands and each stack is effectively a single Norris' strip of $\sim m\times I_{c}$ but also magnetically coupled to the other via the strong demagnetisation effect in the narrow gap in between, and (b) full decoupling into isolate tapes is only achieved in the single strand in transposition “flights” from one stack to another. Consequently, the magnetisation of Norris' strip can be extended straightforwardly for loss calculations in simple algebra forms to achieve satisfactory agreement with experimental results. On the other hand, the ac losses measured on Roebel cables with striated REBCO exhibited significant differences which are yet to be fully understood. Using qualitative arguments together with 1D analytical results as well as numerical modeling, this work show the “filaments” in a striated strand are also magnetically coupled hence behave considerably differently from a set of isolated filaments of Norris' strips. It then explains that small random misalignments among the striated strands when assembled into a Roebel cable would alter significantly the magnetic coupling within the stacks and result in the ac loss behaviour observed in experiments.