Abstract
We present experimental data on the ac losses in Roebel cables with striated 2G REBCO strands. Electromagnetic measurements by pick-up coils were carried out at different temperatures between 10 K and 85 K at frequencies between 2 Hz and 20 kHz in transverse ac fields up to 0.5 T. The Roebel cable samples were assembled from laser cut 2G REBCO strands, which consist of striated filaments prepared with laser scribing. Measurements at different temperatures show that the Roebel cables with striated strands have a lower losses by a factor of strand filament number over the whole temperature range. For analysis, a semiquantitative approach for unstriated Roebel was found to work satisfactorily. However, the Striated Roebel cables appears to have a different field penetration from those of the unstriated Roebel. The coupling current among the filaments has a time constant shorter than 0.1 ms and do not appear to increase in the striated Roebel cables, hence insignificant for most magnets related applications.
Highlights
H IGH current cables are essential for some superconducting devices such as accelerator magnets, superconducting magnetic storage coils and superconducting fault current limiters
One of the few possibilities is that of a Roebel bar [1]–[4] which was originally invented more than a hundred years ago for reducing the eddy current losses in high current cables/bus-bars for conventional machines
We attempt to a semi-quantitative analysis of the ac losses of Roebel cables based on the applied field losses of a thin strip
Summary
H IGH current cables are essential for some superconducting devices such as accelerator magnets, superconducting magnetic storage coils and superconducting fault current limiters Such cables are required to consist of several transposed superconducting strands to ensure uniform current distribution for field quality and cryogenic stability. With the growing availability of 2G REBCO tapes in long length, several laboratories [2], [3] have been producing prototype Roebel cables with punched or laser-cut strands for the important studies on the critical current performance and manufacturing optimization. The hysteresis loss in superconductors is most effectively reduced by subdivision into fine filaments In addition these filaments must be twisted at a sufficiently short pitch in order to ensure that they are magnetically uncoupled under desired ac conditions. The results, together with data from single striated strands and standard Roebel cable, confirm that striated filaments are uncoupled in Roebel cables with further loss reduction from that found in an earlier study [10]
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