Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Abstract

Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

Magnetization Losses of Roebel Cable Samples With 2G YBCO Coated Conductor Strands

“…While the potential for high current and the strong flux pinning in high magnetic fields favor the YBCO 2G coated conductors, the effectiveness of current homogenization and magnetization reduction by strand uncoupling using Roebel transposition remains to be optimized and verified. While there have been a number of experimental studies [6][7][8][12][13] on the ac loss in Roebel cables, their scopes have been limited to 77 K. The published results are not quite sufficient for the accelerator applications, in particular for quantifying strand coupling and identifying the transposition effects.…”

High temperature superconductor (HTS) cables

“…A method to enhance the number of tapes over N max is to assemble stacks of punched tapes instead of single tapes as strands of a cable (Goldacker et al , 2009a;Terzieva et al , 2010). Figure 3.10 shows such RACC cables with the same width (∼4.4 mm) and transposition length (109 mm) but with different numbers of assembled tapes.…”

Application of high temperature superconductors for fusion

“…Reduction of the tape width or filamentarization of the superconducting layer (e.g. laser striations) can help to reduce the hysteresis losses [14]. Coupling losses of superconducting cables can be reduced by enhancement of the inter-strand resistance.…”

“…By enlarging the dimensions of former and twist length and by increasing the tape number in each Roebel strand (e.g. by assembling stacks of tapes instead of single tapes [14]), the total number of punched tapes can easily be enhanced far above 1000. With the average current carrying capacity I c (77 K, self field) ∼62 A of each punched tape, the J c (B, T)/J c (B = 0, 77 K) dependence in Fig.…”