Bi-2212 is the only high field, high-temperature superconductor (HTS) available in the macroscopically isotropic, multifilament high J _c round wire (RW) form capable of generating high uniformity fields with minimum-screening current errors. However, the heat treatment that enables impressively high J _c (4.2 K, 30 T) values that can attain ∼5000 A mm⁻² also produces significant filament bonding (bridging). Filament bridging appears to significantly enhance hysteretic losses of the filaments themselves by coupling neighboring, nominally independent filaments, enabling shielding currents to flow across multiple filaments as though they were one filament of much larger diameter. Wire twisting can be employed to reduce filament-to-filament eddy current coupling losses due to induced currents flowing across the matrix, but twisting is less effective in reducing increased losses from bridging. Here, we compare the twist-pitch dependence of the losses of overpressure processed (OP) high J _c Bi-2212 RWs with partially bridged filaments to those found in OP Bi-2212 RWs with discrete, not-bridged filaments. We show that filament sub-bundles in standard, partially-bridged wires that have some superconducting connections between filaments can exhibit significant coupling (much larger effective filament diameter), but twisting still reduces their hysteretic losses to values close to or below the ITER Nb₃Sn wire loss specification, even though Bi-2212 wires have significantly larger J _c values. Although it has been reported that twisting can reduce wire J _c by damaging filaments, we found no reduction in transport J _c , even for nominal twist pitches of 12 mm in 0.8 mm diameter wires. Evaluation of more-recent, higher J _c Engi-Mat powder wires showed that their reduced filament bridging and improved longitudinal connectivity significantly improved transport J _c and reduced the J _c normalized losses, signaling that J _c can be further improved without commensurate increase in losses. This important result strengthens the argument for production of high field, low loss HTS magnets made with Bi-2212 RWs.

中文翻译：

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捻距对超压处理的高 J c Bi-2212 圆线中滞后损耗和传输 J c 的影响

Bi-2212 是唯一可用于宏观各向同性、多丝高场的高场高温超导体 (HTS)杰 _C 圆线 (RW) 形式能够产生具有最小屏蔽电流误差的高均匀性场。然而，使令人印象深刻的高热处理杰 _C (4.2 K, 30 T) 值可以达到～5000 A mm ^-2也产生显着的灯丝粘合（桥接）。灯丝桥接似乎通过耦合相邻的、名义上独立的灯丝显着增强了灯丝本身的滞后损耗，使屏蔽电流能够流过多根灯丝，就好像它们是一根直径大得多的灯丝一样。由于感应电流流过矩阵，可以采用绞线来减少灯丝到灯丝的涡流耦合损耗，但绞线在减少桥接增加的损耗方面效果较差。在这里，我们比较了超压处理 (OP) 损失的扭距依赖性高杰 _C 具有部分桥接灯丝的 Bi-2212 RW 与 OP Bi-2212 RW 中发现的具有分立、未桥接灯丝的那些。我们表明，标准的部分桥接导线中的灯丝子束在灯丝之间具有一些超导连接，可以表现出显着的耦合（有效灯丝直径大得多），但扭曲仍然可以将它们的滞后损耗降低到接近或低于 ITER Nb 3 的_值Sn 线损耗规格，即使 Bi-2212 线的损耗明显更大杰 _C 值。虽然有报道称绞合可以减少电线杰 _C 通过损坏灯丝，我们发现运输没有减少杰 _C ，即使对于 0.8 毫米直径导线中 12 毫米的标称绞距。评价越新，越高杰 _C Engi-Mat 粉末线材表明，它们减少的细丝桥接和改进的纵向连通性显着改善了传输杰 _C 并减少了杰 _C 归一化损失，表明杰 _C 可以在不相应增加损失的情况下进一步改进。这一重要结果加强了使用 Bi-2212 RW 生产高场、低损耗 HTS 磁体的论据。