Influence of twist pitch on hysteretic losses and transport J _c in overpressure processed high J _c Bi-2212 round wires

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 similar to 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.