Prediction of the J_C (B) Behavior of Bi-2212 Wires at High Field

Critical Current Distributions of Recent Bi-2212 Round Wires

“…2(b)) that the older and lower-J c Nexans wire has about 40% higher σ/µ (∼ 0.1 vs. 0.14). As noted earlier by Brown et al [6], many wires made with Nexans, Engi-Mat and other powder sets had the same normalized J c (H) characteristics, even though the magnitude of J c varied by almost 6. We conclude that the effective percolative connectivity of wires is what is really controlling the J c magnitude.…”

“…In this work, we have compared the I c distribution of two Bi-2212 RWs made by B-OST prepared with newer and better and older and worse quality powders from Engi-Mat and Nexans. We were motivated to better understand the conclusion of Brown et al [6] that the magnitude of J c in Bi-2212 wires was almost completely determined by the effective connectivity of the current path which we interpreted to be very small since the ratio J c /J d is of order 1% or less. Even if the vortex pinning in Bi-2212 is weak (it is…”

“…In a well-optimized material like Nb − Ti where current is believed to flow uniformly, magnetization and transport measurements typically agree well when corrections for electric field (E) differences are made (and filament sausaging is avoided). Bi-2212 is more complex: the current path is percolative and uncertain with a fractional occupancy of as little as 1% [6]. Moreover, most transport measurements typically characterize only ∼ 1 cm of conductor due to use of straight wires in narrow-bore magnets.…”

“…Although its J c (16 T, 4.2 K) = 6500 A • mm −2 far exceeds the Future Circular Collider (FCC) specification of J c (16 T, 4.2 K) = 1500 A • mm −2 , J c for state-of-the-art Bi-2212 is still lower than 1% of the depairing current density [4] (J d ∼ H c /λ ∼ 3 × 10 6 A • mm −2 , where the thermodynamic critical field H c ∼ 1 T [5] and the penetration depth (λ) of Bi-2212 ∼ 240 nm), as opposed to the 20 -30% values achieved with Nb − Ti or REBCO. Our recent comprehensive survey of many Bi-2212 wires made between 2009 and 2019 has shown that while J c defined by I c /A, where A is the total Bi-2212 cross-section, varied by almost a factor of 6 they actually all had almost identical normalized J c (H), leading us to conclude that very similar vortex pinning properties were shared by all wires and that |J c | was determined by the effective filament connectivity [6]. Filament connectivity is affected by multiple factors on many length scales, including filament cross-section variations before reaction and post-reaction defects such as, cracks, voids, blocking secondary phases, and grain-to-grain connectivity variation.…”

The Conundrum of Strongly Coupled Supercurrent Flow in Both Under- and Over-doped Bi-2212 Round Wires

“…The round-wire form is very desirable for magnet applications because it allows the conductor to be produced in a low-hystereticloss, multi-filamentary, twisted state in a macroscopically isotropic architecture that provides high magnetic field quality. The paradox of round wire Bi-2212 is that it is now possible to make it with some 6 times higher J c [12][13][14] than carefully textured Bi-2223 tapes [15,16], a conundrum that seems to violate much of our hard-won understanding of HTS grain boundaries over the last 30 years.…”