Development of general expressions for the temperature and magnetic field dependence of the critical current density in coated conductors with variable properties

Abstract

REBa 2 Cu 3 O y coated conductors have recently become viable for high field superconducting magnets and this use may be the principal present driver of coated conductor development. Driving the transport critical current density (J c ) as high as possible has become one of the principal goals of CC manufacturers but this can only be done by developing highly engineered nanostructures that may not be easy to control in quantity production of long lengths. Protection of high field (B) magnets operating in the temperature (T) of 4-20 K range is challenging and one key data set needed for accurate quench modeling is a wide-ranging J c (B, T) data set. At the National High Magnetic Field Laboratory (NHMFL), 12 km of REBCO tapes were purchased for the allsuperconducting 32 T user magnet that successfully reached field recently. They were characterized at 4.2 K with field orientation B perpendicular to tape and at 18°off the tape-plane axis. Of the tapes selected for 32 T, three were chosen for additional J c (B, T) characterization from 4.2 to 75 K in the B^tape orientation in fields from 1 to 15 T. Although all tape lengths were bought to the same advanced pinning specification, in fact there was substantial variation of more than 2 in the low temperature, high field J c . Here we probe the reasons for this variability in the context of measurements of the transport J c (B, T) dependence of 3 representative samples from this distribution with Ginzburg-Landau models of vortex pinning using a power law for J c (B) and an exponential temperature dependence for T<45 K and 3 T<B<15 T. A fourth tape from the 32 T magnet procurement with J c outside this range was then selected to test the validity of our modelling. Using this extensive data set, the correlation between J c (B, 4.2 K) and J c (B, T) enabled us to predict J c (B, T) for tapes procured for the 32 T magnet with an expected accuracy of 10% or less for T<40 K and B up to 15 T. Transmission electron microscopy made clear that the BaZrO 3 (BZO) size, volume fraction and density varied significantly across the range of conductors studied, suggesting that nano-structural control is difficult during coated conductor manufacture and that the resulting J c variations may have to be accepted in procurement practice.

Development and large volume production of extremely high current density YBa2Cu3O7 superconducting wires for fusion

“…1 a, inset), as well as for the entire production as we show below. This verifies the applicability of the lift-factor approach in the case of this simplified YBCO wire, in contrast to previous reports for other, more complex 2G HTS wire formulations 26 , 27 .

Figure 1 ( a ) Critical current, I c , in magnetic field ( B//c ) at 4.2 and 20 K of three YBCO wire samples measured at University of Geneva (red curves), Tohoku University (black curves) and the NHMFL at Florida State University (blue curves).

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“…The samples differed in substrate thickness: 40, 60 and 100 µm and YBCO layer thickness: 2.55, 2.82 and 3.28 µm, respectively. In all three samples very high values of critical current were achieved, among the best reported to date for commercial 2G HTS wire 7,26,[30][31][32][33][34] . In particular, an I c at 20 K, 20 T in the 220-270 A/4 mm range and an I c at 4.2 K, 20 T in the 450-570 A/4 mm range were measured.…”

“…For in-field experiments up to 15 T, we used the Oxford Instruments 15 T/17 T magnet system with a 52 mm cold bore. Samples were immersed in liquid helium during experiments at 4.2 K. Samples were in helium gas during experiments at 20 K 26 In experiments up to 31.2 T we used the NHMFL resistive magnet system (cell 7) with a 50 mm bore magnet; 38 mm in Janis cryostat. More experimental detail can be found in 38 .…”

“…We used the so-called “lift-factor” methodology in our result analysis. This methodology is accepted among 2G HTS wire manufacturers 27 , 28 , 36 ; however, it has certain applicability constraints and thus is often criticised 26 , 27 . Lift-factor is a simple empirical I c scaling parameter: it is defined as the ratio of a sample’s I c at a specific temperature and magnetic field to the I c of the same sample at 77 K in self-field.…”

“…(2) Employ uniformly distributed Y 2 O 3 nanoparticles, native to YBCO, as vortex pinning centres, thus keeping the composition and microstructure simple to facilitate reproducible fabrication. This contrasts with the common approach of introducing extrinsic nano-columns aligned about the REBCO c-axis as pinning centres [21][22][23][24][25] , a proven challenge to industrial implementation [25][26][27][28] . (3) Take advantage of the very low neutron cross-section of Y of 1.28 b compared to those of Gd (49,000 b) and Eu (4570 b), which is particularly important for application in a fusion reactor.…”