The strain and temperature scaling law for the critical current density of a jelly-roll Nb3Al strand in high magnetic fields

Abstract

The engineering critical current density (J E ) and the index of transition, N (where E = αJ N ), of a Nb 3 Al multifilamentary strand, mass-produced as a part of the Fusion programme, have been characterized as a function of field (B), temperature (T ) and strain (ε) in the ranges B 15 T, 4.2 K T 16 K and −1.79% ε +0.67%. Complementary resistivity measurements were taken to determine the upper critical field (B C2 (T , ε)) and the critical temperature (T C (ε)) directly. The upper critical field defined at 5%ρ N , 50%ρ N or 95%ρ N , is described by the empirical relation B ρ N C2 (T , ε) = B ρ N C2 (0, ε) 1 − T T ρ N C (ε) ν . The upper critical field at zero 0953-2048/02/070991+20$30.00 © 2002 IOP Publishing Ltd Printed in the UK b 1 2where the Ginzburg-Landau (GL) parameter κ is given by the relation, γ is the Sommerfeld constant and t = T/T C (ε). At an applied field equal to the upper critical field found from fitting the Kramer dependence (i.e. at B C2 (T , ε)), the critical current is non-zero and we suggest that the current flow is percolative. The functional form of F P implies that in high fields the grain boundary pinning does not limit J E , this is consistent with J E -microstructure correlations in other superconducting materials.

Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet

“…1,22 Access to variable-temperature and variable-strain measurements (in one probe) on a single sample enabled a resolution of the apparent contradiction between strain scaling (found at fixed temperature) 18 and temperature scaling (at fixed strain). [23][24][25][26] This led to a general scaling law 23,24,27 for LTS materials which includes a 1/κ 2 term 28 (κ: GinzburgLandau constant) that gives best fits to the data for Nb 3 Sn 23,29 and Nb 3 Al 30 and is consistent with computational data 31 and general analytic forms. 32,33 An excellent bending beam apparatus was also developed at the University of Twente for short samples of LTS wires.…”

Reversible and irreversible effects of strain on the critical current density of a niobium–tin superconducting wire

“…These results confirm that, to achieve high accuracy, A in Eq. (1) cannot be a constant but must be a function of strain [2,4,9].…”

“…The wire was heat-treated in an argon atmosphere on a stainless steel mandrel and then transferred, copperplated and soldered to a copper-beryllium alloy spring sample-holder using a procedure well documented elsewhere [8,9]. The strain was applied by twisting one end of the spring with respect to the other, where the magnitude of the strain had been previously calibrated using standard cryogenic strain gauges [8].…”

Visualizing the mechanism that determines the critical current density in polycrystalline superconductors using time-dependent Ginzburg-Landau theory

“…The prefactor A is weakly width dependent, but for the data shown is 1.6ϫ 10 −3 , which is within 20% of the experimental values in Nb 3 Sn, 8 and Nb 3 Al. 30 Figure 3 compares J C for equivalent 2D and 3D systems ͑an inner grain boundary resistivity N = S was chosen to speed the computation, effectively giving monolayer grain boundaries͒. In three dimensions, the Kramer dependence of J C extends to lower fields than that in two dimensions.…”