There is currently no consensus on how best to parameterize the large volume of data produced in measuring the magnetic field (B), temperature (T) and strain (ε) dependence of the engineering critical current density (JE(B, T, ε)) for A15 superconducting strands. For the volume pinning force (FP) and the upper critical field BC2(T, ε), we propose given b = B/BC2(T, ε) and t = T/TC(ε) where TC(ε) is the critical temperature. FP (or JE(B, T, ε)) includes three strain-dependent variables α(ε), BC2(0, ε) and TC(ε) and four constants, n, p, q and v. The form is different to that proposed by Summers et al by a factor T2C(ε). We suggest that the form is sufficiently general to describe superconductors whether the electron–phonon coupling is weak or strong and find that α(ε) is proportional to where Δ(ε) is the superconducting gap and γ(ε) is the Sommerfeld constant. Comprehensive JE(B, T, ε) data are presented for a modified jelly-roll (MJR) Nb3Sn conductor that are consistent with the form proposed with n ≈ 5/2, p = ½, q = 2 and v = 1.374. Hence the scaling law proposed leads to a critical current density for the MJR Nb3Sn given by Comparison with data in the literature suggests that α(ε) ≈ 3 × 10−3μ0γ(ε). Furthermore, the volume pinning force (FP(S/C)) within the Nb3Sn superconducting filaments alone can be described in terms of superconducting parameters in the form where κ(T, ε) is the Ginzburg–Landau parameter.

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.

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Persistent current joints are critical components of superconducting magnets, and improvements in their properties are key to the production of next generation devices. Research into joints is presently constrained by the inability to easily characterise their superconducting properties both quickly and with high precision. Demonstrated here is a novel inductive resistance testing (IRT) technique performed using a widely available commercial magnetic properties measurement system (MPMS). The technique offers a simple, rapid and highly sensitive means of obtaining the key superconducting properties pertinent to joint performance; I c (B,T) and R(I,B,T). This method enables the extremely fast characterisation of joints in unprecedented detail, which can help to improve our understanding of their superconducting properties and ultimately accelerate the development of new jointing methods.

In order to establish the test method of mechanical properties of oxide composite superconductors, an international round-robin test (RRT) has been carried out under the programme of VAMAS activity, for which eight research groups participated. The present RRT made the following guideline clear for assessing the mechanical property of Ag/Bi2223 multifilamentary tapes. Owing to the constitution of the brittle superconductive oxide layer embedded in the ductile metal matrix, the stress-percentage extension (R-A) curve shows a three-stage behaviour, that is, the true elastic region is very narrow and is followed by the quasi-elastic region before reaching the macroscopic plastic region. For assessing the elastic constant, it is recommended to look for the maximum slope carefully by enlarging the initial part of the R-A curve. The following three quantities, elastic constant, yield strength and tensile strength, can be reasonably determined with good accuracy by the procedure reported in this paper. The percentage extension after fracture is, however, excluded from the standard procedure, because it scatters to a great extent owing to the nature of the test sample as well as the experimental limitation.