The unified strain and temperature scaling law for the pinning force density of bronze-route Nb_3Sn wires in high magnetic fields

摘要

Detailed measurements of the critical current density (J_c) of a bronze-route niobium-tin wire are presented for magnetic fields (B) up to 15 T as a function of temperature (T) from 6 K up to 16K in the strain (epsilon) range between -0.7 percent and + 0.7 percent. The data for this technological wire are described by a unified strain and temperature scaling law for the pinning force density of the form F_p(B, T, epsilon) = J_c X B = A(epsilon)[B_c2~*(T, epsilon)]~n b~p(1-b)~q, where A(epsilon) is a function of strain alone, B_c2~* is the effective upper critical field at which F_p extrapolates to zero, b = B/B_c2~* is the reduced magnetic field and n, p and q are constants. It is demonstrated that were A(epsilon)(B_c2~*)~n replaced by F(T)(B_c2~*)~m where F(T) is a function of temperature alone, the strain index m is a strong function of temperature and strain, and in high compression m = n. The effective upper critical field can be parameterized using the expression B_c2~*(T, epsilon) = B_c2~* (0, epsilon) (l - (T/T_c~*(epsilon))~v), where v is a constant and T_c~*(epsilon) is the effective critical temperature at which B_c2~* at a given strain extrapolates to zero. The strain dependence of the ratio B_c2~* (0, epsilon) / T_c~*(epsilon) and the slope (- partial deriv B_c2~* (T, epsilon) / partial deriv T)_(T = T_c~*(epsilon)) is reported. The data presented are useful for cryocooled high field magnets and for identifying the mechanisms that determine J_c in niobium-tin superconducting wires.