Superconductive Transitions in Fast-Pulsed Magnetic Fields

Values of $H_{c2\mathrm{}}$ for Nb-25% Zr and ${\mathrm{Nb}}_3$Sn measured in longitudinal pulsed magnetic fields having values of $\frac{\mathrm{dH}}{\mathrm{dT}}$ of ${10}^9$-${10}^{11}$ Oe/sec are found to be less than 25% of that of the static field values. The increasing- and decreasing-field values of $H_{c2\mathrm{}}$ are essentially the same. Measured values of $H_{c2\mathrm{}}$ increase rapidly as sample diameters are reduced below 0.04 mm. These values of $H_{c2\mathrm{}}$ are independent of temperature between 1.4 and 4.2°K. A discussion of these results shows that neither eddy-current Joule heating nor generation of critical currents by the changing field can explain the differences observed in values of $H_{c2\mathrm{}}$ measured in fast-pulsed and static magnetic fields. Measurements on Cu-clad Nb-25% Zr support these conclusions. Possible explanations of the results of these measurements are discussed in terms of present theories, which are shown to be inadequate to explain the phenomena.