A review is given on recent topics on experimental studies of flux pinning in superconductors, while theoretical problems were discussed in the preceding review article. Here we compare experimental results with theories on the elementary pinning force and the summation problem. Quantitative comparison of the elementary pinning force is not possible at this moment because of ambiguity in the summation theory. The dependence of the specific pinning force of grain boundaries on the impurity parameter shows a similar tendency to the calculations by Zerweck, Yetter et al., and Welch based on the electron scattering mechanism. This observed tendency is different from the prediction from the microscopic theory of Thuneberg. The summation theories are compared with experiments within a regime of strong pinning where conventional calculation of the elementary pinning force holds safely. Observed pinning forces seem to obey the linear summation derived from the mean field approximation theory. The other topics such as the saturation phenomenon and the strain scaling are also discussed.
An apparatus to measure fracture toughness at cryogenic temperatures was made, and elastic-plastic fracture toughness tests were carried out on an aluminum alloy (A5083-0) at 4K and 77K. The apparatus consists of a testing machine, a cryostat and various measuring systems. The acoustic emission measurement method was applied to detect a critical value of fracture toughness. There was found good agreement between JIC by the multispecimen method and that by the acoustic emission measurement method at 77K. Fracture toughness values for the aluminum alloy, were 22kJ/m2 and 20kJ/m2 at 4K and 77K, respectively.
Design of a superconducting magnet to be coupled with a disk MHD generator in the closed cycle MHD experimental facility FUJI-1 is described. The superconducting magnet consists of two separated windings whose shape is of the circular Helmholtz type. A new structure between the windings has been proposed which supports magnetic force induced between the two windings and, at the same time, minimizes the heat leakage to the windings.