A fatigue testing machine for long-term continuous operation at liquid helium temperature using a refrigerating system was developed and installed at the National Research Institute for Metals in 1983. The system is equipped with a recondenser in the test machine cryostat, where helium mist is transferred from the refrigerator and evaporated gas in the cryostat is recondensed to maintain a constant liquid helium level without an additional supply of liquid helium during testing. The system has been operated almost 15, 000h over 17 years, and fatigue properties of various kinds of structural materials have been obtained, including operating techniques and practical specifications of the system. The results of long-term operation of the refrigerating system, technical information on fatigue tests, and fatigue properties of structural materials at cryogenic temperatures are summarized.
Together with the problem of the boundary energy in the intermediate state described in the previous chapter, one of the main objects of the Ginzburg-Landau (GL) theory was the analysis of the nature of the transition between superconducting and normal states by the magnetic field. In the London theory, the number density of the superconducting electrons, ns, which plays the role of the order parameter is assumed to be constant throughout the superconductor and depends only on the temperature. In contrast, the order parameter in the GL theory, which by difinition is related to the number density, ns, may change over the GL coherence length and also depends on the magnetic field, as described by the GL equation. In this chapter, calculations made by GL on the magnetization and the nature of the magnetic transition of thin superconducting films will be described.
A new system was developed for measuring losses electrically in short, straight HTS tapes under various external conditions of transport currents and transverse magnetic fields. In this method, Poynting vectors around the straight sample were measured to get loss values by using two movable sets consisting of both a potential lead pair and a miniature pick-up coil on each flat face of the tape. In order to confirm validity of this system, loss measurements on short samples of Ag-sheathed Bi2223 multifilamentary tapes were carried out in liquid nitrogen for three cases as simultaneous sweep of ac transport currents and transverse ac magnetic fields and two single sweep cases of one of them. The results obtained show that this is extremely useful for measuring these ac losses as long as longitudinal change in the magnetic field can be neglected.