The London equation involves the penetration depth of the magnetic field as a parameter. As the properties of the penetration depth may provide important information as to the nature of the superconducting state, a number of experiments were performed in the period between 1935-1950 to determine its magnitude and temperature dependence. The earlier experiments were performed on micro-size specimens, in which one (thin films), two (thin wires) or three (small spheres) dimensions are comparable or smaller than the penetration depth. Later on, the a.c. methods utilizing the change in mutual or self inductance which allows the use of macro-size samples were performed with frequencies ranging from low frequencies, radio frequencies and ultimately up to microwaves. In this chapter, the experiments and results obtained during this period are described.
The cryogenic interlaminar fracture toughness of woven glass-epoxy laminates was measured under mode I loading using double cantilever beam (DCB) tests. The tests were performed at room temperature (R. T.), liquid nitrogen temperature (77K) and liquid helium temperature (4K) to evaluate the effect of temperature and geometrical variations on interlaminar fracture toughness. The fracture surfaces were examined by scanning electron microscopy to verify the failure mechanisms. A finite element model was used to perform the delamination crack analysis. The results of the finite element analysis are utilized to supplement the experimental data.
This paper describes the results of a visualization study of the phase transition process and heat transport phenomenon in supercritical nitrogen. An experimental apparatus for the visualization study is designed to observe the heat transport phenomena. The experimental cell is a rectangular parallelepiped surrounded with two optical windows, a ceiling, two side-walls made of Bakelite and a planar heater at the bottom. Heat is applied in step functions from the planar heater. The phase transition from the vapor/liquid equilibrium state to the supercritical state is observed by the use of a shadowgraph technique. On the other hand, the heat transfer phenomenon in supercritical nitrogen is investigated using a laser holographic interferometer. The initial temperature gradient, which suppresses the generation of convection, is made in the experiment. We can successfully observe the heat transport phenomenon that the convection and piston effect carry thermal energy in supercritical nitrogen.
The superconducting magnets on Maglev vehicles when running vibrate mechanically due to electromagnetic disturbance from the ground coils. Therefore, the current leads fixed on the superconducting coil, which is vibrating at about 98m/s2 (=10g), are also required to endure the vibration. We executed dynamic vibration tests for two types of current leads (straight & arc types) which had a multi-stacked configuration of Ag-sheathed Bi2223 tapes impregnated with epoxy resin in a concaved reinforcing gutter. We evaluated the straight-type lead for an operating current of 700A after every vibration test in liquid nitrogen for one hour under the dynamic strain deformation of 50-100μ on the surface of the reinforcing material covering the current lead. We could not detect degradation of the current lead by vibration through the total cycles of 3.9×106 after continuous vibration tests. We also executed vibration tests for arc-type current leads which were combined with an actual energized superconducting coil for a Maglev. Large accelerations of these current leads occurred at frequencies of 308 and 375Hz. In spite of the maximum acceleration of 600-700m/s2 generated by actuating the excessive force on a superconducting coil for two seconds, which occurred on these current leads carrying 500A, the superconductivity of the current leads did not shift to normal conductivity. There was no damage to either type of current lead during these vibration tests. So we confirmed the good prospect for the application of these current leads to actual Maglev superconducting magnets.