This paper outlines materials testing at cryogenic temperatures. The evaluation of standard mechanical properties of domestic materials is essential for designing cryogenic devices which are necessary for the development of fusion reactors, magnetic suspension trains, superconducting generators and so on. Typical examples of equipment and techniques used by various researchers in the field of cryogenic testing are described.
Superconducting magnets for fusion device are greatly influenced by the radiation environment and the principal radiation-induced trouble is to arise from the change of mechanical fracture behavior in organic materials used as insulating and/or potting materials. Small compression test specimens of epoxy resin are tested after gamma and pile irradiations at low temperatures. Three kinds of mechanical properties i.e. (a) breaking stress, (b) breaking strain and (c) elastic modulus are investigated. A statistical analysis of the data from a large number of small specimens revealed the followings: (i) The effects of gamma and neutron irradiations on the fracture behaviors are observed to be different. This fact suggests that the simulation study by gamma is not safe for neutron irradiation. (ii) In the case of reactor irradiation overall softening are induced accompanied by local hardening and embrittlement. A qualitative explanation to the above results is presented and their effect on the performance of the magnets is stated.
A combined superconducting coil is proposed for the purpose of increasing the strength of magnetic fields generated by large scale superconducting magnets where alloy type (NbTi) wires of the fully stabilized type is utilized as main windings. The coil is provided by numbers of high field composite superconducting wires produced by the compound type (Nb3Sn, V3Ga) material which are arranged as partial by-passes of the exciting current in regions exposed to strong magnetic field higher than the critical value of the superconducting material of the main windings. The arrangements of the high field wires are made in such a manner as they are in tight contact with (parallel type) or in place of (series type) the portions of the main windings in the respective turns in the high field regions. In either type, the Joule heat is generated by the current flowing through the normal conducting matrices of the connecting regions, and this heat is removed by the cryogenic Helium directly contacting with the winding surfaces. The amount of this excessive heat loss is in the range of 1kW-10kW for the large scale (1010-1013J) magnet practically used in the fusion reactors or the energy storage devices where the average field is about 8T and the maximum field is in the range of 14-17T.
A device for making indium wires and their use for low temperature vacuum seal are described. As an example of the application, a design for indium-sealed beryllium windows is shown, which are tight to liquid He II even after repeated thermal cycling and can be used in cryostats for Mössbauer-effect or X-ray experiments.
A modified version of an indium seal with a very small loss area, in which pure In solder is used instead of pure In wire, has been tried for a glass window of 20mm in diameter. It has turned out that this seal is vacuum tight against the liquid helium at 4.2K. A seal of a 10mmφ sapphire window with STYCAST 2, 850GT was also successfully tested at 4.2K.