This review article summarizes the recent advances in the investigation of multilayer insulation in the light of cryogenic radiation heat transfer. Brief summary for the previous investigation of multilayer insulation is first made after “Advances in Cryogenic Engineering” volumes and next some problems that are peculiar to cryogenic radiation heat transfer systems are pointed out after Tien and Cravalhos' article. Discussion is expanded to the recent study for the prediction of heat transfer rate in multilayer insulation through the introduction of the author's own investigation both experimental and of analysis. Other related investigations in this field are also introduced and discussion as to the relationship among those studies is made. Some prospective research areas in the cryogenic insulation technology are pointed out.
In order to realize a stable and continuous cooling system of superconducting magnets for magnetically levitated high speed trains, design study of the cryostat with liquid helium condenser has been performed. Boil-off helium gas inside liquid helium vessel of the cryostat is to be reliquefied by liquid helium condenser in which helium mist coolant from a external helium refrigerator is flowing.
A cryostat coupled with helium refrigerator has been constructed and tested in order to establish an on-board refrigeration system of superconducting magnets for magnetically levitated high speed trains. The cryostat (1975mm in length×745mm in hight×185mm in width, and 320kg in weight) is composed of a liquid helium vessel in which a coil and a liquid helium condenser are mounted, two layers of radiation shields and an outer vessel. A modified Sumitomo's 4l/h helium liquefier is used as a refrigerator which provides its helium coolant to two radiation shields and to the liquid helium condenser, simultaneously. The helium mist coolant in the condenser suppresses an increase of gas phase pressure inside the liquid helium vessel. This clearly indicates the condensation of evaporated helium gas in the vessel. All the system showed smooth, stable operation throughout the experiments. A series of tests provide us useful design data for the on-board refrigeration system.
An acoustic emission (AE) from the superconducting magnet system provides a new method for monitoring the operating condition of the magnet. From our experimental results, we are able to utilize the acoustic emission for the prediction of the “Magnet Quench.” The signal-to-noise ratio of AE transducers at liquid helium temperature has been found to be about 18dB higher than that at room temperature. All the way from cooling down to the running of the magnet, we can monitor the whole system of both superconducting magnets and cryostats. From more detailed analysis, the acoustic signal is affected by electro-magnetic induction noise, and further, we are able to identify the acoustic emission of the superconducting magnet from that of the liquid helium shower, because of so much difference of the wave form and frequency spectrum. For theoretical discussion, we have introduced an elastic medium model for the superconducting magnet to explain some stages of different acoustic emissions.