The development of a superconducting Maglev system in Japan was started more than 30 years ago. Since 1997, the running tests have been successfully continued on the Yamanashi test line. A manned run in 1999 attained a speed of 552km/h, The Yamanashi test results proved a remarkable stability of the onboard superconducting magnet system, In these 30 years, we have encountered many difficult problems during this development. In this paper, the history of the Maglev development will be introduced with focus on these problems, especially in regard to the superconducting magnets and onboard refrigerators.
We fabricated 2.1GHz TM010-mode microwave resonators with (Bi, Pb)2Sr2Ca2Cu3Ox (Bi2223) superconducting thick films on a Ba(Sn, Mg, Ta)O3 dielectric disk with a relative dielectric constant of εr=24. The Bi2223 thick films were screen-printed on both sides of the dielectric disk and subjected to a double repetition of cold isostatic pressing at 0.4GPa and sintering at various temperatures ranging from 820 to 840°C. An increase of the sintering temperature raises the Bi2223 phase purity up to 98% on the surfaces of the thick films, but high-temperature sintering above 830°C causes a chemical reaction at the interface between the Bi2223 thick film and the dielectric disk. An optimization of the sintering temperature to 830°C gives the unloaded quality factors Qu as much as 74, 000 at 70K and 158, 000 at 25K, which correspond to surface resistances Rs of 0.34 and 0.15mΩ for Bi2223 thick films, respectively. These values are approximately 20 times higher than those for the resonator using Ag electrodes with the same structure.
Although the thermoacoustic theory of the 20th century has succeeded in making a variety of thermoacoustic phenomena understood and contributed to the development of thermoacoustic devices, the application is shown to be limited to the ideal gas of the first kind; if the theory is applied to nonideal gas, the result violates the first and the second laws of thermodynamics in some limiting cases. A consideration of the postulated relationship between the entropy flow and the oscillating entropy leads to a proposal of a new relationship, which is an extension of the old relationship to general fluid, of which thermal expansion coefficient is not equal to the inverse of temperature. The new relationship modifies expressions of both progressive and standing wave components of the heat flow. Some effects of the extension on the explanation of refrigeration power at low temperatures are discussed. Also discussed is the possibility of thermoacoustic heat engine with the working fluid of a negative thermal expansion coefficient.