In the past decade, great progress has been made in submillimeter receiver technology. The superconductor-insulator-superconductor (SIS) mixer has demonstrated ultra-low noise performance in a large frequency range up to 1, 000GHz. A breakthrough has been made with a new superconducting hot-electron bolometer (HEB) mixer that realizes a low-noise mixer even beyond 1, 000GHz. Such progress in submillimeter mixers, combined with the progress in submillimeter signal sources and low-noise IF amplifiers, has made many scientific programs in astronomy and earth atmospheric research feasible. Because of the large opacity of the atmosphere, submillimeter observation will be successful only on high mountains with extremely low humidity or in space above the earth's atmosphere. In Japan, a small submillimeter telescope, which was developed to find the distribution of interstellar neutral atomic carbon in the Milky Way Galaxy, is now under construction and scheduled to go into operation on the top of Mt. Fuji. Towards the future, on the other hand, there is a big plan for a next-generation radio telescope, called the Large-Millimeter and Submillimeter Array (LMSA), which is being investigated by Japanese astronomers. The LMSA, when built on a high mountain site overseas, under good atmospheric conditions, will have the remarkable capability to take a fine image of distant astronomical objects with a resolution of 0.1 to 0.01 arcseconds. Submillimeter observation from space will be quite fruitful both for astronomy and for atmospheric research. A small satellite, SWAS, was developed by NASA to observe the oxygen molecules and water vapor in interstellar space. European Space Agency (ESA) has a long-term project, FIRST, in which the objective is to build a 3-m telescope in space for far-infrared and submillimeter astronomy. Swedish scientists have built a small satellite, ODIN, for astronomy and aeronomy. The JEM/SMILES, which is proposed by NASDA and Communications Research Laboratory (CRL) as an experiment for the International Space Station, is a challenging mission to demonstrate the advantages of using a combination of submillimeter SIS mixer and closed-cycle cryogenic cooler for atmospheric observations.
Activity on the international standardization of the test methods for industrial superconductors is introduced, which is authorized by IEC/TC90 (Superconductivity). A brief history and strategy of the organization are explained. The substantial work involved to discuss and prepare the draft for each item is left to the respective international working group. At present, 8 working groups cover the field of superconductivity and related properties; terminology, critical current, residual resistance ratio, copper ratio, mechanical property and surface resistance. One of them, the test method of critical current for Cu/Nb-Ti composite superconductors, will soon be published as an international standard. The domestic activity in Japan supporting international standardization is supervised by the Japanese National Committee of IEC/TC90. The mission of the JNC is the following three activities; support of the international activity as a secretary country, translation of international standards to JIS and supervision of domestic activities. The actual test methods are proposed and carried out by the New Materials Center and Japanese Fine Ceramics Association. It is emphasized that establishing good liaison with groups such as VAMAS concerning the fundamental aspects of standardization is crucial to promote effective international collaboration.
Measurements of thermoacoustic prime movers with stacks made of copper wire mesh are presented. The resonance frequencies were below 100Hz, which means the movers can be used to drive pulse-tube refrigerators. Onset temperature and resonance frequency were studied. It was found that, under certain conditions, the second harmonic component becomes dominant and the prime mover essentially works on the second mode, which should be avoided. The influence of gas properties, frequency, mean pressure, mesh size and stack length on the overall performance are measured and expressed in terms of normalized input power, heater temperature and pressure amplitude. A maximum output power of 26W from the thermoacoustic prime mover was achieved with the frequency, input power and heater temperature 73Hz, 833W and 700K, respectively.
We have performed precise, experimental and theoretical studies of the quench propagation velocities of the YBa2Cu3Ox (YBCO) high-temperature superconducting film prepared on oxide single-crystal substrates by pulsed laser deposition. In the experimental studies, we determined the quench propagation velocities using strip-shaped YBCO films prepared on LaAlO3 single-crystal substrate with many voltage terminals. That is, the quench propagation velocities were measured from the time that the voltage occurred at each terminal after quenching in part of the strip by a heater. We previously examined the influence of the temperature increase by the heater in order to estimate the correct value of the quench propagation velocity. We then used the data obtained in the region of small temperature increase by the heater. The quench propagation velocities of YBCO film obtained in this study were lower than that of the metallic superconducting wire by an order of 2-3. In a theoretical analysis, the quench propagation velocities were found to be expressed by a function of the specific heat, the density and the thermal conductivity of the substrate, a temperature difference between the operation temperature and Tc, the strip width of the superconducting film and the power of joule heating for the unit area. The theoretical estimations were in good agreement with the obtained quench propagation velocities from this experiment. Therefore, the theoretical analysis in this study is appropriate for the quench propagation characteristics of the superconducting film, and the quench propagation velocity can be estimated when the experimental conditions are determined.
The characteristics of heat transfer in a simulated channel for a large superconducting coil have been investigated. Subcooled He I layer on the heated surface exposed to the pressurized superfluid helium (He IIp) expands the non-boiling region beyond the Kapitza region up to qn, above which nucleate boiling sets in. As the bath temperature decreases, the orientation-independent qn is increased more rapidly than qλ, at which the superfluidity is broken at the center of the heated surface. A decrease in the temperature of the coolant in contact with the heated surface has been detected in He IIp as well as in the saturated He II. Enhanced stabilization can be established for superconducting coils by taking qn into account; the temperature rise at the hot spot on the superconductor may be restrained below 4.5K without bubbling.