The principle of pumping mechanism and basic construction for the cryopump cooled by a small refrigerator are described. Also some applications of the cryopump are described. Two-staged Gifford-McMahon cycle or modified Solvay cycle refrigerator are commonly used for the cryopump. The first stage cold station cools a baffle and a radiation shield which condenses water vapour and blocks thermal radiation to the cryopanel. The cryopanel cooled by the second stage cold station condenses most gases except neon, hydrogen and helium which are adsorbed by charcoal attached to the cryopanel. Pumping speed for a cryopanel is led from the statistical mechanics. An operating procedure based upon a typical cryopump system is also described.
An enormous number of papers on high Tc superconductors have been reported since the discovery of Ba-La-Cu-O superconductor in 1986. It becomes difficult for many researchers to read out the papers related to their field. In this article we report the explanation of numerical database for high-Tc superconductors “SUPERCON” which has been constructed in NRIM. The crystal structures of high-Tc superconductors are classified using the common name used in many papers. The references are represented by a newly developed reference number. Numerical data on superconducting properties, Hall coefficient, thermal conductivity, thermoelectric power and specific heat are collected from the published papers. Metadata and the usage of “SUPERCON” are described.
Fundamental techniques of superfluid cooling of superconducting magnets and electronic apparatuses are exemplified by an AC magnet. Superfluid helium, as a cryogen for magnets, has excellent properties such as superfluidity and super-thermal conductivity. Despite these properties, its heat transport capacity is restricted by narrow cooling channel configurations of a magnet winding and by the critical heat flux for a surface of a superconducting wire. To obtain the maximum heat transport and maximum critical heat flux, the optimum operational temperature should be between 1.8 and 1.9K. Also mentioned is the heat transfer to superfluid helium under transient state, where one can obtain a critical heat flux that is higher than that under the steady state but with a lifetime less than a one millisecond.
The conventional electric quench-detecting system of AC superconducting coil is vulnerable to the electromagnetic noise, so the systems have problems in quench-detecting sensitivity and reliability. Recently the systems combined with physically different methods for the purpose of increasing reliability were reported. If oxide superconductor in R & D are applied to current lead of superconducting coil, such type of current lead is supposedly composed of a simple bulk structure. It is potentially brittle and catastrophic under thermal shock by rapid temperature rise when quenching occurs. Moreover, because of thermal insulation, the fully stabilized design is not applied to the current lead. So, higher speed quench-detecting system is required. This paper presents the development of high speed quench-detecting system using personal computer and digital signal processor (DSP). The DSP is a processor to realize the fast calculation of addition (subtraction) and multiplication by pipe line processing. Basically the four-terminal method is used. The quench-detecting is performed by evaluation of the mutual correlation factor for current through the current lead and its potential difference. In the superconductive state the voltage is measured which leads by π/2 radians to the alternative current, then the factors is ideally equal to zero. On the other hand, in the quenched state the voltage is deformed due to superposing the resistive voltage component emerged, the evaluated factor results in having some nonzero values, and thus the quenching can be detected. In this paper, it is indicated that the calculation of the factor can be processed for every one cycle of current and voltage, and the quenching recognition is performed at least within 17ms for current of 60Hz. This value would be reasonable for the protection of the coil system from catastrophy. It was also discussed that the method using the mutual correlation factor can well recognize the quenching state, even if the higher harmonics due to the electromagnetic noise were included in the measured voltage.
Non-metallic thermal insulators are investigated in order to replace “metallic super insulations” as radiation shields for cryo-cooling systems of superconducting devices, which emit and/or receive microwave radiation, such as antennas. As candidates, layered refractive wafers and multilayered thin film filters are demonstrated. Layered Germanium wafers with spacings indicate that the total infrared reflectance increases with the number of wafers, but gradually saturates because of the slight infrared absorption of Germanium. Thermal insulation experiments show that the reflection efficiency, η, assuming black body radiation is incident, correlates well with experimental thermal insulation efficiencies. On this basis, infrared reflection characteristics are obtained for a variety of filters of (Ge/BaF2)m/Si. Infrared spectra of these filters have demonstrated that single band filters have a η-maximum of-72%, and that extended filters composed of double hand filters lead to a reflection efficiency of-82%.