A miniature cryocooler is used to reduce thermal noise in infrared sensors and laser analyzers. An infrared sensor is applied to industry, security and defense systems and space technology. Application to space technology has led the technology of cryocooler. In EOS-AM1 (Earth Observing System) by NASA, long-life (operating life 50, 000 hours that is 20 times longer than present one), low disturbance (0.1N that is 1/20 of present one) and low power consumption are required to the miniature cryocooler. These technologies have dramatically improved the miniature cryocooler. Here, the word “miniature cryocooler” means a cryocooler that works in 30-80K of cooling temperature and cooling capacity lower than about 1W. (1) Trend of R & D, (2) refrigeration principle, (3) stirling cycle cryocooler technology, (4) pulse tube are presented in this paper.
Void formation is a serious problem for Ag-sheathed Bi-2212 superconductor wires produced by a partial melt process. We established a gas pressure melting (GPM) method which utilizes a combination of -1.0atm oxygen partial pressures and ≤6atm of total gas pressure. The method has proved very effective for obtaining a homogeneous dispersion of small voids and good superconductivity. A solenoid coil fabricated under usual atmospheric pressure showed no zero-resistivity due to the presence of large 50-mm-long void agglomerates. On the other hand, the newly developed GPM method reduced agglomerate size to -10mm, and enabled solenoid coils fabricated using 9 and 5 pieces of 25-m-long round wire to generate a maximum magnetic field of 0.67T and 1.1T, respectively, at 4.2K.
The high performance of the superconducting joint of NbTi multifilament superconducting wires is achieved by using a sleeve that is reinforced and is tapered at the inner corner of the sleeve. The critical current of the joint is 1, 800A at 5T. This value is equal to the critical current of the superconducting wire. The superconducting joints are adopted as a superconducting magnet for synchrotron radiation. The critical currents of the joints between the wires for the coils are over 1, 100A at 5T for all joints, and those between the wires for the coil and persistent current switch exceed the quench current of the persistent current switch. By testing the superconducting joint using two filaments, it is suggested that the superconducting current passes at the joint when the interface of the NbTi filaments are slipped and deformed.
To measure total hemispherical emissivities εTH(T) of metals at low temperatures, a new transient calorimetric technique is developed to improve its accuracy. The accuracy of this technique mainly depends upon heat loss Qt through a thermocouple (TC) which suspends a specimen and measures its temperature. To reduce Qt, an Iron. Constantan TC with diameter of about 20μm is used, and the temperature profile of TC is controlled by a guard heater attached to the other sides of TC. εTH(T) of aluminum specimens which shaped wire (99.9994% purity) and plate (99.988%) are measured in the temperature range from 125 to 350K. The specimens are mechanically polished. The roughness of the plate surface is measured. Qt, evaluated by a combined radiation and conduction heat transfer analysis, is less than 1% of the total power loss of the specimen. εTH(T) for wire and annealed plate are in good agreement with each other, and close to the values extrapolated from the date obtained above room temperature. An increase in εTH(T) of the plate without annealing may be due to the Beilby/damaged surfaces formed by mechanical polishing.
A laser holographic interferometer was successfully applied to the visualization of the phenomena in cryogenic liquids, in particular the visualization of natural convection in liquid nitrogen and of thermal and pressure wave propagations, transient boiling and transient evaporation from a free surface in superfluid helium (He II). This technique has the advantage in yielding straightforward quantitative data. Counting interference fringes which appear in a holographic image provides quantitative information on the temperature or pressure variation in the liquid of interest. This method was also found to be applicable to even a very small variation of temperature, especially in thermal shock waves.