Refrigerator is a device that absorbs entropy at the low temperature end and ejects it at the high temperature end. Transporting mechanisms of entropy from cold to hot temperature by fluid flow are discussed. Thermal effects due to fluid oscillations, which are called thermoacoustic effects, are described and their applications to refrigerators are discussed. Systematic discussions based on thermoacoustics involve, in particular, the pumping loss in regenerator, the shuttle loss of displacer, the dream pipe, the basic pulse-tube, the Stirling engine, the Stirling refrigerator, the Gifford-McMahon refrigerator and the modified pulse-tube. Long history of investigating theroacoustic phenomena, including those of combustion system and Wheatley's natural engines, are reviewed briefly. Better understanding of regenerator would result in further development of more efficient regenerator and refrigerator. A pulse tube refrigerator of progressive wave type would be one of small cryocoolers in future.
Low temperature operation of MOS devices improves performance of LSI circuits because of the increase of carrier mobility, steep subthreshold slopes, elimination of latch-up, decrease of electrical resistance and improved noise behavior. A supercomputer using CMOS circuit technology cooled at liquid-nitrogen temperatures operates at double the speed of the circuit chips as compared to room-temperature operation. A further improvement of the speed in CMOS chips will be possible by optimizing the device design.
This paper reviews the development of superconducting RF cavities for particle accelerators during the past several years. Development of material and fabrication technique is described. Some of the accelerator project in the world using superconducting RF cavities are introduced. At present the most serious problem to get higher accelerating gradient is to understand and cure the electrons by field emission. If this problem is solved and the accelerating field of 20-30MV/m is obtained, superconducting cavities could be used for a linear collider of TeV energy region.
The stability of a forced-flow cooled superconducting coil is investigated by use of the numerical simulation. The numerical code to integrate the simultaneous partial differential system composed of the 1D hydrodynamic equations and the 1D thermal conduction equation has been developed and stability margins are evaluated as functions of coolant mass flow rate, operation current and imposed magnetic field. The results of computations show that the stability margin is multi-valued with respect to these operation parameters, as expected from the experimental results. It is also shown that the appearance of the first unstable regime is closely related to the existence of the stagnant region located at the upstream side of the heated zone and that the second stable regime appears because the heat transfer is appreciably enhanced by the fast induced backflow due to the thermal expansion of coolant.
A superconducting power cable is one of the promising ways for transmitting huge electric power efficiently in the future. We have proposed the new design of superconducting power cable with a similar electrical insulation structure to the conventional extruded cross-linked polyethylene cable (CV cable). The prominent feature of the design is to exploit excellent electrical properties of polymers in the cryogenic temperatures and to separate helium coolant from the electrical insulation. The cooling test of the cable showed that the most serious problem is a crack of polymer insulation due to a large difference in thermal contraction between the polymeric insulation and a conductor. Contractions, tensile properties and the influence of heat cycles on them were measured for some polymeric insulating materials. Furthermore, mechanical stress of the polymeric insulation of the cable at low temperature was calculated using the data of mechanical properties obtained. As a result, it is found that ethylene propylene rubber has superior mechanical properties in the cryogenic temperature region.
Methods to protect superconducting transformers from premature quench due to local disturbances and excess current at accidental short-circuit were studied with regard to structure of the conductor and construction of the transformer. Average variations in temperature of the winding were calculated by a heat balance equation under the conditions both of constant current for the premature quench and of constant voltage for the quench due to the excess current. The superconducting transformer is protected from the quenches by suppressing the increase in the temperature of the winding from the initiation of quench to the interruption of the power supply. A permissive level of the maximum temperature restricted the volume fraction of copper in the conductor for the former type of quench and the length of the winding for the latter. The superconducting transformer protected from the above two types of quenches was designed and compared with conventional transformers concerning the ‘one-turn voltage’ of the winding. In the comparisons, the main characteristics of the superconducting transformer, efficiency and % impedance, were equivalent to the conventional one. In the range of capacity less than 300 MVA, the one-turn voltage can be larger than that of the conventional one for the maximum temperature of 200K. The maximum one-turn voltage acceptable for the protection of windings is not compatible with the % impedance for the capacity more than 300 MVA. The increase in the permissive temperature of the quenched winding may be neccessary for the superconducting transformer with high performances.
The effect of uniaxial stress onto Ag-sheathed BiPbSrCaCuO on the Jc-B characteristics was investigated. It was shown that uniaxial stress was effective for the increase of Jc and n-value by the improvement of packing density and grain alignment. Larger Jc was obtained for the sample which included the low-Tc phase before pressing and sintering process. The existence of the low-Tc phase is expected to be useful for a good contact between grains after the growth of the high-Tc phase. The pinning potential was calculated from the measurement of the decay of the remanent magnetization at 4.2K, and it was revealed out that sample including the low-Tc phase at the initial stage had higher pinning potential.
Metal-sheathed tapes with Tl-(Ba/Sr)-Ca-Cu-O (TBSCCO) system were fabricated using the drawing-rolling method combined with a pressing process in order to improve the weak link properties. The Tc's of the tapes were measured to be 115±2K. The Jc was measured at various temperatures and in the presence of external magnetic fields. At 77K the Jc was 15, 900A/cm2 in the absence of magnetic field and slightly above 1, 000A/cm2 at 1T. The effect of magnetic field becomes much smaller at the lower temperature, for example at 20K the Jc was 8, 000A/cm2 at 1T. A tape fabricated by the drawing-rolling method without the post-pressing process gave the Jc of about 2, 000A/cm2 at 20K and 1T. An effect of the pressing is thought to be the densification of TBSCCO core. Another effect which could be thought is the orientation of TBSCCO crystals through the pressing process, although the orientation is not clear in the SEM observation. The relatively higher Jc is thought to be caused by an improved junction between grain boundaries and by an introduction of pinning center from various defects and unknown crystal phases. We believe that there exists flux pinning center in the TBSCCO system which is not yet identified.
BiPbSrCaCuO silver sheathed high-Tc superconducting wires were fabricated through the solid reaction method. Maximum critical current density is 3.0×104A/cm2 at 77.3K in a zero magnetic field, 1.5×104A/cm2 at 0.1 Tesla and 3.3×103A/cm2 at 1 Tesla. We propose that the magnetic field dependence of Jc of the wire is dominated by the Lorentz force which works perpendicular to the c-axis, caused by the transport current with c-axis component crossing grain boundaries when the magnetic field was applied parallel to the plane of the wire. While, when the magnetic field was applied perpendicular to the plane, it was dominated by the pinning force in the grains.
High critical temperature superconducting assembly of 50cm length have been developed using silver sheathed Bi1.8Pb0.4Sr2Ca2.2Cu3Ox superconducting tapes. A critical current of 150A at 77.3K was achieved over 40cm with the resistivity of 4×10-13Ω·m, which was much lower than the resistivity of silver, 3×10-9Ω·m. The average critical current of every 5cm was 177±9.4A. It suggests that the superconducting tapes of 10m length used for the assembly have a uniform transport property. Larger transport current will be attained by increasing the number of strand. Multifilamentary taped superconducting wire showed the good bending strain vs. critical current property. The critical current was decreased by 30% due to about 3% bending strain. Transport current properties of single core taped superconducting wires were stable for 50 times heat cycles between room temperature and 77.3K.