Most of the low temperature systems such as superconducting magnets require the efficient cooling system. The irreversible loss due to the entropy generation at the low temperature must be minimized to overcome this requirement. The concept of exergy gives a useful method to know where is the main source of this entropy generation. This paper describes the fundamentals of exergy analysis for cryocoolers or simple cryogenic systems. It includes the definition of exergy, exergy loss and thermodynamic efficiency of the system. Several examples of calculation method for the simple cryogenic system are also given.
This paper describes a new calorimetric method for AC loss measurement of superconductors. In this method, AC losses of superconductors can be detected as the capacity change. The bubbles generated by the AC losses move into the parallel electrodes and cause the capacity change of it, because the dielectric constant of helium gas is different from that of liquid helium. Then this capacity change is converted to the frequency change by a RC-oscillation circuit. Consequently, AC losses can be calculated by the frequency change, which is calibrated by a reference heater. This measurement system has some advantages, such as the shorter measuring time compared with that of other calorimetric ones and the less affection by the recondensation of bubbles. The resolution of this measurement is 4mW, however, it can be improved by the optimization of the electrode structure and the application of higher oscillation frequencies.
We have developed Fiber-Reinforced-Superconductors (FRS) for an application of high-field pulsed magnets. FRS is a superconducting filament reinforced with a fiber of high-elastic modulus. Tungsten fibers have been used as reinforcement so far. On the other hand, tantalum or titanium fiber has relatively high elastic modulus (even though each is lower than that of tungsten), and moreover, each fiber is expected to function as a resource to supply tantalum or titanium element into Nb3Sn during the thermal diffusion process, which will improve the superconducting properties. Hence we prepared FRSs with tantalum and titanium fibers to measure their superconducting properties. The results of Tc and Jc measurements show that the FRS using the titanium fiber has low critical temperature compared to the FRS with a tungsten fiber. We suppose that this is caused by excessive addition of titanium into Nb3Sn. On the other hand, critical current density and upper critical field of the FRS using tantalum fiber are improved because of appropriate supply of tantalum into Nb3Sn. The superconducting characteristics are studied with taking intrinsic strain in Nb3Sn layers into consideration.
Investigation has been conducted on the following subject: low temperature creep deformation behavior of SUS 316LN austenitic stainless steel under constant load which is expected to be the material of superconducting toroidal coil case in Tokamak fusion reactor. Round bar specimens were extracted from SUS 316LN, and several tensile tests were carried out: monotonic loading tests at a room temperature, 77K and 4.2K, and constant loading tests at a room temperature and 77K. Increase in strain under constant load occurs over proportional limit of the material. Critical stress not to allow the increase in strain is 75-100% of 0.01% proof stress at the test temperature. This is the same value as 0.01% proof stress of extremely low rate monotonic loading test when no creep deformation occurs; thus, this is the same value as critical stress where plastic deformation initiates. Based on the result at 77K, the critical stress at 4.2K not to allow the increase in strain under constant load is estimated to be about 90% of 0.01% proof stress at 4.2K, which is about 75% of 0.2% proof stress at 4.2K.
The flow structure of a thermal counterflow jet is investigated by direct measurement of the normal fluid velocity with a laser Doppler velocimeter (LDV). The temporal and spatial variation of the normal fluid velocity is measured to investigate the detailed nature of He II thermal counterflow jet. The velocity profile and the variation of the center-line velocity with the axial distance are also obtained to discuss the flow structure in a downstream region. It is found that the thermal counterflow jet in the far field behaves like a fully developed turbulent jet in ordinary fluids. It, however, seems that the way of development differs from that of ordinary fluid jets.