This paper describes the influence of the driving frequency on the oscillating behavior of gas pressure and temperature inside a pulse tube refrigerator and the ultimate wall temperature at the cold end. Two types of pulse tube refrigerators, basic and orifice pulse tube refrigerators, are investigated. The length of the pulse tube is 300mm and its diameter is 13mm. The oscillation of the gas pressure and temperature at various points along the pulse tube refrigerator and the wall temperature distribution is obtained as a function of the driving frequency in the range from 1 to 18Hz. The gas pressure oscillation is quite similar at all measuring positions through the pulse tube for both types. However, the gas temperature oscillation differs from location to location along the pulse tube. The wall temperature at the cold end depends on the driving frequency and there exists an optimum range of the frequency within which the lowest value of the wall temperature is attained. It is found that typical frequencies in the optimum range are 3Hz for the basic type and 6Hz for the orifice type. The gas temperature oscillation at lower frequencies can be clearly distinguished from that at frequencies higher than the optimum values. Moreover, it is also shown that the differences in the oscillating behavior between both types are more noticeable at the lower driving frequencies.
Microstructure was primarily determined by the composition ratio in Ag-sheathed Bi2Sr2Ca(1-x)Cu(2-x)Oy superconducting tape. The cation ratio of the raw powder was kept low in Ca and Cu compared to the stoichiometric Bi-2212 composition in order to suppress the second phase such as the coarse Sr-Ca-cuprate phase during the partial melting process. A highly aligned Bi-2212 phase with a small amount of Bi-2201 phase was obtained for the composition of x=0.36. No impurity phases were observed in the boundaries of the Bi-2212 phase by transmission electron microscopy. The best Jc of 1.67×105A/cm2 at 4.2K in zero magnetic field was obtained for the Ag-sheathed tape with the x=0.36 composition. The pancake coils were prepared by the wind-and-react technique. At liquid helium temperature, the sixteen-stacked pancake coils generated a magnetic field of 2.25 Tesla (T). The Ic of the magnet at temperatures from 4.2 to 30K coincided with the Ic of the short tape up to 6T. The evaluation of stability for the steadystate operational current was carried out using the quadruple pancake coil in various temperatures. The maximum stable operational current in a cryogenic atmosphere was a value between Ic defined as the criteria of 1μV/cm and 10-13Ω·m.
In this study, ac losses in BSCCO silver-sheathed wires, both single- and multi-filamentary, were investigated by means of ac-magnetization and ac-transport techniques. It was shown that the longitudinal magnetization ac loss in a single-filamentary wire could be quantitatively explained by the simple sum of the magnetic hysteresis loss of the superconducting core and the eddy current loss in the silver-sheath, while those in multi-filamentary wires behaved quite differently, possibly being affected by electromagnetic coupling between filaments. On the other hand, the self-field ac loss of all the wires employed was found to be well described by a simple expression derived for a uniform superconducting cylinder based on Bean's model. Namely, the self-field ac loss was determined solely by Ic and Ip/Ic (Ip: peak value of the transport current), regardless of their cross-sectional structure, and the distribution of the current flow is considered to have an elliptical symmetry even for multi-filamentary wires, where the critical state is established from the outermost filaments toward those in the interior. It is suggested that the self-field loss of multi-filamentary wires can be reduced if the distribution of the current flow can be made equivalent for all filaments.