This paper presents a commentary on the test method for the residual resistance ratio (RRR), which is one of the most significant properties for superconductor application, of Cu/Nb-Ti composite superconductors. International standardization of the RRR has been carried forward by IEC-TC90 and now the third working draft (WD) has been prepared. The WD shows scope, definition, apparatus, data acquisition and correction of bending strain. It will be issued as IEC standard 1788-4-1 in the near future.
This paper presents a commentary on a method for testing the DC critical current of Cu/Nb-Ti composite superconductors up to 1, 000A. For direct current superconducting applications, Cu/Nb-Ti composite superconductor is the most popular material of many superconducting materials. The DC critical current is the most significant property in practical superconductors, and the establishment of this test method will contribute significantly to popularize superconductivity. Additionally we are anxious to make an immediate publication of this test method as a Japanese Industrial Standard (JIS) that corresponds to international standard IEC 61788-1, and to utilize it extensively.
This paper describes the numerical and experimental superconducting levitation forces of a single-grained Y-Ba-Cu-O bulk superconductor approximately 48mm in diameter. The experimental levitation forces were measured by changing the magnet radius systematically. It was experimentally found that the levitation force shows a maximum when the magnet radius is slightly smaller than the superconductor radius. Comparison of the experimantal and numerical levitation forces showed that even large bulk superconductors have macroscopic Jc as high as 2.0×104A/cm2, not much different from the microscopic Jc.
Self-field AC loss of cylindrical multi-layered conductor, which was an assembly of Bi-2223 Ag-sheathed superconducting wires, was investigated. The self-field loss of the conductor derived from the electric field measured by the 4-probe method could be explained theoretically by the critical-state model. It was also explained that the self-field loss could be measured by the pick-up coil which formed a loop surrounding all the superconducting wires in the conductor. To compare these methods, a cylindrical 2-layer conductor with two types of taps to detect the voltage was fabricated: one was a common voltage tap by solder and the other a pick-up coil surrounding all the superconducting part of the conductor. AC losses obtained by the two methods mentioned above were nearly identical when the transmission current was smaller than the critical current. Thus, we suggest that a pick-up coil is a useful tool for AC loss measurement of multi-layer conductors where no soldering is required.
The heat transport characteristics of a closed two-phase cryogenics thermosyphon are experimentally investigated using nitrogen as the working fluid. The thermosyphon was operated over a wide dynamic range, from near the triple point to the critical point. The thermal resistance and maximum heat transfer rate (critical heat flux) were the main points investigated in this study. The maximum heat transfer rate was compared with several theoretical predictions. It was found from this experiment that the maximum heat transfer rate is determined by the interaction between the vapor flow and the returning liquid film flow near the wall in the evaporator section.