Korea's superconductivity technology in the power sector has progressed rapidly since the launch of the 21st Century Frontier R&D Program for superconductivity. Under this program, large-scale active research has been conducted. The program supports HTS cable, motors, transformers, fault current limiters, and basic materials for 2G wire development. Current technology is somewhat less advanced compared to developed countries such as the USA, Japan and Europe. It is expected that the efforts and strong motivation of the Koreans will soon make the country competitive with advanced nations. In addition, the power industry has also begun to support the research and development of superconducting power technology via ETEP. Including such support, we review the recent progress of applied superconductivity research and development in Korea, mainly in the power sector.
The first 920 MHz high-resolution NMR magnet has been operating at the Tsukuba Magnet Laboratory (TML) of the National Institute for Materials Science since April 2002. It has proved its effectiveness by determining the three-dimensional structures of protein molecules. To accelerate studies in structural biology and solid-state NMR, a second high-field NMR magnet was developed and installed at TML. Although its basic design was based on that of the 920 MHz NMR magnet, some parts have been improved. We applied a 16 wt.% Sn bronze-processed (Nb,Ti)3Sn conductor for the innermost coil, replacing the 15 wt.% Sn conductor. This enabled a size reduction of 3.50 mm x 1.75 mm in the first magnet to 2.80 mm x 1.83 mm in the second because of an improvement of more than 20% in critical current density. As the magnetic field generated by the innermost coil was increased, at the same operating current of the first magnet, operation at 930 MHz was expected. The magnet was energized up to 21.6 T without any quenching and operated in a persistent mode at 920 MHz for more than one month. On March 24, 2004, it was excited up to 930 MHz. After the central field was increased to 21.89 T once, it was decreased to 21.86 T, and the persistent-mode operation started from a frequency of 930.7 MHz. This is the highest field that the magnets made of NbTi and Nb3Sn coils have ever achieved. The field stability and the field homogeneity were measured and it was confirmed that this magnet can be applied as an NMR magnet.
Discussions based on the first and second laws of thermodynamics confine the temperature gradient in a regenerator within some region. Experiments, however, show that forced fluid oscillations induce stable temperature distribu-tions in regenerators. This fact suggests that it is possible to discuss stable temperature distribution by employing the law of minimum entropy production rate in addition to the first and second laws. In order to discuss stable temperature distribution analytically, a short regenerator where the local entropy production rate is homogeneous is supposed. Minimizing the entropy production rate of the short regenerator gives a thermodynamically stable temperature gradient, which depends on both oscillation mode and amplitude. Necessary conditions for realizing a stable temperature gradient in a short regenerator are also discussed.