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.
Radial-type superconducting magnetic bearings (SMB) have been developed for a 10kWh-class flywheel energy storage system. The bearings consist of an inner-cylindrical stator comprised of YBCO bulk superconductors and an outer-rotor of permanent magnets. The rotor is suspended without contact via the pinning forces of the bulk superconductors, which are arranged such that the c-axis of each bulk is aligned parallel to the radial direction of the cylinder. After the whole system was completed, various tests focusing on both its static and dynamic behaviors were carried out to confirm that the SMB has sufficient performance for 10kWh-class flywheel systems. In this paper, we describe the implementation of the stator part and report the SMB test results; namely, that the rotor was successfully suspended up to 7,500 rpm. Thus, we succeeded in an energy storage of 2.24 kWh.
A15 Nb3Al has gained much interests for high-field and large-scale applications due to its excellent critical current density Jc and strain tolerance. However, the shift of Nb3Al composition from stoichiometry at temperatures below 1800°C causes difficulties in the fabrication of conductors with inherent Nb3Al characteristics. In this study, Nb3Al powders prepared by arc melting in an Ar / H2 atmosphere were encased in a sheath and fabricated into tapes through the ex-situ PIT (Powder in Tube) process. The present study revealed that the oxygen content in the Nb3Al powder plays a crucial role in yielding large Jc in the tape. Furthermore, the addition of Ag powder to the Nb3Al powder was found to appreciably enhance the Jc of the tape in lower fields. The superconducting performance of the tapes is discussed in relating to their structures.
This paper describes a persistent current HTS magnet that has been developed for Maglev applications. The HTS coil for the magnet consists of 12 single-pancake coils wound with four parallel Ag-sheathed Bi2223 wires. The HTS coil is connected with a persistent current switch (PCS) made of YBCO films and cooled below 20 K by a two-stage GM pulse-tube cryocooler. Detachable current leads are used to reduce heat leakage to the 1st stage of the cryocooler. The initial target for the current decay rate of the magnet was 10 %/day. In order to achieve the target, wires with n-value of more than 14 in a range of 10-9-10-6 V/cm at 77 K, 0 T were selected and coil winding processes were improved. As the result, a current decay rate of 0.44 %/day was obtained under persistent current operation of the HTS magnet. To observe the mechanical capability of the magnet, mechanical vibration tests up to ±15 G and electromagnetic vibration tests were carried out.