Magnetization method for long high-Tc bulk superconductors (study on swing-field-cooling method)

We have developed a new water-treatment system that consists of a membrane separator and a magnetic separator using long (>200 mm) high-Tc bulk superconductors (SC's) as permanent magnets. For the magnetization of the long bulk SC's, we developed a new magnetization method, i.e., a swing-field-cooling method. This method involves applying a field, and cooling the bulk SC's below their transition temperature (T_c). The bulk SC's are then moved inside and outside of the applied field repeatedly. As a result, the bulk SC's experience high and low fields under the gradually cooled ambient, and the trapped field strength gradually increases. The advantage of the method is that it can magnetize bulk SC's with larger area than that of the applied field magnet. To evaluate the method, we fabricated a bulk magnet system more than 200 mm long, and magnetized it by split solenoid superconducting magnets with a diameter of approximately 100 mm. We then performed field measurements and found that a wider area could be magnetized by the present method. In addition, we also developed a simple model-based on the Bean model-of the magnetization process. Taking account of the temperature dependency of critical current density (J_c), the model could successfully simulate the resulting magnetic field produced by the present method.