Study of High Stable Magnetic Field by Superconducting Transformer

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In 2004, DC transport current was tried to be induced by 500A class superconducting transformer. However, since the superconducting transformer was design to obtain high current, it was difficult to hold DC current for long time. In 2005, a new superconducting transformer was designed and fabricated in order to hold DC current for long time even though the magnitude of the current was small. The transport current induce in secondary winding was measured by the voltage at the low resistive shunt resistor inserted in the secondary winding. The current at the first winding of the superconducting transformer was controlled by PID method. For the result, the controlled time was extended to about 1 second. The control accuracy was within plus minus 0.4%. This value is far from 1 ppm which is required by NMR and MRI.In 2006, the accurate of current was tried to be improved by the knowledge obtained in previous studies. It is necessary to correctly measure the current induced in the secondary winding in order to obtain the stable current. Therefore, the magnetic field induced in the secondary windings was measured by a hole sensor and a pickup coil. The current of the first windings was controlled by the measured current of the secondary windings. As the result, the pickup method was better than the hole sensor method, and the more accurate control was established.In summary, the DC mode of the current induced in the secondary winding of the superconducting transformer is performed and confirmed. The methods to obtain stabilized magnetic field and current were proposed and performed. It is considered that these methods will be applied to many kinds of superconducting apparatuses. For example, the present knowledge will be applied to NMR and MRI facilities to obtain high stability magnetic field.