The development of helium free high-temperature superconducting (HTS)-MRI magnet is expected. An HTS-MRI magnet made of HTC tape superconductors has difficulties in the persistent current exciting mode for producing a highly stabilized magnetic field for MRI imaging, due to a screening current in REBCO tape. Furthermore, development is necessary in non-resistive joint and persistent current switch of REBCO MRI magnet.
One solution for these problems is an active magnetic field stability control by use of a power supply instead of the persistent current operation. We have developed the power supply system consists of a exciter power supply and a small trimming current supply which compensates the magnetic field deviation due to the screening current. The MRI magnetic field feedback/feedforward control test was carried out, that is, the overshooting and the current trimming control, using a HTS-MRI test magnet.
In this paper, we measured and evaluated the magnetic field stability, using a HTS-MRI test magnet in the power supply driving mode using a dropper-type and of a switching-type direct-current exciter system. A high stable direct current power supply is normally designed using the dropper-type regulator which is suitable for the constant current operation of less current ripple. However, it is of large power loss and is not suitable for charging/discharging operation. On the other hand, the switching-type regulator is suitable for the variable current control operation and of less power loss. However, it is of large current ripple in the constant current operation which would degrades the magnetic field stability in HTS-MRI imaging operation.
The HTS-MRI magnet which is fabricated by REBCO tape superconductor was charged up to 20.5A with ramp rate of 0.05 A/s and then discharged to target current 20A (0.27 T), that is the overshoot charging operation for compensating the screening current effect on the magnetic field stability. The magnet current was kept 20 A for about 3 hours. The magnet field stability at the center of imaging area, the magnet current and the magnet terminal voltage were examined. In case of the dropper-type exciter, the terminal voltage instability was observed in the charging operation, and the magnetic field stability was evaluated as 3.70 ppm/h in 3 hours of the constant current mode. In case of the switching-type exciter, the magnet current was smoothly built up without any voltage instability, and the magnetic field stability was evaluated as 6.69 ppm/h in 3 hours of the constant current mode but the voltage ripple was observed.
The availability of the power supply system was discussed in MRI imaging of a mouse using an experimental HTS-MRI system. The MRI image of the mouse was obtained under the power supply driven operation of the dropper-type regulator. The S/N ratio of the image was 6~7 in the imaging time of 15 minutes and 19 of 120 minutes which is similar level to that of the commercial LTS-MRI system with persistent current mode. On the other hand, the clear MRI image of the mouse was not obtained under the power supply driven operation of the switching-type regulator due to the small but repetitive fluctuation in the magnetic field due to the voltage ripple.
A resistor of 1.2 ohm was set between the magnet terminal in parallel to the MRI magnet in order to improve the magnetic field stability in the constant current mode for the clear MRI imaging, at the same time to alleviate the current instability in the charging mode with the dropper-type power supply and to reduce the magnet current ripple due to the voltage ripple in the constant current mode with the switching-type one. As a result, the magnet current was smoothly built up and the magnetic field stability was improved to 1.98 ppm/h with the dropper-type power supply. The voltage ripple was still observed in the constant current mode but the magnetic stability was improved to 5.53 ppm/h with the switching-type power supply. The MRI image of the mouse was obtained under the power supply driven operation of the switching-type regulator, since the magnetic field ripple due to the voltage ripple was reduced by the parallel resistor. The S/N ratio of the image was almost the same (19) as with the dropper-type one in the imaging time of 120 minutes. The power loss of the switching-type power supply is about 1/6 of that of the dropper-type one.
This work was supported by NEDO “development of an HTS magnet system with highly stable magnetic field” as part of the “promotion technology development for HTS practical application.”
Keywords: High Temperature Superconductor, MRI magnet, Exciter system, magnetic field