超電導電磁石励磁電源システム

抄録

Electric power supply systems for superconducting magnets are reviewed. Recent technical developments concerning main power supply circuits, magnet protection systems against quenches and magnet excitation controllers are described.
Requirements for performances of main power supply circuits have been changed to large current output, high voltage output and magnet energy regeneration reflecting the needs for large scale superconducting magnets such as fusion application or pulsive operation.
Main power supply circuits of silicon diode/transistor type have been changed to those of thyristor/transistor type due to less energy losses in circuits. However as far as transistors are used in the circuit, these power supply circuits are not appropriate for large current and pulsive operating magnets. Thyristor type power supply circuits meets all performances mentioned above. As this type of power supply circuit is able to send back magnets energy to AC line, it is suitable for pulsive operating magnets. In near future power supply circuits for the output more than 10, 000A will be changed to transformerless type, because the winding of such large current transformers is nearly impossible.
The power supply circuits of silicon diode/transistor type, thyristor/transistor type, thyristor type and transformerless type would be useful for the output current of less than 1, 000A, 1, 000A to 10, 000A, approximately 10, 000A and more than 10, 000A respectively.
Several methods of quench detection and magnet protection against quenches have been applied. We should select the most appropriate method corresponding to the magnet system considering inductance, current density and operating current of the load magnet.
As for quench detection, the most general method bases on the principle that the bridge circuit consisting of two resistors and a magnet with a center tap loses voltage balance when a part of the magnet has resistance due to quench.
In order to discriminate the quench voltage from noise voltage, the following consideration should be paid for the detection circuit. Namely the circuit, which recognizes as magnet quench when the unbalanced voltage keeps longer time than preset duration with higher voltage level than preset value, will be effective.
As for magnet protection against quenches, a DC circuit breaker to cut off the load magnet from the main power supply circuit and a magnet energy absorber are necessary. The combination of a thyristor circuit breaker and a varistor absorber is one of the most efficient system for magnet protection. However the magnet protection system consisting of a no-fuse breaker and a resistor is fairly welcome from the economical point of view.
A magnet excitation controller has a current sweeper which sends the output current signal to main power supply circuit. Recently electronic type sweepers instead of mechanical type are used because of easy control. Both digital type sweeper and digital control circuit of main power supply are now tried to obtain much higher control precision. By using a specially designed excitation controller, a superconducting magnet with a superconducting switch for persistent current operation can be excited speedily suppressing heat generation in the superconducting switch.