At the National Institute for Fusion Science we are aiming to achieve fusion energy. In addition to conducting research experiments in high-temperature plasma using the Large Helical Device (LHD), we also are moving forward with conceptual designs for a future helical fusion reactor. At that time, a fusion reactor that actually produces energy will have features such as device composition and operating methods that differ from the LHD, which is a device for experiments. Thus the features in the LHD that can be applied to the reactor design are limited. An example is the power supply for sending an electrical current into the superconducting electromagnets (coils) that generates the magnetic field that confines the plasma. Here we will introduce one example of research on the design for the power supply for superconducting coils that will be used in the fusion reactor.
In order to confine high-temperature plasma, we send a large electrical current through the superconducting coils that have been set near the plasma and generate a powerful magnetic field. In the LHD and in a fusion reactor the methods for sending an electrical current through the coils differ. The LHD uses six superconducting coils and generates a magnetic field. To each coil is attached one power supply, and through regulation individually of each electrical current being sent it is possible to change variously the shape of the magnetic field “container” that confines the plasma. Through this regulation, in the LHD, which is an “experiment device,” we can conduct experiments that optimize plasma performance through various configurations of the magnetic field container. Based upon these results, we can make the most suitable design for the future fusion reactor.
In this way, in the fusion reactor so designed it will not be necessary to regulate individually the electrical current of each coil, as is now the case with the LHD, because we will have the most suitable conditions for the magnetic field and for the placement of coils with respect to the confinement of high-temperature plasma, like the LHD. We will be able to adjust all the coils together. Thus, even if we do not prepare a power supply for all of the coils, if we compose a design that skillfully combines the superconducting coils and can connect all of them in a series, we can operate all of the coils at one time through the electrical current of just one power source.
On the other hand, because the superconducting coils have no resistance, we can gradually increase the electrical current and slowly achieve a large electrical current. In the LHD it takes thirty minutes after turning on the electrical current before that day’s experiments can be conducted. In the fusion reactor that generates electricity, because electric power can be operated continuously for more than one year, there is no difficulty in gradually increasing the current through the superconducting coils late at night. If the length of time that the electrical current is being increased is extended, the amount of power being used can be reduced.
A future helical fusion reactor will be three times larger than the LHD and the energy sent to the superconducting coils will be more than 100 times that sent to the LHD. Thus, simply scaling up the power supply utilized for the LHD will result in great difficulties in realizing our goals. Therefore, in the helical fusion reactor, when applying the design guidelines for slowly increasing the coil current using one power supply, the amount of the necessary capacity was four times that required for the LHD, and was able to be delivered in a practical volume.
In the future, in addition to further advancing our examination of the superconducting coil power supply for the fusion reactor, we will examine energy generation when converting the heat from the fusion reactor into electric power and other issues, and we plan to advance design research for the helical fusion reactor as a power plant.