National Institute for Fusion Science

At the National Institute for Fusion Science (NIFS), we are advancing conceptual designs for the Helical-type Fusion Reactor, which will aim at generating fusion energy in the future. Because this fusion reactor will be three or four times the size of the Large Helical Device (LHD), together with designing a structure that will be strong enough to support the equipment, we must design this considering how to support an object with a helical-style unique twisted shape. However, if we think only of making it sturdy, then the ports necessary for placing parts inside the structure and replacing parts will lessen, and the entire structure will become too heavy. Here, as we move forward in design research for the Helical-type Fusion Reactor, considering such issues we will introduce how the structure that will support the superconducting magnets is being designed.

In the future fusion reactor, in order to confine plasma we will generate a strong magnetic field and send an extremely large electrical current to the superconducting magnets. The electrical current and the magnetic field, as is known from Fleming’s Left-hand Law, work together and generate force, and the force gets stronger as the current and/or the field larger. In the case of a helical fusion reactor, this force equals several thousand tons per meter. In order to cope with the deformation of the superconducting magnet due to the force, we reinforce it by surrounding the whole of the superconducting coil magnets with structural materials. Because the superconducting coil magnets are being operated at an extremely low temperature, the structural materials that support the superconducting coil magnets must similarly be at a low temperature. Thus, in reinforcing the structural materials we use high-strength stainless steel even at the low temperature. Further, in order to construct an extremely large structure by welding the parts together, the parts’ material must be of a thickness that permits welding. On the other hand, in order to ensure the strength it will be necessary to thicken the parts. But because the parts must directly contribute to the weight of all the building materials, if these are too heavy, supporting them and cooling them becomes difficult. Moreover, among the constructing materials, it is necessary to secure space for placing the vacuum vessel necessary for generating plasma as well as large windows for taking out objects from inside the chamber and placing objects inside.

In order to design the structure that will satisfy various demands such as these, it is necessary to calculate in detail how much strength is working upon what parts in the building. More concretely, we will construct a model building using a three-dimensional CAD and divide it into extremely fine elements, and using a computer calculate the strength and shape required for each and every element. In the case of the helical fusion reactor, for its three-dimensional and complex structure, we will divide it into several millions of elements and are conducting calculations. As a result of the calculations, in the event that the strength required of the materials and the shape, for example, are too large, we will sometimes reinforce those places and sometimes convert to configurations in which strength does not concentrate, and repeatedly re-calculate searching for better models for the structure’s objects. In this way, by improving the design, we were able to safely support the great power generated by the superconducting magnets and at the same time we were able to decide the shape of the structure for the helical fusion reactor, which will be lighter and have larger openings. In particular, that we were able to enlarge the openings is an important point in terms of considering the maintenance of the helical fusion reactor’s three-dimensional configuration.

We will examine next such questions as: Can this not be made lighter? Can this not be made in a more simple way? And, can the strength not be increased and the magnetic field raised? Moving toward making the most appropriate helical fusion reactor, we plan to advance our research.