At the National Institute for Fusion Science we are advancing design research for the Force-Free Helical Reactor (FFHR) in moving toward the future fusion power plant. In the FFHR because the size of the superconducting conductor that will generate the magnetic field for confining plasma will be approximately four times that of the Large Helical Device (LHD), numerous engineering issues must be solved in developing this type of immense superconducting conductor. Here we will introduce research relating to one such issue, cooling the superconducting conductor.
We can generate a powerful magnetic field because the superconducting coils have no electrical resistance and a large electrical current can flow. But we can first demonstrate that capacity after having cooled the coils to an extreme low temperature. In the LHD we use two types of superconducting coils, the helical coil and the poloidal coil. The helical coil uses liquid helium and the poloidal coil uses supercritical pressure helium to cool the coils to -269 degrees Celsius. Supercritical pressure helium is in a state that is neither gas nor liquid. We changed low-temperature helium to a state higher even than the pressure called critical pressure by several barometric pressure. Like gas, it is easy to treat. Moreover, as a coolant it has extremely outstanding qualities because, like liquid, it can absorb much heat.
In the helical coils the superconducting conductor is cooled by immersing it in the can filled with liquid helium. In the poloidal coils, supercritical pressure helium is forced to flow through the conductors called the cable-in-conduit (CIC) conductors to directly cool the conductors. The CIC conductor houses numerous extremely fine conductor wire rods inside the stainless steel tubes. Recently, in the superconductor experiment device, due to its high cooling qualities, a CIC conductor is adopted like the poloidal coils in the LHD. However, in the future fusion reactor, it is necessary to improve the cooling qualities because the heat generated inside the superconducting coils increases. At ITER, in France, in order to secure the flow of supercritical helium in a CIC conductor for the purpose of enhanced cooling by keeping a flow path that is 1 centimeter in diameter in the conductor core. In the FFHR superconductor coils, too, a similar type of superconducting conductor is a powerful candidate, and we are progressing with design research.
In the fusion reactor’s superconducting coils, the heat generation inside the superconducting conductor grows large in areas near the plasma. Thus, in the FFHR’s helical coils, heat generation in the conductor closest to the plasma will become extremely great. In order to take even more heat it is necessary to increase the flow rate of the supercritical pressure helium. But the forced flow through the path of about 1 centimeter diameter results in a large pressure difference between the exit and the entrance. If the flow path expands, the flow quantity can be increased, but because the space for the superconductor wire becomes narrow, it will become difficult to secure the necessary number of the superconductor wire. In order to balance them, we had to change the shape of superconducting conductor and conditions for cooling and conduct calculations, and have moved forward with design research. As a result, it will be necessary to have an electrical current flow of 10,000 ampere for each superconducting conductor in the FFHR. For that reason, while securing space for the necessary superconducting conductors we were able to find possible design conditions for flowing the supercritical pressure helium necessary for cooling.
From here we are moving forward with the most appropriate design, conducting trials of superconducting conductors, and planning confirmation of those qualities through experiments.