National Institute for Fusion Science

In a magnetic confinement fusion experimental device such as the Large Helical Device (LHD), we confine high-temperature plasma through the strength of the magnetic field. In experimental devices using the inertial confinement method, powerful laser lights from all directions are aimed at a fuel target composed of solid hydrogen, and that solid hydrogen is converted into a high-temperature, high-density plasma state. This is thus called laser fusion. In Japan, laser fusion research is advancing at Osaka University. However, in addition to the important fuel target in this method being in an uncommon state, in order to harden the hydrogen, extreme low temperature technology is necessary. At the National Institute for Fusion Science, as one facet of joint research being advanced together with universities around the country, development of this fuel target is moving forward in joint research with Osaka University. Here, we will introduce technological developments in composing hydrogen ice, which is the target of the fuel. Research is advancing using advanced extreme low temperature technology improved upon through the development of the LHD’s superconducting coils magnet and their operation.

The fuel target, which is the object of current advancing development, is 0.5 millimeters in diameter and hollow, and has the thickness of the external fuel layer of several tens of microns, is of a structure similar to a soap bubble. The soap bubble is composed of soap suds, but the fuel target is composed of hydrogen ice. To produce a soap bubble, breathe in and inflate, and the bubble is complete, if one can do so. However, if one does not make the hydrogen -260 degrees Celsius, it will not become ice. Thus producing ice is not so simple. Research that has developed technologies for making hydrogen ice began by producing vessels that could be cooled to extreme low temperatures. Then, using plastic that could not be broken at low temperatures, in order to be molded into the pellet shape we built a hollow sphere that was slightly larger than the fuel target, and we attempted to affix hydrogen ice of several tens of microns in thickness to the inside.

The target itself is quite small. Because the target is but the size of the ball at the end of a ball-point pen, if we do not use a microscope we cannot see the conditions inside. Further, because this target is inside an extreme low-temperature vessel, we cannot freely touch it. In such a condition, how are we to construct the fuel target? From the results of research we developed a method that uses phenomena that one can see in one’s daily life. Dry ice is a solid matter that freezes carbon dioxide. When the carbon dioxide becomes a liquid while melting that typically cannot be seen. This is because carbon dioxide changes from solid matter directly to a gas. We call this phenomenon in which carbon dioxide does not pass through the stage of becoming liquid, but rather from solid matter to gas, or from gas to solid matter, as sublimation. Under certain conditions, hydrogen too undergoes sublimation and changes directly from gas to solid matter. Using this phenomenon of sublimation, we are attempting to develop technology for beautifully affixing hydrogen ice.

If we place hydrogen while in a gaseous state inside the plastic mold, irrespective of the gravity, the gas will fly around freely inside. Then, if the mold freezes, the gas will sublimate into hydrogen on the inner surface of the mold. Becoming ice, it should adhere beautifully to the inner side of the mold. The goal is for it to become smooth and assume a uniform shape. However, at present, in development there still are numerous irregularities remaining. Our approach is to develop a plan appropriate to the experiments at Osaka University, establishing fabrication methods for this fuel target and developing the research over several years.