National Institute for Fusion Science

In the generation of fusion power in the future, the fuel to be supplied from outside will burn in a high-temperature plasma state. For that reason, it is important to investigate the circulation process of particles (plasmas and neutrals) inside the vacuum vessel, such as the ratio of fuel particles supplied that become plasma, the ratio of plasmas that return to being neutrals, and the ratio of neutrals that again become plasma. The key to the circulation process of such particles is the behavior of particles in the peripheral region. In the Large Helical Device (LHD), we are investigating through experiments the circulation process of particles in a plasma’s periphery using gas, such as hydrogen and helium. We are replicating those results through simulations using a supercomputer. Here, through simulations of peripheral plasma, we will report on research that clarifies the circulation process of particles supplied to plasma.

In the generation of fusion power, similar to the generation of thermal power, power will be generated by powering the generator using the heat generated by the burning of fuel. In the generation of thermal power, of the natural gas and the petroleum ignited, all of the fuel supplied to the combustion chamber will burn. In the generation of fusion power, however, because fuel gas is burned in a high-temperature plasma state, it is not the case that all of the fuel particles supplied to the vacuum vessel will be burned. A portion of them will not burn although having become plasma and will return to being neutrals. Those neutrals often again become plasma or as neutrals escape from the vacuum vessel. In this way, particles, through plasma, are circulating through outflow and supply.

Because the high-temperature plasma is confined in the magnetic field container it is not touching the vacuum vessel wall. However, the low-temperature plasma that has emerged from the high-temperature plasma region led to the place called the Divertor and terminates there. When plasma touches the wall and terminates it returns to being neutrals. Some of these neutrals are evacuated by the pump, and other of these neutrals become the plasma again. Such circulation of particles is called “recycling.” Deeply involved in such recycling is the plasma in the peripheral region, including the area near the Divertor. In experiments we are investigating the circulation process of particles in the periphery. However, it is difficult to measure quantities everywhere in the periphery in an experiment. Further, the ability to undertake direct measurements is limited. Thus, we have utilized simulations from supercomputers, and have made it possible to seek for the distribution of plasmas and neutrals in the LHD.

Using this simulation of peripheral plasma, we investigated the influence of differences in the configuration of Divertor plates that eliminate plasma. In the simulations such comparative research is extremely useful because by changing only the configuration of the Divertor plate and no other conditions we can seek density and temperature in all places. Regarding the results of the simulations, changing the direction of the Divertor plate from a direction in which the plasma is visible to one in which the plasma cannot be seen directly, plasmas that returned to being neutrals on the Divertor plate became more readily able to re-enter the plasma and plasma regenerated more near the plate. As a result, it has become clear that plasma which had touched the Divertor plate and returned to being neutrals increased, and both the plasma density and the neutral particle density on the plate’s front face each influenced the other and increased.

In order to burn fuel efficiently in the fusion power plant of the future, research not only on peripheral plasma that strongly influences the circulation process of fuel particles, but also on only high-temperature core plasma that cause burning is important. In the future, not only cooperation among researchers of core plasma and researchers of peripheral plasma but also comparative research on simulations and experiments is recommended, and complementary work is sought all the more.