In the generation of fusion power in the future, in addition to maintaining the burning, a high-temperature plasma will be controlled by supplying fuel gas from outside into that high-temperature plasma. Establishing an effective fuel supply method thus is an important issue. The conditions of how plasma is produced are being investigated by supplying gas particles, which are the source of plasma, through various methods. Here we will report on particle supply experiments conducted in the Large Helical Device (LHD) in which hydrogen ice (solid hydrogen pellets) is injected into a plasma.
In plasma experiments currently underway, typically gas is puffed into a plasma and particles are supplied to that plasma. However, this method is not efficient, and only one in ten of the puffed particles becomes a plasma. The high-temperature plasma is confined so that it does not escape from the magnetic field container, but this is because the magnetic field container obstructs particles externally supplied from entering the plasma core. Thus, the “solid hydrogen pellet injection method” that uses hydrogen ice has been conceived as an efficient method for supplying particles. By lowering the temperature of hydrogen gas to the extreme temperature of -260 degrees Celsius and converting the gas to hydrogen ice (solid hydrogen pellet), and injecting a solid hydrogen pellet into a plasma at the extremely high speed of approximately one kilometer per second, which exceeds the speed of sound, the solid hydrogen pellet can supply hydrogen particles to the core part of the high-temperature plasma while melting and flying through the plasma.
In the solid hydrogen pellet injection method it is important that particles be supplied to the plasma core before the solid hydrogen pellet melts. In a simple calculation, because a solid hydrogen pellet at a temperature of minus 260 degrees Celsius will completely melt in 1/1,000,000 of a second from the heat of a high-temperature plasma whose temperature is tens of millions of degrees, the pellet will advance but several millimeters into the plasma, thus barely even entering the plasma. However, in experiments, the lifespan of a solid hydrogen pellet will be 1,000 times longer and the pellet will fly several tens of centimeters into the high-temperature plasma. And it has been observed that the pellet can supply particles to the core area of the plasma. Why is the life of a solid hydrogen pellet so long? The solid hydrogen pellet can create a gas layer nearby when its surface melts inside a high-temperature plasma. The melting of the solid hydrogen pellet slows considerably because this layer of gas absorbs heat from the plasma. To use a familiar example, even if one spills water on a hot frying pan the water does not readily evaporate, and the phenomenon in which that water becomes water drops and roll around the pan will occur. The heat cannot easily turn into water drops because the surface of the water heated by the frying pan evaporates, and a film of vapor is made between the frying pan and the water drops.
Because the solid hydrogen that has melted is of a higher density than the nearby plasma, it becomes a lump of high-density plasma called a plasmoid when the solid hydrogen is heated still further by the high-temperature plasma. We have clarified in experiments using the LHD that, next, solid hydrogen in a high-temperature plasma expands and fades, and is then absorbed, and that in this absorption process the plasmoid receives influence from the magnetic field and moves. The plasmoid has been observed moving more than 10 centimeters in a direction in which the magnetic field is weak at a highest speed of five kilometers per second. Because we can improve the particle supply characteristics if the direction in which the plasmoid is moving can be changed toward the core part of the plasma, we are at present moving forward with research that combines experiment observation and simulation calculations.
Through such research we aim to establish a method for supplying the highly effective fuel particles that are necessary for the future fusion power generation.