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August 25, 2014
Efficiently Confining High-pressure Plasma: Research on the Amount of Plasma Leakage

In the Large Helical Device (LHD), research is advancing on stable confinement of high-pressure plasma for a long duration in a nested container composed by magnetic field lines. Broadly stated, there are two reasons why plasma escapes from the magnetic field container. One reason is the phenomenon in which part of the magnetic field container collapses and plasma escapes to the outside as a result of large differences in pressure from the core area to the edge area. This phenomenon of plasma moving outside occurs in an extremely short time, and was introduced in Research Update no. 231, “Let’s Try to Destroy a Plasma” (uploaded June 2, 2014). The other phenomenon is one in which high-temperature plasma slowly escapes through a “void” and a “tear” in the magnetic field lines container. Fluctuations inside the plasma are thought to be a cause of this plasma leakage. To the extent that the amount of leakage is small, we can reduce the size of the future fusion power generation plant. Here we introduce research on the degree of plasma and heat losses.

When we are able to raise the plasma pressure in relation to the pressure of the magnetic field that confines the plasma, we will be able to construct a more economical fusion power generation plant. In the LHD, in plasma of a low pressure value and in plasma of a high pressure value it has been discovered that the quality of the degree of plasma and heat losses differ. In plasma of a low pressure value, the degree of losses is almost completely non-dependent upon the plasma pressure. In contrast, in plasma of a high pressure value, the degree of losses differs greatly in proportion to the plasma pressure. The heat energy that will be generated by fusion will be in proportion to the square of the plasma pressure. In the future fusion power generation plant, from an economic perspective, confinement of high-pressure plasma using the smallest possible magnetic field is desired. Thus, increasing the degree of losses in proportion to the plasma pressure will make a large magnetic field necessary, and may become an issue in realizing an economical fusion power generation plant.

In the LHD, changing the shape of the magnetic field lines container that confines the plasma, we are gradually coming to understand what causes the leakage of plasma by investigating the degree of plasma and heat losses at high pressure value. The most powerful candidate is fluctuations inside the plasma. In that context, on the one hand, the degree of losses increases in proportion to the plasma pressure. On the other hand, we can project that the degree of losses will become smaller when the electrical resistivity of the plasma decreases. Because the electrical resistivity of plasma is in inverse proportion to the plasma temperature to the power of 1.5 and in proportion to the plasma density, the electrical resistivity of plasma assumed for the fusion power generation plant will become much smaller compared to the LHD plasma. For that reason, this projection, in addition to the realization of the future LHD-style fusion power generation plant, is becoming a desirable projection. In order to verify whether this projection will be accurate or not, currently in the LHD we are investigating qualities of the degree of losses at a high pressure level that is equivalently comparable to the future fusion power generation plant for plasmas of electrical resistance values lower than those of plasma used heretofore. In recent experiments, we have confirmed that the observed degree of losses nearly matches the predicted value. This indicates that in the future fusion power generation plant there will be a sufficiently small degree of losses. We will make advances our research, and we expect to bring to life our design for the helical-type fusion power generation plant.