National Institute for Fusion Science

At the National Institute for Fusion Science (NIFS), in seeking the realization of the fusion energy of the future we are moving forward with research on high-temperature plasma using the Large Helical Device (LHD). However, in order to clarify in a scholarly manner the complicated behavior of plasma, we are conducting fundamental experiments using small-scale devices. One such machine is the linear type high-density plasma generation device, or HYPER-1. University professors and graduate students from all over Japan have come to NIFS to use this device, and they are engaged in joint-research projects that examine the fundamental qualities of plasma. Further, the results of their experiments are enlivened for research using the LHD. Here we will introduce the phenomenon in which the temperature of the electrons in the plasma change suddenly, which was discovered through HYPER-1.

The magnetic field container of HYPER-1 is not of the doughnut-shape like the LHD. Rather, it is linear, and because it has an edge, it cannot confine high-temperature plasma. The electron temperature of plasma generated by using the same microwaves as a household electric oven is, at 100,000 degrees, of a low temperature compared to the LHD, but we can regularly generate in a space 30 centimeters in diameter and 2 meters in length comparatively high-density plasma that are equal to edge plasma in the LHD. Thus, beginning with fusion plasma and interstellar plasma, we can use this comparatively high-density plasma in various types of basic research experiments including spectrum research and light source research.

We call a sudden phenomenon that occurs repeatedly over a period of time “intermittent.” For example, a hot spring that releases vapor and boiling water over a certain time interval is called “an intermittent hot spring.” Because that interval when the hot spring spouts is not fixed we cannot accurately predict when the next spout will occur. A similar intermittent phenomenon was discovered in plasma in HYPER-1 through joint research with Kyushu University and Nagoya University. When one observes that the temperature of electrons in a plasma that are generated in a stationary manner by microwaves suddenly increases, the phenomenon in which the temperature returns to its original 1/100,000 of a second later is intermittently repeated.

When we insert an electrostatic probe in a plasma in the HYPER-1 and measure the electric signals from that plasma, intermittent changes are detected in the same way. Further, in order to investigate the quality of the random and irregular signals, using statistical method, we investigated how long would be the time interval between a one-time sudden phenomenon and the next such phenomenon. According to the results of the statistical investigation of this phenomenon during which it occurs some 100,000 times during the twenty-second long generation of plasma, we learned that when the time interval gets long the frequency falls drastically. Further, the frequency of the fall clearly matches that which is called in mathematics an “exponential fall.” In the background of this phenomenon in which we saw disorder with absolutely no regularity was hidden this type of beautiful mathematical structure.

This phenomenon of generation intervals discovered through the HYPER-1 is measured as a phenomenon in which, even in the LHD’s doughnut-shaped plasma confinement device, particles intermittently eject out from the magnetic field container. This may suggest that there are common characteristics between plasmas with quite different temperature and density. From here on, too, we will move forward with research that aims to clarify this phenomenon through joint research with universities abroad and in Japan.