World's first observation of
high-speed moving plasma turbulence
In the Large Helical Device (LHD), turbulence that moves faster than heat as the heat escapes in plasma was observed for the first time in the world, providing new insight into the understanding of turbulence in fusion plasmas. This characteristic of turbulence makes it possible to predict plasma temperature changes, and it is expected that observation of turbulence will lead to the development of a method to control plasma temperature in real-time.
In order to achieve functioning a fusion power plant, it is necessary to stably confine a plasma of more than 100 million degrees Celsius in a magnetic field and maintain it for a long time. The LHD research group observed heat and turbulence using newly developed measuring instruments developed in collaboration with the University of Wisconsin, USA, and observed for the first time in the world that turbulence moves faster than heat when heat escapes in plasmas in the Large Helical Device (LHD). One characteristic of this turbulence makes it possible to predict changes in plasma temperature, and it is expected that observation of turbulence will lead to the development of a method for real-time control of plasma temperature in the future.
In high-temperature plasma confined by the magnetic field, "turbulence," which is a flow with vortexes of various sizes, is generated. This turbulence causes the plasma to be disturbed, and the heat from the confined plasma flows outward, resulting in a drop in plasma temperature. To solve this problem, it is necessary to understand the characteristics of heat and turbulence in plasma. However, the turbulence in plasmas is so complex that we have not yet achieved a full understanding of it. In particular, how the generated turbulence moves in the plasma is not well understood, because it requires instruments that can measure the time evolution of minute turbulence with high sensitivity and extremely high spaciotemporal resolution.
An "internal transport barrier (ITB)" can form in the plasma, which acts to block the transport of heat from the center outward. The ITB makes a strong pressure gradient in the plasma and generates turbulence. Assistant Professor Kenmochi and his research group have developed a method to break this ITB by devising a magnetic field structure. This method allows us to focus on the heat and turbulence that flow vigorously as the ITBs break, and to study their relationship in detail. Then, using electromagnetic waves of various wavelengths, we measured the changing temperature and heat flow of electrons and millimeter-sized fine turbulence with the world's highest level of accuracy. Previously, heat and turbulence had been known to move almost simultaneously at a speed of 5,000 kilometers per hour, about the speed of an airplane, but this experiment led to the world's first observation of turbulence moving ahead of heat at a speed of 40,000 kilometers per hour. The speed of this turbulence is close to that of a rocket.
This research has dramatically advanced our understanding of turbulence in fusion plasmas. The new characteristic of turbulence, that it moves much faster than heat in a plasma, indicates that we may be able to predict plasma temperature changes by observing predictive turbulence. In the future, based on this, it is expected that methods will be developed to control plasma temperatures in real time.
This research result was published in Scientific Reports, a scientific journal of the Nature publishing group, on May 16, 2022.