In order to construct a fusion power plant, it is necessary to generate a stable plasma at a temperature of more than two hundred million degrees Fahrenheit in a magnetic field and maintain it as long as necessary to generate excess energy. A research group led by Assistant Professor Naoki Kenmochi, Professor Katsumi Ida, and Associate Professor Tokihiko Tokuzawa of the National Institute for Fusion Science (NIFS), National Institutes of Natural Sciences (NINS), Japan, used measuring instruments developed independently and with the cooperation of Professor Daniel J. den Hartog of the University of Wisconsin, USA, in plasma experiments. They discovered for the first time that turbulence moves faster than heat when heat escapes in plasmas in the Large Helical Device (LHD). One characteristic of this plasma turbulence makes it possible to predict changes in plasma temperature. It is expected that observation of turbulence will lead to the development of a method for real-time control of plasma temperatures in the future.
When high-temperature plasma is confined by a magnetic field, “turbulence” is generated. This means that there is a flow with vortexes of various sizes. This turbulence causes the plasma to be disturbed. The heat from the confined plasma flows outward which results in a drop in plasma temperature. In order 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 scientists have not yet achieved a full understanding of it. One important question is how the generated turbulence moves in the plasma. This is not well understood. It requires instruments that can measure the time evolution of minute turbulence with extremely high sensitivity and extremely high spaciotemporal resolution.
A “barrier” can form in the plasma. This acts to block the transport of heat from the center outward. The barrier causes a strong pressure gradient in the plasma and that generates turbulence. Assistant Professor Kenmochi and his research group have developed a method to break down this barrier by devising a magnetic field structure. It allows researchers to focus on the heat and turbulence that flows vigorously as the barrier breaks to study their relationship in detail. Using electromagnetic waves of various wavelengths, the researchers measured the changing temperature and heat flow of electrons and millimeter-sized fine turbulence with the highest levels of accuracy in the world. Previously, heat and turbulence had been known to both move at a speed of about three thousand miles per hour which is about the speed of a passenger aircraft. The new research led to the world’s first discovery of turbulence moving at speeds of twenty-five thousand miles per hour.
Assistant Professor Naoki Kenmochi said that “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, we expect to develop methods to control plasma temperatures in real-time.”