For nuclear fusion to occur, plasmas of more than 100 million degrees Celsius need to be stably confined in a magnetic field for long periods of time. A research team led by the National Institute of Fusion Science in Japan and the University of Wisconsin in the United States has discovered for the first time in the world that the turbulent flow of the plasma in a large-scale spiral device moves faster than the heat when the heat escapes. This turbulent feature makes it possible to predict changes in plasma temperature, and its observation may lead to the development of a method to control plasma temperature in real time in the future. The findings were published recently in the journal Nature Scientific Reports.
In hot plasma confined by a magnetic field, "turbulence," a flow with vortices of different sizes, is created. This turbulence causes the plasma to be disturbed, and heat from the confined plasma flows outward, causing the plasma temperature to drop. To solve this problem, it is necessary to understand the thermal and turbulent properties in the plasma. However, turbulence in plasma is complex and not fully understood by researchers. In particular, how the resulting turbulence moves in the plasma is unclear, because instruments that can measure minute time evolutions with high sensitivity and very high spatial and temporal resolution are needed.
A "barrier" forms in the plasma, preventing the transfer of heat from the center to the outside. The barrier creates strong pressure gradients in the plasma and creates turbulence. A Japanese research group has developed a way to break this barrier by engineering the structure of the magnetic field. This approach makes it easier to study the heat and turbulence that flow violently when barriers are broken, and to study their relationship in detail. The researchers then measured changes in temperature, heat and turbulence of the electrons with the highest accuracy in the world, using electromagnetic waves of different wavelengths. Previously, it was known that heat and turbulence moved almost simultaneously at 5,000 kilometers per hour, about the speed of an airplane, but this experiment found for the first time in the world that turbulence moved before heat at 40,000 kilometers per hour, which is close to a rocket speed.
The research has greatly improved the understanding of turbulence in fusion plasma, said Shohui Kenji, assistant professor at Japan's National Institute of Fusion Science. The new feature of turbulence, that it moves through the plasma much faster than heat, suggests that temperature changes in the plasma can be predicted by observing the predicted turbulence. In the future, scientists hope to develop a method to control the plasma temperature in real time on this basis.
