Discovery of high-velocity moving plasma turbulence that exceeds heat movement

Left: Formation of a barrier in the plasma to confirm the heat inside. Right: Breaking the barrier revealed turbulence that moves faster than heat, as heat escapes from within the plasma. Credit: National Institute of Fusion Sciences

To realize 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. 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, using independently developed measuring instruments and with the cooperation of Professor Daniel J. den Hartog of the University of Wisconsin, USA, discovered for the first time that turbulence moves faster than heat when heat escapes in plasmas from the Large Helical Device (LHD). A feature of this turbulence allows the prediction of plasma temperature changes, and it is expected that the observation of turbulence will lead to the development of a method for real-time control of plasma temperature in the future.

At high temperature plasma confined by the magnetic field, “turbulence“, which is a flow with vortices of different sizes, is generated. This turbulence disturbs the plasma and the heat of the confined plasma flows outward, which causes the temperature of the plasma to drop. To solve this problem , it is necessary to understand the characteristics of heat and turbulence in plasma. However, turbulence in plasmas is so complex that we have not yet fully understood it. In particular, the way the turbulence generated moves in plasma is not well understood, as it requires instruments capable of measuring the time evolution of minute turbulence with high sensitivity and extremely high spatio-temporal resolution.

A “barrier” can form in the plasma, which acts to block the transport of heat from the center outwards. The barrier creates a strong pressure gradient in the plasma and generates turbulence. Assistant Professor Kenmochi and his research group have developed a method to break this barrier by designing a magnetic field structure. This method allows us to focus on the heat and turbulence that flow vigorously when barriers break, and to study their relationship in detail. Then, using electromagnetic waves of different wavelengths, we measured changes in temperature and heat flow of electrons and fine millimeter turbulence with the highest level of precision in the world. Previously, heat and turbulence were known to move almost simultaneously at a speed of 5,000 kilometers per hour, or about the speed of an airplane, but this experiment led to the world’s first discovery of turbulence moving past the heat at a speed of 40,000 kilometers per hour. The speed of this turbulence is close to that of a rocket.

Assistant Professor Naoki Kenmochi says “This research has significantly advanced our understanding of turbulence in fusion plasmas. The new characteristic of turbulence, namely that it moves much faster than Heat in a plasma, indicates that we can predict the plasma Temperature changes by observing predictive turbulence. In the future, based on this, we plan to develop methods to control plasma temperatures in real time.”

The research has been published in Scientific reports.

The first experimental proof of the propagation of plasma turbulence

More information:
N. Kenmochi et al, Previous Turbulence Pulse Propagation in Avalanche Events in Magnetically Confined Plasma, Scientific reports (2022). DOI: 10.1038/s41598-022-10499-z

Quote: Discovery of high-speed moving plasma turbulence that outpaces heat motion (May 19, 2022) Retrieved May 21, 2022 from plasma-turbulence-exceeds.html

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